Jack Stall
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From: In your head.
Jack Stall
...I think you call it.
As far as I can recall (being a mere fixed winger), in a helicopter, you can reach an airspeed at which your rearward moving blade doesn't have enough speed to generate lift. Although I stand to be corrected!
What happens next then? Is it controllable?
Cheers
Chesty
As far as I can recall (being a mere fixed winger), in a helicopter, you can reach an airspeed at which your rearward moving blade doesn't have enough speed to generate lift. Although I stand to be corrected!
What happens next then? Is it controllable?
Cheers
Chesty
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From: London
What you’ve described is Retreating Blade Stall
Just as the stall of an aeroplane wing limits its low speed, the stall of a rotor blade limits the high-speed potential of a helicopter.
The airspeed of the retreating blade slows down as forward airspeed increases. It has to produce an equal amount of lift to that of the advancing blade so, as the airspeed of the retreating blade is decreased with forward airspeed, the angle of attack of that blade must be increased to equalise lift throughout the rotor disc area.
If the angle is increased further and further to equalise the lift, the retreating blade will eventually stall - at some high forward airspeed.
First indication is often a vibration - the equivalent of buffet in a fixed-wing.
The nose lifts and there's a tendency for the helicopter to roll in the direction of the stalled retreating blade.
If the pilot didn’t reduce the pitch of the blades, the stall would become worse, vibration would increases and control might eventually be lost.
Is it controllable?
Yes, in the same way that you don't allow a fixed-wing to stall - by recognising the warning signs and acting accordingly.
Just as in a fixed-wing you take action at the buffet stage to prevent an incipient stall developing into a full stall, so you take action in a helicopter.
In both cases, you reduce pitch.
High forward speed is only one of the circumstances in which RBS can occur; there are other causes.
NB: There are hundreds of expert pilots on this forum - I'm not one of them.
This is just a 'broad brush' answer until one of them explains it more accurately.
FL
Just as the stall of an aeroplane wing limits its low speed, the stall of a rotor blade limits the high-speed potential of a helicopter.
The airspeed of the retreating blade slows down as forward airspeed increases. It has to produce an equal amount of lift to that of the advancing blade so, as the airspeed of the retreating blade is decreased with forward airspeed, the angle of attack of that blade must be increased to equalise lift throughout the rotor disc area.
If the angle is increased further and further to equalise the lift, the retreating blade will eventually stall - at some high forward airspeed.
First indication is often a vibration - the equivalent of buffet in a fixed-wing.
The nose lifts and there's a tendency for the helicopter to roll in the direction of the stalled retreating blade.
If the pilot didn’t reduce the pitch of the blades, the stall would become worse, vibration would increases and control might eventually be lost.
Is it controllable?
Yes, in the same way that you don't allow a fixed-wing to stall - by recognising the warning signs and acting accordingly.
Just as in a fixed-wing you take action at the buffet stage to prevent an incipient stall developing into a full stall, so you take action in a helicopter.
In both cases, you reduce pitch.
High forward speed is only one of the circumstances in which RBS can occur; there are other causes.
NB: There are hundreds of expert pilots on this forum - I'm not one of them.
This is just a 'broad brush' answer until one of them explains it more accurately.
FL
Last edited by Flying Lawyer; 4th July 2006 at 23:38.
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From: Warrington, UK
Jack stall is associated with the hydraulic servos/jacks that operate the main rotor blades. In certain helicopter(AS350/355, Gazelle are ones that I know of), during hard manouvering, the aerodynamic loads on the blades can become high enough to prevent the hydraulics from being able to move the blades further.
In the Gazelle, there is an uncommanded pitch up and roll to the right. Simply reducing the severity of the manouver will reduce the load and free the controls.
In the Gazelle, there is an uncommanded pitch up and roll to the right. Simply reducing the severity of the manouver will reduce the load and free the controls.
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From: In your head.
Cheers guys, just what I was after
Is it wrong to assume that when the nose pitches up, as a result of RBS, you actually gain height?
FL I'm just trying to get my head around the physics of your spinning bits. When you say you reduce pitch to prevent, or recover, from RBS, I take it that this is the pitch of the blade and has the effect of reducing the AoA of the stalled blade. I understand that bit as that's what we do on fixed wings.
The thing that's confusing me - When we stall it, usually, happens to both wings simultaneously. Assuming you had entered a fully developed stall, as I picture it, when you carry out your stall recovery why does it not aggravate the symptoms of the stall and increase the roll rate or AoB?
Chesty (Confusing myself) Morgan
Is it wrong to assume that when the nose pitches up, as a result of RBS, you actually gain height?
