Blade stall at high speeds (Attention, physics inside..)

[MasterZelgadis]

Hi,

 

I noticed something while trying to keep that damn thing flying where I want it to fly :D

 

The higher my speed is, the higher will be my bank angle to the right side.

 

Well, nothing noticable here, fully logical behavior, because my speed adds to the speed of the advancing rotor blade cutting through the air, and is substracted from the retreating blade. Et voila... different lift on each side of the helo.

 

What I was wondering is the following. When I have two counterrotating rotors, there should be no different lift on each side, or am I wrong here?

 

I fly with an airspeed of 300. I have an advancing blade on the left side of the helicopter, and a retreating blade on the right side. But from the second rotor, I have an advancing blade right, and a retreating blade on the left side...

 

I would understand, if the overall lift generated would be decrease, but in a somehow balanced way

 

Hope you can understand, what I want to say :)

[sobek]

IIRC, the problem is that one rotor operates in the downwash of the other. Additionally, the rotors sit at different lenghts from the center of gravity, thus the moments generated are transferred over different lever lenghts, resulting in the total moment being different from zero.

[Bucic]

Ka-50 is only more symmetrical aerodynamically than classic tail rotor helicopters but not completely symmetrical.

 

The key factor here, I think, is the aerodynamic influence of one rotor blades on the second rotor blades. And not only in a 'upper rotor influences the flow of the lower rotor' way as one may initially think. On top of that there's the phenomena of increasing angle between the upper and the lower rotor disks (i.e. increasing distance between blade tips of the upper and lower rotor disks) resulting in even stronger asymmetry.

 

EDIT:

Aaand the different momentum arms, as Sobek said :)

 

Is anything from what's been said missing from the respective manual chapter? Just curious.

[MasterZelgadis]

That makes sense..

 

I am at work now, don't have the manual here, but I read the chapter yesterday. As far as I saw, there was only mentioned, what I said above, that the airspeed adds/substracts to/from the rotor blade speed, causing different lift on each side of the heliocopter. If I remember correctly, the special situation with a coaxial rotor design wasn't mentioned, but I will check that as soon as I get home

[Bucic]

That makes sense..

 

I am at work now, don't have the manual here, but I read the chapter yesterday. As far as I saw, there was only mentioned, what I said above, that the airspeed adds/substracts to/from the rotor blade speed, causing different lift on each side of the heliocopter. If I remember correctly, the special situation with a coaxial rotor design wasn't mentioned, but I will check that as soon as I get home

Get the mobile version of the manual (PDF split into parts) dude! :)

[EtherealN]

What I was wondering is the following. When I have two counterrotating rotors, there should be no different lift on each side, or am I wrong here?)

 

You are wrong. :)

 

L=½*Cl*A*r*V²

 

L= Lift

½= A constant

Cl= Coefficient of lift for the airfoil

A=Area of the airfoil

r=Air density

V=Velocity/Air Speed

 

As you can see, the Velocity factor is squared. This means that the gain in lift of the advancing blade will be greater than the loss in lift for the retreating blade.

[bogusheadbox]

You are wrong. :)

 

L=½*Cl*A*r*V²

 

L= Lift

½= A constant

Cl= Coefficient of lift for the airfoil

A=Area of the airfoil

r=Air density

V=Velocity/Air Speed

 

As you can see, the Velocity factor is squared. This means that the gain in lift of the advancing blade will be greater than the loss in lift for the retreating blade.

 

I think i am getting what the original poster is asking and i think SOBEK hit the nail on the head with it.

 

If we break down Cl (co-efficient of lift). It basically for low speed flight comprises of wing shape (planform) and alpha.

 

As the lower rotor will be influenced by the higher rotor, it makes sense to note that alpha will be changed for the lower rotor.

 

Using the formula of lift it makes sense that if we increase alpha (not beyond critical alpha) we will increase lift.

 

Therefore if the lower rotor has a changed alpha from the higher rotor then there would possibly be asymetric lift from the two seperate rotors even though they are contra rotating.

 

Add to this the different moment arms and yes, it is far from symetrical.

[MasterZelgadis]

Yes, the formula and the conclusion is correct for a single rotor helo.

But for a coaxial rotor helo, there should be for a retreating blade on one side also an advancing blade on the same side, generating the same amount of lift like the advancing blade on the other side.

 

So far for my assumption at the moment I opened this thread.

 

So if the asymmetrics are based on the fact that one rotor operates in the downwash of the other one, let's say we have a tandem configuration, like the CH-47 Chinook. There we should have the effect I described above, because both rotors can effectively use their full blade lengths for generating the lift..

 

edit:

Get the mobile version of the manual (PDF split into parts) dude! :)

That was exactly what I thought today, loading the manual pdf to my Android. ;)

Edited December 17, 2010 by MasterZelgadis

[bogusheadbox]

So if the asymmetrics are based on the fact that one rotor operates in the downwash of the other one, let's say we have a tandem configuration, like the CH-47 Chinook. There we should have the effect I described above, because both rotors can effectively use their full blade lengths for generating the lift..

 

 

Also don't forget the moment arm. The top rotor asymetry will have a greater effect than the lower due to the greater moment arm.

 

The Chinook is a good case to look at and would be an interesting topic all by itself.

 

I believe that the rear rotor is higher than the front rotor and though they don't officially overlap the blades do intermesh and this would/should have some effect?

 

Interesting discussion

[MasterZelgadis]

Yes, the rear rotor is higher. Also more power for the rear rotor is needed, to genereate the same amount of lift like the front rotor, because the airflow of the front rotor affects the rear rotor...

 

The question would be, does it affect, and if yes, how much does it affect the rolling effect at higher speeds?

[Bucic]

IIRC, the problem is that one rotor operates in the downwash of the other. Additionally, the rotors sit at different lenghts from the center of gravity, thus the moments generated are transferred over different lever lenghts, resulting in the total moment being different from zero.

The different momentum arms is only a secondary factor to the aerodynamic interaction between upper and lower rotor discs / blades.

 

 

Hence the primary cause to asymmetry comes from aerodynamic interaction between upper and lower rotor discs / blades, unless there's some other phenomena the existence of I'm not aware (like some fancy gyroscopic, usually sneaky, phenomenas).

 

NOTE: The rotor mast deflection visualization is irrelevant to the case, drawn for personal use only!

 

Yes, the formula and the conclusion is correct for a single rotor helo.

But for a coaxial rotor helo, there should be for a retreating blade on one side also an advancing blade on the same side, generating the same amount of lift like the advancing blade on the other side..

Correct evaluation of the insufficient 'velocity only approach' :)

[walker450]

Great topic! I first learned about it reading this great article I found on simhq.com:

 

 

DCS: Black Shark

and Coaxial Rotor Aerodynamics

 

 

((((article by Erik "EinsteinEP" Pierce))))