A study of the flow field about an upper surface blown wing and its effects on longitudinal static stability of a small jet airplane

The effects of powered lift technology on the longitudinal stability characteristics of an upper surface blown (USB) jet-flapped aircraft have been studied. Both theoretical and experimental methods were employed to investigate the flow field in the vicinity of the lifting wing and its influence on pitching moments of the aircraft in motion.

A nonlinear vortex-lattice theory was used to model the lift induced airflow in the near flow field about the wing. Comparison with literature data showed excellent agreement, giving confidence in the method used.

Flight tests were conducted with the "Ball-Bartoe Jetwing" technology demonstration aircraft to visualize flow angles on the fuselage surface and at selected points in the flow field near the horizontal tail. To save cost, simple data acquiring techniques were used. Also, the theoretical flow computations were restricted to a simplified Jetwing model with a relatively small number of panels. Still, there was fair agreement between theoretically and experimentally determined flow angles at the tail of the Jetwing aircraft.

The vortex-lattice routine was then used for a parametrical study of flow angles and longitudinal stability parameters. The results show that the powered lift strongly affects the flow field about the wing. With increasing blowing coefficient the flow field has a more destabilizing impact on the pitch-stability of the aircraft. The data also indicate that for certain conditions the presence of the jet sheet improves the stability contribution of the horizontal tail by reducing the downwash derivative in this area. This may be attributed to the particular geometry of the trailing wake and tip vortices. Further research is needed to substantiate these findings.

The vortex-lattice routine developed as part of this study is considered a valuable tool for the flow analysis and the preliminary design of powered lift aircraft with jet-flapped wings.