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.
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.
The power for the on-board electronics is provided via an axial flux generator. Permanent magnets will be fixed to the vehicle frame. 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. A great example of a DIY axial flux generator is shown below.
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.
A simple power budget was compiled for the major components to get a rough estimate of the power required.
The axial flux generator example shown in the video was outputting 45W at about 2000 RPM. 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.
For a complete system deployed on a vehicle, the total powered required for a quadcopter would be about 50W. For Goliath, the gas engine is capable of providing 22kW (30 Hp). So the percentag of power used for the rotors would be 0.23%. Even if the power budget quadruples, it still only be 1% of the total power available.
The primary structure of the hub consists of two aluminum plates with the mechanical components sandwiched in-between. Each blades has a steel shaft which is held in place with an Acetal bushing. The blades are actuated using the servos.