What happens when there are waterdrops on the wing? Will they create extra drag (because the wing surface will not be smooth)?

From : Don Stackhouse

It depends on a number of factors, especially the airfoil and on the Reynolds number ("Re") involved.

At full-scale Re's, the problem is in trying to keep the boundary layer flow laminar as long as possible. The drops tend to turbulate the flow, causing an increase in drag. In some cases they can also cause enough disruption to cause flow separation, which causes a very large increase in drag and a loss of lift.

At Re's more typical of model sailplanes, the problem is in keeping the flow attached. It would rather be laminar and separated than turbulent and attached. For smaller, slower models, if you turbulate it, the flow might even return to laminar later on the airfoil. The turbulating effect of water drops on the airfoil might actually help in at least some cases. However, in others, depending on the airfoil, the location of the droplets on the airfoil and the amount of disruption, it could hurt.

Another consideration is the location on the airfoil where the droplets tend to accumulate. On some parts of the airfoil they will tend to be blown off, while on other parts they will accumulate. The areas where this happens are not necessarily likely to be the same places where bugs tend to accumulate. The regions involved also depend a lot on things like angle of attack and airspeed. If the conditions are cold enough that the water can freeze, it really gets complicated, as the accumulating ice alters the airfoil shape. B.F. Goodrich (the big name in full-scale aircraft de-icing systems) even has a refrigerated wind tunnel where they can test for ice accumulation patterns and use that to help determine the required deicer boot design.

As far as painting or waxing, once again it depends on the Re's involved. Many years ago NACA (the forerunner of NASA) did some wind tunnel studies of this. In their case they were looking at full-scale Re's, where the problem is in trying to keep the flow as laminar as possible. They found that painting and waxing BOTH tended to INCREASE drag, even on clean (i.e.: no bugs or water droplets) airfoils, in comparison to bare sheet metal airfoils. They never completely sorted out why this was, but the best theory proposed that the paint and/or wax introduced "micro-waviness" to the surface that caused the boundary layer to transition to turbulent sooner than the smoother bare metal surface.

At our Re's, the turbulating effect of paint and/or wax could hurt or help, depending on the details of the individual case involved.

Don Stackhouse
DJ Aerotech