FL I'm just trying to get my head around the physics of your spinning bits. When you say you reduce pitch to prevent, or recover, from RBS, I take it that this is the pitch of the blade and has the effect of reducing the AoA of the stalled blade. I understand that bit as that's what we do on fixed wings.
The thing that's confusing me - When we stall it, usually, happens to both wings simultaneously. Assuming you had entered a fully developed stall, as I picture it, when you carry out your stall recovery why does it not aggravate the symptoms of the stall and increase the roll rate or AoB?
Chesty (Confusing myself) Morgan
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From: USA
The classis signs of RTS are the loss of cyclic control, usually a gradual thing (because RTS is a gradual thing) where the lateral cyclic trim (where the cyclic needs to be to keep the wings level) begins to move rapidly toward the retreating side. This is accompanied by some increase in n/rev vibration, usually. Eventually, the cyclic hits the stops and the aircraft rolls toward the advancing side. At the same time, the nose tries to lift, so the cyclic needs to go somewhat forward. The reason why the aircraft behaves assymetrically, as compared to airplanes, is that the rotor is losing lift on one side only.
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From: Denver, CO and the GOM
Originally Posted by NickLappos
<<trim & snip>>
...where the lateral cyclic trim (where the cyclic needs to be to keep the wings level) begins to move rapidly toward the retreating side. Eventually, the cyclic hits the stops and the aircraft rolls toward the advancing side. At the same time, the nose tries to lift, so the cyclic needs to go somewhat forward.
...where the lateral cyclic trim (where the cyclic needs to be to keep the wings level) begins to move rapidly toward the retreating side. Eventually, the cyclic hits the stops and the aircraft rolls toward the advancing side. At the same time, the nose tries to lift, so the cyclic needs to go somewhat forward.
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From: The Dark Side
According to my recollection and my RAF CFS notes the helicopter rolls towards the retreating blade and pitches up. The retreating blade commences to flap down as it passes through the 12 o'clock and reaches a maximum rate of flap down at 9 or 3 o'clock depending upon direction of rotation reaching its lowest point at 6 o'clock.
GAGS
E86
GAGS
E86
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From: USA
Reversed bits, sorry guys. Hasty response! Didn't re-read my response, had to run to a Gulfstream and fly around the world....
The cyclic drifts toward the advancing side, with a pitch-up! The effect is actually gradual if you approach the stall via low g and high airspeed. It can be very sharp if you experience it via rapid cyclic pulls to create higher g.
The cyclic drifts toward the advancing side, with a pitch-up! The effect is actually gradual if you approach the stall via low g and high airspeed. It can be very sharp if you experience it via rapid cyclic pulls to create higher g.
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From: UK
Originally Posted by Chesty Morgan
I'm just trying to get my head around the physics of your spinning bits.
Get hold of a book called 'Principles of Helicopter Flight' by a guy called W.J.Wagtendonk.
His book will probably answer all of your questions concerning the physics of our 'spinning bits'. His is certainly the volume that 'we' turn to in order to answer the questions ourselves.
HTH,
B73
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From: Belgium
Correction on the fixed wing side, one blades usually stalls a fraction earlier and the airplane 'drops' a wing, if not recognised you'll enter a spin.
On the rotary side: it's no rule that there's is buffeting, it can just pitch up and roll without any warning. However as menitioned before the only recovery is to lower collective and reduce speed.
On the rotary side: it's no rule that there's is buffeting, it can just pitch up and roll without any warning. However as menitioned before the only recovery is to lower collective and reduce speed.
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From: UK
Originally Posted by Heliport
There's enough expertise on this forum to answer his question.
Better to go straight to the source, I reckon. FleaBay might turn up cheaper copies (if 20 quid is too steep for Chesty).
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From: Australia
Originally Posted by Chesty Morgan
I'm just trying to get my head around the physics of your spinning bits.
Ask Bell why the blades flap, get a million answers, most anyone can tell you how to make them do it and what happens - when etc. but the physics, fluid hydraulics etc. - ???
Another book from the archives; 'The helicopter and how it flys' by John Fay, an Englishman - lent my copy eons ago - was not returned. Read it twice and passed the OZ theory easy.
Gatvol
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From: KLAS/TIST/FAJS/KFAI
Excuse me for not being an engineer, but wasnt the subject about Jack Stall, not Retreating Blade Stall?? If I recall Jack Stall was something found in the AS350 after lots of cyclic movement and may commonly be known as a Hydraulic Hiccup or Cyclic Lock. Leave it alone for a few seconds untill hydrualics get back to normal??.
OK, now make me look stupid..........
OK, now make me look stupid..........
