Abstract: For the complicated structure and limited power of pneumatic ejection of active unmanned aerial vehicles(UAV)，a hydrodynamic ejection scheme is proposed，in which the reverse thrust generated by high-pressure water jets as power is used to take off the VAU quickly.A non-linear ejection dynamics model is established, which couples the air chamber state equation and water chamber momentum equation of ejector，the energy and momentum equations of jet，and the force balance equation of catapult load. The variations of ejection speed and displacement，spurt speed and reverse thrust，ejector air chamber pressure，and the influences of ejector diameter，jet diameter and inflation pressure on catapult performance are numerically analyzed. The results show that, as the catapult time increases，the catapult speed and displacement increase approximately in a logarithmic curve and an exponential curve, respectively，and the ejection speed，the reverse thrust of jet and the air chamber pressure decrease significantly in a parabolic state. When the ejector diameter is 200 mm，the ejector length is 2 400 mm，the jet diameter is 20 mm，and the inflation pressure is 31.5 MPa，the 150 kg UAV can be accelerated to 25.02 m/s in the range of 16.62 m. Increasing ejector diameter adversely affects ejection performance.The increase in jet diameter has little effect on the maximum ejection speed，but it significantly shortens the catapult time and distance，and increases the catapult overload.Under the premise of catapult overloaded，an appropriate increase in inflation pressure can reduce catapult time and distance，and help to increase catapult speed.

Key words: unmannedaerialvehicle, hydrodynamicejection, jetreverse, dynamiccharacteristics, numericalsimulation