The suckdown induced on a vertical takeoff and landing aircraft by the lift jets and the equal but oppositenreduction of jet thrust have been studied to develop a physics-basedmodel for predicting these forces. Static pressurendistributions and jet velocity profiles have been computed and compared to surface pressure and jet surveynmeasurements to understand the origin of the forces on the aircraft and jet. It is concluded that the suckdown force onnthe aircraft in the absence of flow separation is caused by the acceleration of the flow entrained by the jet; thisnacceleration reduces the static pressure on the bottomof the fuselagemore than on the top of the fuselage. The equalnbut opposite reduction of jet thrust is due to the increased loss of jet kinetic energy to turbulence in the high-pressurenregion created at the nozzle exit by the turning of the entrained flow. It is shown that both forces can be computedncorrectly if the static pressure at the nozzle exit is allowed to adjust to the local static pressure imposed by thenentrained flow..
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