Autos and Rotor Stall
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When drooping the rotor in a 3 foot hover even to the point that it touches down does nothing but show that the aircraft will settle to the ground from rapidly decreasing rotor effeciency because of the loss in lift associated with the slowing rotor RRPM like in a hovering auto. The rotor does not stall in that scenario. in order to create a full unrecoverable stall you would need a high up flow velocity through the disk like in a high descent and a decreasing rotor RRPM. The up flow can an primarilly does increase the AOA on the blades untill it reaches a stall . Pulling on the collective makes this happen faster only because critical angle of attack is reached sooner. The large angles of attack remain even at flat pitch and create huge amounts of drag wich you can not over power with the engine. Forget about the auto the rotor meets the up flow at a time when RRPM is below normal and the driving autorotation part of the disk has already been aerodynamicly destroyed . So there is no way to demostrate this due to that fact that it is fatal when encountered.
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Just thinking about the loss of centripal strength in the blade due to running at low RPM in these demonstrations in an R22 (any type for that matter), the abnormal flight load placed upon the structure of the blade would be enormous. Also I am told that brinelling of the main rotor pitch change bearings is mostly caused through underspeeding. I dont reckon I would be to keen on flying a ship that had done a few of these demonstrations.
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The rotor can stall.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.
Warragee
A while ago I was told by a flying mate of a demo he had been shown on his R44 conversion with the RRPM down at 70% in the hover, I was horrified and emailed my tech man at Robinson whose reply was quite succinct and went along the lines of
Why did they do it ? and We recommend replacing the blades now.
GS
A while ago I was told by a flying mate of a demo he had been shown on his R44 conversion with the RRPM down at 70% in the hover, I was horrified and emailed my tech man at Robinson whose reply was quite succinct and went along the lines of
Why did they do it ? and We recommend replacing the blades now.
GS
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I'm always reminded of this very sad incident when thinking about rotor stall.. Although I think the AAIB are still investigating, so we can't draw conclusions, you can clearly see how the blades have clapped up.
BBC NEWS | UK | England | Lancashire | Helicopter crash pair identified
What worries me is that this seem quite possible with drooping Nr and fully artic head.
BBC NEWS | UK | England | Lancashire | Helicopter crash pair identified
What worries me is that this seem quite possible with drooping Nr and fully artic head.
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Mungo:
Nearly all (new design) articulated heads have flap restrainers to prevent the blades from flapping up too high.
Even the S-61 had them, and it goes back quite a long time…
Nearly all (new design) articulated heads have flap restrainers to prevent the blades from flapping up too high.
Even the S-61 had them, and it goes back quite a long time…
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A while ago I was told by a flying mate of a demo he had been shown on his R44 conversion with the RRPM down at 70% in the hover, I was horrified and emailed my tech man at Robinson whose reply was quite succinct and went along the lines of
Why did they do it ? and We recommend replacing the blades now.
GS
Why did they do it ? and We recommend replacing the blades now.
GS
What's the difference between holding a 70% hover on the cushion with power and
when you perform a throttle chop in the hover, you hold the lever where it is then raise it to cushion the landing. Both maneuvers would probably see 70% rrpm before touch down.
yes but i would guess the stresses on a blade going through 70% quickly ,and sinking , not under power would be very different to hanging there off the blades under power at 70%. I am not an engineer but would agree that there could be some real damage done to blades and hinges if operated like that . I certainly would never do that in mine ...and can see no good reason to do it !!!!
