A Mathematical Model Of Tilt-rotor Flight Dynamics And Investigation Of Flight Control In Transition Mode

Tilt-rotor aircraft combine the vertical takeoff and landing (VTOL) capability of helicopters with the high speed performance of turboprop airplanes. Located at the tips of the wing, the prop-rotors act as helicopter rotors when flying vertically and as airplane propellers in cruising flight. During the procedure, the airspeed continues accelerating (or decreasing) and the aircraft configuration is rapidly changing. The aerodynamic interactions are complex and lead to dynamic instabilities. The shift between helicopter and airplane flight modes necessitates a change in control strategy. Thus the flight dynamic characteristics and control features in the conversion mode are the key technologies in the whole tilt rotor design. The paper focused on the conversion mode in the longitudinal plane which is the more challenging case for the conversion procedure.Because of the aircraft symmetry, the longitudinal motion of the aircraft is uncoupled from the lateral motion. A mathematical model reflecting the tilt rotor unsteady and nonlinear aerodynamic characteristics in the longitudinal channel is presented in the paper. In the full flight envelope, the rotor thrust continues tilting and the aerodynamic forces of each component change as a function of airspeed and nacelle angle. Furthermore, because rotors are mounted on the tips of the wing, the rotor/pylon/wing aerodynamic interactions are also included in the model. Such factors reflect the unsteady and nonlinear characteristics in the model. The model developed here is suitable for the tilt rotor aircraft during hover, transition and cruise. The longitudinal equations of motion provide a foundation for further investigation on tilt-rotor flight dynamics.Based on the mathematical model developed by the paper, the trim calculations are carried out in the conditions that the example aircraft converts from helicopter to airplane mode in level flight with zero longitudinal acceleration and angular rate. The conversion corridor and the minimum rotor thrust required and the corresponding control settings for the conversion mode are given. Compared with the XV-15 flight test data, the results are reasonable and effective.Improved guidance schemes will be needed, especially in the terminal area for safety and economic viability. A perturbation guidance technique is then used; the equations of motion in the longitudinal channel are linearized around the nominal trajectories and linear quadratic control laws are developed on the deviation of the aircraft states from these nominal values. A simulation of the guidance scheme applied to the example tilt rotor vehicle exhibits satisfactory behavior in the presence of wind and initial errors.