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In aircraft, an airfoil comprising in combination a fixed surface having a rearwardly opening housing for receiving a portion of a control surface, pivotally supported relative to the fixed surface and having a nose section extending forward into the housing, said nose section being vertically spaced from and between the rear edges of the housing, the control surface being spaced from and substantially faired into the contours of the fixed surface to form a continuation thereof; means for statically balancing said control surface comprising weights attached to the nose section and entirely confined within the housing through‐out the range of movement, and means for aerodynamically balancing said control surface comprising a flexible impervious membrane connecting and scaling the nose of the control surface to an interior surface of the housing whereby to divide the housing into two chambers each communicating with one surface of the airfoil through the vertical spaces between the rear edges of the housing and the nose section of the control surface, said flexible membrane having sufficient slack at all normal positions to progressively increase the area thereof in contact with the wall of the housing as the control surface is tilted in cither direction from neutral towards its extreme positions with the nose section approaching the corresponding wall of the rearward extension of the fixed surface, whereby the effective area of the aerodynamic balance is decreased.