Pitch-heave dynamics models for an air cushion vehicle

The pitch-heave dynamics of an air cushion vehicle with an uncompartmented segmented skirt system were investigated through experiments on two craft and through simulation. The experiments show that this type of craft can exhibit a pure heave instability at higher fan speeds and a combined pitch-heave instability when released from large initial pitch angles. The response in both cases is strongly affected by the type of material used in the skirt. Results presented here show that the heave instability is strongly affected by the skirt material extensibility which allows cushion volume and exit-airflow-area modulation with cushion pressure. The volume modulation gives an additional cushion capacitance which is seen to decrease the stability whereas the airflow area modulation increases stability.;The important skirt related factors in the dynamics of the segmented skirt cushion are determined through a simulation of the nonlinear dynamics and a linearized analysis. Both the nonlinear and linearized analyses include a detailed model of the skirt geometry which uses an elastic membrane model of the skirt. Both also include different models of the lift-air system that can account for combinations of pneumatic and flexibility capacitance of the plenum box and cushion, and for the inertance of airflow in the ducting. In addition, the nonlinear model incorporates a simple skirt force model to account for hysteresis seen in vehicle static stiffness characteristics. The simulation results show that the pitch-heave instability occurs as a result of coupling which arises from the dependence of the exit-airflow area geometry on both the pitch and heave position. The static hysteresis models produce enough damping to suppress the instability at small initial angles but not at larger ones--as seen in the experiments. However for the skirt materials used, the static hysteresis model is inadequate to provide all the damping seen in the dynamics experiments. The lift-air inertance and capacitance effects are unnecessary for the prediction of the pitch-heave instability; the capacitance however is necessary for the heave instability and gives a major qualitative improvement in the agreement of the simulation and experimental pressure histories.;Exploitation of the flexibility capacitance is advanced as a means of scaling the capacitance of full scale craft in scale models.