Daylight On Dreamships Parts 4, 5 & 6

DAYLIGHT ON DREAMSHIPS (Part 4)

By Fred Lindsley

(This article is from SAAA's "Airsport" magazine May/June 1988 edition)

Part one of this series covered general requirements, and parts two and three outlined some simplified methods of estimating performance. In continuing this series, written by Fred Lindsley to assist Airsport readers, we escape calculations, move back to basic drawing, get some cautionary advice plus recommendations for helpful books.

Aerambodynamics

‘Car Wars’ is one of the features at Brisbane’s celebratory Expo 88. It is, of course, in the entertainment section and the irony of its relevance to ever-increasing car insurance rates may have escaped a number of people.

Also, in the small Queensland backwater where I reside, we now have no less than three video rental establishments in addition to the sole electrical appliance shop which sells video tapes. Perhaps other readers have noticed similar developments in their areas. Scrutiny of the wares offered for hire, seven days a week, reveals that few of them do anything to promote the ‘educational and recreational’ activities by which ANOs 100.18 and 101.28 benevolently grant amateur aircraft builders freedom from the much more rigorous requirements of the Air Navigation Regulations.

What is increasingly ominous is the evidence that people are patronising these video rental shops. To such educational affect that little Shine ‘n Whine ‘n Jyson can now tell the difference between an M-1 carbine and an AK 47 at what might be defined as ranging shot distance.

Those who can find time between changing video tapes to do a little reading may have noted a recent item. The long-established Airfix organisation is re-issuing no less than 126 of its plastic model aeroplane kits. The punch line is that not one of the kits features a civilian aircraft.

Evidently the times they are achanging. Today Rambo-ism is a growth industry. If you want further evidence of how newly acquired mental images can affect uninvolved innocent parties, and even dreamship doodlers; consider in retrospect two recent Melbourne events. Enthusiastically publicised by the media as massacres and the first one, just to demonstrate that we are not deviating from an aviation oriented flight path, occured within weapon range of SAAA HQ at Clifton Hill.

Now dreamships are usually motivated by mental images of realisable reality. So it is not too difficult, with the portents so clearly evident, to envisage a couple of near future dreamship scenarios.

In the first scenario you are flying south to Mangalore. With the sun at your back you are therefore theoretically in a position of tactical advantage. The sabre scar on your cheek wrinkles as, with a sardonic smile, you raise a monocle to your eye to scan the sky and surrounding terrain. Ritter von Ochskrapp searches for victims worthy of a well-acclaimed predatory skill.

You disdainfully ignore all those white fibreglass aircraft as totally unworthy of your experienced finesse. Those poor misguided dupes are flying aircraft where they are not permitted to paint even registration letters on the top of all their numerous wings. Even worse is the fact that those craven curs have spent countless weeks adding a hundredweight of bog and then rubbing it down to a very peculiar approximation of the designer’s original (and maybe critical) laminar flow airfoil. Probably at more detriment to the lungs than any tobacco manufacturer’s advertising at any televised sporting fixture. All a most deplorable waste of time which could more pleasantly have been expended on speculation involving paint manufacturer’s catalogues.

Another reason for ignoring those white fibreglass airplanes is, of course, the fact that they cruise a lot faster than you do, have sneakily crept up from behind and are now rapidly disappearing in the distance towards Mangalore.

But the von Ochskrapps are duty bound to seek more subtle adversaries. Namely all those Corby Starlets sufficiently well camouflaged to justify their participation in any of the 25 conflicts currently in progress around the globe.

Achtung! There’s one of them! On the same course, about one nautical mile ahead and 2000 feet lower.

The Starlet pilot is flying as if he didn’t have a care in the world except the gnawing anxiety that the light-earth camouflage at his port wing tip does not exactly match the local Victorian landscape.

Muttering “Befehl ist Befehl” you dive on the enemy and execute a smart barrel roll around him. The Starlet pilot gets a startled glimpse of lozenge camouflaged wings, dazzle painted fuselage, and German crosses everywhere else including the canvas wheel covers.

As you zoom past your victim with a derisory rocking of your wings you glance in your rear-view mirror and observe, with chagrin, that the Starlet pilot is not playing the game. He hasn’t got four dummy cannons disrupting the airflow at the leading edge of his wings. A real dummkopf or, at the very least, a pusillanimous poltroon. You convince yourself that honour is satisfied, proving once again that the Jagdgeschwader Juvenile is not to be trifled with.

With a lightning quick change act from aggressive biplane to menacing low-wing monoplane we zip to scenario two. You taxi up along the barrier at Mangalore, switch off, wind back the canopy, nonchalantly peeling off your flying gloves and stand up in the cockpit. The noise you hear through your partially deafened ears is the tumultous applause of the multitude.

Before you can step out of the cockpit the crowd has broken down the barriers and surround your aircraft with a rising crescendo of excited twittering with an accent on the first syllable. Particular attention is focused on the pressure can fake exhaust streaks which artistically avoid besmirching the name painted on each side of the fuselage. Walter Mitty. With the panache of a Picasso you located the fake exhaust streaks aft of the exhaust stubs which are responsible for your partial deafness, the carbon monoxide which is causing your headache, and the lack of a carburettor heater attached thereto which may get you into a heap of trouble one day.

More than half the admirers are crouched under the wing, crying “Ooh, Aah” as they caress all the hollow fibreglass armaments which encumber the lower surface from wingtip to wingtip. Of course you virtuously put these in with a modification in compliance with ANO 100.6. Because of the current rapacity of the Department, who are now charging fees, you put the whole lot in as one modification. In a triumphant example of using the ANOs to beat the system you described the dummy Sidewinder missiles on each wing tip as wing anti-torsion dampers.

The above comment in lighter vein is merely to draw attention to a more serious matter. The adverse effects of 365 days per year exposure to coloured electronic images is being increasingly commented on by an impressive number of sociologists throughout the world. Rambo needs to be recognised, by garb or similar equivalent to the war tattoos of primitive tribes. Should his self-esteem be spumed he may, with a weapon in his hand, take his revenge on civilisation which includes you and me.

In the field of sport aviation, for dreamship developers and existing amateur builders, there are signs of what might be called aerambodynamics being on the increase. It essentially consists of giving priority to the establishment of external images. The quasi-military camouflage (not exactly the best anti-collision device ever invented) is more important than the physical laws controlling the flow of fuel through what is alleged to be a carburettor hidden by the engine cowl.

The two scenarios also draw attention to one other important point. With dreamships it is always YOU in the cockpit, juggling with the shining levers. Which shows that we were actually on track all the time because we are now back to the drawing board. There is no other effective way to ensure that, when your dreamship is finally built, YOU will be able to fly it in comfort and with safety.

Back to the Drawing Board

In some modern jet airliners the total passenger weight, with every seat occupied may be as little as 10 percent of the Maximum Take Off Weight (MTOW). Providing the passengers are comforted with in-flight movies, and adequate toilet facilities, their accommodation does not present any notable design hassle.

In the sort of aircraft we are interested in, the situation is very different and a bit more awkward. For one thing the occupants can be more than 30 percent of MTOW. (In two-seaters to ANO 95.25, two average weight occupants amounted to 38 percent of MTOW). Their precise location involves considerations affecting centre of gravity travel and stability.

Furthermore, in our sort of aircraft the occupants are usually less than an arm span from lots of significant hardware. Such as wing and strut attachments, landing gear attachments, engine mountings, flying controls, a battery, instrumentation, assorted avionics, and maybe a fuel tank. The pilot not only has to sit there and not fall out, he or she has to see where they are going, particularly on landing in taildragger machines, but additionally be able to reach or grab in a hurry any shining lever needing to be juggled.

You will also be interested in ensuring that the fuselage is not too narrow or, if a cabin type, too low. This affects cross-sectional area (among other things) which is related to drag and hence eventual performance. So this is a good time to look at this most important part of your dreamship.

Let me tell you at once that sitting in a chair clutching a tape measure will not work. There are only two effective methods of tackling this task. The first is to make a full size mock-up out of firewood, cardboard and fencing wire. If you are of average height you will also have to enlist two assistants closely aligning with the statistical dimensions of large and small occupants. The second and best method is careful scale drawing, in pencil, and on fairly robust tracing or ordinary paper because you will not believe the amount of rubbing out you are letting yourself in for.

Figure 1. Dimensions in millimetres for a set of pilot layout templates at a scale of one fifth full size. Other scales can be made by proportional change of the tab­ulant dimensions.

Make from thin cardboard. Insert a drawing pin at each articulation. Add a 12mm diameter piece of cardboard to the pin and then bend the pin over at 90 degrees.

The first article in this series suggested the use of small size paper, down to around A4, for preliminary dreamship layout at rather small scale. Unfortunately small scales are hopeless for effective cockpit layouts and I would suggest that 1:5 be regarded as a minimum. You will definitely need some articulated pilot templates. Figure 1 shows their general layout and how to make them. To do the drawing properly you must have a set of three templates. One of my sets, at 1:4 scale, is old enough to qualify as an exhibit in a toy and doll museum, but they still work when borrowed by some of my Sunshiner colleagues.

While the side view is the most important, because that’s where the templates get used, you must also give attention to plan and front (or rear) views. In a side-by-side seater can you reach something on the starboard side of the instrument panel? Effective arm reach in plan view is a good bit shorter than in side view. In any pusher with a fashionable needle nose, will a foot go through that superb double curvature skin long before a rudder pedal, adjusted for a tall man, gets to full forward travel? In a side-by-side seater does a cross section of that expensive blown canopy reveal that the top of a tall pilot’s head is liable to be frost bitten out in the slipstream? Does forward stick bang your knuckles on the instrument panel? Or backward stick arouse surmise about impotence? At the cost of a few pencils (and erasers!) you will learn much to your advantage from this sort of layout drawing.

Note that Figure 1 shows the approximate location of the horizontal and vertical CG of a seated person which is in the vicinity of the navel. Occupant CG is extremely important when it represents a big chunk of disposable load. With tandem two-seaters it is customary to perch the passenger so that only pilot weight (neglecting fuel use or retractable gear) influences aircraft CO travel. There are exceptions like the Magister and Moth Minor which were flown solo from the front cockpit for reasons outside the scope of this article.

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Side-by-side seaters with pusher engines are invariably a bit of a CG hassle and, if you are not very careful, can finish up being unable to be flown solo without ballast - like one well-established commercial amphibian. Figure 2 shows basic requirements for downward vision, seating and lapstrap/harness layout. The minimum downward vision sightline applies in azimuth. In other words it’s no good giving a port side pilot the minimum angle straight ahead because he is looking over a low part of the instrument panel. The pilot has to have the minimum angle when he is looking to starboard over the hump in the middle of the instrument panel.

