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HeliTorque :: View topic - retreating blade stall
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HeliTorque Forum Index » Wannabes

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retreating blade stall
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Logan
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PostPosted: Tue Nov 11, 2008 5:58 pm    Post subject: retreating blade stall Reply with quote

i was reading an article yesterday about retreating blade stall in turns, is this due to the fact that the outside part of the disc on a turn is moving faster through the turn than the inner side? kind of like wheels in a car going around a corner?
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PostPosted: Tue Nov 11, 2008 6:00 pm    Post subject: Reply with quote

Sounds like good logic to me! Very Happy
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PostPosted: Tue Nov 11, 2008 6:12 pm    Post subject: Reply with quote

so if im turning right and the rotors and turning anti-clockwise then the accelerating left side of the disc will have enough wind speed to stall the retreating blade.....???? At a certain IAS which could cause such an event correct? im just checking myself!!!
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PostPosted: Tue Nov 11, 2008 6:20 pm    Post subject: Reply with quote

Well if your blades are rotating anti-clockwise, your accelerating blade is the right side and retreating blade is the left side. So turning to the right you would get a slower accelerating blade and faster retreating blade.

I guess high pitch angle might make it worse.
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PostPosted: Tue Nov 11, 2008 6:25 pm    Post subject: Reply with quote

sorry thats what i meant! the decelerating blade!
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PostPosted: Tue Nov 11, 2008 7:05 pm    Post subject: Reply with quote

Logan

It sounds as though some one is confusing fixed wing and helicopter principles. I'd be very surprised if a turn at speed could cause enough of a speed difference between the inside and outside of the turn to cause a stall.

Stalling happens because the angle of attack passes a critical angle for that shape of aerofoil and lift decreases with further increase of angle of attack.

At high speeds the angle of attack on the retreating blade passes the critical angle and the blade flaps down, causing a further increase in the angle of attack deepening the stall.

Rolling towards the retreating blade can tip an unstalled blade over the edge at high speed, causing the stall.

Be careful what you learn from books without guidance, and equally be careful what books you read, there are some well accepted text books with sections in them which are completely untrue, but seem to fit the circumstances.

Gary
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PostPosted: Tue Nov 11, 2008 7:09 pm    Post subject: Reply with quote

i know thats exactly why i come and post questions here! sorry i have so many, its entirely because i want to make sure what im learning is correct and i have a deep understanding!
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PostPosted: Tue Nov 11, 2008 7:56 pm    Post subject: Reply with quote

veeany wrote:
Logan

It sounds as though some one is confusing fixed wing and helicopter principles. I'd be very surprised if a turn at speed could cause enough of a speed difference between the inside and outside of the turn to cause a stall.

Stalling happens because the angle of attack passes a critical angle for that shape of aerofoil and lift decreases with further increase of angle of attack.

At high speeds the angle of attack on the retreating blade passes the critical angle and the blade flaps down, causing a further increase in the angle of attack deepening the stall.

Rolling towards the retreating blade can tip an unstalled blade over the edge at high speed, causing the stall.

Be careful what you learn from books without guidance, and equally be careful what books you read, there are some well accepted text books with sections in them which are completely untrue, but seem to fit the circumstances.

Gary


I agree that its highly unlikely. I didn't include it in a previous post but i was going to ..

Do you think it is more likely to occur during a climbing turn with a much higher pitch angle?

I still think that you can't really get enough forward speed in a level turn to make a vast difference between the outer edge of the disc and the inner edge, perhaps in a tight descent turn.
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PostPosted: Tue Nov 11, 2008 8:10 pm    Post subject: Reply with quote

Its not the turn its the forward speed and blade flapping that will do it, the act of rolling, not bank angles will matter.

I believe the certification standards are supposed to protect us from RBS and it is one of the reasons most helicopters have a reducing Vne with increasing DA. I'd need to get them out and have a look to be sure.

I doubt climbing turns cause problems related to RBS, high pitch angles yesr, high AoA yes, high enough to stall at the speeds we can climb at no.
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PostPosted: Tue Nov 11, 2008 11:24 pm    Post subject: RBS Reply with quote

Logan,

Veeany is essentially correct. The turning or climbing/descending of a copter has little to do with the blade stall.

The RBS occurs because of the high angles of attack required to compensate for Dissymmetry of Lift, and begins at or near new the tip of the helicopter blade.

Dissymmetry of Lift is the unequal lift across the rotor disc resulting from the difference in the velocity of the air over the advancing blade half and retreating blade half of the rotor disc area.

Think of it this way: Ever been on moving stairs? (We call them escalators).
If you move with the strairs, your speed is increased by the speed of the stairs, but if you walk down or against the stairs, your speed is reduced.

Same with airflow over the advancing and retreating blades. The copter reaches a point in forward flight, where the "opposite airflow" over the retreating blade virtually cancels any lift it can produce and the flapping can no longer adjust the angle of attack to match the advancing blade.

In a 2 bladed counter rotating system, the blades flap as a unit since there is no flapping (or horizantal hinge), such as in the Bell 206. RBS still will occur, and the copter usually pitches up sharply and drops off to the left. (Pitches up due to Gyroscopic Precession, and drops left because there is insufficient lift on that side.

Hey.....I get $75.00/hr US for this giving this kind of lesson. Laughing [/i]
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PostPosted: Wed Nov 12, 2008 1:07 am    Post subject: Reply with quote

haha $75 it is! thanks AB, veenay, and windswept!
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