Design and performance prediction of swashplateless helicopter rotors with trailing edge flaps and tabs

This work studies the design of trailing edge controls for swashplateless helicopter primary control, and examines the impact of those controls on the performance of the rotor. The objective is to develop a comprehensive aeroelastic analysis for swashplateless rotors in steady level flight. The two key issues to be solved for this swashplateless control concept are actuation of the trailing edge controls and evaluating the performance of the swashplateless rotor compared to conventionally controlled helicopters. Solving the first requires simultaneous minimization of trailing flap control angles and hinge moments to reduce actuation power. The second issue requires not only the accurate assessment of swashplateless rotor power, but also similar or improved performance compared to conventional rotors. The analysis consists of two major parts, the structural model and the aerodynamic model. The inertial contributions of the trailing edge flap and tab are derived and added to the system equations in the structural model. Two different aerodynamic models are used in the analysis, a quasi-steady thin airfoil theory that includes arbitrary hinge positions for the flap and the tab, and an unsteady lifting line model with airfoil table lookup based on wind tunnel test data and computational fluid dynamics simulation.;The design aspect of the problem is investigated through parametric studies of the trailing edge flap and tab for a Kaman-type conceptual rotor and a UH-60A swashplateless variant. The UH-60A model is not changed except for the addition of a trailing edge flap to the rotor blade, and the reduction of pitch link stiffness to imitate a soft root spring. Study of the uncoupled blade response identifies torsional stiffness and flap hinge stiffness as important design features of the swashplateless rotor. Important trailing edge flap and tab design features including index angle, aerodynamic overhang, chord and length are identified through examination of coupled trim solutions in wind tunnel conditions at high speed. Flap and tab configurations that minimize both the control angles and hinge moments required to trim are developed for both the Kaman-type and UH-60A models, and the rotors are successfully trimmed across the range of forward flight speed.;The conventionally controlled UH-60A rotor model is validated with data from the UH-60A Flight Test Program. Excellent correlation is obtained for rotor power in hover and in forward flight. It is shown that the magnitude of the predicted power, but not the trend versus forward speed, is affected by the calculation of inflow distribution. Both uniform inflow and a pseudo-implicit free wake model are used to calculate the inflow distribution for the swashplateless rotor. Using the free wake model, the predicted swashplateless rotor power is sensitive to the pattern of trailed vorticity from the rotor blade. Trailed vortices are added at the inboard and outboard boundaries of the trailing edge flap, and the flap deflection is used to calculate an effective angle of attack for the calculation of the near and far wake. This wake model predicts the swashplateless rotor requires less main rotor power than the conventional UH-60A helicopter from hover to mu = 0.25. As the forward flight speed increases, the swashplateless predicted power increases above the conventional rotor, and the rotor lift-to-drag ratio decreases below that of the conventional rotor.