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It is desirable to have adjustment of the rudder pedals rather than the complication of moving the seat fore and aft. Otherwise a tall pilot will have his knees so high that it is both uncomfortable and can restrict emergency sideways movement of the stick. If there are vision problems with small pilots it may be desirable to arrange for vertical adjustment of the seat. The provision of thick cushions to solve this problem is now frowned upon. They may not be available when needed and there are warning notices that thick resilient cushions can cause spinal injuries.

In our sort of aircraft, belt attachments must be made to the aircraft structure and not to the seat. Seat attachments feature only in airliners, which usually have lapstraps only, and the seats are specially designed for the purpose. Full harness attachment to the seat applies to military aircraft which have ejection seats, massive and extremely expensive hunks of engineering.

With harness you should work to the proper design requirements and develop a lively awareness of the G loads which apply in various directions. In certain cases the positioning of the lapstrap may require an additional fifth belt as installed in some sailplanes and aerobatic aircraft.

With varying size pilots (which is what the templates are for), keeping belt angles within the prescribed limits can be a bit of a problem. It is much easier to sort this out on the drawing board than to face the prospect of having to modify an already built fuselage. With tall pilots it is absolutely essential that the shoulder angle is not more than the 5 degrees below the horizontal shown in Figure 2. Otherwise under forward G loads, even without any vertical velocity component, the much despised triangle of forces will apply downward compression loads to the spine.

This has too frequently caused death or severe permanent spinal damage in accidents which otherwise would not have caused injury. Later in this series we shall be hearing more about the triangle of forces which is fundamental to stressing.

Beware the Bewdy Boshkosh Syndrome

What’s this heading doing here? Because it deals with a matter equally important to dreamship doodler and active or intending amateur builder.

The ailment causes money to be wasted, projects to be delayed or abandoned, and acts against the best interests of the most technical of all leisure time activities. It exists, is ignored, and may yet emerge, belatedly, as a generally recognised contagion. Not too dissimilar to another malady, virtually unheard of four years ago, which is today a daily news item in most branches of the media.

For a good many years I’ve been saying that I have no recollection of meeting any traveller to Oshkosh who would not have learnt just as much, if not more, by sitting at home and reading about it. I am now confirming this view in writing. By the same token I am suggesting that if you really want to learn something about the Pyramids, as distinct from using them as a conversation piece, you would be better off with an hour in your local library than in enduring the expense, heat, dust and flies of that short journey south of Cairo.

It should be made quite clear that no denigration of Oshkosh fly-ins is intended. Since the early 1950s EAA, voluntarily supported by a vast amount of aviation expertise, have done a magnificent job in re-activating USA amateur aircraft building from a previous 20 year limbo. The cause of which could well repay study because the 20 year hiatus commenced a good many years before the USA got involved in the 1941-45 hostilities.

EAA also expended large resources, including the introduction of new publications, to assist the ultralight community before reducing such support. The USA AOPA also spent US$750,000 in similar assistance before opting out. Both organisations may have made the same re-discovery, the reason why USA amateurbuilt aircraft activity came to a standstill in the 1930s (except for the State of Oregon), that it is almost impossible to educate the irresponsible. More restrictive legislation is the only socially acceptable solution, and the ‘goodies suffer because of the baddies.

Should you contest this viewpoint, just first take a good hard look at Australian ultralight history since the introduction of ANO 95.10 in September 1976.

There are no problems with Oshkosh fly-ins, and if visits there were made on the same basis as a trip to the Taj Mahal there would be no problem back here in Australia. Unfortunately it doesn’t seem to have worked out that way. What the vigorous EAA and their well-supported fly-ins represent gets totally distorted by a misplaced faith in education by geographical association. The returned pilgrims become gurus. sure sources of technical wisdom because they’ve been there. This inversion of reality is the Boshkosh Syndrome.

The reasons and symptoms are not difficult to detect. Unless you have an understanding accountant, are subsidised by the taxpayer. or have some eager shareholders convinced that your visit to Wisconsin will yield them the Yale key to making a fortune in aviation, it is very probable that your trip will cost you a substantial amount of personal cash. Over your dead body are you going to admit that one cent of this cash was wasted.

Any of the following can be recognised as warning signs:-

An enormous quantity of coloured photos not identified on the back as to aircraft type, category or construction. I can’t remember what that one was but, gee, it looked neat”.

Should the designer of your favoured aircraft have been present, back here we get a deluge of name-dropping. “So I said to Elmer” and “Elmer told me

There will have been contact with a little old man who welds up variable pitch propellers from Coca-Cola cans and Buick differentials. There will be a stash of coloured photos, or even a video tape, adding credence to this miracle. Last year it was a different little old man who welded up aircraft wheels from Oldsmobile front axle stubs and Chevrolet hub caps.

The Boshkosh Syndrome victim does not have a worthwhile technical book in the house, despite his ex cathedra advice on what ought to be done about every part of every aeroplane. But he has piles of magazines loaded with coloured pictures. They remain unread and the thought never occurs to extract any useful technical information, file it, and give the rest to a worthy cause like the boys of the Air League.

And so on.

Now let us look at some supporting evidence, always a commendable philosophy when the intention is to avoid future contretemps in aviation. Although SAAA has been instrumental in obtaining type approval for around 80 different amateur built aircraft, there are a large number which have been submitted which have not yet made the grade and some never will. Too many of these are mouldering or corroding away all over Australia and there may be few readers of Airsport who cannot point their finger at one such backyard monument in their area.

These, to put it kindly, dormant projects, represent significant inputs of enthusiasm, labour and money. In cases where the authorities have given written warning that the builder proceeds at his own risk, and then have very decently leaned backwards to conduct stage inspections, these dormant projects also represent expenditure of the taxpayer’s money too. This is where the Boshkosh Syndrome may one day begin to hurt.

If you check on the dormant types what are you apt to find? The best of Boshkosh most decorated wheel hubs. The best of Boshkosh tricycle to taildragger conversion. The best of Boshkosh space shuttle replica. (The names have been changed to protect the guilty).

Not a cane toad’s migration march from this typewriter we have three examples resulting from pilgrimages to Wisconsin. In total they comprise quite big bickie investment. One of them, to the tune of five figures, is never going to get anywhere. Another, all ready to fly for a long time, does not comply with any ANO. The third, which has been on the shelf for 14 years and is currently being assessed for kiss of life resuscitation was, not surprisingly, the best of Boshkosh tiny nosewheels one year. The builder, not surprisingly either, has made no less than four pilgrimages to Wisconsin and may now be impervious to any suggestion that he might be a clinical textbook example of the dreaded Boshkosh Syndrome.

Dreamship developers beware. Enthusiasm is essential in this business. Aviation is the only major industry founded by enthusiasts and indeed, up to 1914, was almost entirely controlled by amateurs. But a notable example of enthusiasm in free flight is the spectacle of lemmings jumping over cliffs, a well-authenticated periodical event in northern Scandinavia.

So let us temper enthusiasm with a bit of technical reality. Steer clear of the Boshkosh earbashers.

Recommended Reading

At Mangalore I was very pleasantly surprised by the number of people, including a charming lady, who voiced kind and congratulatory words about this ‘Daylight on Dreamships’ series.

There were also a good few requests for of titles of suitable books. This poses quite a problem because it was the lack of useful modern books which prompted this series in the first place. Now while hundreds of books have been published on technical aspects of aviation, very few of them are in what I would consider the ‘approachable’ category, meaning books that don’t frighten me too.

Out of print books can usually be tracked down in the archives of State Libraries and there is an inter-state lending system where they can usually get what you want from another State Library. It is recommended that you initially read, instead of buy books which still may be available for sale. Many books prove different to what you might anticipate from the title, and I’ve made a few expensive boobs because of that. Roughly speaking, the older a book is, the more helpful it will be for dreamships.

The most useful all round book, mentioned in the first of this series is:

Man Powered Flight by K Sherwin, Argus Books 1971. Believed to be out of print but I’m advised there are still a few around in specialist bookshops.

Known to be in print:-

The Design of the Aeroplane by Darryl Stinton, Granada 1983. Big, extremely sound, all aerodynamics and performance and little on structures. There is also The Anatomy of the Aeroplane, by the same author, about 20 years earlier and recommended.

Understanding Aircraft Structures by J Cutler. Granada 1981. Will not tell you how to stress your dreamship but may be considered essential in order to understand why the aircraft engineer has to start looking at structures in a rather different way to the engineer who designs steel or concrete bridges.

Theory of Wing Sections by Abbott and Doenhoff, Dover 1958. The standard textbook.

Ultralights by F G Bailey. Macdonald Futura Australia 1985. The best book on the subject.

Design for Flying by D B Thurston. McGraw Hill 1978. Unreservedly recommended. Design for Safety by D B Thurston. McGraw Hill 1980. Equally recommended.

Modern Boatbuilding by Steve Sleight. Conway Maritime Press 1985. Possibly the best book on all the composites, adhesives and finishes. (There is a fair amount of potentially dangerous rubbish talked about composite aircraft by people who don’t seem to have read anything about the engineering applications and problems associated with composites. Including Notsoeze builders who have the exhaust pipes rubbing on the fibreglass engine cowls).

Flight Without Formula and Mechanics of Flight, both by A C Kermode. Also by the same author, The Aeroplane Structure. All have been published by Pitmans in numerous editions since 1932. The latest editions are metric so go for the earlier ones.

Out of print:-

Aircraft Structures by D Peery. McGraw Hill 1950. The 1st edition. The 2nd edition of 1982, by Peery and Azar, is not so useful.

Introduction to Aeronautical Engineering. Volume II Structures. Volume III Properties and Strength of Materials. Both by J D Haddon. Pitman 1934.

Materials of Aircraft Construction by F T Hill. 3rd edition. Pitman 1937.

Aircraft Design, Vols I and II by C H Latimer-Needham. Chemical Publishing Co 1939. Very easy to follow. Written by the designer of the Luton Minor and Major.

Fundamentals of Aircraft Flight by F K Teichmann. Hayden Book Co 1976. A most excellent introduction with numerous worked out examples.

Landing Gear Design by H G Conway. Chapman Hall 1958.

Airplane Performance, Stability and Control by Perkins and Hage. John Wiley 1949. The standard textbook.

Engineering Aerodynamics by W Diehl. Ronald Press 1936. Another standard classic and still right up to the minute for our sort of aircraft.

The How and Why of Aircraft by Stevens and Allward. Putnam 1952. The best book ever published to explain things in simple language, including jet engines.

It is re-emphasised that this ‘Daylight on Dreamships’ series is merely a simplified guide to the basics. Any cautionary advice given, no matter how coloured to stimulate reading, is very firmly focused on the essential function of amateur aircraft building. Namely the saving of money or more importantly perhaps, not wasting your money on dead ends or aborted projects.

