Retreating blade stall

[Airway]

Is a retreating blade stall possible on a coaxial rotor based helicopter?

 

http://www.helis.com/howflies/maxspeed.php#2

[Yo-Yo]

Surely yes, but due to the coaxial rotors it is not the same classic scheme has. No significant banking moment.

[d0ppler]

no

(based on my common sense, which might be wrong from time to time :D)

 

edit : like now :D

[Airway]

Surely yes, but due to the coaxial rotors it is not the same classic scheme has. No significant banking moment.

 

I would expect that both rotors would have a retreating blade stall on exactly the opposite site (lower rotor left side, upper rotor right side as they were counter rotating?) then and that would compensate the effect.

Is that right?

[d0ppler]

based on my common sense, yes, it would compensate the effect (that was actually what I was answering in my last post)

[DarkWanderer]

I would expect that both rotors would have a retreating blade stall on exactly the opposite site (lower rotor left side, upper rotor right side as they were counter rotating?) then and that would compensate the effect.

Yes, and the ultimate result of it is blade collision. You're right.

[Airway]

Blade collision :huh:

I hope I will never experience that.

 

@all:

 

Thank's for you reply :thumbup:

[nemises]

^^ hehe...you will .. I'm sure everyone who has flown the sim has clipped blades at least once by now :)

[d0ppler]

My first blade clip came after my very first 7 seconds in the air..

[Vortex]

If one of the DCS staff answers this, can they also tell us if "compressibility" in the advancing blade is also modelled?

 

Just did a run with immortal mode.

 

RBS is likely to be modelled given I had un-commanded nose up attitude when at speeds way over VNE, I didn't get any nose down moments. But just because I couldn't incite compressibility doesn't mean it isn't there.

 

BTW RBS in a conventional heli induces a nose up attitude, so I can't see why a co-axial should be any different given the effect is from gyroscopic precession (or whatever the terms given to the effect 90 degrees after the event).

Edited December 29, 2008 by Vortex

[Aser]

^^ hehe...you will .. I'm sure everyone who has flown the sim has clipped blades at least once by now :)

 

Amen! :joystick:

[Spartan1-1]

Your right about the gyroscopic precession. This is a great question. With coaxial rotors, i assume it would be an equal loss of life on both "wings". Would it act as a conventional stall. That is if the blades dont kiss each other.

[RvETito]

BTW RBS in a conventional heli induces a nose up attitude, so I can't see why a co-axial should be any different given the effect is from gyroscopic precession (or whatever the terms given to the effect 90 degrees after the event).

 

It isn't. The lower rotor RBS is at 3 o'clock while for the upper is at 9 but maximum deflection due to precession for both is at 6 o'clock (over the tail). So both rotors tilt backwards hence the pitch up moment.

[Airway]

Товарищи !

 

This is a bit too complex for me :D

 

The blades profile of the upper and lower rotor are turned 180° (meaning both rotor blades have their front profile leading into the flight direction of the helicopter)

So the one rotor rbs is at 3 o'clock, the other one's at 9 o'clck, as AirTito said, but doesn't that mean that the deflection is exactly on the other side of the rbs?

[RvETito]

Due to gyroscopic precession the maximum deflection occurs 90 degrees ahead in the direction of rotation.

[Airway]

90° ahead - I see. Thought of 180°

 

Whatever gyroscopic precession means - but might be the same what I think of and had no word for it. :D

[Frederf]

They aren't turned 180 degrees, they are mirrored horizontally. If the top blade has effect XYZ at the 3 o'clock and 6 o'clock positions then then the bottom blade would have the effect at the 9 and 6 positions.

 

I figure a double retreating blade stall wouldn't be biased for bank since each rotor would have a bank tendency in an opposite direction but you'd definitely get an overall lift decrease yes? The pitch up tendency I don't know about, I assume it's some side effect of RBS.

 

Also, not blade collision here.. yet :D

[RvETito]

There is a bank momentum because despite the lateral components of the rotors thrust are almost the same, the upper rotor has twice the arm to the CG.

[Airway]

lever rule?

[RvETito]

Of course :D

 

Gimme a fulcrum and a lever long enough and I'll move the Earth.

[Airway]

:megalol:

 

That's a good one.

 

At least something was left in my brain from engineering school. :lol:

[Tango]

The pitch up tendency I don't know about, I assume it's some side effect of RBS.

http://www.dynamicflight.com/aerodynamics/retreating/

 

Best regards,

Tango.

[GGTharos]

THANK YOU!

 

Darn it, sometimes the solution is so simple, yet so difficult to see, even though it's staring you blatantly in the face :D

 

 

There is a bank momentum because despite the lateral components of the rotors thrust are almost the same, the upper rotor has twice the arm to the CG.

[Vortex]

It isn't. The lower rotor RBS is at 3 o'clock while for the upper is at 9 but maximum deflection due to precession for both is at 6 o'clock (over the tail). So both rotors tilt backwards hence the pitch up moment.

 

Yep, or are you disagreeing with me? Either way that's what I was talking about. I was confusing myself a little with the terminology as I was trying to remember "Phase Lag" but that's different.

Edited December 30, 2008 by Vortex

[Airway]

So it seems to be a pretty dangerous effect when you are low and fast, as combat helicopters normally are.

I see that you have to reduce collective pitch and neutralize the cyclic to get out of this dangerous situation what is almost impossible when low and fast, or?