24-ft tunnel tests on a high-lift model downwash and velocity measurements at the tailplane

Abstract

Pitching moment measurements in the 24 ft. tunnel have shown that the highlift model with double Fowler flaps down and slat open, although reasonably stable without slipstream, becomes unstable at the higher thrust coefficients required for level flight and climb. In some cases the tailplane contributed nothing to the longitudinal stability of the model. The present tests have been made to investigate the airflow in the neighbourhood of the tailplane. Pitching moment measurements in the 24 ft. tunnel have shown that the highlift model with double Fowler flaps down and slat open, although reasonably stable without slipstream, becomes unstable at the higher thrust coefficients required for level flight and climb. In some cases the tailplane contributed nothing to the longitudinal stability of the model. The present tests have been made to investigate the airflow in the neighbourhood of the tailplane. The effect of the increased velocity is main]y confined to the region of the slipstream, while the increase in downwash angle with thrust coefficient extends over a wider range in both directions. The variation in downwash angle and velocity is such as to make the tailplane a destabilising rather than a stabilising member, at constant throttle with the flaps and slat in operation with the tallplane in any practicable position. This is due to the large downwash angles associated with the relatively low aspect ratio wing. The stability could be improved by the use of a higher-aspect ratio wing. The effect of slipstream on the complete aeroplane, however, is not necessarily destabilising with flaps down because of the favourable effects of a high thrust line and of the slipstream velocity over the wing and flaps.