EVPR: Electric Variable Pitch Rotor

An electrically actuated variable pitch rotor with a wireless interface

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The EVPR is a variable pitch rotor driven by servos placed directly in the hub of the rotor. Control is provided via an ESP32 and power for the servos is provided via an axial flux generator, both of which are placed inside the rotor hub.
The EVPR could enable new multi-rotor vehicle designs, enhance current multi-rotor designs and possibly be used to bring variable pitch propellers to small conventional aircraft.


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.

Power Source

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. The hardware in the below video serves as the starting point for this project.

  • 1 × ESP32 Controller
  • 2 × Tower Pro MG996R Servo, Metal Gear

  • Wireless Data Proof of Concept

    Peter McCloud4 days ago 0 comments

    The EVPR will use wireless data transmission to control the servos embedded inside of the rotor hub. An ESP32 will be at the heart of the on-board electronics, receiving the data signal and controlling the servos.

    To test whether the wireless data concept will work on a spinning rotor, a Sparkfun ESP32 Thing Dev Board was placed on a fixed-pitch rotor with a battery for power. A test was run to confirm whether an LED on the board could be turned on and off remotely over WiFi.

    Below are shots of the dev board setup before and after being placed on the fixed-pitch rotor. Since the built-in LED on the board would be hidden inside the rotor hub, an extra LED(rectangular blue) was connected and placed in a location that would be visible externally.

    The test was successful (video below), demonstrating that the wireless system should work for controlling servos for the EVPR.

  • Blade Torque and Servo Sizing

    Peter McCloud03/22/2017 at 04:38 0 comments

    One of the driving parameters for a variable pitch rotor is the torque required to actuate the blades. The servo size required directly translates to the torque required to maintain the blade position. As part of the #Inexpensive Composite Propellers/Rotors project, I recently covered the RC series of airfoils which are specifically designed for rotorcraft. This airfoils were specifically designed to have low pitching moments.

    How low? The two plots below show the moment coefficients for the rc5-10, designed for inboard rotor section and the rc6-08, designed for rotor tips. For the region of interest the moments coefficients are within +/- 0.005. Comparing this to the NACA 2412, which was the airfoil used previously, the pitching moment is 5-10 times greater in magnitude for comparable angles of attack.

    The shape of the blades for the EVPR will be identical the 3rd generation rotors currently being built for #Inexpensive Composite Propellers/Rotors minus the hub.

    The overall blade torque required at maximum thrust was calculated. The per blade torque requirement ends up being 1.52 N-m or 1.12 ft-lbs. However the hobby servo world works in kg-cm, so the requirement is 15.5 kg-cm. However, the travel required to go from zero thrust to full thrust for the rotor blades is much less than the full travel for a servo. The gearing ratio is TBD, but a factor of 2-3 is likely, so the servo torque needs to be greater than 5-8 kg-cm.

    The initial servo chosen was the Tower Pro MG996R. They were chosen because they were what I have on hand, but they have sufficient torque (10 kg-cm) and they are fairly light (55 g)

  • Project Inspiration

    Peter McCloud03/21/2017 at 03:26 0 comments

    Variable pitch rotors have been in the back of mind ever since starting #Goliath - A Gas Powered Quadcopter. The default control scheme was always vanes underneath the rotors, but variable pitch would be a more elegant solution.

    The inspiration for this project started after coming across Solid-State Rotor: A Demonstration, an AIAA paper by Onur Bilgen and Thomas E. Alberts. The paper is behind an AIAA pay wall, but the the gist of the paper is that they built solid-state rotors using piezo-electric materials that could change the rotor shape. The electronics for the materials were embedded inside the rotor hub. Power was provided via a generator built into the hub.

    This project differs from the prior work in a few ways. The actuation is provided via servo instead of piezo-electric actuators. It'd be really cool to go completely solid-state, but the actuators are pretty expensive and they'd make the rotor cost more than $1000. It's also for bigger rotors, and this project will be open source.

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daigakusei wrote 03/21/2017 at 11:10 point

Hi Peter,

is the power generator not a little bit over engeniering. The generation of electric power in the hub reduce the rotation for the rotor. Every change of magnetic field in the coils produce an reverse force on the magnet. 

Why not an Qi power induction like the loader for mobilephones. 

The sender coil axial around the axis and receiver coild in hub parallel.

The AC of the system whouldn't interferenc with the rotation of the hub.

P.S.: Excuse my bad english its not my native language.

  Are you sure? yes | no

Peter McCloud wrote 03/21/2017 at 18:53 point

I had steered away from wireless power induction because I thought it'd be less efficient. Looking at Qi after you mentioned it, that may not be the case. I'll have to look into that more.

One advantage to a generator based system is that it could potentially be more fault tolerant. If the vehicle suffers a power loss the rotor could still operate and a backup flight control system could attempt to autorotate by feathering the rotors.

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