Conventional helicopters that lose engine power are able to autorotate and achieve a relatively gentle landing.
What about quad-rotor drones that have become so popular in recent years? If one motor craps out then the whole beast becomes unstable, so the operator would be forced to cut power to all four motors. They appear to have fixed-pitch props, so there would be no ability to flare just before touchdown. But would the spinning props at least slow the descent rate to a degree that usefully mitigates crash damage?
There are a few which use controllable pitch props driven by a central motor, so they might be capable of autorotation, although I’m not sure how much control the pilot would have. Most are fixed pitch props and can’t auto.
The rotors of a typical quad-rotor drone have very low inertia, and fixed pitch. They’ll come to a stop almost immediately if the power fails. If you want to be able to land safely in case of a motor failure, you design your drone with 6 or more props and autopilot software that can compensate for loosing a motor.
They have very recently come out with new software algorithms that can maintain control of a quad-rotor drone when one motor fails. This is fairly new and probably isn’t out in the field much (if at all at this point) but I imagine it will catch on quickly, especially since the alternative is that your quad-rotor drone goes SPLAT in a motor failure.
My understanding is exactly that – one motor per rotor with the rotor attached directly to the motor’s output. The controller steers just by controlling the speeds of the various motors.
Right, and they’re generally brushless DC motors. So when the controller stops pulsing the field coils, the rotor/prop assembly should be free to windmill/autorotate.
Mostly I wondered if they are generally stable during descent, and if the props can actually arrest the descent in a meaningful fashion, given that the blades can’t be adjusted to a downward pitch angle.
It’s well accepted that a “windmilling” prop has more drag than one that’s stopped. But there’s no way the total drag would be enough to yield a gentle landing.
And loss of stability is likely - a quadrotor without power has no way to stay upright.
The reason quadcopters work so well using such cheap parts is the same reason they autorotate only slightly better than a brick.
The fixed pitch rotors are far cheaper and mechanically simpler than the mechanism a conventional helicopter uses. However, in order to make a fixed patch aircraft hover so well, you need to be able to respond to changes in roll/pitch/yaw extremely rapidly. So you need extremely low inertia rotor blades, relative to the inertia of the quadrocopter body and the torque from the motors.
This low inertia means if you lose power, they will slow to a speed at which they provide negligible thrust very rapidly, and the quadcopter crashes like a brick.
This is also why large quadcopters don’t use longer blades, but instead become octocopters and larger.
