LOW SPEED AERODYNAMIC CHARACTERISTICS OF JET VTOL AIRCRAFT AT ANGLES OF ATTACK.
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF AERONAUTICS AND ASTRONAUTICS
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A VTOL model incorporating a lifting fan mounted vertically in the fuselage was tested in the M. I. T. Wright Brothers Wind Tunnel to examine the variations of the pitching moment and longitudinal forces with changes in the angle of attack and forward velocity. The model was tested at angles of attack between 90 degrees, but the results were considered reliable only up to 45 degrees due to stalling of the model fan blades. Moment was found to be unstable between the measured angles of attack from -45 degrees to 10 degrees, increasing moderately as angle of attack increased. An increase in the ratio of forward velocity to fan efflux velocity also produced an increased moment. A theory developed by A. R. Kriebel based upon a Fourier analysis of the vortex distribution on a thin cylindrical ducted fan was employed to predict the results of the experiment. Considering the simplifying assumptions used in the theory, the correlation was found to be reasonably good except at high ratios of free stream to fan efflux velocity. At these ratios, an uncompensated for low pressure area aft of the exhaust duct resulted in considerably erroneous predictions. Thrust generally increased with angle of attack and varied with the velocity ratio, the variation being related to the sign of the angle of attack. Author