Rob a Frise Aileron is one that has the nose portion projecting ahead of the hinge axis, the lower surface being in line with the lower surface of the wing. when the trailing edge of the aileron is raised the nose portion protrudes below the lower surface of the wing and increases the drag on that wing tip.

this was an attempt to combat the adverse yaw of a simple hinged aileron which produces more drag on the downward moving aileron causing the airplane to yaw uncomfortably in the opposite direction of the desired turn. It was invented sometime in the late thirtys or perhaps the early forties and I beleve the name came from the inventor and early patent holder but can,t confirm that point.

One of the more common ways to minimise the effects of adverse yaw is aileron differential which mechanically or electronically prevents the downward moving aileron from traveling as far down as the upward moving one goes up thus combating adverse yaw.

John