Unpowered flaps

Introduction

A flap is a movable portion of the trailing or leading edge of an airfoil which can be deflected downward to increase the maximum lift coefficient of the airfoil. Many different types of flap have been used on aircraft, the selection of which involves considerations, in addition to aerodynamic performance, of mechanical complexity, cost, and weight. The effectiveness of a flap can be increased by injecting high-velocity air over its upper sur­face near the nose of the flap. This arrangement is referred to as a blown flap. The same effect as that produced by a flap can be accomplished by replacing the physical flap entirely with a jet of air deflected downward from the trailing edge of the airfoil, a scheme known as a jet flap. Sketches of unpowered flaps, the blown flap, and the jet flap are presented in Fig. 6-1. The leading-edge flaps, of course, are normally used in combination with a trailing edge flap.

Considerable data are available on flaps, but most of them are in a rather disorganized state. Reference 1 is an admirable attempt to bring order out of chaos. In addition to its summary curves, Ref. 1 contains an extensive bibliography of sources of information on specific flap configurations. How­ever, it falls somewhat short in that it treats the increment in the lift co­efficient below stall and not the increment in C, itself. This chapter does not present extensive data on the different types of flap, but there are enough to substantiate some of its general conclusions and observations.