we can write
This equation shows that the speed depends on the wing loading (W/S), density, and lift coefficient. The speed does not depend on the drag a result that does not seem to make the perfect sense we expected. The equation is correct, but the approach is incorrect.
The flaw is related to what is meant by "flies faster." When we directly compare two sailplanes in flight side-by-side, we fly them at approximately the same glide slope (or L/D). In other words, we do not match the lift coefficient CL but instead the L/D, which equals the ratio CL/CD. For the two aircraft, this requirement gives
where the subscript (1) refers to the sailplane with the lower drag and (2) the one with the higher drag. Equation (3) gives:
we get the result that
Thus, if we assume that the two sailplanes have the same wing
loading, then through equation (1) we find that the speed V of the
sailplane with the lower drag (flying at CL1) is higher
than that of the sailplane with the higher drag (CL2)
a result that agrees with the "right answer" our expectation.
RC Soaring Digest on-line articles
RC Soaring Digest Homepage