Does the rough surface of a wing improve performance?

I cut a little foam wing for a E-400 sport plane. I used 1.25 oz glass on it and soaked up the excess resin with toilet paper. Will the roughness of the FG create a better or worse boundry layer, less or more drag?

From : Don Stackhouse

Lynford, that depends on the model, the operating envelope and the airfoils. The roughness of the glass surface should act a a turbulator, causing the boundary layer to transition from laminar to turbulent. Laminar boundary layers have lower drag than turbulent ones. They also tend to separate much easier than a turbulent boundary layer, and once separated, their drag is much higher than a turbulent boundary layer.

Some (not all) airfoils are prone to separation at certain ranges of angle of attack and Reynolds number. Speed 400 models often operate in this range. If the airfoil you're using is one of these, the addition of a turbulator of some sort can help the flow remain attached where it would otherwise have separated. This can improve both lift and drag. For best effect, the turbulation should occur just before the point where the flow would normally have separated. This way you get the low drag benefits of a laminar boundary layer over as much of the surface as possible, but turbulate it just in time to prevent separation. Turbulating ahead of that point, and keeping it turbulated (such as with your rough wing skin) will incur some penalties from unecessary turbulent flow, but will still prevent separation. The overall effect might still be positive.

On more modern airfoils that are designed to avoid separation without using turbulators, there is no benefit to turbulation, only extra drag. Whether or not your particular model falls into this particular category is hard to say without going into a lot more detail about the entire design.

It's possible that your rough wing skin helps a little, but also possible that it could hurt. In any case, it's not likely to be a big problem, if it's a problem at all. I wouldn't worry about it too much. If your wing skin has ripples in it, even small ones, that more likely to be a troublemaker. If it's a little rough but reasonably wave-free, I'd recommend you leave well-enough alone.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/