The obvious question is, how does a flat wing have so much "dihedral effect?

I've built and am flying a Lacey M-1 elctric R/C. It flies well in it's present configuration, but I need to know why. Background: originally I had designed the model to have ailerons of the same size(to scale) as the original full size airplane. I'd checked with Mr. Lacey (now retired and living in Hawaii ) about the aileron chord. He told me 5".

My model had terrible adverse yaw problems. I coupled rudder into the aileron channel which helped, and finally I disconnected the ailerons altogether. The model flies very well using rudder for roll control. I remember Mr. Lacey saying that the full size aircraft flew well with rudder alone. As you probably know the lacey is a high wing cabin airplane with absolutely no dihedral, and my model is the same...

From : Don Stackhouse

This is not unusual for a high-wing cabin fuselage arrangement, particularly with a big, slab-sided fuselage with sharp corners. It's caused by interaction of the underside of the wing with the flow around the fuselage during a yaw.

Think of the fuselage as a very short-span, low aspect ratio wing set vertically. If we yaw such a fuselage to the left, it now has an angle of attack from the right, and begins producing lift to the left. This means that the air pressure on the left side of the fuselage is reduced, and the pressure on the right side of the fuselage is increased. The end-plate effect of the wing sitting on top of the fuselage will make the effect especially pronounced near the top of the fuselage near the wing.

The reduced pressure on the left side of the fuselage subtracts from the pressure on the underside of the left wing, which reduces the lift of the left wing. Meanwhile, the increased pressure on the right side of the fuselage increases the lift of the right wing. Voila! we have a difference in lift between the two wings, causing the aircraft to roll in the same direction it was yawed, which is functionally equivalent to "dihedral effect".

Note that for a low wing the effect is reversed, so that the response of the aircraft to this effect is to roll the opposite direction from the yaw. This is why low wing airplanes typically use more dihedral than otherwise similar high wing aircraft.

Don Stackhouse
DJ Aerotech