Experimental Evaluation of a Quadrotor Biplane with Variable Pitch Rotors

Vertical Takeoff and Landing (VTOL) capable air vehicles such as quadcopters are being chosen for missions requiring higher payload capacities, longer endurance in hover, longer range, and most importantly faster travel time. To meet this need, hybrid air vehicles that incorporate the design features of fixed wing vehicles and rotorcraft have come to the forefront. One such hybrid vehicle, the quadrotor biplane, is being explored especially for package delivery capability. This vehicle configuration has four proprotors beneath which two wings are placed in a biplane configuration parallel to the rotor shafts. For control of the vehicle in both hover and forward flight mode a conventional quadrotor control methodology is utilized. The quadrotor biplane takes flight in tail-sitter configuration and uses differential thrust vectors to pitch forward into forward flight configuration. This control method takes the place of conventional control surfaces, therefor the system is more mechanically simple. The vehicle design and package delivery functionality have been validated experimentally, in both hover and forward flight. An experimental investigation of a variable pitch proprotor in a wind tunnel revealed large gains in efficiency at higher airspeeds. Furthermore, a relationship between free stream velocity and pitch angle required for lowest power consumption was determined. As such, a test vehicle incorporating a variable pitch mechanism on each rotor was devised. The vehicle has a gross takeoff weight (GTOW) of 2.86 kg (6.31 lbs). Each variable pitch mechanism’s independent actuation was verified using VICON motion capture cameras. To control the vehicle, three different flight control modes were devised. The first mode holds the servo position and varies the motor RPM, and is the typical control used for quadrotors. The second mode is the opposite of the first, holding RPM constant and varying each proprotor's blade pitch. This mode was tested on a commercial quadcopter with fixed RPM and independent blade pitch control. Finally, the third mode is a novel combination of differential RPM and variable pitch. The response of each mode was validated in hover testing.