Rotors keep on whipping along (track)

[seikdel]

I just finished campaign mission 3 (I think?) At the end, I shut everything down. My rotors drop to 25% and just keep spinning until I finally engage the rotor brake. Is that a bug, or is the wind just brisk enough to keep them there forever?

 

And yes, my flying isn't great. I'm afraid I'm a med student who can only dream of being a rotary pilot. It took me 3 tries to realize I was going through my waypoints backwards =/

 

Edit: Attachment failed

Flappity rotors.zip

Edited January 4, 2012 by seikdel

[Griffin]

Yup had the same thing happen to me in a different mission.

That's just how it is in the sim I guess :)

[msalama]

It's the wind. But is that a problem, since you should put the rotorbrake on anyway once the RPM drops below 30%?

[KosPilot]

It's the wind and another amazing example of the outstanding job that ED have put into the modelling of the rotor system.

[Griffin]

Would that be realistic? I won't say what is and what isn't because I have very little experience on helicopters (and none in co-axials) but I feel like it would not happen in real life. It's just because of simulation. Or does anyone know for real?

Not a big deal for me. Nothing is perfect and this is just how TFCE handles physics. It's still second to none!

Edited January 4, 2012 by Griffin

[Devrim]

If I'm not wrong; -in a video (footage?)- I've seen that a man (technician probably) could turn the blades with his hands.

 

I think if wind speed is enough, blades can be spinned.

[Griffin]

Yes you certainly can even from the tail rotor. But would ~ 5 m/s wind be able to spin the rotor at 20 % RPM with zero pitch?

[Devrim]

No way. :)

 

In my opinion; 5m/s wind may not able to start to spin rotor. But, may be, it extends the spin time. So, it delays the stopping. :p

 

:bomb: My English! I know words but can't get together!

[msalama]

Hmmm, we better ask our experts if rotors windmilling at zero pitch is realistic I think. Heeeey AirTito, Alpha16 & GGTharos!!! Help us clueless barstages here willya :D

 

PS. Your English is fine Devrim BTW.

[AlphaOneSix]

5 meters per second? That's roughly 10 knots.

 

In stiff winds, rotors are more likely to flap up and down rather than start rotating. It's a hard question to answer for me because if I leave a helicopter outside in any kind of wind for more than a few minutes, we will at least turn on the rotor brake and usually we'll also tie down the blades. I have seen rotor blades windmilling in a breeze before, but nowhere near 20% rpm, they were just barely moving.

[EtherealN]

I have seen rotor blades windmilling in a breeze before, but nowhere near 20% rpm, they were just barely moving.

 

I suspect the key to 20% rpm there is inertia - that is, this wouldn't happen unless the rotors were already spinning.

 

And of course, for how long were they monitored afterwards? (I don't quite have time to watch the track atm.) Might be that the rotors are still slowing down, just very slowly due to the healthy wind component.

[KosPilot]

Windmilling at "zero" or any other collective pitch setting is expected. With the heli standing on the ground and the wind coming directly from behind, the forward tilt of the rotor mast ( and missing nose wheel strut :D ) will enable the rotors to windmill just as they would for autorotation. The difference in this situation is that the heli is stationary and no energy is consumed to maintain safe rate of descend.

 

When the rotors are spinning down and the wind is blowing up through the rotordisks, at some point the sum of lift and drag for the entire rotorsystem including resistive torque from drives and gearboxes may become equal. At this point RRPM is maintained.

If the relative wind is at a vector angle within usable AoA limits for the given profile, there is always a section of a rotating rotor that is going to produce lift. Most profiles have the most efficient lift/drag ratio at about 4 degrees AoA.

Looking at the lift formula; Lift=Cl*1/2p*Vexp2*S (Cl:Lift Coefficient, p:density, V:Velocity, S:Surface Area), V is the major factor for lift production and the reason the rotor must already be spinning in order to enter the regime where windmilling is possible.

 

100% rotor RPM for the Ka-50 should be something about 300, give or take, so RRPM@20%=60.

Say the rotor tilt is 4 degrees. Total swept area is 330.3m2 and even at a small angle as 4 degrees, the wind is working an area of about 23m2. (Disregarding further tilting / exposed area due to flapping action)

Wind Power (W) = .5 * swept area (S) * air density (p) * Wind speed exp3

.5 * 23m2 * 1.23kg/m3 * 5*5*5 = 1770W

Betz limit and loss ~60% = 700W of rotation power to overcome drag, in theory that is.

If 700W is enough to maintain 60 RPM? I honestly do not know but I think it's plausible as the power is only going into maintaining a constant RPM under minimal load.

 

ED has put a lot of effort into rendering real time rotorsystem behavior and I have no doubt their calculations are more accurate than mine.

Edited January 5, 2012 by KosPilot
details details...

[msalama]

Thanks Kospilot, sounds plausible enough to me at least. Although my engineering maths are admittedly more than a little rusty some 15 years after college it has to be said :D

[Griffin]

Damn man that's convincing to say the least!

I have no doubt YoYo has done a great job. It's just that simulation engines differ from the real world and cause debatable stuff no matter how accurate the input values are. Thanks for putting it into numbers :)