The compound helicopter gains interest as operational needs push future rotorcraft capabilities beyond current standards. The compound helicopter is investigated as part of the Future Vertical Lift Program to replace the entire U.S. Army helicopter fleet. The compound helicopter resembles a mix between a fixed-wing aircraft and a conventional helicopter. It features a controllable rotor as well as wings, elevator, ailerons and a push propeller near the tail. Increased agility is achieved by the unique combination of controls and the maximum flying speed is expanded by unloading the rotor lift and redistributing it over the wings. However, manoeuvring at faster speeds comes at a cost. High loads in the rotor hub are expected.

Because of this unique helicopter configuration, some of the controls can be seen as redundant. This enables multiple combinations of control inputs to generate a (near) identical helicopter state. Therefore, the redundant controls can be used for a secondary objective next to manoeuvring the rotorcraft. The thesis will investigate the feasibility of using the redundant controls of a compound helicopter to alleviate loads in the rotor hub during an aggressive roll manoeuvre. This cuts down maintenance costs of highly loaded components and increases their reliability. The focus lies on understanding the physical phenomena leading up to alleviating loads.

A multi-body dynamics model of the compound UH-60A Black Hawk was constructed to simulate manoeuvring flight. The main rotor is represented as a blade element model with a Peters-He inflow model. Aerodynamic coefficients are found from quasi-steady look-up tables. Blades are assumed to be rigid and feature a feather and flap hinge. The fuselage aerodynamics are interpolated from test data. The empennage is modelled using 2D look-up tables to compute the aerodynamic coefficients. The wing and push propeller, unique to the compound helicopter type, are modelled by a non-linear lifting line and a point force acting near the tail respectively. A flight controller was implemented as the fly-to-trim method was used to find the trim condition. The model was validated against FLIGHTLAB for trim and a rolling manoeuvre. The main wing lifting line was separately validated against a vortex lattice method.

The first experiment varies the control strategy to alleviate loads during the roll doublet. Either a pure lateral cyclic, pure aileron or a 50% cyclic - 50% aileron input are investigated. When a pure later cyclic input is used, the rotor will lead the roll and the fuselage will follow. This effect is reversed as the rotor lags when a pure aileron input is used. The rotor smoothly follows the fuselage’s roll motion when both controls are combined. This is caused by the reversed lateral flapping response switching from a pure cyclic to a pure aileron input. The combined input levels out the flapping response. As the moment measured in the hub is linked to the flap angle, loads are reduced from a factor > 7 for a pure cyclic or aileron input, to a factor ∼ 2 for the combined input.

A second experiment investigates the effect of different trim settings prior to the roll manoeuvre. The horizontal tail deflection, compound thrust, rotor rpm and constant aileron input at trim are varied separately. This enables the helicopter to offload both the lifting and propulsive function of the main rotor and reduce the power required in cruise. Depending on which controls are used to achieve the trim state, loads in the hub are increased or decreased. Longitudinal hub moment loads are decreased when the required longitudinal cyclic is alleviated using the horizontal tail or compound thrust. The effect on the lateral hub moments scale with the offloading of the main rotor, except when a constant aileron input is applied at trim. A constant aileron input will lower the power required by pushing the lift more outboard over the advancing blade. The required cyclic input to counter this aileron deflection increases the lateral hub moments.

The final experiment combines the two others by defining a suboptimal trim condition and varying the control strategy during the roll doublet, according to the first experiment. It was confirmed that the 50% lateral cyclic - 50% aileron input reduces lateral blade flapping which lowers the lateral bending moment in the hub. Longitudinal bending loads are alleviated as both the lift and propulsive function of the main rotor are alleviated. This also reduces the power required in cruise. The 50% lateral cyclic - 50% aileron input increases the control power and shows to be beneficial for handling qualities.