Model plane designer?
Its so many years since I aw the original that I hardly remember anything about it. But the "twist" at the end that the aero-engineer only designed model planes didn't ring the faintest bell.
Such a premise, and the character's claim that the principles are identical were (a) ludicrous and (b) irrelevant.
Ludicrous because the building of life-size planes involves engineering principles and practice that goes way beyond anything required for building miniatures. No-one has to use calculus to determine the tensile strength and the amount of flex of a piece of balsa wood. If the proto-type breaks, no-one got hurt. You just use a thicker bit.
Irrelevant because the equipmeny necessary for making sensitive measurements and calculations wasn't available under the conditions they found themselves in. For example, an over-simplistic rule of thumb in miniature-design is that the balance point is around the main spar. So how would they have calculated the positioning of the wings to give the right balance point? Put them too far forward and the plane becomes unflyable. Too far backard and it'll nose-dive.
Oh, and a tendency to lift as the wind hits the plane head on is not an indication that it will be flyable. Try flying a piece of flat cardboard that lifts in the wind!