Monday, November 20, 2006

The Best Airplane

England. World War II. An 8th Air Force bomber crew gets a chance for local leave. After visiting several museums -- including going to see the original, one-and-only Wright 'Flyer' (*) their leave ends with a swing past Stonehenge on their way back to the base.

The pilot, a young lieutenant with an aeronautical degree from Purdue says "It's amazing how they managed to do all this without higher mathematics."

The copilot, another young lieutenant with an ME from Georgia Tech gazed up at one of the surviving lintels dimly seen in the fog. "To me the amazing thing is how they knew which stones to use, without any strength-of-materials data."

The flight engineer, a grizzled sergeant a bit the worse for wine whose first radial was also a rotary took another nip from his flask and gave one of the huge stones a friendly pat. "What amazes me is that the sonofabitch ever flew at all," and wanders off muttering, "Best damn airplane ever built."

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In response to several messages from newbies asking "What's the best airplane for me to build?" I've mentioned a number of homebuilt aircraft including John Taylor's 'Titch', Kenny Rand's KR-series, Pete Bower's 'Fly Baby' and Calvin Y. Parker's 'Teenie-Two'. The 'Titch' is a plywood-covered design, similar to the Miles and other examples of that genre including the Hughes 'Hercules' (generally known as the Spruce Goose). The Fly Baby, with it's wire-braced fabric-covered wings is thought of as a fabric-covered aircraft even though the fuselage sides are plywood. The KR's use a plywood fuselage and composite wing-covering although the spars are wood. The Teenie is of course an all-metal design, assembled with drug-store grade pop-rivets and one of the few designs that flys quite well behind a stock VW engine.

There's really no such thing as ‘best' when it comes to homebuilts. I mentioned these particular planes because I'm familiar with them, having flown all but the 'Titch' and have contributed, great or small, to the construction of each type. I believe they offer a good cross-section of what is available and consider them relatively easy to build. Indeed, asking which plane is best is a bit like asking if you should marry a girl who is pretty or one who can cook. (If you can't find a pretty girl who's a good cook, there's no correct answer... it depends on your appetites :-)

Offering an opinion on the Internet always draws a bit of flak. One fellow took the trouble to explain why 'nobody' builds aluminum airplanes anymore... and spent the remainder of a rather tiresome message expounding on the virtues of his particular choice for the ‘best' airplane, a KR, which he referred to as an '...all-composite design.'

Does everyone understand the definition of 'composite'? Plywood is a composite material, as is reenforced concrete. If you mean a structure using some form of re-enforced plastic resin, you'd have to go back about 3000 years -- the re-curve bows used by Mongol horsemen were composite structures. And if you meant with regard to airplanes, molded composites have been in use since before the First World War (using plywood) and the first ‘plastic' airplane flew in the 1930's.

The KR's are a nice example of a moldless composite structure. As with the fabric covered balsa fairing from which they evolved, moldless composites allows an inexperienced builder to easily produce aerodynamically clean surfaces by carving or sanding a core material to match a template. The load is carried by the skin that is bonded to the core and while the skin is usually a composite of resin and fiber, it may also be metal.

The key point here, in my opinion, is not the use of composites but the use of a inexpensive, easily shaped core rather than a massively expensive mold.

Okay, so you all know about composites. That means you understand the limitations of one-off, hand lay-ups when it comes to specifications. Which is that their weight and strength can be all over the ball park. To ensure uniformity you need a good deal of experience or a good deal of tooling, such as vacuum bagging or even pre-molded skins... or you must accept a certain increase in weight, trading that weight -- in the form of extra resin or whatever -- for your lack of experience. And like it or not, most first-time homebuilders are not very experienced.

A majority of those who respond to the ‘best airplane' question insist wood is the only way to go, citing the ready availability of suitable wood - - in the United States - - and the fact every American boy has built a bird house or gun rack. As a matter of fact I never built a bird house until I had grandchildren but the point is valid if ‘best' means access to materials, since you can build a pretty good airplane from any reasonably well-stocked lumber-yard. Cheap, too, compared to most other alternatives. Of course, you need to know your onions when it comes to the characteristics of wood.

With aluminum, here in the States the newbie is effectively subsidized by our aerospace industry. Aluminum is also smart in engineering terms because the newbie starts off with more known, valid information - - there is a spec for the aluminum and for most of the fasteners you will use. Of course, if you elect to use pop-rivets from the hardware store it would be wise to set up a simple testing rig and get some idea as to the quality of each batch. (Just pop-rivet a couple strips of .020 together and use them to lift a hunk of 3/4" ply used as a pallet for your weights. That will give you shear and tear-out strength. For the bearing load, make up your test-strip in double-shear and hoist using the center test-strip.)

In most cases, if you copy a proven design there is a reasonable likelihood it's strength and weight will be within reason. But as soon as you depart from those norms, unless you are using certified materials -- materials having known specifications for strength -- you'll find yourself back in the days of the Wright brothers, facing all the problems and all the unknowns they had to face. The Wright's addressed those problems in a logical fashion, testing before risking. It's a lesson well worth learning.

