LIFTING SURFACE THEORY AND TAIL DOWNWASH CALCULATIONS FOR V/STOL AIRCRAFT IN TRANSITION AND CRUISE
AIR VEHICLE CORP SAN DIEGO CA
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A large-tilt-angle lifting-surface theory is developed for tilt-wing and tilt-propellerrotor VSTOL aircraft. The method is based upon an inclined actuator disc analysis in which closed-form solutions are obtained for the velocity potential at large distances behind the actuator surface. Both the normal velocity and the nonlinear pressure boundary conditions are satisfied exactly across the slipstream interface. The inclined actuator disc analysis is combined with a discrete-vortex Weissinger-type lifting surface theory. Wing- propeller combinations at arbitrary wing angle of attack, propeller tilt angle, and thrust coefficient are considered. Multiple slip-stream effects including slipstream rotation are introduced. Agreement between theory and experiment is shown to be satisfactory for small slipstream inclination angles. However, at large angles the theory with an undeformed, but displaced, slipstream and wake predicts significantly lower downwash angles in the tail region than shown by the test data, possibly due to slipstream deformation and wake roll-up. Use of only one-half the calculated wake displacement gave improved agreement at these conditions. However, insufficient data are available for making a general evaluation of the theory at large angles. Extensive digital computer results are given in chart form, showing span loading, downwash angle, stability parameter, and dynamic pressure at arbitrary points behind the wing for VSTOL configurations with two and four slipstreams.