Autorotation is when a helicopter loses power to its engines and the rotors begin to spin due to air resistance. Pilots can then safely land the helicopter because of this
But I just don’t understand how it works. Surely if the helicopter is falling then the air will push the rotors in the opposite direction to how they normally spin and that won’t create lift? Can someone explain?
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One really cool thing about helicopters is that the pilot is continuously adjusting the angle of the blades. They can adjust them all the way from “zero lift” (upwards force) to “hovering” to “accelerating me upwards”. But they can also go in the opposite direction, essentially making the blades push them down.
When a pilot autorotates, they change the blade angle to “push me down” mode, and as they fall, this speeds up the rotor speed (it spins faster). Then, as they get near the ground, they switch back to “push me up” mode, and use the already-spinning rotor to slow their fall.
Your best bet at understanding is this SmarterEveryDay video.
Rotor blades don’t create lift by generating downwash alone. They create lift because their blades are shaped like wings with the top of the blade curved and the bottom flat. The mechanism that controls the pitch of the blades attaches to the front of the blade, that means when air pushes up on it like when it’s falling the trailing edge of the blade moves up which would cause the blades to rotate the correct direction when falling.
I’m a helicopter pilot. Autorotation has to do with the relationship between lift and drag. During normal flight, the lift generated by the rotor blades will also create some drag on the blades. This is called induced drag, and it’s due to the downward motion of the air through the blades. When you are in an auto, the air is moving up through the blades, reversing the direction of the induced drag so that it drives the blades forward, maintaining rotor RPM. Then, at the bottom of the auto, you trade that RPM for lift to cushion your landing.
If you look at the blades at minimum pitch, they are adjusted to do autorotation, aka, the blade is pointing down.
In powered flight you increase the pitch to go up. In autorotation you slam the collective control down and you have “reversed” the blades to be pushed by the air coming from below, from your fall, (instead of the usual pushing air down to go up).
The rest is pilot skill. As you need to use the yaw pitch and roll controls to fly a bit nose down to convert your fall into a down&forward flight, then trade the forward momentum into lift during the final flare.
Been there as a passenger in a demo, pretty cool and safe.
If you are not familiar with helicopter collective control, watch a video. It’s easier than explain it with words.
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