What is auto-rotation?

Question

I am 15 so I don't fly anything besides simulated aircraft on my computer.I hae a friend one year younger than me who has 3 planes,well they are his dad's,but he flies them completely on his own with his father just enjoying the flight.He was at this town fair and they had helicopter rides.He didn't have enough money but since he was a pilot himself the pilot gave him a discount and extra time.
He told me they practiced something called auto-rotation which u use in case of engine failure to glide to a safe landing spot.He told me that you try to have as much RPM as possible and the air itself spins your rotor and prevents the aircraft to drop like a spinning s**t,like I thought it did when engine failure occured.

Can someone explain to me the procedures with more detail and if possible give me a link to a video of someone doing it?

Answer 1

autorotation is one of modes of operation of ANY propeller.

please remember... it is NOT windmilling. - you'll need this trivia later.



now, we wlll be talking about ACTUAL helicopters, not RC models.



the main rotor of a helicopter has its momentum, when spinning. if the power supply stops (engine failure) the momentum is transferred into the mass of air being blown downwards, while the rotor RPM droop.



so.. the pilot of the helicopter lowers the collective control lever, which reduces the pitch of all main rotor blades. NOT INTO THE NEGATIVE VALUES (negative values are nonexistent in real helicopters).



by reducing the pitch to certain value, the compound vector of the vertical speed AND the rotational speed produces aerodynamic reaction at the blade. the reaction is always perpendicular to the compound vector. now, AS LONG as the reaction vector stays pointing parallel to rotor axis, the rotation is stable and you are NOT trading momentum for lift. if the reaction is pointing forward of the rotor axis, the blade is accelerated, if pointing behind, the blade is decelerating (and RPM are drooping).



this way, you keep the momentum of the rotor for your last pull, during the flare in appropriate height over the terrain. there you use up the momentum to blow enough air downwards to stop the descent and you land safely.



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people often think that the rotor would need negative pitch to autorotate.. that is false, as this setup would only be necessary for spinning up the rotor from ZERO RPM.- then, the rotor would be windmilling, and WOULD need negative pitch to spin the proper way.

Answer 2

I'm not a pilot but I remember a lot of helicopter sims.



You can actually do this in the game enemy engaged Apache vs Havoc and the sequel Commanche vs Hokum. A German helicopter pilot told me about this while we were on the ground on a windy rainy day.



Effectively when the engine dies there is nothing to keep the rotor turning therefore you start to fall as there is no thrust. Which keeps you up in the air. You disengage the blades from the engine as the engine provides resistance to the blades.



If you pitch the blades and the whole machine forward the blades get turned by the relative wind of you falling. Before you hit the ground you level off and pitch the blades to maximum lift. This trades the spinning of the blades into lift for a brief moment......



Too early ~ you are too high and hit the ground fast and get hurt.



Too late ~ you hit the ground fast and get hurt.



Just right the blades bite into the air and start to decelerate a few ft above the ground resulting in a survivable landing.







http://www.youtube.com/watch?v=pzWw5U3eCok





The problem is with military tank killer helicopters like Apaches and Super cobras. In that they fly so low to avoid anti aircraft stuff there often is not enough time to do this.

Answer 3

Must just add something here. Negative values of collective control are available in real helicopters and pilots landing on ships use it to stick them to the deck, after contact as the ship rises and falls. Plus to get momentum into the main rotor prior to auto rotation the effect is to windmill.

Answer 4

its not something you can se.