Control Surface Flaps for Trim and Balance

Abstract

W I T H I N the past few years the application of small flaps to the movable control surfaces of airplanes has become extensive. That these flaps give a quick, light and highly effective control is shown by the fact that on the CurtissWright Condor a 90° adjustment of the elevator flap control wheel changes the longitudinal trim of the airplane as much as 28 turns on the stabilizer adjustment control. On airplanes of more than one engine the control flap on the rudder has proven to be of great assistance to the pilot when flying with one engine dead and is largely responsible for the fact that modern twin engine airplanes can be designed satisfactorily with a single vertical tail. For example, the original Curtiss Condor, built in 1929-1930, had two fins and rudders located in the slipstreams from the two propellers to enable straight flight to be maintained on one engine. The new Condor is built with a single vertical tail mounted rigidly on the fuselage, and the rudder with its control flap has sufficient power to hold the airplane on a straight course with one engine out of commission. When used to give control in lieu of an adjustable stabilizer or vertical fin, the control flap may be called a trimmer and when used to reduce the hinge moments of a control surface, the term balance is appropriate. In the following notes the theoretical formulae for the effect of these flaps are presented as well as the results of wind tunnel tests on several types of trimmers and balances.