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So, I was learning about helicopter and how they auto rotate and I learned that when the engine fails the helicopter starts to fall and the air is thrown up at the blades which turns them like a windmill. I though about this for a little and found a issueenter image description here

This image shows that the blade is being pushed back .......

In the wrong direction

The blade would have to be moving the other way the create lift. wouldn't it?

I also found another issue. If this force was applied to every blade wouldn't the forces equal out and the blades stay at the same speed.

The video I was watching about this said that the blades and span like a windmill is span in the wind, but the way a windmill spins would push the blades the wrong way.

I tried to google windmills to see if another voo doo magic happened in them but, most results explained how a wind turbine makes electricity which was not very helpful.

I am sorry about the quality of the picture I drew

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  • $\begingroup$ It's a really good question! $\endgroup$ – Koyovis Jul 9 '17 at 23:04
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You are missing one essential detail: The incidence of helicopter rotor blades can be changed. This is called collective pitch.

When power fails, the pilot changes the blade angle such that the spinning direction is maintained but now the rotor extracts energy from the upwards airflow instead of pushing air down. Note that the blades will still create lift, but because of the fact that the flow into them has an upward component, the lift force on the blades is tilted forward such that the horizontal component keeps the blades spinning. Now the helicopter flies much like a glider, continuously trading altitude for forward and rotor speed. This is the same for all blades, and each blade will pull the rotor forward.

Close to the ground the pilot will now pull on the collective pitch lever and increase lift on the blades momentarily. This is a dynamic maneuver and needs to be well coordinated. Now the lift force increases, but the lift vector on each blade will point slightly backwards, so the rotor spins down. Now the pilot is trading the stored rotational energy of the rotor for increased lift in order to reduce sink speed and come to a soft landing. This requires a minimum amount of rotor inertia to have enough energy available for deceleration.

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    $\begingroup$ No wonder you've developed a reputation for clear and concise answers over on Aviation SE. $\endgroup$ – Emilio Pisanty Apr 27 '17 at 13:40
  • $\begingroup$ So, a flat helicopter blade would not be able to auto rotate because as you lowered the pitch to negative the blade would not make any lift. Just sorta curious. $\endgroup$ – Crafterguy Apr 27 '17 at 18:03
  • $\begingroup$ @Crafterguy: No, even a flat plate would produce lift, but less efficiently than a proper airfoiled wing. Lift is perpendicular to the flow direction, and when the flow comes slightly from below, the lift force is tilted forward. The upward component is keeping the helicopter from falling any faster and the forward component keeps the rotor spinning. Flow direction must be sufficiently up to maintain positive lift even with negative blade pitch. $\endgroup$ – Peter Kämpf Apr 27 '17 at 18:38
  • $\begingroup$ Have you personally done this manoeuvre? I guess helicopter training includes this as a must, no? $\endgroup$ – WetSavannaAnimal Apr 28 '17 at 1:18
  • $\begingroup$ @WetSavannaAnimalakaRodVance: No, I don't have a helicopter license. $\endgroup$ – Peter Kämpf Apr 28 '17 at 5:09
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I'll keep my answer short and sweet, because the single best answer you can get is to watch the Smarter Every Day video on how autorotation works. It's 9 minutes, with great visuals helping explain the process, and far better than any text answer could ever be.

For those who don't want to hit the link, one of the major factors in autorotation is the linear velocity of the air across the blades. Air actually travels faster across the parts of the blade further from the center. Helicopter pilots can leverage this trick to create power while falling. The inner parts can be pushed by the air (because the inner parts of the blade are moving slow), while the outer parts actually push air as though the engine was still running!

A pilot will actually adjust the pitch of the blades to balance these effects out to decend slowly while maintaining a bit of extra spare energy to use right near the end to land safely.