Should it come to my attention that this series is being treated as entertainment and being read for a giggle by people who haven’t the least intention of sharpening a pencil or tapping a pocket calculator, the series will abruptly cease. It can be taken over by the gigglers who, I rather imagine, will waste a lot of time discussing how to put it on video.

It would not be surprising if a nation-wide survey revealed that the video addicts among us today also rank highly among the perpetrators of the too many aborted backyard monuments, aircraft types both approved and otherwise over a period extending more than 20 years. If you want to transform a dream into a nightmare just try to make an estimate of the time and money which has been wasted. Then make an additional estimate of how much taxpayers money has gone into the too frequently maligned Department’s assistance with ANOs, advice, correspondence and stage surveys. Then re-read this article.

And then read one of my articles in the December 1983 Airsport. I have some confidence that serious dreamship devotees may detect a prophetic glimpse of aerambodynamics on page 17.

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Dreamship NSW I.

Contributed at Mangalore by W.J.S. of Berowra Heights. NSW.

This reduced scale sketch, redrawn to suit magazine layout purposes, does less than justice to the thoughtful work put into the original drawings.

Except for the 6” x 6” square aluminium alloy tail boom, intended construction will be foam/fibreglass. The wings are detachable to give a width of 7 feet for trailering. The rectangular panels in the cabin roof are the location of a recover parachute which is ingeniously anchored by HT steel cables to the firewall and via the hollow front windscreen support, to a forward fuselage bulkhead. A simple brake will stop the propeller rotating before deployment of the parachute. The transparencies are wrap-around flat panels and require no moulding.

An observation is made about wing-tip fuel tanks which can introduce fuel management problems. Just one degree of one wing low will give 5.5 inches difference in tank levels, and fuel may exit from the vent of the low tank. This can entail more sophisticated vent systems.

Additionally. rolling in turbulence causes the tip tanks to act as parts of the rim of a big flywheel. The outward centrifugal force on the fuel in the tank, plus the negative head from the resistance of long fuel pipes, plus the suction head of a high mounted fuel pump can result in vapour looks, notably with mogas fuel.

Congratulations to W.J.S. who will solve these problems the proper way, with a pocket calculator at the drawing board. The “bugger-the-poofter-drawings-I-can-do-it-all-at-the-anvil”, real practical men may be less fortunate. The reality of the problems may be revealed to them, in flight, under circumstances accompanied by acute alarm.

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Dreamship WA 1.

Contributed by an experienced pilot who, quite reasonably asks why should many amateur built aircraft confine themselves to speeds within the capability of modern family cars?

Outside the limitations of ANO 101.28, this dreamship should meet the experimental requirements of ANO 101.31. It will be a pressurised 3 seater, the seat behind the pilot being rear-ward facing. Performance is quoted as “300 knots IAS at 20.000 feet or higher, therefore with a ground speed of 400 knots or more”

Wing construction would be of wood with full span flaps, the outer sections acting as flaperons as on the Victa Airtourer. Aluminium alloy is favoured for the pressurised fuselage. The use of an automotive engine turning at 6000 RPM and driving a shrouded fan is being investigated as an option for the pure jet.

As received the horizontal tail surface aligned with the centre-line of the jet tail pipe. In this sketch the component has been raised to minimise unauthorised heat treatment of the tailplane spars. Otherwise the sketch is essentially as originally submitted.

FOUR PLACE DREAMSHIP

(This article is from SAAA's "Airsport" magazine May/June 1988 edition)

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Many thanks to Fred Lindsley for the ‘Dreamships’ articles. For some time I have nursed the glimmer of a design. Not that I am ever likely to actually BUILD anything, but others might like to use some of my ideas.

When I first became interested in Amateur Building, my dream was to build a four seater something for the family. There are plenty of designs for one and two-seaters, but such aircraft are essentially ‘selfish’.

Let us commence the design process from the ‘wrong’ end. It is a topic which has used much Airsport ink in recent months — what do we use for an engine?

Given that we wish to avoid the now hard-to-obtain and expensive aircraft engines, and provide good reliability, and we need 130HP plus, we seem to have run out of options. But wait. In most previous articles on the subject a SINGLE engine is assumed.

Many will argue that VW conversions are not reliable enough for an aircraft which may be expected to travel cross-country. But look what happens with TWO engines. Some simple statistics.

First, let us define engine failure. For my purposes it is a complete loss of power while airborne, which with a single engine results in a forced landing. Exclude Alan Taylor’s ‘automatic rough’ which results in only reduced power, and exclude faults which are, or should be, found on the ground during runup.

Let us assume that our engine has a Mean Time Between Failures (MTBF) of 500 hrs. (I know it should never be that bad.)

Now when we fly our twin engined aircraft, every 250 hrs ON AVERAGE we will have one engine die. We must now continue on one engine to the next convenient landing place. Assume an average of one hour’s flying is required when this happens.

The MTBF tells us that in this hour, there is a 1/500 chance that the other engine will die too. This means that on average we will be faced with no noise at all once in every 125,000 flying hours! What is the MTBF of your Lycoming?

This analysis demonstrates why two moderately reliable engines are a far safer bet than one engine which is ‘twice as reliable’.

For the analysis to be true we must have:—

1) Independent fuel systems

2) Independent Ignition (of course) and electrics.

3) Independent props and reduction drives, if used.

4) A single engine service ceiling of at least 3000ft

5) Good ‘engine out’ handling characteristics.

One aircraft now being built in Australia does have all of these characteristics. It is of course ‘maestro’ Rutan’s DEFIANT. About the only events which could force a Defiant down with simultaneous engine loss are:—

Run out of fuel — read Pilot Error.

Dirty fuel - read bad ground handling/pre-flight.

Massive icing — read Pilot Error.

Finger Trouble.

So why not simply build a Defiant? Because it is too big, too heavy, too fast and too powerful! It should not require 300hp to fly four people — 150hp is plenty, even 130 if one accepts a more modest - and affordable - performance.

Is it possible for two VW’s to carry four people and some light luggage at around 120-125 Kts for 500-600 miles? Easy! All one needs is two Dragonfly’s or KR-2’s in formation!

These two very efficient designs give us base figures for wing loading, power loading and achievable performance envelope.

A Canard because it is inherently more efficient - the stabiliser provides lift instead of increasing the wing load and drag. We cannot use Dragonfly’s equal area Canard layout though. It is intolerant of fore and aft movement of the CG. It requires the passenger(s) and fuel all to be essentially at the CG.

The Defiant planform is a better guide. The rear seat passengers and the

fuel are at the CG (the fuel in wing leading edge tanks of course). The major variation in CG is caused by the presence or absence of the co-pilot.

A smaller and lighter aircraft with a much lower wing loading than Defiant, my design has a number of other differences.

Why the weight and complication of retractable nose gear, plus a rhino rudder? Save weight by using a pivoted wheel fairing which acts as the rudder. Pilots can’t forget to lower it either.

Now for directional stability. The wing tip fins on EZEs act as both tail and rudders. Do they work in the con­ventional way?

We usually think of the ‘lift’ forces acting on a fin providing directional stability. But with an EZE, the wing tips are not very far behind the CG, and they are very small. DRAG forces on these little fins are also important.

Graph the drag force on a symmetrical airfoil at changing angles of attack. It forms a ‘bathtub’ or roughly parabolic curve. Now draw the curves for two such fins angled inwards by 2 or 3 degrees. The drag forces are small but equal at no yaw.

As this pair of fins is yawed left, the drag on the left fin will reduce while the drag on the right fin increases sharply. If the two fins are wide apart, the difference in drag forces will have a significant moment providing yaw stability.

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EZE’s wing tip fins may not point inwards - some do, some don’t - but drag is a consequence of RELATIVE airflow, and the air flow inwards across the top of any wingtip, giving the same effect as pointing the fins inwards in an undisturbed airflow.

The main spar supports landing loads, with oleos in a streamlined fairing angled to give directional stability.

To permit the use of Mogas, the leading edge tanks would be aluminium ‘wet’ tanks.

The Canard is tapered to increase strength to weight. The Aspect Ratio of most GRP designs (9-10) has been reduced a bit to 7.5 to reduce the overall span.

That’s enough from me, or I’ll be asked to put up as well as shut up!

Martin Greenwood W.A.

DAYLIGHT ON DREAMSHIPS (Part 5)

By Fred Lindsley

(This article is from SAAA's "Airsport" magazine September/October 1988 edition)

Continuing this series written for Airsport readers, Fred Lindsley contributes an elementary introduction to the strength of aircraft structure.

This subject will be amplified in subsequent articles.

Getting With The Strength

One of my remoter relatives, now retired, got involved with the march of progress from two-bay biplanes to four-jet airliners.

Before accelerating speed in the march of progress ensured that every minute of X-ray exposure, or squinting into a bore-scope, resulted in nearly as many hours being subsequently expended on completing the associated paperwork, he had spent a considerable number of years dealing with the practical nitty-gritty on quite a large number of aircraft types including gliders.

Such routine chores as standing perched on high trestles, in bleak places, removing and replacing assorted pieces of engine. Or contorted down the back end of oven-temperature fuselages, contemplating suitable methods of torture mayhem to be inflicted on the character who wrote the section on flying controls in the manufacturer’s manual.

In those days he had, on the lid of his toolbox, a painted notice of some fame. It read, “Before opening toolbox, use brain”.

Most assuredly, where the strength of aircraft is concerned, that notice is particularly good advice to both dreamship enthusiasts and those amateur builders who are in the throes of construction. Even perhaps a few builders who have already completed their aircraft and are suddenly wildly enthusiastic about departing from the approved drawings by doing an innocuous teeny-weeny little modification which, they decide, is obviously so minor that they don’t need to tell anybody about it. Except all their mates who are expected to grovel in admiration of such superlative design genius. Moreover, marvellous to relate, all done without laying pencil to paper.

Let us suppose that you can at Mangalore convene all the professors and all the computers in Australian universities, plus a semitrailer load of software. Confront these pillars of Academe with as many amateur built aircraft as can be flown in. Then ask these professionals to start producing a stress analysis on any single aircraft, and see what happens.

The experts will, in an intellectual sort of way, give you the finger. The more compas­sionate ones, accustomed to suffering fools gladly in modern universities, will gently explain that without drawings and data they are disinclined to start up the diesel engined alternator to drive their million dollar computers. Because only the drawings will tell them whether that Boshkosh Bummer has its spar flanges made out of two inch by one inch California Redwood, or an unknown dimension using the Constitution of the United States reconstituted as recycled pulp.

Indelicately exposing another finger these devoted exponents, of the experimental process, might raise queries about what sort of design requirements would have to be manipulated into their computer programmes.

