bulged control surfaces?

[Bob Chiang]

Hi all,

I recently read something about the rudder on the full sized P-51. According to the thread participants, the thickness of the rudder at the hinge line is wider than the adjacent vertical fin. As I understand it, the intent is to keep the boundary layer airflow against the rudder. They didn't mention if the other control surfaces were constructed this way.

Is this something that would be helpful on our models?
-Bob

[Ollie]

I think it is interesting to note that Dr. Mark Drela's AG4- series of airfoils do just the opposite. Their thickness is reduced in the vicinity of the flap and aileron hinge lines. With slight reflex, for high speed, the bottom surface of the wing is fair to reduce drag. When the trailing edge is drooped slightly to increase camber, at low speeds, the top contour is fair in the vicinity of the hinge line to avoid thickning of the boundary layer that would increase drag.

It is just a guess but, the thickened rudder just behind the hinge line might have been motivated by a desire to improve control power rather than to reduce drag.

I wonder when the design decision to thicken the rudder leading edge was taken. Was it part of the original design or did it come later? The answer might be a clue as to its purpose.

[Antonsoarer]

I wonder if it was designed to tackle flutter in high-speed dives. If you mask a control surface at high-speed it can end up oscillating (bouncing) in and out of the layers while inside of or near the neutral position.

Hmmm… just described one of many precursors to flutter.....

Tony.

[BMatthews]

The idea was that the slight rounded bulge would encourage the airflow to stick longer.

Actually some of Mark Drela's very thin sheet balsa tail airfoil shapes that use the separate movable control surfaces use the same bulge at the hing line for much the same reason I suspect.