Wing efficiency
Thanks for the drawing Bob.
Steve, how does this design compare to an ellipical such as on a spitfire wing? Does it give the wing the same efficiency.
I wonder how much length could be added to keep the rv-6 wing in the utility catagory?
The key to wing efficiency is to achieve a nearly elliptical distribution of lift. It is not necessary to have an elliptical planform to do that -- there is an infinite combination of planform + airfoil camber + twist combinations that can achieve that. Although difficult to fabricate, an elliptical wing with no twist is one way to do it. Ironically, my pal R. J. Mitchell kind of missed the point on the Spitfire, which had a beautiful elliptical planform, but then he added 3 degrees of twist. One problem with the elliptical planform is that it tends to stall near the tip first because of the shorter chord lengths operating at lower Reynolds no. (thats beyond the scope of this discussion) and he wanted to prevent that.
It does turn out that an elliptical planform with a straight trailing edge is better than an elliptical planform with a straight 25% chord line like the spitfire - for some subtle reasons, you get a more elliptical loading when the trailing edge is straight. For a good reference on this, see the following article:
"Computation of Induced Drag for Elliptical and Crescent-Shaped Wings" by Stephen C. Smith and Ilan M. Kroo, Journal of Aircraft, Vol. 30, No. 4, July-August 1993.
Putting nice tips on an untwisted rectangular wing is better than one might expect, but it is not going to magically make the whole lift distribution elliptical either. The rectangular wing has a lift distribution that can be approximated by the average between an ellipse and a rectangle (Schrenk's rule). Some of the 'extra' induced drag comes in the last few feet of the wing, where the lift doesn't drop off as quickly as it should. Nice tips help a lot. Triangular tips like Paul Lipps' help drop the lift more quickly, but don't do it in a very smooth way because of the abrupt transition from rectangular to triangular shape. Sheared tips are not too bad, but the primary purpose of sheared tips is to influence the tip flow at high angles of attack, they shed the tip vortex farther forward and it rolls over the upper surface of the wing. This produces some extra lift, especially in the region near the ailerons, maintaining good roll control into the stall. The fact that the sheared tips also help drop the lift distribution more quickly and smoothly is a bonus -- but it they are not as good as an elliptical tip for low induced drag because the effective span is lower. This is evidenced by the newer stock Van's tips that increased the span from 23' to 24' on the RV-8 and had very little influence on the performance.
I can help you with the structural aspects (keeping within Utility catagory loads) but that should be done off-line.