MORAL

You can’t stress aeroplanes just by looking at them. Where strength and airworthiness are concerned, no amount of computer, hand tool or machine tool equipment is the slightest bit of use until the brain is switched on. Plus a pencil, some paper, and a dedication to either very tedious longhand arithmetic or some time-saving device like a sliderule, pocket calculator, or a Cray computer. With the reflection that with computer printouts it can sometimes be very difficult, and extremely time-wasting, to check exactly where the errors occur in the programme.

Dreamship devotees, whose family bud­gets seldom extend to Cray computers, might take heart from a study of the North Ameri­can X-15. First flown in June 1959, three of these research aircraft over the next ten years logged 199 flights, all descents and landings being made without power. Maximum speed Mach 6.72 or 4,520 mph. Maxi­mum altitude 354,200 feet (and still a world record).

Designed with the use of ye olde worlde sliderules and, despite its experimental cate­gory, not just by building around some chalkmarks on the workshop floor. By 1968 the X-15 programme had resulted in 766 technical research reports, which suggests that computers were beginning to rear their ugly paper-consuming heads.

The Bittaboolchit Hangar Structure

The Sunshiners, famed in sad song and apocryphal story, started scratching plans for a new hangar. The existing hangar, noticeably paying homage to a long-deceased architect in Pisa, maintains its inelegant outward form by virtue of a rare biological phenomenon. Not only are the termites holding the structure together by linking their arms and/or legs, but they are simultaneously clasping them in prayer. Because even the dimmest witted termite can perceive that starvation is nigh.

Naturally the primary purpose of the new hangar will be protection from the scorching sunshine of all the interminably discussed beaut finishes (now conversationally drifting into the ‘neat finish’ mode). Unfortunately the Sunshiners, vociferous advocates for an Experimental Category, have discovered that the land on which a new hangar is proposed entails many awkward requirements. Such as to virtually prevent the Sunshiners from building an experimental category hangar in any manner or from any material which takes their individual passing fancies.

The Sunshiners are beginning to discover that this is where structures begin, that basics are the foundation of the business. Plus, their shattering disbelief, more building regulations than all the ANOs they have for long and so proudly boasted of evading. Worse still, there are now some building inspectors, new to the locality, whose ambitions for continued employment and promotion are cultivated by a devout adherence to the official requirements.

Apparently no scope for experiment at all. There shall be no quicksands under one corner of the hangar. The concrete slab will need so many little bits of bent iron, dimensioned on a piece of paper forsooth, in order to cope with specified area loading. Plus the concentrated parking load of the gift steamroller used for trying to extend the somewhat restricted runway by rolling it longer, like pastry.

Moreover, as soon as the projected hangar starts projecting its structure into the atmosphere, there are building requirements about aerodynamic loads which, a few minutes of latitude further north, miraculously increase to cyclone velocity.

The Sunshiners are very lucky that they are not living in one of those inclement parts of the world where the local requirements spell out high winds plus the weight of two metres of snow on the hangar roof.

So far the Sunshiners are just glumly postponing the structural strength part of the deal because there is a farcical requirement for drawings. Before they spend money and start building, if you ever heard of anything so ludicrous! The Sunshiners’ views are going to be quite interesting when they discover the additional requirements for electrical reticulation, stormwater drainage, and ventilation in toilets for the disabled which is mandatory requirement under certain circumstances. It seems that no way are they going to be permitted to build an experimental category hangar.

The crushing blow that really subdued the Sunshiners (only temporarily of course), was the sheer agony of having to read something, just to find out what they couldn’t and shouldn’t do. For Pete’s sake, it’s only a hangar. It’s not as if it was a home made aircraft!

Structures In Motion

It is acknowledged that many readers will see the above section on a notional hangar as a blinding glimpse of the obvious and “Everybody knows this”. Do they? The Bittaboolchit edifice was chosen as an essential starting point for any reader hoping get to serious grips with the subject instead of just glibly quoting from magazines G factors which sometimes are suspect.

The point is that hangars have to be thought of in terms of function, the ability to retain an accepted standard of strength and safety over a reasonable period of time, and that it isn’t going to invite insolvency due to maintenance costs. 5000 years of building have resulted in Building Codes for various types of building. Together with the enforcement of these codes by authorities and penalties, social and financial - particularly from insurance companies - following failure to comply with the black and white.

No concentration of welding gear, concrete mixers or hired cranes can get over the salient fact that initial application of logical thought usually works out better than dashing into the paddock and deploying all the constructional equipment.

Building structures are fairly static, except in earthquake zones, and there are building codes for these places too. Notably the New Zealand requirements which are used in other volcanic areas of the Pacific zone.

When structures are required to move a bit more or faster the situation begins to get a bit more complicated, although the fundamental laws of physics and mechanics remain the same.

Ships are an example. The hull has to be basically stressed for two conditions. When it is lifted by a wave in the middle and the front and rear ends, with lesser buoyancy, sag down due to their weight. Or when two waves lift the bow and the stem so the hull sags down in the middle.

In ships with high and relatively heavy upper works at the stem, like most of the modem container vessels, rolling can produce inertia loads which additionally twist the forward part of the hull. Cases have also occurred, in quite up-to-date ships, where the sections have been designed to produce more than usual increased buoyancy when heeled and so reduce rolling angle. To such effect that under certain sea conditions the roll has stopped so abruptly that people at the higher levels have suffered whiplash injuries as in a car rear-end shunt.

And so to aircraft which, structurally, are not always aligned with what some people visualise in their heads. It is quite impossible to deal with aircraft structures, even for dreamships, without some basic knowledge of aerodynamics. And while model aircraft may help you with aerodynamics (only providing you do some reading), they are almost without exception extremely poor guides to structures. Being an aeromodeler from long before the days of balsa I can probably cope with the predictable cries of outrage from aeromodelers who are rapidly going to tell me how many times their models emerged unscathed from heart-stopping disasters.

An aircraft in flight is entirely supported by air. All the in-flight loads result from air blowing or sucking on bits and pieces. Including the tiny load which pushes the pointer of the ASI and the even tinier load which puts the altimeter needle where you want it. All the passengers in an airliner are normally a static load so when they make a mass move to the rear toilet, the aircraft Centre of Gravity moves aft. This has to be compensated by a change of trim; either the wing angle of attack changes, which alters the wing loads, or control movement is made at the tail (in conventional aircraft), which alters the tail loads.

Additionally there are inertia loads in turbulence which are trying to shake the navigation lights off the wingtips, the rudder off its hinges, the engine out of its mountings, and you out of your seat. Inertia loads also feature in aerobatics. So how do they come about? Because of the aircraft’s reaction to imposed aerodynamic loads resulting from turbulence or manoeuvres.

If the aircraft has a very elastic structure like the Voyager it will, up to a point, absorb some increase in loads by flexing its structure. It will probably be a bit of a handful to control as well. If aircraft structures do not flex to some degree they will break much sooner than a mind geared to five-bar gates is likely to comprehend.

All, repeat all, aircraft structures must be regarded as being made of rubber. Perhaps a very tough rubber which under load only deflects a thousandth of an inch or less. Without this basic understanding, which is hard to visualise in very small movements, it is extremely difficult to get across the basic principles of all structural engineering, but particularly on aircraft where this understanding is vital. It has been said that aeronautical engineering is just ordinary engi­neering made a bloody sight more difficult.

Even when the aircraft has its wheels on the ground there are aerodynamic factors operating which result in effects different to the behaviour of a wheeled vehicle. When a car leaps into the air over a hump-backed bridge, and comes down ker-rumph on its springs, its weight does not significantly change. Not so the aeroplane. As it accelerates down the runway the weight on the landing gear decreases as wing lift increases with speed. At maximum rolling velocity there is no weight on the gear because the aircraft is just airborne.

On landing the reverse applies. At touch down speed any installed high lift devices are doing their job and the pilot is attempting to reduce sinking speed to minimum. As the aircraft slows down the weight on the landing gear tends to increase, quite suddenly if lift-dumping spoilers are used.

Consequently aircraft landing gear design is a rather more complex affair than the suspension of land vehicles. Plus the problem that the landing gear has to be built for minimum weight because, whether fixed or retractable, the gear is just a damned heavy nuisance used for a very small percentage of the aircraft’s operational life. So quite a number of aircraft like the Boeing 747, and notably those operating from aircraft carriers where the runway can move upwards without the courtesy of a NOTAM to the pilot, didn’t get finalised on the drawing board until the project engineers clinched the arrange­ment of the landing gear. And how its loads - which are many and variable - could be transmitted to the rest of the structure.

Nor can an aircraft even be directly com­pared with a submarine, another type of vehicle which is surrounded by the medium in which it operates. And even with titanium alloys it is so rubbery that the hull reduces in diameter when submerged. As the hull has then less displacement but still has the same weight, if the crew and computers are not very careful they can fall to the bottom of the ocean. So they have some not immediately apparent structural problems too.

Perhaps the only valid comparison of an aircraft are those types of sharks (and stingrays) which, unlike the majority of fish, have no neutral buoyancy adjustment air bladders. Such sharks must keep swimming because if they slow down, or swim into water of a lower density, they can get into a stalled condition. It is also interesting to see that the stingray, when moving, makes maximum use of ground effect to minimise the otherwise high induced drag resulting from its low aspect ratio configuration. Can this be an example of the computer aided design we hear so much about nowadays?

Stress And Strain

Suppose you have a piece of speargun elastic about ¾ inch diameter and 12 inches long. Anchor the top end to a suitable support and have some uniform weights like bricks at the ready. Measure the free length. Then attach one brick to the lower end and measure the extended length.

Remove the brick and the elastic should return to its original dimension. Keep on adding bricks, measuring the extension and removing the bricks to repeatedly check on the original length.

There will come a point when you remove the load and discover that the elastic has not returned to its original length. It is just a little bit longer. The test material has reached its elastic limit and it’s time to be a bit cautious about adding weights. So continue with half bricks or smaller. Keep your toes out of the way because, sooner than you would think, that speargun rubber is going to snap, particularly if the higher load is sustained for any length of time, and the bricks are going to come tumbling down. This is when the material has reached its ultimate tensile strength.

If you weigh the bricks one at a time on the kitchen scales you can plot a real neat graph. The original cross section area of the rubber was, near enough, half a square inch. More precise measurement and calculation will of course give you a more accurate figure. If at any stage of the experiment you divide the weight of the load by the cross section area you will get the ‘Stress’ in the material.

For dreamship purposes it is definitely best to stick with Imperial measure in lb per square inch although many American books refer to ‘Kips’. One ‘Kip’ is one thousand lb per square inch. Metric units are varied; Kg per sq cm, atmospheres, and pascals or bars with a weird assortment of Greek and Latin prefixes. I’ve had occasion to work with all of them, plus Imperial pressures in inches and feet of water or mercury, and recommend that lb per sq inch is still preferable for dreamship applications. There is much less chance of a cock-up.

