Multi-rotor vehicles are opening up new possibilities everyday. While much of what's promised regarding agriculture, shipping, flying cars, etc. is years, or even decades away, if just some of their capabilities can be achieved, the world will be a different place. Variable pitch rotors can make the promises of multi-rotors come to fruition.
Fixed pitch rotors are typically optimized for a single condition, namely hovering. This means that for other conditions, the design is less than optimal. Variable pitch rotors change the pitch of the blades as required and provide better overall efficiency.
Variable Pitch Examples
Most multi-rotor vehicles use fixed pitch rotors, but there are a few exceptions. The Stingray 500 and the MIT ACL Variable Pitch Quadrotor are two such examples. The maneuvering capabilities of these two vehicles are simply amazing.
EVPR: Electric Variable Pitch Rotor
The EVPR is a self contained mechanism. Servos inside the hub are used to actuate the blade pitch. Power for the servos is provided by an axial flux generator built into the hub. Control is provided via wireless signals. Once completed, the project will be demonstrated on #Goliath - A Gas Powered Quadcopter.
Unlike traditional variable pitch rotors, the blade actuation hardware is self-contained inside the rotor hub. Each blade is mounted to a shaft, held in place by two pillow-blocks. The shaft is actuated using a geared servo.
Below is the gear drive in action.
The servos and gear ratio should be application specific. For #Goliath - A Gas Powered Quadcopter, the required torque and a variety of servos are shown in the figure below.
The gear ratio chosen was 5:3 and the servo chosen was the Hitec HS-5685MH.
The heart of the EVPR will be the ESP32. The chip will receive the data signal wirelessly from the flight controller and be used to command the servos via pulse with modulation (PWM) signals.
A simple test was conducted to ensure that data could be transmitted to the spinning rotor. The test was successful and the same wireless system should work for controlling servos for a variable-pitch rotor.
Primary power for the on-board electronics will be provided via an axial flux generator with battery for startup and backup power.
The axial flux generator will have permanent magnetsfixed to the vehicle frame. and the coils will be built into the rotor hub and provide 3-phase alternating current. The current will be rectified to provide a DC power source for the on-board electronics and servos. An example of an axial flux generator is shown in the below video. The axial flux generator for the EVPR will be based off of that design.
The power required will be dependent of the blade torque requirements for a specific vehicle. A simple power budget is shown below for the Goliath quadcopter as an example.
The axial flux generator example shown in the video was outputting 45W at about 2000 RPM. For Goliath, the EVPR rotors will be operated nominally between 3000 and 4000 RPM, so the design will need to be scaled down to keep too much power from being generated.
The total power required will be about 160W. Since the gas engine is capable of providing 22kW (30 Hp). So the percentage of power used for the rotors would be 0.7%. Even if the power budget doubles, it still only be 2% of the total power available.