what makes the rotors spin in an autorotation?

rubber314chicken

rubber314chicken

Pre-takeoff checklist
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Rubber314chicken
title says it all. I know how auto rotations work, but what would cause the blades to keep spinning with no engine? and would the spinning rotors also drive the tail rotor?
 
When the helicopter is dropping towards the earth, the relative wind is up, through the rotor blades. They spin as the air moves through them, just like these do:


I'm not sure about the tailrotor part of the equation.
 
okay, so if the downward motion is what cause the spin, would the blades spin faster with a negative angle of attack? I see how the pin wheel thing could work, but I can't see it happening if the rotors are parallel to the earth, almost like holding a piece of paper and expecting it to pinwheel.
 
rubber314chicken said:
title says it all. I know how auto rotations work, but what would cause the blades to keep spinning with no engine? and would the spinning rotors also drive the tail rotor?
Click to expand...

Same thing as what keeps a propeller spinning when the engine quits.


And the tail rotor is mechanically connected to the main rotor through the gear box.
 
okay, I see now. A combination of the airfoil shape and negative pitch (uncovered by more googling) will keep the blades spinning.

Thanks, that makes sense now.
 
rubber314chicken said:
title says it all. I know how auto rotations work, but what would cause the blades to keep spinning with no engine? and would the spinning rotors also drive the tail rotor?
Click to expand...

Air flowing upward through the rotational plane being deflected by the angle of attack on the rotor blade (same way that a sail boat sails, the airfoil exerts a force on the air, redirecting it. That force has an equal and opposite reaction of propelling the airfoil the opposite direction.) As soon as you loose power, you ditch the collective as quickly as you can since the flatter the pitch, the greater the angle of redirection, the faster it makes the rotor spin, and the faster you keep it spinning , the more inertia (kinetic energy) it has so as you near the ground you can pull up on the collective increasing the blade pitch and converting that stored energy into thrust. The tail rotor is connected directly to the main rotor through the gearbox, so it is powered by the same inertial force.
 
rubber314chicken said:
okay, I see now. A combination of the airfoil shape and negative pitch (uncovered by more googling) will keep the blades spinning.

Thanks, that makes sense now.
Click to expand...

The pitch is still positive (angle of attack is positive, that is) in the autorotation. The AOA has to be low enough, though, to get the lift vector foward enough over enough of the span that the rotor still turns. The helicopter doesn't glide as nicely as the airplane, but comes down really quick.

Dan
 
Dan Thomas said:
The pitch is still positive (angle of attack is positive, that is) in the autorotation. The AOA has to be low enough, though, to get the lift vector foward enough over enough of the span that the rotor still turns. The helicopter doesn't glide as nicely as the airplane, but comes down really quick.

Dan
Click to expand...

really, because I read that some heli's go into negative for their auto rotation.
 
rubber314chicken said:
really, because I read that some heli's go into negative for their auto rotation.
Click to expand...

Not a heli expert, but I never heard of that, and it doesn't make sense since the rotor's are still providing lift to keep you from dropping like a rock rather than like a 2x4 as they do while autorotating as I'm used to in R-22s and UH-11s
 
I assume by negative you mean negative G's. No, helicopters with semi or fully articulating rotors cannot safely go to 0 G let alone negative G. When you enter an autorotation you may go slightly less than 1 G. I've never had an engine failure in a helo but rolling off the throttle and entering an auto is usually a very gentle process, and once you are established it is a constant speed glide (1G). In the helos I've flown the glide ratio is anywhere from about 5:1 to 7:1.

Joe
 
Areeda said:
I assume by negative you mean negative G's. No, helicopters with semi or fully articulating rotors cannot safely go to 0 G let alone negative G. When you enter an autorotation you may go slightly less than 1 G. I've never had an engine failure in a helo but rolling off the throttle and entering an auto is usually a very gentle process, and once you are established it is a constant speed glide (1G). In the helos I've flown the glide ratio is anywhere from about 5:1 to 7:1.

Joe
Click to expand...

no, I mean AOA of the rotors. See the link above.
 
rubber314chicken said:
no, I mean AOA of the rotors. See the link above.
Click to expand...
I see.

First of all remember AOA is very different from Pitch Angle. AOA is the angle of the airfoil and the relative wind while Pitch Angle is the angle between the blade and the rotor disk (plane of rotation).

I looked through my JetRanger (Bell 206B POH and couldn't find anything on the blade pitch angle range. Full down looks like zero to me but I couldn't tell the difference if it was -0.5°.

AOA on the other hand is definitely negative during autorotation. During powered flight the main rotor takes air from above and in front and forces it down and back giving a thrust vector into the plane of rotation.

During autorotation (straight ahead glide) the air is coming from below and in front giving a drag vector back and up. Same range of pitch angles on the blade but in one the relative wind is coming from above and the other the relative wind is coming from below.

Hope that helps.

Joe

Edit: My first flight got delayed so I have more time.

Nothing found in the R44 POH, but the Schweizer 300C manual says this:
Blade twist -8°39', Collective pitch full down (3/4 radius): 2.5° ± 1.5°. Collective pitch travel: 12° ± 1°.

Thinking about this I don't see how a <1° pitch angle difference could make a sign difference on AOA. Entering an autorotation under the hood involves a pitch up of about 10°. I've never seen the AI entering an autorotation without the hood.
 
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The answer to the pitch in autorotation as with everything is, it depends. With light helicopters like the Hughes 300 and Bell JR the disk loading is pretty light, so that a near zero pitch angle is suitable to maintain inertia. The same cannot be said of the bigger birds, which almost always have a negative pitch setting at the tip for those exciting autorotations near the dead zone.

Some collectives have a detent, and/or a slip collar to get the full down travel of the collective for autorotation. Although I believe this has gone out of favor in more modern aircraft. As disk loading goes up, the ability to maintain good rotor inertia becomes problematic. Slight negative pitch in these heavy birds helps to maintain the inertia. It also become harder to handle in the flare, and it's not unusual to see a rotor overspeed slightly just before the collective is raised if it's hot, heavy, low, etc.
 
Just as I suspected: PFM

We had this 'disc'ussion in school about a week ago. We got a very good explanation of it, but I still concluded that it was some sort of magic. The same kind of magic as the Coriolis Effect.....

--Matt Rogers
 
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