The effect of compressibility on the attitude of aircraft in rectilinear flight

Abstract

The attitude of aircraft (i.e., the angle between the aircraft datum and the flight path) is of considerable importance in the aiming of certain airborne armament. An investigation was therefore made of the effect of compressibility on the attitude of aircraft in flight in a straight path. The application of the results of linear perturbation theory to the problem was examined, and the deductions made compared with the results of attitude measurements on a Spitfire IX over a wide range of altitude and air speed. As is well known, linear perturbation theory indicates a reduction of the slope of the curve of attitude against lift coefficient with increase in Mach number. The theorv indicates, however, that in straight flight at a constant ratio of wing lift to air pressure the variation of Mach number with lift coefficient is such that to a first approximation the slope of the curve of attitude against lift coefficient remains unchanged at the low Mach number value, only the intercept, or apparent no-lift angle, being altered (Fig. 1). This reduction in no-lift angle is proportional to the ratio of the lift to the air pressure but is not directly affected by Mach number or air speed. The experiments show a reduction in no-lift angle which agrees with that predicted by theory for the aspect ratio of wing tested. The change in apparent no-lift angle is of the order of half a degree between sea level and 40,000 ft. The above conclusions should not be applied to wings over 15 per cent thick.