Research On Mathematical Modeling Method For Tilt Rotor Aircraft Flight Dynamics

The tilt rotor aircraft is a new aircraft type which combines the advantages of vertical takeoff andlanding capabilities, inherent to the helicopter, with the high forward speed and range of a fixed wingturboprop airplane. The tilt rotor aircraft consists of three primary modes of flight that are thehelicopter flight mode in hover and low forward speed flight, fixed wing aircraft flight mode in cruiseand high forward speed flight and the conversion mode from the helicopter flight mode to fixed wingaircraft flight mode. Conversion or reconversion of the tilt rotor aircraft can be made within a corridorhaving a range of airspeeds, conversion angles, and fuselage attitudes. This dissertation developed asimple mathematical model to investigate the conversion corridor of the tilt rotor aircraft’s transitionflight, a non-linear flight dynamics mathematical modeling method of tilt rotor aircraft for all flightmodes and the simulation of the dynamic tilting process.The conversion corridor of the tilt rotor aircraft is the nacelle conversion envelope. For conversionmode of tilt rotor aircraft from helicopter mode to fixed wing airplane mode, an analysis method basedon the transition flight’s characteristics of morphing and speed changing is established to investigatethe nacelle conversion envelope. The method included the lower speed envelope and the higher speedenvelope. It made a detailed analysis to the matching relationship between the rotors aerodynamicforces and the wing aerodynamic forces during the conversion mode. The lower conversion boundarywas restricted by wing stall and the upper conversion boundary was restricted by power availablelimited by engines. The XV-15tilt rotor research aircraft was taken as an example to analyze theconversion envelope and the design parameters of the rotor and the wing were analyzed to enlarge theconversion corridor.In hover and low speed flight, the wing is immersed in the rotor wake and a strong aerodynamicinteraction exists between the rotors and the wing creating a penalizing wing download in terms ofhover performance and pay load capability. A mathematical model for calculating the wing downloadwhich was created by the wake of the two rotors impinging the wing below was developed. The tiltrotor aircraft geometry scale, the tilt angle of the nacelle and the flight condition were considered inthis model. This model for the main phenomena of the rotor/wing interaction is relatively simple andefficient in order to be used for the tilt rotor aircraft engineering calculations, modeling in flightmechanics and real time calculations.The tilt rotor aircraft’s flight control system blends the basic elements of a conventional helicopterand a fixed-wing airplane. The flight controls in helicopter mode resemble those of a lateral-tandem rotor helicopter. While the fixed-wing control surfaces (ailerons, elevator, and rudder) remain active inall flight configurations, the rotor controls are automatically phased out as the nacelles are tilted towardthe airplane configuration. The control strategy of tilt rotor aircraft that is the hybrid of helicopter andfixed wing airplane is extremely complicated. In this dissertation, a control strategy of tilt rotor aircraftin various steady level flight conditions is studied in helicopter, transition and fixed wing airplanemodes. The displacements of the controllers are among their motion limits and the variations of aircraftcontrols and corresponding attitudes are continuous and monotonous.The flight dynamic mathematical model differs from that for a conventional fixed wing aircraftprincipally in the added requirements to represent the dynamics and aerodynamics of the rotors, theinteraction of the rotor wake with the airframe, and the rotor control systems. In this paper, generalaerodynamic forces and moments of tilt rotors, wing, fuselage, horizontal and vertical stabilizers aremodeled. Together with the control strategy, an optimization method is utilized to obtain the trimmingvalues of three flight modes and aircraft dynamic response to different controls. The XV-15tilt rotorresearch aircraft is taken as an example to demonstrate the effectiveness of the methodology. Theresults were validated against generic tilt rotor simulation model results and compared to flight testwhere available.The dynamic tilting process of the tilt rotor aircraft was investigated in this paper based on thedynamic optimization. The time marching solution of the controls, the tilt angle of nacelle, the forwardspeed and the attitude in the transition from helicopter mode to fixed-wing airplane mode werepresented based on the objective function of the dynamic tilting process. It is proved that the algorithmsdeveloped in the paper will contribute to the further investigation of the tilt rotor aircraft’s dynamictilting process and the design of corresponding control law.This method for calculating conversion corridor of tilt rotor aircraft, a non-linear flight dynamicsmathematical model of tilt rotor aircraft and the inverse algorithms for tilt rotor aircraft’s dynamictilting process based on the dynamic optimization provide a platform for further investigation on tiltrotor aircraft flight mechanics and can be used to support the aircraft design, handling qualitiesevaluation, pilot training and as a flight test support tool.