Jet Entrainment Theory for Vertical Takeoff and Landing Aircraft Suckdown

The suckdown induced on a vertical takeoff and landing aircraft by the lift jets and the equal but opposite reduction of jet thrust have been studied to develop a physics-based model for predicting these forces. Static pressure distributions and jet velocity profiles have been computed and compared to surface pressure and jet survey measurements to understand the origin of the forces on the aircraft and jet. It is concluded that the suckdown force on the aircraft in the absence of flow separation is caused by the acceleration of the flow entrained by the jet; this acceleration reduces the static pressure on the bottom of the fuselage more than on the top of the fuselage. The equal but opposite reduction of jet thrust is due to the increased loss of jet kinetic energy to turbulence in the high-pressure region created at the nozzle exit by the turning of the entrained flow. It is shown that both forces can be computed correctly if the static pressure at the nozzle exit is allowed to adjust to the local static pressure imposed by the entrained flow.