A six freedoms flight dynamics model for a helicopter with pylons was built based on the theory of component-level. Trim calculation was conducted

and the result compared with the existing conclusion. Stability was analyzed for the helicopter casting objects with different patterns at different flight velocities. The result indicated that there was great effect on rolling angle and lateral control-stick when the helicopter cast objects by one-side due to the asymmetry of the recoil force. With the increase of the forward-flight velocity

casting objects had a greater effect on Dutch mode. Dutch mode evolved into two divergent aperiodic modes as the helicopter cast objects at a velocity higher than 50m/s

flight stability became worse obviously. The helicopter should cast objects at a lower velocity.