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Among the more difficult problems encountered by the Wright brothers was the fact Aircraft Spruce & Specialty Company wouldn't answer the phone. Since they were building an airplane they naturally wanted to use aircraft certified parts but in 1903 such things were hard to find. Nor did the Dayton library hold any tomes on airfoils. And when they went shopping for an engine, the local Volkswagen dealer wouldn't talk to them.

It was all very frustrating.

So they built their own wind tunnel and developed their own airfoil data. And they set up a simplistic materials- testing lab to find out how strong was strong-enough, testing wood and wire and fabric. And of course, they had to build their own engine.

Each step along the way they proved what they had discovered with experiments, progressing toward their goal of a powered, man-carrying aircraft in a years-long progression of logical steps, even to the selection of the closest thing they could find to Muroc Dry Lake for their flight-test facility... the sandy dunes along the Carolina shore and a particular spot among them near a village called Kitty Hawk.

After the Wright brothers had flown, things got much better for the aviation industry which had been in a bit of slump until then. For one thing, you no longer had to build your own material's testing laboratory to find out which size of music wire to use, nor figure out how to make a ferrule of the pesky stuff or which grade of canvas made the best wings. The bicycle brothers from Dayton had figured it out for you and offered the information to all who asked.

Having flown one airplane it wasn't too hard to build another. But oh, those Wright's were the very devil to work for! They insisted every part be made just so. It didn't matter if you'd been carving ash wagon spokes for thirty years, the Wright boys insisted wing-struts were different and would fire you on the spot if they found you adding just a bit more beef or trying to make those curvey things they called 'ribs' a little prettier.

Remarkably, when the new batch of copied parts was assembled into an airplane, the copy flew almost exactly like the original. Which of course was the whole idea. And the idea behind standards and certification of those standards was to insure that same thing would happen, over and over again.

As the Russians discovered with the B-29 and innumerable 'designers' with the Teenie-Two, when you steal a design and copy it, the copy will fly pretty much like the original... if you are both a good thief and a good copy-cat.

Aircraft standards are a monument to copy-cat-ism. Buy a pound of AN470B3-5's (those are rivets, by the way) from a manufacturer on the west coast and sonofagun if they don't work exactly the same as a batch of AN470B3-5's purchased from a manufacturer on the east coast! The same holds true for certified aluminum and steel and all sorts of stuff. Even plywood. The 'certs' are reasonable assurance the stuff will meet certain minimum specifications as to strength, weight and dimensional tolerance. Very handy stuff to know when you're building flying machines.

Then there are guys like me, who occasionally make a box spar outta pine shelving and doorskins. (Or landing gear struts out of exhaust pipe.) Then, like the Wright brothers, I break it. By the time it breaks, I know how strong a spar I can make out of that particular batch of pine shelving using plywood from that particular pallet- load. Ditto for foam and fiberglas. Make it, break it and go on from there. Because the last time I checked, there were no certs for spars made from pine shelving. No aviation certification for fiberglas fabric from the local boat shop. No guaranteed strength figures for pop-rivets bought in bulk from J.C.Whitney. When you elect to use such materials in an airplane, like the Wright brothers, you must become your own material's-testing-laboratory.

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Psst! Hey, kid. Wanna build an airplane?

Because you can, you know... others have, meaning you can too. You can build it from composites or plywood or fabric or aluminum... each has certain advantages. And those materials may come from the hardware store, fabric shop and local lumber yard... and may prove perfectly suitable for your flying machine... if you are wise enough to test them.

Plywood, aluminum, fabric, composites... What do you want? A pretty face or a good cook? An expensive party-girl or a steady, stable home-body? Do you wanna build it quick... and lose interest just as quickly? Or are you planning on something you can use to commute to work, year in, year out. Need to get in & out of that strip of beach down in Baja? Or do you plan to use that abandoned SAC base outside of town?

Wanna know the best plane there is? It's the one you decide to build.

The Democratic Process does not apply. The fact all your buds are driving RV's or glasbackwards plastic planes doesn't mean you should too. The plane you select will be the one that most closely matches your particular mix of money, space, tools and experience.

The truth is, yammering about the best this or the best that is mostly a waste of time. You must decide; you are the Mechanic in Charge. But once you've picked a design, stick with the plans! If you do something every day to further the project you'll be flying in a matter of months. Seriously. The secret is in the habit of doing something on the airplane every single day and in NOT deviating from the plans.

No one does, of course. Everyone is smarter than the original designer. Or fatter. Or taller. Or richer or poorer. So be it. If you depart from the plans, the Wright brothers have laid a clear trail for you to follow in how to ensure your new design -- for that is what it is, once you start tinkering with someone else's plane -- will be strong enough. And light enough. And smart enough to fly.

-R.S.Hoover -EAA 58400

(*) So how did the epitome of American mechanical genius come to be dangling in a British museum? Look it up. It's a lesson in how power, wealth and politics may be used as a weapon against the common man, then as now.

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The Best Airplane is a recurring theme among wannabe homebuilders. This (edited) article was first posted in July of 1999. Same question. Same answer :-) Steel tube & rag wings were not mentioned here because they were addressed in an earlier article (‘Flying On The Cheap')

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