The dimension that the rubber stretched above its original length, at any stage, is the ‘Strain’. If you plot a new graph with the ‘Stress’ vertically on the left and ‘Strain’ horizontally to the right along the bottom, you will find that the plot goes up in a slanting straight line until it starts to bend to the right. This is the elastic limit where the material does not return to its original dimension and is sometimes referred to as the ‘Yield Point’.

A bit further along the plot it comes to a stop, where the bricks broke the test piece. Now it will have been observed that as the rubber gets stretched it also decreases in diameter and so has less cross-sectional area. But the stress is always related to the original area in the unloaded condition.

This type of graph is fundamental to all types of structural engineering. The experiment described above merely used rubber to get over the absolutely essential message that you MUST visualise ‘strain’ when dealing with aircraft, even when the component is a mighty solid looking steel wingroot fitting attached with hi-sheers to an aluminium alloy spar. Because steel and aluminium alloys stretch differently the hi-sheers may carry unequal loads. This may be critical when designing to meet the high G loading cases of whatever design requirements apply, e.g. FAR 23 etc. That root fitting is not what you might think you see. It is what the designer’s figurings say you have to trust your neck with. Always supported by proof loading.

In proper material testing the strain is measured in thousandths of an inch or less and the loads are applied at a steady speed. The shape of the stress/strain graph is very important. For most steels the graph tends to flatten horizontally around the yield point and then get back on track before bending to the right to the point of failure. With most aluminium alloys there is no such clearly indicated yield point and it is extremely important on aircraft that reference be made to the published data. Even in the same alloy and in the same temper the yield strength varies with the thickness of the material.

A famous fighter which had an unanticipated manoeuvre excursion in the course of a high altitude gun-firing exercise managed to land with about four degrees more dihedral than when it took off.

The Basic Stress Conditions

Before they starve to death I have enlisted the aid of the Bittaboolchit termites in Figure 1 to indicate the main conditions which may apply in any part of an aeroplane. Even the beaut finishes are stressed by expansion or contraction loads and flexing of the underlying surface. In a fabric covered aerobatic aircraft the fabric may have to support nine times the weight of the aircraft. The fabric, between the ribs, flexes into aerofoils which never saw the light at the end of a wind tunnel.

FRED41.jpg

In the diagram the arrows show the direction in which the loads are applied. Generally speaking, except for tension in cables, tie rods and streamline wires, loads in aircraft components seldom occur singly. In a typical wing spar under flight loads there is a combination of tension in the lower flange, compression in the upper flange, plus sheer from the wing lift loads and additional sheer loads from the fuselage, a wing fuel tank or an outboard engine installation may apply.

Also note that in a heavy landing the loads in the spar flanges may be reversed (as may also occur in down gusts or inverted flight). The down load from a full fuel tank will increase by the G factor and there may be completely new upward sheer loads at the location where the landing gear is attached. If this sounds complicated don’t worry about it, because all these awkward circumstances are defined in the airworthiness requirements as, for example, in FAR 23 which applies to powered aircraft up to 12,500 lb (5669Kg) MTOW.

You just have to figure out what in the hell is pushing or pulling on various parts of your dreamship, multiply these loads by the stipulated factor in the requirements, look up the material you intend using and then apply the numbers to ensure that any part doesn’t distort too much at the associated stress and strain.

The sometimes favoured method of ‘eyeballing’ is inherently unsafe. Its proponents should be regarded with a justifiable degree of apprehension, even when they are building an approved type of amateur aircraft. They also need to be restrained, by forceful deterrence or ridicule, should they, without seeking advice, develop bright ideas about making the slightest change involving any part of the aircraft structure or control system. Since aerodynamics are so fundamental to aircraft strength and behaviour, care must also be exercised when modifying specified canopies or engine cowlings in pursuit of eyeball appeal.

Figure 1 also indicates the molecular deformation which occurs in any material to which loads are applied. In the tension and compression diagrams the difference in length, compared with the unloaded dimension, shows the strain. (Exaggerated here, of course, in order to get the message across to a few acquaintances where recent experience suggests that the need is dire).

Compression loads, normally straightforward, are not so in the case of struts loaded as in the case of something like a Piper Cub lift strut under landing loads. Long before the material gets anywhere near its compressive yield strength the strut will buckle. There is no way out of this problem by eyeballing either. You either have to mess about with calculations involving radius of gyration, or take the easy way out and look up some of the funny S shaped curves which the aviation literature has kindly provided since before I flew my first paper dart.

Sheer loads need a bit of thought in timber (and certain composite structures) because timber sheer strengths are very different across or along the grain. Also in any piece of ply there is an infinite number of sheer strengths of which only four need concern us. Along the outer grain, across it, at 45 degrees to the grain, and a cunningly hidden one - which shouldn’t apply in any respectable dreamship - across the thickness of the ply.

Sheer strength is also important in parts subject to torsion as, for example, a control tube poking out of a fuselage to waggle an outboard aileron up and down.

Bearing loads may be more significant than is often assumed. Rivet strength tables usually have cautionary notes on the subject. Thin sheet will fail in bearing, the material being pushed out of the way like plasticine long before the rivet sheers. Even when bearing failure seems unlikely the situation is compounded by the fact that aircraft rivets have increased sheer strengths when in thicker sheets.

Bearing loads are frequently the critical ones in aircraft timber structures and at assembly joints, for this very reason, you can finish up with more big diameter bolts than you otherwise need. There are also definite limitations for bolt length to diameter ratio, to prevent sheer loads bending the bolt inside the timber and applying excessive beading loads close to the bolt head and nut. Furthermore, bearing strengths are seldom the same as for compression and there is varying correlation between different timbers. So just be very careful if you think you know a secret store of a really cheap substitute for aircraft spruce.

There is some more bearing load perspective to be added to your visualisation of aircraft loads. Bearing strengths vary according to the direction of load to grain as in, for example, the angle of the spar fittings at the upper end of a lift strut on a braced high wing monoplane. Don’t be dismayed. All the data is in the books. And somebody is bound to put it on video if you have a certain amount of patience.

Just spare a thought for the people who in a few months designed, stressed and built the wooden English Electric Wren. A flying competition winner in 1923, with an 8hp ABC motorcycle engine. It is still flying years later, proving that doing the figuring may be more of a help than a hindrance.

Prelude To Further Progress

The subject of aircraft strength entails only a relatively few basic principles and, for the sort of aircraft in which we are mainly interested, no number-crunching complex mathematics. On the other hand the diversity of application using the basic principles is not small. Each dreamship has to start on a clean sheet of paper, and requires individual analysis as applicable.

In actual practice it may be found that the really sticky problems arise in the detail design with items like awkward control pulley attachments and similar superficially minor items. Bearing in mind that factors like weight, availability of materials, or the need for maintenance access more often than not enforce revision of attractive preliminary ideas.

The essential requirements for the subject are a clear understanding that aircraft are different, that even diamonds flex under load and that it is important to nut most things out from a background of aerodynamics. Nothing required except pencil, paper, published data and logical thought. Use brain.

Some of the books recommended in the previous part of this series (Airsport May/June 1988) have useful sections dealing with strength. The following books, which are just a few of the many published, may help to clarify some problems:-

Aeroplane Design

By E W C Wilkins, Griffin and Co Ltd. 1938.

Practical Stress Analysis

By D R Adams, Pitman. 1938.

The Principles of Aircraft Stressing

By W.L. Morse, Griffin and Co Ltd. 1941.

Stress Analysis for Aircraft Draftsmen

By Greenwood & Silverman, McGraw Hill. 1943.

Aircraft Layout and Detail Design

By N H Anderson, McGraw Hill. 1946.

Design of Wood Structures ANC-18

2nd edition 1951. USA Government Publications.

Fundamentals of Stress Analysis

By Deyermond and Arslan,

Aero Publishers. 1966.

Analysis and Design of Flight Vehicle Structures

By B F Bruhn, Tn-State Offset. 1973.

Synthesis of Subsonic Airplane Design

By B Torenbeek,

Deift University Press. 1976.

Since the strength of aircraft is essentially associated with aerodynamic effects - includ­ing model aeroplanes and parachutes which, uniquely, have most of their loads in tension - it is nice to be able to recommend a book which is admirable, available, cheap, and Australian. It is ‘Aerodynamics for Soaring Pilots’, written by eminent SAAA member Henry Millicer, and available through the Gliding Federation of Australia at:- GFA Sales Department,

GPO Box 1080, Adelaide, SA, 5001.

Price $10.50 plus mailing.

Some of the above reading should keep you going until the next article when we shall look at aircraft load factors and the flight envelope. The crux of the dreamship problem lies in ensuring, when the dream is fulfilled, that the structure is not so heavy that it won’t get off the ground, or so flimsy that it dismembers itself just after take-off.

To solve this problem you need both eyeballs. Just use them, in conjunction with the brain, to read some books. The next crucial step is to close the toolbox and pick up a pencil.

FRED40.jpg

DAYLIGHT ON DREAMSHIPS (Part 6)

By Fred Lindsley

(This article is from SAAA's "Airsport" magazine November/December 1988 edition)

Continuing from the September 1988 issue of Airsport, Fred Lindsley outlines some more basic aspects of strength in aircraft structures. Like riding a bicycle for the first time, the only requirements are an acceptance that there is a long history of other people without previous experience demonstrating ability to cope, and a genuine willingness to learn.

Getting Down To Earth With Structures

“If a builder builds a house for a man and does not make its construction firm and the house which he has built collapses and causes the death of the owner of the house - that builder shall be put to death.”

This is just the first of several similar clauses, all of them well phrased to subdue any unseemly jollity in builders.

It appeared in the above form in 1922, a translation from the Code of Hammurabi who was King of Babylonia around 2100 BC. So we now have evidence covering about 4000 years of building codes on record, and this very early one got structural failure straightaway into the picture.

The original was carved in stone and the photocopies were on baked clay tablets. So they were probably joking in the Hanging Gardens of Babylon that when the weight of building materials equalled the weight of the regulations, you could be confident about arranging a house-warming party.

You could be fairly confident too that the builders working to this code were not exactly keen on participating in any experimental category.

So, what about subsequent progress? Well, as recently as 1960 the New York chapter of the American Institute of Architects held a meeting. This followed several conferences on serious construction failures in civil engineering involving all sorts of large projects, in steel, timber, and reinforced concrete. It is interesting to see that the first item in their summary of causes was:- “A frequent faulty design practice is

changing the design without the knowledge of the original designer”.

Where have we read this before? In Airsport of course, and in many other aviation publications too. Usually in connection with accident reports.

Back in the Sept/Oct 87 Airsport I wrote an article on timber which mentioned trestle bridges. Not a few of these, in a manner of speaking, had their depressing moments. So what about the metal bridges in the USA?

