My Own Private Wichita
Reading Ken Scott’s very sound advice to would-be homebuilders in Flying's May issue got me thinking about my own experiences, and how things have changed since I got involved in homebuilding.
I started thinking about building an airplane around 1963. At that time there were no prefab kits on the market and only a limited number of plans. Most homebuilts were sport biplanes or small wood or tube-and-fabric cruisers like the Wittman Tailwind. Like most young men I was entranced by speed and fighter-like looks and flying qualities, and so I leaned toward something like the Long/Bushby Midget Mustang, a handsome single-seat taildragger of all-metal construction. But although plans for such airplanes could be bought, I don’t recall ever considering them; I wanted to design an airplane myself. This was a project for which my only qualification — at the time I was on a two-year hiatus from college, where I majored in English — was apparently boundless self-confidence. I knew nothing about engineering, stress analysis, calculus, aerodynamics, materials, construction methods or aircraft systems.
In various college engineering libraries I pored over NACA technical reports and the one or two books on practical construction that existed then. I remember that K. D. Wood’s Aircraft Design, in a floppy red binding, was full of fascinating details, many of which were either over my head or irrelevant to the kind of airplane I wanted to build. Despite all the reading, however, I think that much of the real learning consisted not of facts systematically absorbed from study but of insights that came to me from random and often unexpected sources.
For example, the phrase “load path” is often heard in discussions of structures. Forces acting upon a structure can be thought of as three types: tension, compression and shear. The first two correspond to pulling and squeezing; the third results from forces acting in opposite directions on an object, like the blades of a pair of scissors on a sheet of cardboard. If a wing, for example, supports the weight of an engine that is far away from it, there has to be some structure connecting the two, and it should be possible to say what sort of compressive, tensile, and shearing stresses and strains are occurring between one and the other.
This idea seems pretty basic, but my ideas about its practical application remained nebulous. Then I happened to pick up a magazine that contained one of those wonderfully detailed see-through drawings, this one of a Thorp Sky Scooter. The Sky Scooter’s engine rested on a narrow bed mount rather than a weldment of steel tubes. The tension in the upper part of the bed mount had to get into the upper fuselage longerons, which were much farther apart than the bed mount was wide. The load path was through a shelf in the firewall: Tension became shear, shear turned back into tension. This picture brought me my little load-path epiphany: The shelf was analogous to a spar. It is so obvious now that it seems bizarre that it could ever have seemed like a revelation; but that is how learning is done.
Another example: the Scooter’s designer, John Thorp, whom I later got to know and who helped me a great deal, liked to point out the counterintuitive fact that all steel, whether it be a wire hanger as soft as cookie dough or a tempered spring that you can’t scratch with a file, is equally stiff. Now, this is something that engineering students learn in the first week of Materials 101, but I never took Materials 101, and so notions of elasticity, stress and strain had to come to me by other routes.
The concept of stiffness, or more precisely, of “elastic modulus” — how much a material changes size under a given load — is very important in design. Here is why: Suppose you have a 100-pound weight, and you have some 75-pound-test steel wire and some nylon monofilament, also of 75-pound-test, to hang it from. Together the numbers add up to a 50 percent margin of safety, but actually both components will break, because the resistance of each support is only as great as the stretch put into it. The stretchy nylon will provide hardly any support at all while the steel wire reaches its limit and breaks; then all the weight falls upon the nylon, and it breaks as well.