DYNAMICS OF A HELICOPTER WITH A SLING LOAD (ROTOR DOWNWASH, MILVAN, UNSTEADY AERODYNAMICS, CH-53, TRIM)

The operations of helicopters carrying externally slung load are often hampered by stability and control problems. To develop control schemes for alleviating these problems, it is necessary to have a good dynamic model for the helicopter and sling load system. Therefore the goals of this work are to improve the existing dynamic models, for single point suspensions, especially in the area of load aerodynamics, and to investigate the open loop characteristics of the system. The model is intended for use in designing control schemes for helicopters with sling loads.;The equations of motion (EOM) of the helicopter-load system are derived for a single point suspension. The dynamic model treats both the helicopter and the load as rigid bodies, each subjected to inertial and aerodynamic forces and moments. The formulation is applicable to various types of helicopters, loads, and versions of a single point suspension. The nonlinear EOM are derived first, and then separated into two sets: Nonlinear trim equations, and linearized EOM for small perturbation about the equilibrium. The linearized equations are symmetric--the equations for the helicopter and the load are similar, being as close as possible to the standard form of EOM of a single flight vehicle.;The dynamic model takes into account the effect of the downwash of the rotor on the dynamics of the load. Momentum theory is used to approximate the downwash and to predict the wake boundaries.;The results of static and dynamic wind tunnel tests of a typical bluff body load, the Milvan (a standard container), were collected and analyzed to construct an aerodynamic data base. The aerodynamic coefficients of a bluff body are strongly dependent on the frequency of the motion. They are modeled here as aerodynamic transfer functions, whose numerical constants are found by curve-fitting the empirical results.;The trim conditions are calculated using nonlinear equations for the load and linearized equations for the helicopter. The linearized equations use tables of nominal trim conditions, and stability and control derivatives about them, which were calculated by the helicopter manufactures using a complicated non linear model of the helicopter.;The dynamic model is programmed in a computer code EOMPROG, which has been verified by comparison with independent calculations. The program is used to investigate the open loop characteristics of a typical configuration--a CH-53D helicopter carrying a Milvan on a single point suspension. The main new observations are discussed. (Abstract shortened with permission of author.)...