ORIGINAL: ptxman
banktoturn> There is really no benefit (that I know of) to an elliptical planform per se. An elliptical lift distribution gives the minimum induced drag, everything else being equal. An elliptical planform is one way of getting an elliptical lift distribution. The important thing to note is that induced drag is only important for a lifting surface (wing, stabilizer, ...) which is operating at high coefficient of lift.
This brings up another grey area for me, maybe more related to wing design. Does this eliptical planform / lift distribution relationship have any (unmentioned?) underlying airfoil assumptions? In other words, if you had 2 radically different root & tip airfoils, would an eliptical planform still be better (meaning minimized induced drag) than a trapezoid type planform of similar aspect ratio? Or is there some 1940's Spitfire folklore built in there & the advantage is only confined to constant airfoil family accross the wing panel? (Gee, I dont even know, did they blend airfoils on those eliptical warbirds back then?)
banktoturn> A good airfoil would have low CD at zero and low CL. It might make sense to have a slight inverted camber, as you mention, but certainly not much. I'm not sure about this tradeoff either. There are some sites on the web that list favorable stabilizer airfoils, and I don't know offhand that any of the URLs are. A Google search may turn them up. If I find any, I'll let you know. You definitely want a real airfoil shape, rather than a blunt flat plate.
Nope, the blunt plate was never an option! The Avionik D99 (F5D pylon rcer above) references a naca0006. The Ariane crowd (link =
http://www.delago.de/ariane/EProfil.htm ) favours an HD800 & HD801 for similar application. From what I can see on the polars seem pretty comparable.
- Assuming the max thickness is defined, what about the high point placement? With the elevator proportion like shown in the D99 & maybe 10 deg max deflection, is there any pros/cons of the resulting polars by having the high point occur at 25% vs 30% vs 40% etc?
ptxman,
The effect of the elliptical lift distribution (minimum induced drag) is independent of how you accomplish it. If everything else is held constant along the span (same airfoil, no twist, etc.), then an elliptical planform gives an elliptical lift distribution. If a non-elliptical planform is used, then twist, or varying camber (blending different airfoils), etc. can be used to get the elliptical variation in lift from the root to the tip. The elliptical planform itself has no value, only the elliptical lift distribution. In any case, a trapezoidal planform, with the appropriate taper ratio, gives a lift distribution close enough to elliptical that it is hard to justify the trouble to build an elliptical one.
I'm not sure what all the issues are regarding the location of the high point. One major issue is laminar flow. A high point which is relatively far back (40% to 60%, say) is a primary characteristic of a laminar flow airfoil. This reduces drag by delaying the transition of the flow from laminar to turbulent, because laminar flow results in lower skin friction than turbulent flow. Generally, the flow can be kept laminar up to the high point (if the surface is very smooth & not wavy), but not much past it. I'm assuming that a high point fairly far back is better for this reason. It is not uncommon to use a turbulator somewhere near the high point, to cause the laminar flow to become turbulent. This helps prevent separation, which is more likely for laminar flow than turbulent. One thing to keep in mind: it might be hard to get the benefit of a laminar flow airfoil on the stabilizer, with propwash and other 'dirty' air hitting it. I don't know offhand how likely it is.
banktoturn