NACA-RM-L55E23 REV A

Abstract

INTRODUCTION <br>The analysis of the downwash behind wings given in references 1 to 3 has provided a good basis from which the horizontal-tail contribution to the static longitudinal stability can be estimated for wing-body combinations having thick unswept wings of moderate to high aspect ratios. This analysis was concerned largely with the conditions of unseparated flow and little rolling-up of the trailing vortex sheet, which conditions are applicable to most of the useful flight range for the type of wings considered. The corresponding problem for current high-speed airplane configurations is considerably more complicated than the problem studied in references 1 to 3. The increased complexity of the wing-body-tail interference problem is due to (1) the presence of flow separation over the wing for a considerable portion of the lift-coefficient range, which results from the use of sweep and airfoil sections having small nose radii, (2) the faster rolling-up of the vortex sheet resulting from the use of low-aspect-ratio wings (ref. 4), and (3) the greater importance of the fuselage because of its larger size. Early investigations of wing-tail interference for swept-wing configurations (refs. 5 and 6) showed that the tail had a powerful influence on the variation of stability through the lift-coefficient range and that this influence varied greatly with the vertical location of the tail. Numerous subsequent investigations have been conducted at both low and high speeds to study the wing-tail interference problem for various swept-wing configurations. In reference 7 a number of the important factors affecting the horizontal-tail contribution at low speeds were examined, and the problem of combining a tail with wing-fuselage combinations to provide good longitudinal stability characteristics was discussed. <br>The purpose of the present paper is to a more comprehensive review and analysis than was given in reference 7 of present knowledge concerning the low-speed horizontal-tail contribution for sweptback-wing airplanes. The characteristics of the flow behind sweptback wings and wing-body configurations are described and related to the downwash characteristics of specific tail locations for unstalled and stalled flow conditions. The effects of variations of tail position, variations of wing plan form and airfoil section, trailing-edge flaps, stall-control devices, and ground interference on the air-flow characteristics and tail contribution are analyzed and demonstrated. The more favorable tail arrangements are emphasized and their application to specific configurations are illustrated. <br>A brief analysis of the tail contribution to stability of thin unswept-wing configurations of small aspect ratio is included. Swept-forward or composite pl n forms are not considered; however, data for such configurations are given in references 8 to 12. The analysis of the factors affecting horizontal-tail characteristics is, to a large extent, descriptive. It was not possible to present quantitative design charts for estimating the tail contribution, but it was possible to correlate a number of significant parameters affecting the tail contribution and to suggest rough design procedures based on this empirical correlation. The experimental data on which the analysis in this paper is based were obtained mostly at Reynolds numbers greater than 4X10⁶. <br>In addition to the analysis presented herein, an index to published data on the air-flow characteristics and stability contribution of the horizontal tail obtained from tests of models at Reynolds numbers greater than 4 X 10⁶ has been prepared and is presented in tables I and II. For convenience, an index to figures is presented as table III.