Stability and control issues associated with lightly loaded rotors autorotating in high advance ratio flight

Interest in high speed rotorcraft has directed attention toward the slowed-rotor, high advance ratio compound autogyro concept as evidenced by the current DARPA Heliplane project. The behavior of partially unloaded rotors, autorotating at high advance ratio is not well understood and numerous technical issues must be resolved before the vehicle can be realized. Autorotation in helicopters usually indicates an emergency loss of power. For the concept vehicle autorotation is the normal working state of the rotor. The necessity for a reduction in rotor speed with increasing flight speed results in high advance ratio operation where the retreating side of the rotor is dominated by the reverse flow region. Further, rotor speed changes also affect the rotor dynamics and the associated hub moments generated by cyclic flapping. The result is rotor characteristics that vary widely depending on advance ratio. In the present work, rotor behavior is characterized in terms of issues relevant to the control system conceptual design and the rotor impact on the intrinsic vehicle flight dynamics characteristics. A series of trim, stability, and control analyses, based on features inherent in the concept vehicle, are performed. Trends are identified through parametric variation of rotor operating conditions, augmented by inclusion of the sensitivities to blade mass and blade stiffness properties.;In this research, non-linear models, including the rotor speed degree of freedom, were created and analyzed with FLIGHTLAB(TM) rotorcraft modeling software. Performance analysis for rotors trimmed to autorotate with zero average hub pitching and rolling moments indicates reduced rotor thrust is achieved primarily through rotor speed reduction at lower shaft incidence angle, and imposing hub moment trim constraints results in a thrust increment sign reversal with collective pitch angle above advance ratio mu ∼ 1.0. Swashplate control perturbations from trim indicate an increase in control sensitivities with advance ratio, and advance ratio dependent control cross coupling. Hub moment response to rotor disturbances results in transients where rotor damping is reduced due to low Lock number blades and reduced rotor angular velocity. Experimentally identified frequency response shows dominant low frequency modes with advance ratio dependent damping and the frequencies are on the order of typical airframe modes. Rotor speed response to swashplate control perturbations from trim results in non-linear behavior that is advance ratio dependent, and which stems from cyclic flapping behavior at high advance ratio. Rotor control strategies were developed including the use of variable shaft incidence to achieve rotor speed control with hub moment suppression achieved through cyclic control. Flight dynamics characteristics resulting from the coupling of the rotor and airframe were predicted in flight using a baseline airframe with conventional fixed-wing controls, representative of the current interest in the concept vehicle. Results predicted by linearization of the non-linear models were compared with system identification results using the non-linear simulation as surrogate flight test data. Low frequency rotor response is shown to couple with the vehicle motion for short period and roll mode response to airframe control inputs. The rotor speed mode is shown to couple with short period and long period vehicle modes as the rotor torque balance is sensitive to vehicle speed and attitude changes.