Their ‘Railway Gazette’ in 1895 listed 502 cases of iron bridge failures between 1878 to 1895. Nor was it getting any better. The first 251 cases occurred in 10 years. The second 251 in only 8 years, and 162 of these occurred in the three years from 1888 to 1891.

Even as late as 1905 the ‘Engineering Record’, an American periodical, regularly featured the most serious railroad wreck of the week, which was usually associated with a bridge failure.

Something To Get Up In The Air About

All the above are on provable record and must be borne in mind if you are going to take a serious interest in the strength of aircraft. You will need a new and special perspective which is not restricted by tunnel vision of railway tunnels, hydro-electric dams, sheep-shearing sheds or some memorable steel bridges in Melbourne.

Note that the earlier related industrial engineering, at around the turn of the century, was the heritage for the Wright brothers and other aviation pioneers. Over the subsequent years it would seem that the aircraft structural engineers have a comparative record of which there is little to be ashamed; particularly when reading about some of the problems the structural steel boys still seem to have, notably in connection with welded structures, on buildings, storage tanks, bridges and some very large ships. (One of which a few years ago broke its back, berthed at a wharf, merely by unloading oil).

From the very beginning the aviation people were into a whole new ball game as far as assessing applied flight loads was concerned. Of course once they got some realistic loads they could apply conventional analysis to the bits and pieces. They also had to take a new look at factors of safety. No point in making the aircraft so strong and heavy that the tyres were flat, thereby markedly increasing rolling friction and distance on the take-off run.

Also, once they got past the birdcage stage, the aircraft designers had (and still have) a problem more acute than in any other branch of engineering. They were stuck with external aerodynamic shapes, particularly on wings, which just could not be easily manipulated to a different shape without adverse consequences to performance, stability or safety.

Try a little dreamship exercise. You want to manually change in flight the dihedral of a wing from plus 10 degrees to minus 2.5 degrees. You need zero dihedral for take-off because you do not know what the hell is going happen when the dihedral is altered. There is 16.4 feet of wing outboard of the ingenious mechanism you a about to devise at minimum weight. The catch is that you are restricted with an aerofoil which is part of the aerodynamic deal and, worse, it is only 3.07 inches (78 mm) at its thickest between the beaut finishes on the top and bottom of the wing. You are additionally stuck with meeting a strength factor of 8 (eight).

Try it. Should be no problem. There were a pair of them, halfway out along each wing on the Darmstadt D-30 sailplane which had a span of 20 metre (65.6 feet). A collector of world record more than half a century ago, in 1937.

Today, with laminar flow aerofoils and very thin wings on military aircraft, the need to put everything within shape where the external tolerances may be measured in a few thousandths of an inch, makes the problems more acute. They get solved. But only by recognising that this never-ending problem is peculiar to aircraft structures.

Hanging By A Few Threads

The previous ‘Dreamship’ article showed a sketch of the four basic type of loading which, in essence, are all you ever need to learn.

Just as important, but a bit more difficult, is the need to visualise where the loads go and lose themselves in classical Newtonian style by equal and opposite reactions. With aircraft where most things start with aerodynamic loads, which you cannot measure with a spring balance after dashing out on the wing, nutting out the load paths is not always as easy as it might superficially appear.

So we will look at an example which starts on familiar ground, just so we don’t feel too lost to begin with.

The more studious cane toads at Bittaboolchit have the happy idea of studying aircraft design by gazing aloft, on the off-chance that one of the Sunshiner’s projects might actually get airborne.

If, however, the cane toads are seen leaping for cover in a frenzy, it is certain that their sensitive hearing has detected the approach of Der Supremo Nummer Eins Tigermotte. A ‘splat’ as this aircraft touches down sadly confirms that one cane toad was not sufficiently nimble-witted to detect the warning signs, or nimble-limbed enough to get out of the way.

This, alas, is not the original Tiger Moth, the DH77, a low wing monoplane with the prototype Gipsy I engine which established records for speed (186.47 mph) and altitude in 1927. And also, alas, came to a sad end in Australia, at Mascot in 1930.

The aircraft seen is the later Tiger Moth, a DH82A. First flown in 1931 and produced as the DH82 with a Gipsy III. In 1934 it became the DH82A with a Gipsy Major engine. More than 8500 of them were built, in seven different countries (1095 of them in Australia), before production ceased in 1945.

This particular survivor is occasionally flown by Baron von Orlborlz und Holzernkopf, Direktor of Produce from Aloofly Exclusive Vineyards. Its attention-gathering checkerboard pattern distracts attention from the front exhaust stub which is half burnt away, thus producing the distinctive sound which alerts the cane toads into their particular survival. Ditto this writer who, way back in 1936, was certifying Maintenance Releases (Daily Certificates of Airworthiness in those distant days) for Tiger Moths. And with the benefit of an inverted flight fuel system which required gun oil for its maintenance, also persuading them into - and thankfully out of - inverted spins.

Which gets us to the load path situation. Those old-fashioned designers of the Tiger Moth were sufficiently up-to-date to embody some stressed skin in the primary structure.

Stressed skin on a Tiger? Let not a stroke of apoplectic exasperation stress the skin of blood vessels in your brain. Use brain, and read on.

Crouch below the cockpits of that well-known steel tube and fabric fuselage and below each bottom longeron you will see a bevelled length of hard­wood with lots of little bolts going up through the longerons. If, in trying to undo any of these nuts the bolt shears off, you can usually bet money that when exposed to the cold light of day the bottom of those .875” x .875” x .056” square steel tube longerons are going to exhibit some rather discouraging corrosion. (Incidentally the geriatric specification weldable steel tubing in a Tiger is actually a bit stronger than the expensive off-the-shelf 4130 chrome molybdenum tubing that we buy nowadays.)

Those little bolts you are looking at attach a ply floor to the front fuselage lower bay which has no diagonals. Nor has the upper bay, because of the two cockpits. And at the upper centre section triangulation is made by the streamline wires, the bottom ends of which geometrically terminate on the same lateral straight line.

That ply floor is a bit of stressed skin taking the torsional loads at the front fuselage, introduced by tail loads (of which torsional wracking by that tailskid was a stressing requirement separate to flight loads), reaction from the engine mount and one-wheel landings. That floor is more than just something for Real Practical Men to stamp on when they are getting in or out of the cockpits. It is part of the load path deal. Also why those bevelled bits of wood were hardwood (those little bolts are in single shear where it counts). Of course you can use billabongee buggayutu - if you have the numbers.

Still looking at the same Tiger Moth let us trace some loads backwards.

Baron von Orlborlz weighs 81.6 Kg, a nice round 180 lb. In steady level flight he remains seated by virtue of his weight, bum friction, and a prehensile caress on the imaginary gun trigger at the top of the control column. He is then subject to 1G.

In other flight conditions he will certainly be subject to different G loads. For the sake of convenience we will take those imposed by the emergency alighting requirements because they apply to the attachment of the seats and, in the Tiger, full harness. Also, let it be emphasised, in any Aircraft to anything else which might come loose and do a mischief. Like golf clubs in the luggage locker or the temporary car battery somebody thought was secure enough with a leather strap. FAR 23 has two different requirements. Apparently the USA has the rather strange idea that emergency alighting conditions in normal category aircraft will invariably be less severe than in the case of aerobatic category aircraft. Even in the aerobatic category two conditions are less severe than the European JAR-22 and BCAR Section S which are in complete agreement, in racial harmony with the Tiger Moth, and are tabulated below:-

Upwards - 4.5 G

(FAR 23 aerobatic similar)

Forward - 9.0 G

(FAR 23/aerosimilar)

Sideways - 3.0 G

(FAR 23/aero 1.5)

Downwards – 4.5 G

(FAR 23/aero 3.0)

So the Baron, via his seat, harness, and feet on the rudder pedals in the forward G case, could theoretically generate loads of 810 lb up and down, 540 lb sideways, and 1620 lb forwards. So could his passenger if he had one of the same weight. All of them find their way into the longeron structure of the aircraft. Eliminating the forward G case the others could be repre­sentative of flight loads. From the pilot end there are two basic load paths. From the bottom longerons loads travel forwards and do a 90 degree turn at the lower wing spars. From the upper longerons the loads creep up those N centre section struts and also do a smart 90 degree turn, plus a bit of sweepback angle, into the upper wing spars.

From the spars the loads are distributed among those apparently flimsy ribs mostly using bits of .5”x .125” spruce. Less cross sectional area than a pencil. All those funny thin metal clips holding the ribs to the spars were a commercial material used for pen nibs, but don’t worry, it’s OK because it was identified in black and white on the original drawings. (Lesson for amateur builders - unless it is in black and white you are wasting your time complaining).

From the ribs, where? To that fabric. How? With a lot of stitches from top to bottom of the ribs. Three inches apart, except in the propeller slipstream (which meant the tail too) where they had to be 1.5 inches apart.

So that stitching had to be pretty strong hadn’t it? How about waxed Number 18 linen thread which is a whole .030 inches (30 thou or 0.77 mm) in diameter? That is what all the standard Moths and original DH 82s used. On later service aircraft it was replaced with No.1 braided flax cord which has remained a standard practice.

So, whichever way you cut the cookie or pop the vintage corks, the Baron is hanging by a few threads. At 4.5 G, with fuel and a passenger, there is around 8200 lb or 3.67 tons mixed up with those rib stitches. Thank heavens the Tiger Moth was designed back in 1931 to take more than 4.88 tons. The Tiger is a great survivor. Every one still flying may be at least 43 years old.

The load carrying described above has been desophisticated for simplicity (I’m aware that pundits will say “What about wing weight?”) and in normal strength assessment the loads travel backwards, from the air to the fabric, to the ribs, to the spars etc. The procedure is to figure out the load paths, see where they divert or split into two or more paths. Also to figure out what happens with different loading conditions e.g. application of flaps or aileron which usually puts more load on rear spars. Then to nut out each significant component in a load path, so that it is strong enough for a reliable service life without being too heavy or costing the national debt. Manufacturers of aircraft, including ultralights, who do not do things the aeronautical engineering way seldom survive in the commercial market.

With dreamships you can’t go wrong. You will give yourself almost as good as a university education, in your leisure time and at negligible cost. You will help SAAA too by demonstrating the same viewpoint on the real fundamentals of airworthiness as all the world’s regulatory authorities.

Gee, I Didn’t Even Move The Controls.

Because conventional aircraft are gravity-defying mechanisms and are in flight supported by an invisible and elastic gas of varying density, the established convention of defining basic loading conditions in terms of gravity is almost unique to the aircraft business. Although it is beginning to creep into other enterprises with much longer histories of less enterprising tradition.

Note that although we usually think of G factors as being up, positive, or down, negative, they can come from every direction like the detonator pressures in some early nuclear bombs.

