Application of thin aerofoil theory to the design of double flap controls of small chord

Abstract

Recent experimental investigations on small-chord controls in two-dimensional flow suggest that such controls are more efficient than wide-chord controls. The experiments also suggest that a further gain is obtained if the control or flap is broken, hinged and geared at some point along its chord. This Note examines, on the basis of the thin aerofoil theory, the control efficiency of such double flap systems, as ailerons and as elevators. A range of values of total chord ratio is covered, and the optimum arrangement determined in each case. The theory suffers from the limitations of the thin aerofoil theory, which fails to take account of the thickness/chord ratio and the boundary layer effects ; these can be large for the thicker sections. It does, however, provide an indication of the effect of variations of the various parameters and also the ratio of the flap chords defining the optimum. In general terms the problem considered here is to find the minimum control column force to produce a given rolling moment. Throughout the present work the lift is fixed at that produced by the 0.50 chord flap. It is shown that the smaller chord single- and double-flap systems are more efficient than a wide-chord arrangement and that a double flap is more efficient than a single flap of the same total chord.