Autorotation aerodynamics explained?

[eV1Te]

Mathematical models of dynamics, systems, ballistics

 

Does that include aerodynamics for the helicopter as well?:)

 

Now it's time to read about ballistics so I can post more questions for you to answer :thumbup:

[Yo-Yo]

Does that include aerodynamics for the helicopter as well?:)

 

Now it's time to read about ballistics so I can post more questions for you to answer :thumbup:

 

Yes, aerodynamics as a part of dynamics, of course.

[JasBird]

Hi

 

I'll give a go at explaining Autorotation :) First of all the rotor blades does not spin the other way when autorotating. In powered flight the rotor suck air down through the rotordisc. When engine fails the freewheeling unit will disconnect the blades from the engine, so the rotor rotates freely. Now this will cause you to drop altitude. Falling down means you get air through the rotor system which makes the rotor spin. Remember from physics speed and altitude=energy. Hopefully you have altitude when engine fails so you get time to build up rotor speed before you hit the ground.

 

Using the R22 would be a good example. It has a low inertia(light blades) rotor system, which means the will speed up quickly, but also slow down quickly. According to R22 manual you have about 2-3 seconds to lower collective when engine fails, otherwise the rotor will slow down to much, and then you won't be able to recover, get rotor speed back up.(That's why rotor don't spin other way in autorotation, because once rotor RPM to slow, no recover) Off course engine failure in climb(when high angle on blades) will give you less time to react compared to fauilure at descent. Because the R22 has a low inertia systen it's probably the most tricky helicopter to Autorotate. Rotor RPM changes fast and because blades stop quickly you have to time the flair perfect.

 

I would asume the Ka-50 have high inertia(heavy blades) like most "normal" helicopters(R22 is not normal :)) and even though engine fails, the blades have a lot of momentum to keep spinning on their own, which means more time to react, more energy and easier to autorotate.

 

Think of it like this. And R22 and a Ka-50 shutting down side by side. If we let the blades slow down by them self, the R22 would probably spin for a couple of minutes, while the Ka-50 would go on forever. It has much more inertia. As a sidenote in the R22 you only have a short time window to flare before hit the ground, low energy rotor system. In a normal size helicopter you have enough energy(heavy slow blades) to actually flare and if you pull to much on collective when trying to land you can actually go upwards again.

 

Not sure if I'm off point or bad explanation, but to recap. Engine fauilure, means lower collective to take away drag, and rotor blades needs time to change from sucking air getting air from below. The drop will provide air up through the rotor system, which keeps the fan spinning.

 

Hard to explain aerodynamice without drawing :)

 

Cheers

[eV1Te]

Yep that's true.

 

The things that I wasn't sure about was;

For example that in the case of the R22 when your RPM slows down you can not recover below a certain threshold... But this never happen with the Ka-50, the RPM never slows down that much in the Ka-50 to begin with, therefore it's practically impossible to fall out of the sky with the Ka-50. :thumbup:

 

Also the thing with the autorotation and the direction of the rotation... if you would stop the rotors in mid air and then drop the helicopter, then the AoA would be more than 90 degrees (if you have the collective at more than zero degrees) and the rotor blades would rotate backwards. But since you have a rotation of the blades to begin with, the blades can due to the falling velocity and upwards going wind direction maintain the forward rotation of the blades in the "phenomena" called autorotation.

[JasBird]

Well if the rotors stop in mid air you would die, you will never get the RPM back up. Imagine the speed you would fall in, the coning off the baldes etc, it would be a big mess.

 

Not being familiar with Black Shark or the Ka-50 rotor system, I find it strange that you can not stall(stop) the rotor RPM when autorotating? Full collective in auto doesn't drop rotor RPM? I assume it has a lot of inertia in the heavy blades, but If you tried it from high altitude so you get time to slow the rotors down before you touch the ground.

[eV1Te]

Not being familiar with Black Shark or the Ka-50 rotor system, I find it strange that you can not stall(stop) the rotor RPM when autorotating? Full collective in auto doesn't drop rotor RPM? I assume it has a lot of inertia in the heavy blades, but If you tried it from high altitude so you get time to slow the rotors down before you touch the ground.

 

So I thought at first as well, but try it yourself in the game; turn of engines at a relatively high altitude and play with the collective...

 

And as Yo-Yo (developer of the DCS engine) said a few posts up, it's practically impossible to reach such a threshold that the RPM never goes up again when you reduce collective.

[Draco]

The unrecoverable low-RPM state in the R44 or "catastrophic rotor stall" as the R44 POH calls it, occurs at 80% rotor RPM plus 1% for every 1000 feet density altitude. What happens is as the blades slow down below the critical RPM, they stall. This causes a sudden increase in drag slowing the blades down even further until they aren't producing enough centripetal force to hold the blades flat. This allows them to flex upward considerably under the weight of the helicopter, possibly to the point of blade failure. This state is often referred to as "Tuliping" or "Folding the blades up." It is absolutely unrecoverable. there is nothing you can do to fix it. One of my instructors referred to what happens next as "sky diving with a helicopter strapped to your back."

 

The Robinson factory safety course says you have 1.1 seconds after engine failure to enter the auto or you seriously risk this happening. This is why I never put the collective friction on all the way, just enough to hold it in place while I change the radios or set the GPS.