Numerical investigation of the porpoising motion of a seaplane planing on water with high speeds

Abstract During the taking-off and landing processes on water, a seaplane will begin to porpoise under a certain set of conditions, which is a threat to the flying safety. Porpoising motion is an unstable oscillation about the gravity center in the vertical direction. In this paper, the porpoising phenomenon is studied by a modified two-phase flow solver in OpenFOAM through the simulation of a large seaplane with four turbines during take-off. In order to study the porposing motion under real conditions, complex effects are fully considered such as the hydrodynamic forces, aerodynamic forces, ground effect and slipstream. Firstly, an actuator disk method is added to the interDyMFoam solver, and the sixDoFRigidBodyMotion solver is also modified to make the actuator disk move together with the aircraft. By doing this, the efficiency of computation is greatly improved in the unsteady multi-phase calculation. Secondly, verification and validation (V&V) studies are carried out by comparing the results herein to those of the benchmark towing tank experiments of Fridsma and a single propeller wind tunnel test. Stable high speed planing is studied in advance, and the triggering conditions and dynamic characteristics of porpoising are investigated subsequently. The free surface together with pressure and velocity fields are analyzed and discussed in detail. The result shows that slipstream offers a nose down pitching moment, which increases the aerodynamic lift significantly, and the center of hydrodynamic force moves to the front of gravity center, leading to the continuous amplification of the unstable oscillation in porpoising. As a result the heaving and pitching oscillations are mainly determined by hydrodynamic force, while the aerodynamic force only plays an auxiliary role.