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  • $\begingroup$ "The inner parts can be pushed by the air (because the inner parts of the blade are moving slow), while the outer parts actually push air as though the engine was still running!" -- thus, taking into account that the blade pitch is downword, it seems to me that the outer faster moving parts of the blades will create a downward pull. $\endgroup$ – Alexey Dec 25 '18 at 12:15
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It's a good question. The thing with autorotation is, it does not work very well when considered from a standpoint of vertical flight - it's not really a windmill. The best example for explaining the principle of autorotation is the autogyro: it always flies in autorotation, but never vertically. Or at least, never very successfully for long. From Principles of Helicopter Aerodynamics by J. Gordon Leishman:

enter image description here

The picture illustrates a pretty important issue: it identifies lift and drag forces in forward flight. If the autogyro or helicopter would descend vertically, the rotor would create drag only, since drag is defined as the force in the direction of airspeed. Increased drag, like a parachute. The rotor would act like a partially permeable fixed disk. That would not work very well for stopping the autogyro or helicopter from falling.

It is much more effective if the rotor behaves like a wing: forward flight creates a lift force, and drag is overcome by a propeller or by energy absorption from reduction of potential energy. In other words, if it behaves like a glider. Gliders don't glide very successfully in vertical direction either...or at high angles of attack, which is what is indicated in your drawing at 45º. The rotor blades are wings, but sadly with difficult dynamics - please hang on.

In autorotation we have rotating blades, one going forward and one backwards. The forward going blade experiences headwind, the backwards going blade has a tailwind. The rotor is driven by difference in drag, and if the forward going blade experiences less drag than the rearward going blade, the rotor is driven by the wind force - it never changes direction of rotation. That sounds counterintuitive, but it is exactly what happens.

When the first autogyro was put to a flight test (way before the first helicopter, in early 1920s) it started rolling when forward speed picked up, because the forward going blade has higher relative wind speed and therefore more lift. The first successful flight, of the C4 in 1923, fixed this by introduction of blade flapping, really quite a brilliant design. From Wikipedia:

Dissymmetry is countered by "blade flapping": rotor blades are designed to flap – lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift.

The same blade flapping creates a driving torque of the rotor, by difference in drag between forward and rearward facing blades. But it only does that when the collective is fully down. The collective flight handle sets the Angle of Attack of all blades simultaneously, and the first thing the pilot needs to do when the engine stops is to lower the collective. Otherwise the drag in the forward going blade is higher than in the rearwards going blade, and the rotor stops turning.

enter image description here

So autorotation only effectively works with forward velocity. In the last bit, just before landing, the pilot does go almost vertical: he raises the collective, trading rotational energy of the blades for loss of kinetic energy, and he comes to a beautiful soft landing.

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Your picture is fine so far as it goes, but...

If the helicopter rotor has 2 or 4 blades, now draw the picture for the blade that is opposite to the first one. The slope of that blade is in a different direction, so instead of being "pushed back" it is "pushed forwards." The combination of the two "pushes" is what makes the rotor turn.

A wind turbine, or any type of fan, works exactly the same way.

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  • $\begingroup$ Note, @PeterKämpf's answer (posted while I was typing mine) is describing the situation where the helicopter is moving horizontally relative to the ground. I only considered the simpler case where it is moving vertically downwards. $\endgroup$ – alephzero Apr 27 '17 at 13:33
  • $\begingroup$ Moving horizontally isn't strictly required but increases efficiency. The technique also works in a vertical descent, but less well. $\endgroup$ – Peter Kämpf Apr 27 '17 at 13:35
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    $\begingroup$ Okay, but wouldn't this still turn the blades the wrong way? $\endgroup$ – Crafterguy Apr 27 '17 at 17:53
  • $\begingroup$ No, read Peter's answer again. $\endgroup$ – Pirx Apr 27 '17 at 19:35
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It's exactly the same as when an aircraft glides, except the "wings" go in a circle.

As a student pilot in a fixed-wing aircraft you practice this many times. The instructor pulls the power to idle and says "You just lost your engine..." The first thing you do is push the nose down, so you're in a downward glide. Since you're going downward you pick up speed and that keeps you flying. You trim for the speed that gives you the longest glide range, and look for a place to land.

In a helicopter you do it by pitching the blades downward (with the collective lever). This makes them glide, which gives them speed, which maintains lift, so you can look for a place to land.

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