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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  • Moderator

    Mark do you mind sharing the hardware specs of the quad?

  • Hi Chris,

    Thanks for linking to my work.  It is an honor as you are somewhat of a UAV legend in my mind.  

    It's nice to see that there are people interested in variable-pitch quadrotors.  I was a little worried that the response from the DIY community would not be favorable since the variable-pitch propellers are a bit redundant on a basic quadrotor.

    Thanks for all the work in the DIY world.  In our lab at MIT (ACL), we use your forum posts and information a lot.  Also, the UDB4 is awesome!


  • Makes sense.  I was just wondering.  It looks like alot of infrared cameras above the aircraft.  Just wondering if that was what they were using.

  • I think it's a camera system in the room.  In some of these labs, the position and much of the flight-control is done by external computers, and only trajectories are sent to the quads.  In some cases, I think the external systems just crunch the data and feed a position info to the quads, similar to a private GPS system.

  • How are they able to autoflight in the building?  I am new to this and that is really interesting to me.

  • @Bill, this information, and a desire to make a "safe" shrouded quad have let me to this train of thought.

    I know that ducted fans have lower thrust and more problems with control due to the higher speed and thus inertia which slows the thrust response.  But what if you used a variable area nozzle to make quick thrust changes?  I can picture a simple system of flaps controlled by servos for stabilization responses.  But I would make some changes to the control algorithm.  I would route the Stabilize I-term to the motor speed output, and the P-term to the nozzles.  So the nozzles would collapse down to create quick changes in thrust.  But the motors would slowly increase RPM to create longer-term thrust increases.  This would allow the nozzles to open back up for maximum efficiency.

    A similar scheme would be used for the altitude.  Quick responses go to the nozzle control, slower responses go to the throttle.

    I also just found these fans.  I wonder if they might help solve the problem of lower efficiency of ducted fans, since these are designed for lower flow rates.


    This is also really interesting:


    Too bad they don't make it in a 90mm.  Something like this could be used to control yaw.  I also wonder if it could allow "sliding" sideways without tilting?

  • The reason I chose a tri and not a quad is because I could use of the shelf fc board that allows separation of throttle and pitch - hence a flybar less approach and a mock 120 deg swash

    To be taken to the next level - a dedicated firmware for this approach will be needed

  • The AXI motors are only capable of 500g thrust (less than 1/4lb) or so.  That explains the tiny MIT quadcopter... :-(

  • There's a reason why outdoor rc helis, even the smaller ones with main rotors approaching the size of larger quad rotors, have been collective (variable) pitch for decades.  Simply much better control capabilities.

    For roll and pitch the control theory translates almost directly.  Only yaw will be a bit different.

    If your quad is on the smaller side and gravity is good enough for any downward dynamics then a fixed pitch model will be fine.  If you want more control than gravity allows and/or the size of the quad isn't so small you simply need variable pitch.

    Some work in this area by the DiyDrones community would be fun to follow.  My guess would be that adapting the current code to handle variable pitch would be straightforward (I'd never call it easy :)).

    To this end, I'd take a few lessons from the helicopter community.  Use quality components.  The ideas of linkage slop and servo speed/precision just aren't issues with quality components and design.  Also, the governor function on the ESC will be important.  It won't be nearly as taxing a problem as keeping some 800mm blades spinning at the same rpm when you add 5 degrees of pitch, but you might as well let the ESC worry about constant rpm.

    The extra quality components are going to cost more, but there's never anyway around this if you want the best performance.

    I doubt a variable pitch quad will be so popular that DiyDrones has their own standard frame anytime soon, but there will definitely be some interest.  A variable pitch quad should figuratively and literally be able to fly circles around a fixed pitch one.

  • Folks

    Have a look at my tricopter variable pitch build thread here

    I believe it is contolable as a 120 deg traditional heli swash - i have tried an airplane stabilizer with radio swash mix, and a beastx flabarless system with some success.

    I have now moved to using my apm 1 with traditional hil firmware, and will starva thread here shortly after i receive a replacement variable pitch unit. Bench testing works so far, but i had rudder response reversed so ran into a mishap with on prop unit

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