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I get the basics of autorotation when it comes to a helicopter in unpowered flight, but it's a bit unclear to me when it comes to quadrotors.

My objective is to cover some directed horizontal distance while in freefall and resume control before crashing. I want to do this to save battery, rather than starting controlled flight from the start. If I get horizontally closer to a target location without using much any battery, this is optimal.

Let's say the quad has stable attitude and is in free fall. Then I brake one motor, causing the prop to stop. A naive freebody diagram says the quad will pitch since there's no longer any lift on that side.

My question is what happens after that. Does it reach some equilibrium position? Does the decreased upward lift yield horizontal acceleration relative to the ground, or will it just spin in a circle? Do the rotation directions of the remaining props (2CW/1CCW or 2CCW/1CW) affect what happens next? What if I cut off 2 motors, either next to each other or across from each other?

Thank you.

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    $\begingroup$ You may want to follow this article and find the original study (if this is true). Algorithm lets quadcopters keep flying on three or less propellers. It's claimed it can fly with only one rotor working. Must be clever and use inertia / gyroscopic torque. $\endgroup$
    – mins
    Apr 9, 2016 at 20:14
  • $\begingroup$ There are a few videos on youtube of a quad going into autorotation, but I feel like my current motors are too stiff anyway. I'm mainly interested in the theory, though. $\endgroup$
    – MSGhero
    Apr 9, 2016 at 20:19
  • $\begingroup$ hackaday.com/2014/02/06/… In the video at about 1:00, the quad very clearly descends slower than freefall. This is (lower than) the height I'm interested in starting freefall from. But I see I may be mixing up autogyro and autorotation. As far as shutting off the motor, though, if it starts tumbling but is moving even slightly horizontally in the direction I want, that's fine too. $\endgroup$
    – MSGhero
    Apr 9, 2016 at 20:54
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    $\begingroup$ When I brake the one motor, I don't intend on controlling the other 3. Those are still auto-gyro/rotating/doing whatever. I want as much horizontal distance as possible during free fall before I turn all 4 motors on and begin powered flight. This is my plan to save battery, versus falling straight down or beginning controlled flight from the start. I'll edit the question to include that. $\endgroup$
    – MSGhero
    Apr 9, 2016 at 21:52
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    $\begingroup$ It seems to me that autorotation is going to be hard to control without being able to adjust blade pitch. That's how a helo converts the vertical motion into translational movement. A quad uses a completely different method of directional control that requires power to work. I think tipping it without power will mostly cause it to fall faster since it will reduce the airflow through the props $\endgroup$
    – TomMcW
    Apr 9, 2016 at 22:34

1 Answer 1

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After watching the video the problem he was having was in keeping it horizontal. It appears that the software isn't able to control the unpowered motors.

I know absolutely nothing about quads but I'm assuming with a fixed rotor pitch they must use variable voltage to the motor to control everything such as translational movement. I'm also assuming they have an accelerometer to determine pitch. The first question is how accurate the accelerometer is at low g's.

Second, when in free fall if a rheostat that is controlled by the computer could be connected across the sides of the motor then your problem could be solved. The freewheeling motors will create a voltage just like a generator so a rheostat would be able to control the current and thus the mechanical resistance to the motor. The rheostat will have an inverse relationship with the mechanical resistance (more electrical resistance less mechanical resistance). How linear that relationship is you'll have to experiment with. If you have the software switch to controlling that resistance it should be able to direct the freefall in the same way it does everything under power, only it will be more limited In its authority. If you do it right you might even be able to use the power generated to recover a little bit of the energy from the fall to charge the batteries like a hybrid car does.

Like I said, I know absolutely zero about quadecopters so I can't fill in any details, but it seems feasible.

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  • $\begingroup$ I don't think it's ever been documented before, so I wasn't expecting a solution. However, this definitely puts me on the right track. Thank you @mins $\endgroup$
    – MSGhero
    Apr 12, 2016 at 3:46

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