NACA-RM-A53J07

Abstract

INTRODUCTION <br>Adverse variations in the longitudinal stability of swept-wing airplanes during certain phases of flight produce objectionable flying characteristics that have been the subject of considerable study during the past few years. Some of the stability variations have been eliminated or considerably reduced by the use of stall control devices, but other unsatisfactory longitudinal stability characteristics, particularly those occurring at high subsonic Mach numbers, have not been amenable to improvement by use of such devices. One airplane which has typical longitudinal instability at high speed and moderate lift coefficients was the subject of flight investigations (refs. 1, 2, and 3) which indicated that modifications to the wing, such as the addition of vortex generators or fences, did not produce adequate improvements in the stability. An investigation in the Ames l2-foot pressure wind tunnel of a model with a wing similar in plan form to that of this airplane also indicated that addition of a fence and leading-edge chord extensions do not result in satisfactory stability, since they failed to eliminate a forward movement of the center of pressure at the higher angles of attack at Mach numbers above 0.83 (see ref. 4). <br>Although the objectionable stability changes apparent from the data in reference 4 were attributable primarily to the pitching-moment characteristics of the wing-fuselage combination, they were aggravated by variations in the contribution of the tail to stability, indicating that an improvement might be realized by locating the tail in a region where downwash distribution or wake effects would be more favorable. <br>Wind-tunnel tests of various models with swept wings, such as those reported in references 5, 6, and 7, have indicated that the tail contribution to stability at high angles of attack could be increased substantially by locating the tail near the wing chord plane extended. The model described in reference 4 had the tail 22 percent of the wing semispan above the wing chord plane extended, which corresponded to the tail position of the swept-wing airplane that was the subject of flight tests reported in reference 1. For the investigation described in the present report, the same model was tested in the Ames 12-foot wind tunnel with the tail in a low position, 8-percent semispan above the wing chord plane extended. This position was selected in order to duplicate the location of the horizontal tail of an airplane (similar to the one previously mentioned) which was modified for flight tests with the tail at the bottom of the fuselage. <br>The results &scribed in this report include wake surveys at the tail as well as the evaluation of the effective wing downwash at the two tail locations. In addition, the effect of adding one of the leading-edge chord extensions employed in the tests reported in reference 4 was investigated on the model with the tail in the low position.