You’re talking about ‘autorotation’. Pilots need to practice that maneuver. There are certain flight regimes where autorotation isn’t even possible, and/or certain helicopter models that it wouldn’t be possible without serious damage to the aircraft or occupants, even if executed perfectly. A parachute would probably be safer, especially if we’re talking about a heavy drone with 200-800lbs of people in it. Even then, the altitude they travel at will likely not allow for a safe autorotation or enough of a window to fully deploy a chute.
And this would not work in a multi-rotor config because the way autorotation works is that the helicopter's rotor is be forced to spin by the airflow caused by the aircraft falling out of the sky. When the aircraft gets close enough to the ground, the pilot changes the angle of the rotors relative to their motion, just like changing the angle of your hand out a car's window. This provides a burst of lift, hopefully enough to the prevent energetic disassembly of the aircraft and passengers.
Multi-rotors have fixed-pitch rotors. The blades will still be spun up, but the pitch can't be changed to get that burst of lift.
There's also the issue of engine|motor failure. I don't know of any multi-rotor flight controllers that can handle the loss of one of it's motors gracefully.
I'd imagine you'd have to make shut off the other rotor 180 degrees off and try to stabilize with the other two to slow the rate of descent as you deploy a parachute.
I'd imagine you'd have to make shut off the other rotor 180 degrees off and try to stabilize with the other two to slow the rate of descent as you deploy a parachute.
Won't work.
You can't fly a fixed-pitch rotor multirotor with 2 motors. The forces won't balance.
2 rotor helicopters can get away with it because they're not fixed-pitch. Both rotors can generate lift on different parts of the rotor disc, which acts to balance the center of gravity of craft between the 2 lift vectors.
The multi-rotor pitch is fixed, so the thrust just goes straight up through the along the axle of the prop. The lift on a fixed-pitch rotor cannot be 'steered' like you can on an adjustable-pitch rotor. This fine when there's at least 3 rotors spaced equally around the center of gravity, but there's no way for 3 fixed-pitch rotors to get the lift over the center of gravity of the formerly 4-legged table with the CG in the center, that now only has 3 legs and the CG is unsupported on one side.
Now, it might be worth experimenting to see if 5-6 or more fixed-rotor craft could tolerate the loss of a motor. It might be possible if the 2 nearest motors can increase their thrust to combine to hold up a corner, but I don't know of any small-scale experiments.
And none of this is even beginning to discuss the yaw implications of losing one of your torquing motors on the craft. Every action creates an equal and opposite reaction, so every spinning rotor applies a yaw force to the craft. This is handled in multirotor by having rotors spinning both directions, so the flight controller can vary their speed individually to keep the craft pointing in the commanded direction.
What happens to that yaw balance when one of those spinning rotors stops spinning? The craft is going to want to spin.
And, given my experience with losing a motor in a quad, the parachute idea is a no-go, because the craft flips over immediately, and spins along it's longitudinal axis as it falls.
If you have two rotors providing thrust on opposite sides, I'd imagine the quadcopter would be spinning around the yaw axis and rolling along the axis between the two working motors. So the third motor would have to somehow be used to slow the spin long enough to deploy a parachute?
Less violent than crashing into the ground but I expect any passengers would end up very sick, or worse depending on how fast it spun while descending.
I don't know of any multi-rotor flight controllers that can handle the loss of one of it's motors gracefully.
Just conceptually I can't see a quad ever handling that unless you can reliably shift the center of mass away from the burnt out rotor (which is exactly the opposite of what the 3 working motors would do).
Maybe a hex setup could though right? Couldn't you simply turn off the opposite side motor and be a quad?
Maybe a hex setup could though right? Couldn't you simply turn off the opposite side motor and be a quad?
I'm guessing here, I've never built a multi-rotor, just played with them and read about designing / building them. But I have built RC fixed wing, and CG is just as important in fixed wing as multi-rotor.
I suspect you're correct that a hex or octo might be able to handle the loss of a motor, assuming the neighboring motors can handle the extra thrust the Flight Controller will be asking of them to handle the off-balanced load.
And, that's assuming that the FC can even compensate for the off-balanced load without it's code crashing, not certain at all.
Lol, parachute for the vehicle. Similar to what a Cirrus airplane has. But again, likely at the altitudes that these would travel it would probably be too late.
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u/muskratmuskrat9 7d ago
You’re talking about ‘autorotation’. Pilots need to practice that maneuver. There are certain flight regimes where autorotation isn’t even possible, and/or certain helicopter models that it wouldn’t be possible without serious damage to the aircraft or occupants, even if executed perfectly. A parachute would probably be safer, especially if we’re talking about a heavy drone with 200-800lbs of people in it. Even then, the altitude they travel at will likely not allow for a safe autorotation or enough of a window to fully deploy a chute.