3D Robotics

Variable pitch quads fly upside down

Check out those flips! From Hackaday

Straight from the Aerospace Controls Laboratory comes a variable-pitch quadrocopterdesigned by [Mark Cutler] and [Jonathan P. Howe]. While real, full-sized helicopters always have variable pitch rotors, changing the pitch of the blades on remote control aircraft is a fairly uncommon modification. When it’s done right, though, being able to easily change the thrust direction of a propeller leads to very cool flights, like having an airplane hover nose down.

[Mark] and [Jonathan] identified two interesting techniques that a variable pitch quadrotor can bring to the table. The first is trajectory generation  - because of the added maneuverability, their quadrotor can perform more aggressive banking turns when following a preprogrammed path. The second benefit to their design is quick deceleration. In the first video after the break, you can compare the deceleration rates of a variable pitch and fixed pitch quadrocopter. While the fixed pitch quad continues climbing after being commanded to stop, the quadrocopter outfitted with variable pitch rotors can stop on a dime.

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  • Here is some info on the AXI variable pitch system with the setup instructions, it is the units that MIT used . If correctly setup, the motor RPM should remain the same throughout the entire pitch range:



  • T3

    Hi Robert,

    Yes, I have considered both servo lag and linkage slop.

    For small control adjustments, the servo lag time is an order of magnitude smaller than the lag time of changing the propeller speed.

    Servos are slew rate limited. As long as you stay below the slew rate limit, the response time is on the order of the frame period, around 0.025 seconds. For small control changes, the servos will stay below the slew rate. It is only for large control changes that slew rate enters the picture, creating a significant time lag.

    The response time of the propeller speed, on the other hand, is on the order of 0.25 seconds, which is ten times as large as the response of a servo.

    The faster response of the variable pitch should enable you to use more aggressive feedback, which will compensate for a little bit of slop in the linkage.

    By the way, I tested a few ESCs, and found that their response of speed versus PWM is a staircase function, with a small, but finite resolution, which causes control effects that are not much different from the effects of slop in the linkage.

    I have not done any testing yet with variable-pitch. All of my testing so far has been with fixed-pitch, but that testing indicates that there is a fundamental limitation with fixed-pitch control. Analysis suggests that variable pitch should overcome that limitation.

    Regarding running the motors at full throttle, I think that you win in two ways. The first way is a conjecture on my part, that the propeller and motor will be more efficient at higher speed, because the angle of attack will reduce. The second way I am a little more sure about, the effective gain of the tilt control will improve, because the higher propeller speed will produce a greater change in thrust for a given change in pitch.

    Anyway, doing tests with variable-pitch is high up on my list of things to do. I am quite excited about it, I think variable-pitch will enjoy a significant performance advantage.

    Best regards,


  • @Bill, interesting analysis.  But does it include the the lag time required for servo movement?  And does it include any accounting for the slop in the linkage?

    One other consideration, I'm not sure if this is true but I have heard that BLDC motors more efficient when run at full throttle.  Typically fixed pitch quads fly at around... 50-60% throttle in order to leave enough headroom for control.  With a variable pitch, you can run them at full throttle since you are controlling thrust with the pitch.  Might this result in a flight-time benefit?

  • @bill you are definitely getting some very good following ;)

  • T3


    I am not sure about tilt, but I know for sure that for vertical acceleration, Mark is using variable pitch, not RPM, for control. From watching his videos, I suspect that he is also using variable pitch for tilt control as well.

    The last few months I have been doing some research on quad controls that suggests that small changes with variable pitch is a much better way to control tilt and acceleration than changing the motor RPM. For example, I have been able to measure the LaPlace transform of the tilt response of quad frames to small changes in the control input to motor RPM. Theory and my measurements both show that the LaPlace transform for the tilt response of fixed pitch quads is of the form K/(1+a*s+b*s*s), with values of "a" and "b" that produce about 0.5 seconds time lag for typical quad frames. Second order response makes it more difficult to achieve both stable control, and aggressive control.

    The reasons for the second order response of fixed pitch quads is simple: there are two separate inertias to be overcome, the inertia of the motors and propellers, and the inertia of the quad. When you apply a small change to the command to the ESC, it takes a little while for the motor and propeller to settle to the new speed, because of the inertia of the propeller and the motor. Then there is the inertia of the quad itself, which adds to the latency.

    I have not yet measured variable pitch response, but I have analyzed it, and my analysis indicates that variable pitch should be vastly superior to fixed pitch for tilt control of quads. The reason for that is you eliminate the time lag associated with the inertia of the propellers that comes into play for fixed pitch. You can change the pitch of the propellers much more quickly than you can change the speed of the propeller.

    For variable pitch, my analysis shows that the tilt and acceleration response should be first order, of the form K/(1+a*s), which should lead to much more stable and authoritative control than fixed pitch.

    Another data point on the subject: model helicopters typically run with fixed speed and variable pitch. For example, yaw control is done with fixed speed on the tail rotor, and variable pitch. Combined with a heading hold gyro, the combination produces very precise, very stable control of helicopter yaw.

    In other words, for superior tilt and acceleration control, variable pitch is the way to go. I am going to run right out and get a variable-pitch quad frame.

    Best regards,

    Bill Premerlani

  • Those motors are tiny (no thrust value listed in the specs on either of those pages)...explains why the MIT quadrotor is so small...

  • 3D Robotics

    My impression is that quad is flown normally, with motor RPM doing the small thrust changes. The variable pitch is just used to switch modes (upside down or right side up)

  • My impression, not sure if it's true, is that variable pitch is great for being able to make really sharp thrust changes.  But I suspect that it's not as good as fixed pitch at making really small thrust changes.  The servos and linkage don't have enough resolution?

  • A variable pitch rotor isn't just good for aerobatics.  It can make for much smoother flight in windy - especially gusty - conditions.

    A paper in the Related Publications section on the MIT website has some good data on thrust efficiency for the fixed and variable pitch methods specific to their test quad.  As expected, variable pitch is generally better, but sometimes a fixed pitch (motor speed change) wins out.  This is primarily because their quad is fairly small.  The bigger you get the more you really need variable pitch.  Of course, it always depends on exactly what you are trying to accomplish.

  • I am willing to wager that this can be accomplished with the existing board and software with minimal tweeks. I purchased the gear to build a setup like this last year. it is not hard to mate the tail rotor assembly of an rc heli to an outrunner with the same shaft diameter, props are not even difficult to find. The real issue with this setup is run time and weight. This kind of system is a current hog. Single motor power source is the way ahead with this, or low current high torque motors.
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