Elliptical Planform: Spitfire vs. Supra

[rotofro]

When designing an elliptical planform, what tends to influence the x-axis location of the intersection between the major- and minor-axis? For example, with the Spitfire, this intersection occurs towards the leading edge, while with thermal duration sailplanes such as the Supra or Icon, the axes meet toward the trailing edge. I'm assuming this decision is made for reasons other than aerodynamics...

[schrederman]

I think the designers are trying to have the best of both worlds... elliptical plan form with some Schuman effect.

JW

[mlachow]

There is no easy answer. It's a combination of a whole bunch of things. Keeping the hinge line in a straight line at a certain point in the airfoil, keeping a relatively straight spar so that bending does not induce wing twist, local Rn and airfoil selection along the span, and what happens along the span when you are doing things like circling.

It is aerodynamics, and structural.

Now you should be really confused or impressed by the knowledge and expertise of the designers....

[tonyestep]

Schuemann's idea was that sweeping the mean line near the tips would promote spanwise flow, which would blow away detached tendrils that otherwise would propagate inboard. He modified his full-size plane to sweep the tips, and adduced a bunch of neat photos of tufts under the wing of his plane that supported his theory.

Recently Jon Stone posted a link to this article:
http://www-scf.usc.edu/~tchklovs/Proposal.htm
which has a bunch of numerical analyses of planforms that also support Schuemann's theory, showing better lift curves, lower drag, and (therefore) better L/D for crescent planforms.

However, Mark Drela doesn't buy it, arguing that the sweep doesn't do anything and that the twisting moment caused by the sweep requires extra material in the tips to withstand hard launches; this leads to unfavorable effects on the moment of inertia and hence the handling qualities. Numerical analysis done with Drela's software (AVL) does not give the same results as those quoted by the author of the above paper.

Early planes (e.g. Psyko) with straight trailing edges and swept mean lines were that way mainly because it made it easy to make the hinge lines straight, and maybe because the Schuemann planform was then the fashion in full-scale planes.

Further to complicate matters, most of our planes have polyhedral, and the performance in circling flight is the main criterion that determines design success; published analyses that I've seen don't deal with either of these key factors.

Bottom line: Mike sez it all -- you should be confused, or impressed -- your choice.

[schrederman]

Quote> Now you should be really confused or impressed by the knowledge and expertise of the designers....

Is this that "Shock and Awe", thing?

Sorry, in a crazy mood tonight and couldn't pass it up...

JW