If we install on a firewall a battery which weighs 10 lb, and say the aircraft is subject to an upward vertical load factor of + 6 G, it means that under these conditions the battery weighs 10 x 6G = 60 lb, and it wants to slide down the firewall. So it had better be attached with something better than a piece of string.

Although this 60 lb load can easily be taken in shear by a small steel bolt, think carefully before plugging in the electric drill. That bolt, at the base of its thread, may not have enough cross sectional area to take the 9G forward case mentioned earlier in another condition. Nor may it be adequate under repeated fatigue conditions at a substantially lower G.

Getting back to the +6G case, the thin sheet of the firewall may fail in bearing long before your little bolt fails in shear.

Of course you can be practical and make sure, by making the battery mounting out of 1/4 inch steel plate and attaching it to the firewall with ten 3/4 inch steel bolts and heat-resisting locknuts. Just don’t forget the same firewall now has to carry the additional 6G loads from the mounting and those bolts. It is on such trivial detail that the Sunshiners sometimes come perilously close to falling into error. The moral is that a bit of figuring helps to save weight, time and money.

In the above example we saw how G loads can be usefully turned into realistic applied loads, in lb (lbf for the technical purists. Please no “lbs” in the typography). However, it is very important to keep in mind that G factors are not weights. They are accelerations (or decelerations) which means that a time factor is also involved. This realisation is essential in some aspects of aircraft strength even down to milliseconds.

Notably with landing gears where the whole shock absorbtion process may last less than one tenth of a second, during which the tyre is skidding while it is accelerated from zero to rolling speed (doesn’t happen on your racing car, does it?). This skidding imposes backward loads on the whole landing gear much greater than are generally visualised - and all in milliseconds. It is these unavoidable skidding accelerations which put the rubber on the runways.

A bit further on in this article we will have a closer look at basic G factors, but first it may be a good idea to clarify the picture.

There is a tendency to use G factors as conversation pieces. “My Liarbird has 12G, and your Terminal Inexactitude has only l0G” sort of thing. Without a number of qualifications such statements are meaningless and may be misleading. They also prevent clear thinking about aircraft strength.

A G-meter on the instrument panel close to the aircraft’s centre of gravity will tell you what is happening there. In a perfect steady speed roll about the longitudinal axis through the G meter it should read 1G. Out at the wingtips the G figure may be a lot higher. What is more it increases as you move from the cockpit to the wing tip. It also varies with the angular speed of roll, in degrees or radians per second. That’s why they used centrifuges to establish G limits for test pilots and equipment like black boxes.

Similarly in arcuate motion, as in pulling out of a dive, the G loads at the tail and at the propeller/engine may vary from those recorded at the aircraft’s CG.

The truth is that the G factors we see in official requirements are minimum values. They are published by the authorities in many different countries and, outside the Iron Curtain there is general international agreement on most larger aircraft and not much to niggle about on smaller aircraft as the prudent designer, with overseas sales in prospect, will usually play safe by selecting the tougher case unless the weight or cost penalty screws the idea. All these requirements have been refined, sometimes painfully, over a period of nearly 70 years, and alter with the type of operation intended for various civil aircraft. Military aircraft, particularly very fast ones flying at low altitudes, usually have to meet much more severe strength requirements.

A possible hurdle for dreamship dabblers is their association with mates who equate G factors with boisterous handling of flying controls. This is dangerous half-truth and sufficiently misleading as to have been a contributing factor, in quite recent years, to too many ultralight fatalities both here and overseas.

High G factors can occur without any control movement, hence the heading to this section.

FRED42.jpg

See Figure 1. An aircraft in steady level circling flight will impose G loads which are directly related to the cosine of the angle of bank. It doesn’t matter if it is a model aircraft, the Antonov 124 seen at the Bicentennial Airshow, or the pelican which gets the fish I fail to catch. The fundamental mathematical law has remained unchanged for trillions of years. We just had to wait for Sir Isaac Newton (1643-1727) to cough up the associated figuring.

Now I have great news for our Real Practical Man. He doesn’t actually need time-savers like calculators or trigonometry tables to get the answers. With a little plastic protractor and a carpenter’s rule the G factor can be nutted out with some easy scale drawing. If he can’t do it that way his navigation and drift angles are suspect, and if he is in the same bit of sky I hope I see him first and initiate avoiding action.

Figure 2 is another and significant example of the “Look, no hands” method of wings losing all affection for fuselage attachment while you can still get a feeler gauge between the tyres and the real estate.

For our sketched aeroplane we have selected a Clark Y aerofoil which, at the chosen aspect ratio of 6, should stall at 18.2 degrees angle of attack, with a lift coefficient of 1.56.

FRED43.jpg

It has a wing area of 113 sq feet and is shown flying at an AUW of 1200 lb.

Trimmed for hands and feet off cruise at a lift coefficient of 0.39 its speed will be 152 fps or 90 knots. (See Jan/Feb 88 Airsport for how to calculate this).

Out of the blue it flies into an FAR 23 up-gust of 50 fps. (More on gusts later). Before the pilot-in-command can say “Triangle of Forces”, the 152 fps forward speed and 50 fps up-gust, with a perfectly beastly disregard for comfort, join trigonometrical forces to produce a resultant angle forward and below the horizontal of 18.2 degrees.

Hold it there! That’s the stalling angle! It is indeed. What is more, that stalling angle instantly applies the maximum lift coefficient of 1.56. And instantly applies to the wings a lift load, at sea level, of:-

.00118 x 1.56 x 113 sq x 152 fps x 152 fps = 4806 lb.

Near enough four times the aircraft weight. 40, no less, and you didn’t touch a thing except your worry beads. Aren’t you glad you didn’t listen to your eye-balling mates? And that you calculated for the FAR 23 limit load of+3.8 G, which gives you a minimum ultimate of 5.7G. So you still have an aeroplane with no visible wrinkles in it and, we hope, an increasing respect for the advocates of figuring it out.

This little example, as usual in this series, is an over simplification. If you apply Pythagoras to the triangle you will find that the resultant works out to a .53 percent increase in airspeed (to 160 fps) so the velocity squared law ensures that the wing actually gets a 10.8 percent sharper knock. On the other hand there are different alleviating formulas for gusts which relate to aircraft wing loading and other things which reduce the impact of the sharp edged gust used as an example. Alas you can’t just say “It’s bound to be less” because the formulas have to be worked out for your sort of aeroplane and how floppy its wings are. (These help in gusts but are a very big nuisance for nearly everything else). It’s all in the books and there are NO instant answers.

Also note that a similar change in relative angle of attack applies to the horizontal tail surfaces. There are indications, some recent, that tails of high aspect ratio, and some all-flying tails with excessive ‘elevator’ angle limits, have stalled before the wing. This has resulted in loss of control and in some cases, indirectly, structural failure.

Of major importance is choice of aspect ratio and aerofoil. Any given aerofoil at higher aspect ratios stalls at a smaller angle than at aspect ratio 6. On a very high aspect ratio sailplane the same aerofoil may stall at only 8 degrees or even less. With low aspect ratios the stalling angle increases, which is why you see Mirages and Concordes on the approach at nose-up angles which would be lethal with conventional layouts

Also, at any fixed aspect ratio, different aerofoils have different stalling angle. At aspect ratio 6 aerofoil NACA 4406 stalls at 14 degrees, but its aerodynamic bed-fellow NACA 4421 stalls at 24 degrees. An aircraft using either of these aerofoils, with an aspect ratio of 6 and meeting the 50 fps up-gust case quoted in our example above, would have differences in cruising speeds of 52.2 knots (88.23 fps).

Nor have we even touched on the differences between front and rear spar loadings which may result from choice of different aerofoils. Beware listening to the ‘miracle aerofoil’ eyeballers. They are usually not at all well disposed to perceiving all this poofter aerodynamic bullshit as having any connection with the alleged strength of their eyeballed concoctions.

The Flight Envelope

Or, more precisely, the V-n diagram. ‘V’ being the velocity and ‘n’ the G factor, positive or negative.

In the first ‘Dreamship’ article (Nov/Dec 87 Airsport) it was suggested that, before you started sketching out your dreamship, you compiled a written list of what you wanted your aircraft to do. Very definitely the same thing applies to the V-n diagram. Until this is drawn there can be no serious approach to even starting strength assessment on your dreanship. Nor can you steal a diagram out of a book; the books don’t quote actual real speeds. You have to construct a V-n diagram for your dreamship. The reward is that you can fly your dreanship with enhanced confidence.

Now here I have to admit to a problem. It’s a bit like being given a bundle of wool and knowing that it can be disentangled and spooled up, if only you could find the right place to begin unpicking it. Similarly with V-n diagrams. There are probably scores of them in the textbooks and various airworthiness requirements. It is doubtful if any of them adequately explain how they can be followed by a solitary student beyond the Black Stump, no matter how dedicated and enthusiastic that student might be. How to begin such an explanation is a problem, but here goes.

Earlier in this article it was emphasised that G factors are accelerations. An aeroplane, whether it is standing still on the scales or doing 500 knots in absolutely straight steady flight, is subject to a downward acceleration of 1G. The force with which the earth’s gravity is trying to pull the aircraft, and is equal to the weight of the aeroplane at the time. (I’m cuffing out all the academic diversions about mass.)

This applies to all of our sort of aircraft at all reasonable altitudes. If you go high enough, and fast enough around the world, the outward centrifugal force on the circle you make will exactly equal 1G. You will then have a zero gravity situation, weightlessness, and will have great difficulty in drinking a stubby or keeping your feet on the floor. You will be in orbit.

Aeroplanes however are subject to accelerations as soon as they depart from a steady state of motion, no matter how slight. They can pitch, roll or yaw due to control input, or unkind cu-nim clouds which can tip aircraft upside down (and have done) no matter how quickly and adroitly your skill is deployed on the controls. That is basically why there are negative G cases in the airworthiness requirements. Not because an expert pilot like you is clot enough to fall out of the top of a loop. It’s because history has a fair number of cases where whole flight decks of experienced flight crews had very sudden perplexing speculations that their aircraft’s C of A was being transferred to the aerobatic category. Not so long ago a DC-8 in airline service demonstrated that the V-n bottom line on the design team’s bit of paper could be duplicated in metal, by bunting into the inverted flight mode.

It is possible to get 3G accelerations in some landing gear parts just taxiing at modest speeds. If you’re with me so far, you might agree that it’s time to give G figures some serious regard.

FRED44.jpg

Figure 3 shows the application of positive and negative G loads on an aircraft in steady flight, and when manoeuvering in the pitching plane. Like all these over-simplifications it assumes the aircraft is at constant speed and the inside and outside loops are perfectly circular. Which you and I know is divorced from reality, but we have to start somewhere to get the message across that the approach to aircraft strength is somewhat different to the more common forms of engineering.

