In this article, it struck me that the helicopter lander’s blades might be helped considerably by their undersides’ being dimpled like a golf ball. Those dimples supply significant lift during a ball’s flight. The helicopter wings would seem to benefit in the same way. I suspect that the dimples near the outer radius might be different in size and depth from those near the inner radius because of the difference in surface speed.

Danford Anderson
Green Bay, Wis.

Dimples on spinning golf balls reduce air resistance and increase lift by creating turbulence in the air flowing past the balls. In principle, dimples would also reduce drag for aircraft, says Anubhav Datta of the University of Maryland in College Park. However, since neither wings nor helicopter blades spin the way golf balls do, dimples could also wipe out the lift produced by those aircraft parts. So aircraft designers reduce drag, without compromising lift, by precisely placing a few small bumps or dips, called boundary-layer trips, on wings and blades. The Mars helicopter blades that Datta and his colleagues designed include such trips. –P. Weiss