NACA-TN-823

Abstract

INTRODUCTION <br>When an airplane approaches the ground it undergoes a marked increase in static longitudinal stability. This increase in stability results, in a large measure, from the effect of the ground on the downwash angle at the tail. Not only is the downwash angle decreased bat the rate of change of downwash angle at the tail with wing lift coefficient is smaller near the ground than at great heights from' the ground. Thus, at a given airplane attitude, the tail is operating at a higher positive (or lower negative) angle of attack near the ground than at a distance from the ground, and this difference in tail angle of attack increases as the wing angle of attack increases. The rate of change of tail lift with wing angle of attack and the pitching moment caused by tail lift are therefore increased by the presence of the ground. <br>Another factor contributing to the increase in stability is the increase in the effective aspect ratio of the tail caused by the proximity of the ground. This factor, however, is probably a second-order effect. <br>In view of the increase in stability caused by the ground, a much larger elevator deflection is generally required to trim the airplane at maximum lift, particularly with flaps, near the ground than is required to trim the airplane in the same attitude at a distance from the ground. It is quite possible that the elevator, although satisfactory for other flight conditions, may not be powerful enough to trim the airplane in landing. Even when the elevator is powerful enough to trim the airplane, the deflection required may be of such magnitude that the stick force will be prohibitive. The landing condition may well be the most severe criterion in the design of the tail. <br>A means of determining the elevator effectiveness, when landing, from wind-tunnel tests of a model is therefore desirable. In tests of this type the ground may be simulated by an image model, by an endless belt moving with the velocity of the air stream, by a stationary plate, or by a combination of an image model and a plate. Much has been written concerning the relative merits of each method. (See references 1 and 2.) Obviously, the plate method is by far the simplest. The validity of the results obtained with this method of ground representation, is, however, open to question because of the existence of a boundary layer over the plate in the wind tunnel that is not present over the ground. Despite the doubt as to the adequacy of the plate method, it has been used in numerous investigations. (See references 3, 4, and 5.) <br>Although the results of these investigations (references 5, 4, and 5) are essentially in agreement as regards the effect of the ground on pitching moment, they present no direct evidence as to the applicability of the tunnel data to the full-scale airplane. In the present investigation a model was tested over a plate in the tunnel and the tunnel data were compared with 'flight data for the full-scale airplane. The results of the tests and the comparison are presented herein.