Ordinary engineering is mostly based on static loads. A crane has a capacity of 5 tons. A factor of safety of, say, 6 is applied and the crane is made strong enough. Nearly all aircraft forces applied in flight are dynamic loads, originating with rather squelchy air moving at high speeds, and coming from strange directions.

If the crane driver unwinds all his cable, and gets his diesel or other machinery up to maximum revs and power before engaging the clutch he, and the jib of the crane, may get a surprise when dynamic loading is applied at the hook. We know he is not stupid enough to do this, but if he is going to seriously doodle with dreamships he MUST comprehend the basics of why.

Many years ago the aircraft industry, in every country where they had one, moved away from the civil engineering largely static concept and looked at things in a new way. This introduced the concept of G accelerations and the V-n diagram which requires that an aircraft, during its operational life, is expected to withstand all the accelerations it may get from any of the flight conditions it might just possibly get itself into. Including stalling upside down and encountering unpleasant gusts in high speed dives.

The V-n diagram is known as the Flight Envelope because anything inside the boundary lines is considered safe, like the cheque which is in the mail.

Limit Load Factors are the G figures, positive and negative, which define the shape of the V-n diagram. After application of such loads any part of the aircraft which has been stretched, twisted or otherwise subjected to stress and strain shall restore itself to exactly what it was before load was applied. Not a millionth of an inch more (except for tyres which, in aircraft, increase their diameter in service).

Ultimate Load Factors. All airworthiness authorities apply a safety factor, multiplying the Limit Loads of the V-n diagram by 1.5. So a limit load of 40 becomes an Ultimate Load of 60 which aligns with the minimum load which the structure should withstand before failure.

In ultimate loading tests, which only very large manufacturers can afford, it should be noted that structure is expected to withstand the applied testing load for a very short stipulated period, usually a few seconds. (There is a good bit more to proper proof loading tests than turning a wing upside down and piling sandbags on it. Some articles in overseas magazines are highly suspect. If you intend proof loading some major components please get a Reg 40 engineer to check your assumptions before you fill the first sandbag.)

There is general international agreement about limit loads and other load conditions e.g. gusts, landing gears, etc. On civil aircraft they vary with the aircraft’s intended operation, large modem airliners (and some military transport aircraft) having factors which may look dubiously low to people outside the aircraft industry.

With smaller aircraft, and gliders, there are usually two sets of loads, Normal and Aerobatic, plus in some places Utility which in effect overloads the aircraft, reduces the limit loads, and assumes a pilot with ambition to become a senior citizen. It all depends on what you want your aircraft to do, which takes you back to the first article in this series.

In official requirements the defined loading conditions for V-n diagrams have been painstakingly refined on a basis of practical operation over many decades, plus an awful lot of statistical information, much of which has been gained from the installation of V-g recorders in aircraft operating all over the world. In some cases the limit loads are not defined as a number but have to be worked out to a formula, usually involving wing loading or, in the case of gliders and motorised gliders, the minimum drag coefficient. (It will be a very good man who gets that right first time without flight tests). Most military aircraft are a different ballgame.

Apart from our Australian requirements, the best sources of guidance in English are the European JAR-22, BCAR (British Civil Airworthiness Requirements including Section 5 for very light aircraft), and the USA FAR 23 which applies to aircraft below 12500lb (5670 Kg). gross weight. Should you come by any copies of these make sure they are up-to-date.

Flight Envelope Construction

Figure 4 is an artificial creation for the purposes of explanation only. Taking it step by step it is constructed as follows:-

The V for velocity line, from 0G the right hand side has the speeds in knots and feet per second. Because aircraft get nearly all their loads from aerodynamic forces, and are not expected to collapse as frequently as yacht masts, these speeds are in Equivalent Air Speed. This is what you would get from an absolutely perfect ASI, at sea level, providing your Pitot and static locations were also absolutely devoid of error under any operational conditions within the Flight Envelope.

At speeds above Mach 0.5 (which some modern small aircraft could exceed in a dive) compressibility problems upset the simplification. Additionally at higher altitudes the air is less dense and the speed of sound markedly decreases. Thus the aircraft’s Mach number increases and this is a quite severe structural problem with modern airliners. For these it is customary to construct two more V-n diagrams at selected higher altitudes. Before the workshops start cuffing metal and getting rivets out of the stores.

FRED45.jpg

Figure 4 can be assumed OK to at least 15000 feet. Some of the speeds have been given non-standard identities for the sake of clarification.

Reading from left to right they are:-

V+min. (45 knots). The stalling speed of the aircraft in steady flight at + 1G, which equals the weight of the aircraft. Point X in the diagram. Note in this case, to make it easy, we have no flaps or high-lift devices. These usually reduce stalling speeds but further up the speed scale impose structural loads (or adverse handling conditions) which restrict the upper speed for their deployment.

V-min. (51 knots). The stalling speed of the aircraft in steady flight at -1G. Point Z in the diagram. While it is convenient to visualise this condition with the aircraft upside-down, it should be clearly understood that down-gusts can also apply negative G conditions to aircraft which are the right way up.

Except in the case of completely symmetrical aerofoils like NACA 0012, etc., most aerofoils when inverted usually have a different stalling angle accompanied by a much lower CLmax, so the inverted aircraft stalls at a higher speed. Where wings are set at an angle of incidence to the fuselage datum, the latter usually assumed as the flight path under steady conditions, just apply logical thinking because you are dealing with the strength of your aircraft at all speeds up to points F and A on the diagram.

V-as. (72 knots). The negative accelerated stall speed at, in this case, -2G. See next item for explanation.

V+as. (90 knots). The positive accelerated stall speed at, in this case, +4G. In an earlier section there was a description of how gusts could put an aerofoil at its stalling angle. This condition can also result from manoeuvering.

Take an aircraft in steady level flight at +1G. Sharply pull the stick back and the nose will go up very quickly. But the aircraft, due to the kinetic energy its weight moving at speed, has inertia and its mass responds more slowly, having a strong inclination to proceed along its original flight path. So, for a fraction of a second, the wing is at a high angle of attack to the relevant airflow and, obediently and instantaneously, produces CLmax. If the G load exceeds ultimate, which is 1.5 times the limit loads on the diagram, the wings (or something else) may fail.

This is the Accelerated Stall Case.

Vc. (107 knots). The Design Cruising speed. The essential basis of your diagram. It is whatever you decide, but don’t get too hot-rod because of the next two items.

Vne. (135 knots). The placarded Never Exceed Speed which is usually not more than 90 percent of the next item Vd.

Vd. (150 knots). The Design Diving Speed which is usually at least 40 percent more than the Design Cruising Speed Vc above.

Certificated aircraft are tested at Vd. The 10 percent (at least) lower Vne is to give a safety margin and allow for airframe old age and a more than usual carefree attitude to the accuracy of ASI installations.

So much for the speeds, the ‘V’ part of the diagram. The ‘n’ parts, the accelerations, are also what you decide to adopt or follow from official requirements. Those shown on Figure 4 are only there for explanation purposes.

Please don’t get an obsession about wing strength. Start thinking along aviation lines. The aircraft is a self-contained entity, up there in the blue, with a lot of components plus assorted equipment, with you and your family aboard. Not a Lancia or an Alfa-Romeo in sight to distract you, thinking less pure technical thoughts.

Because aerodynamics are fundamental to aircraft strength the lower speeds on the Flight Envelope are catered for by the High Angle of Attack Case, positive and negative (+HAA at ‘A’ and -HAA at ‘F’). At Design Diving Speed Vd the Low Angle of Attack Case applies, also positive and negative (+LAA at ‘C’ and -LAA at ‘D’).

The curved lines OA and OF are the accelerated stall boundaries and are plotted to the simple formulas given.

Don’t mix up V+min with V-mm. Anything outside these lines is incapable of flying because it hasn’t reached its relevant stalling speed.

The horizontal line XY covers the entire operation of the aircraft in steady flight at +1G.

The Gust Cases

First some history which doesn’t go back as far as ULAA and SAAA. The OSTIV International recommendation for sailplanes included a severe up and down gust case of plus or minus 30 M/s (98.4 fps or 58.3 knots which might be described as having the wind up), gliders previously being enthusiastic dabblers into massive cu-nim clouds. OSTIV was, in effect, replaced by JAR-22 in 1980 (Joint Airworthiness Requirements as agreed by most European countries including the UK). This reduced the severe gust case to the near enough metric equivalent of 50 fps.

In September 1969 the USA, for reasons resulting from attention to their own history, decided to align themselves with the strength and increased the FAR 23 gust requirements by a healthy 66 percent, from 30 fps to 50 fps in the strong gust case. OSTIV went down, FAR 23 went up, and now they all have the same basic gust requirements.

Those readers who align their favourite aircraft with FAR 23 might be interested in looking at the date of the original drawings and also the operational weight the designer had in mind when he leaned on his drawing board.

But today all the airworthiness requirements that matter on our sort of aircraft say the same thing. Strong gusts of 50 fps and weak gusts of 25 fps. Both up and down, identified as positive or negative.

While gusts are usually visualised as up and down, people who sail model boats are acutely aware that gusts can be horizontal too, a discovery shared by investigators into wind shear. Side gusts have stalled vertical fins, particularly ones of high aspect ratio which stall at smaller angles, with dire consequences. That is one of the reasons for the dorsal fin which lowers the aspect ratio, so producing a higher stalling angle and extended yaw control for the pilot. With many aircraft the strength of the tail surfaces is possibly controlled more by meeting the gust cases than other strength requirements.

Note in Figure 4 that the gust lines originate at +1G, unlike the manoeuvering loads which are plotted from zero G. The gusts are not sharp-edged, like hitting a railway sleeper with the front wheel of your car, which for description purposes was applied in Figure 2. Gust alleviation formulas are in all requirements and have to be worked out for the sort of aircraft and operation you have in mind. Not difficult for any genuine dreamshipper.

Getting with the strength is like learning to ride a bicycle. After you have applied a few Bandaids and no longer fall off every time, you wonder what all the fuss was about. The professionals, who will spot some things in this article which don’t align with the path of true aeronautical virtue, may appreciate that there is a difference between trying to blaze a scenic trail for tourists and the boring detail of a chartered surveyor’s ordnance map.

Notification of Sheer Defect

In the previous article on strength (Sept/Oct 88 Airsport) the word ‘shear’ was throughout printed as ‘sheer’. Except for the Bittaboolchit Termites who loyally resisted attempts at applying state-of-the-art electronic typography to my spelling.

If readers haven’t already done so, please suitably amend your copies. Other readers of your copy (it might be your family!) would then be reassured that you are not getting into bad company with technical illiterates.

The next ‘Dreamship’ offering may be about engine installations. Should any readers have other preferences priority would be given to their requests. It would be desirable to have some notification within two weeks after publication of this issue.

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