Synthesis of the Longitudinal Control System of a Fully Submerged Hydrofoil Craft with an Auto-Pilot

Speaking of a fully submerged hydrofoil craft, all the hydrofoils attached to the foilstruts of the craft body are fully submerged in the water, so that the disturbing wave forces acting on the hydrofoils are very small in their foilborne cruising compared with the forces on the surface piercing hydrofoils. Therefore, the fully submerged hydrofoil craft gives us a very comfortable riding. On the other hand, the hydrofoils have little to do with stabilizing the height above the water surface and the attitudes of the craft and hence, an autopilot system is needed to stabilize the height and the attitude of the craft. In the present paper, a synthesis method for the longitudinal control system of a fully submerged hydrofoil test craft with an autopilot is described. The test craft (displacement: 4.3 tons) has two flaps attached to the port and starboard fore foils and one flap to the after foil, The height and the attitude (pitch angle) of the craft are affected by the variation of each flap angle. Therefore, the dynamics of the longitudinal motion of the craft are described as a coupled system, which has two controllers (fore and after flaps) and two controlled quantities (the height and the pitchattitude of the craft). The longitudinal control system including the craft dynamics and the autopilot consists of a height control loop and a pitchattitude control loop. These two control loops are coupled together through the craft dynamics. The longitudinal control system is roughly studied first, assuming the system could be separated into two decoupled loops, and then studied strictly as a coupled system. By this method, it has been shown that the synthesis for the longitudinal control system of the fully submerged hydrofoil craft could be done efficiently, and also that the calculated results of this method coincide well with those of the sea-trials by the test craft.