ORIGINAL: The_Pipefather
That clears up some things.
But I dont understand why a tapered wing can act as if it had dihedral. Could someone please explain? Also, is this effect really significant, in light of the fact that banktoturn and dick say that the efficiency advantages of a tapered wing are not really that significant?
My application involves designing a normal sport plane with a straight (i.e. non-dihedral) wing. If it turns out that a tapered wing can give me the self-correcting effect of dihedral (while both upright and inverted) I want to use it.
Dihedral effect is a change in rolling moment as a result of a sideslip. There is a dihedral angle for any airplane that will result in zero change in rolling moment with sideslip. This is what most aerobatic models want, and the geometric angle depends on the position of the wing on the fuselage, among other things. Non-neutral dihedral effect will ALWAYS be assymetrical (opposite) between upright and inverted.
Wing leading edge sweep works the same upright or inverted, assuming the wing also has "neutral" dihedral effect, and affects mainly yaw, not roll. The Giles series are good examples of this. Zero yaw-roll coupling (zero bank stability), yet strong yaw stability, without discernible dutch-roll, and with no penalty against knife-edge performance either. Really groovy. The tradeoff is
relatively poor high-alpha performance, compared with Edges (you have to work more during harriers). Not bad, mind you, just not quite as good as planes with less LE sweep. Exceptional at traditional precision aerobatics, especially rolling maneuvers (ever try a 32 point roll? The plane begs you to try it.), and can be made passable 3D ships if kept very light.
The efficiency advantage of a tapered or elliptical wing are not significant
for models. After all, who cares about efficiency, unless you are going for duration or distance? To make a constant chord wing more efficient (approximate elliptical lift distribution that banktoturn mentioned), it must be washed out. To make an elliptical or taperd wing
less efficient, give it wash out, since the elliptical, or approximately elliptical lift distribution applies only when those planforms are un-twisted. But they are often twisted anyway, at least on non-aerobatic models, to make them handle better at stall, which is a safety consideration and usually overrides a small efficiency loss due to the twist. But aerobatic airplanes, at least ones that will spend a lot of time upside down, do not want wing twist at all, since even though it will make stall more gentle during a positive stall, it will make it vicious when negatively stalled.
Tapered or elliptical wings generally have less drag, roll faster and snap roll better than equivalent area and aspect ratio constant chord wings.