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During fast and furious mustering operations in an R22 it is possible to experience a flight condition that I am not really sure how to explain, I guess it is Blade stalling but there is no loss of RPM, just an onset of shuddering and severe loss of lift. It is easily fixed by reducing the amount of control input but if allowed to continue will end up shuddering onto the ground with no loss of RPM although I have only heard of it happening in severe operations close to the ground so it’s all over pretty quickly, back off the control input and the problem is gone. To encounter it proceed at 60kts+ straight and level or in a dive, introduce the cyclic input first quite strongly so that the airframe is still travelling in the same plane but with the disc in a flared attitude obviously intending to throttle off and control RRPM with collective and if you are able to get a severe enough flare and you are fast enough (60+) it will happen. If more collective is input after the shuddering has started it gets worse. I learned to fly around it by introducing a slight amount of collective first before the cyclic and no problem, basically same manoeuvre, same result. I guess I am probably just a rough pilot but I have 18,000 hrs mustering in R22’s so plenty of time to practice. So the main requirements are
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment
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Main rotor stall
Some data from my R44-I scientific simulator (quite a while since I used it..)
First the autorotation
at 80% rpm, fully loaded 60 knt auto
8-curve shows the angle of attack at a certain radius versus the max angle, based on a 4th order viscous flow stall model. 8-curve should remain below the stall curve.
AoA show the effective angle of attack of the blade segments
So at 80% : acceptable stall zone
Now at 70% rpm, fully loaded, 60 knt auto
so at 70% : fully stalled retreating blade
In between at at 75%
so at 75% : very questionable
Still 93% of retreating blade is stalled
Secondly the flair
Don't have any figures yet, but my guess is that at 75% you will run out of collective. So not so much stall, but collective limited
Hope that helps, convinces me not to go below 80%. Of course different TOW etc may be considered to refine margins etc, but I'll keep the 80% in mind as a pilot.
d3
(edited to get img links correct)
First the autorotation
at 80% rpm, fully loaded 60 knt auto
8-curve shows the angle of attack at a certain radius versus the max angle, based on a 4th order viscous flow stall model. 8-curve should remain below the stall curve.
AoA show the effective angle of attack of the blade segments
So at 80% : acceptable stall zone
Now at 70% rpm, fully loaded, 60 knt auto
so at 70% : fully stalled retreating blade
In between at at 75%
so at 75% : very questionable
Still 93% of retreating blade is stalled
Secondly the flair
Don't have any figures yet, but my guess is that at 75% you will run out of collective. So not so much stall, but collective limited
Hope that helps, convinces me not to go below 80%. Of course different TOW etc may be considered to refine margins etc, but I'll keep the 80% in mind as a pilot.
d3
(edited to get img links correct)
Nigel and chopjock - I would say it is all about the bending stresses at the root with high pitch angles (very high considering the low Nr). The amount the blades can cone on an EOL is limited by the Nr decaying rapidly - no Vsquared = no lift = no coning. But in powered flight at low Nr the engine keeps them going at high coning angles - much more bending at the root I think.
Wargee - I think your rotor is ingesting its own vortices, not unlike VRS - hence why it get worse if you raise the lever. A mixture of BVI (Blade Vortex Interaction) and incipient VRS.
Wargee - I think your rotor is ingesting its own vortices, not unlike VRS - hence why it get worse if you raise the lever. A mixture of BVI (Blade Vortex Interaction) and incipient VRS.
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The amount the blades can cone on an EOL is limited by the Nr decaying rapidly - no Vsquared = no lift = no coning. But in powered flight at low Nr the engine keeps them going at high coning angles - much more bending at the root I think.
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Rotor stall
By looking at data and running more simulations I got convinced that Shawns explanation is right on (that in it self could have been stated without doing the calculation and just trusting Shawns experience..)
What I see is that depending on load say between 75% and 50% usefull load the retreating blade stall will set in rapidily at any significant forward speed at rpm's below 80%. As Shawn explained this will create a blow back, and even moving cyclic fully forward the heli will slow down getting into a vertical autorotation.
I see that at low speeds (say 10 knots) RPM of 70% down to 65% are reachable before main rotor stall sets in. Up to that point coning remains reasonable (below 10°) but of course we run out of steam very rapidly beyond that point and even before the stall will hit the ground with significant vertical speed.
m2c
What I see is that depending on load say between 75% and 50% usefull load the retreating blade stall will set in rapidily at any significant forward speed at rpm's below 80%. As Shawn explained this will create a blow back, and even moving cyclic fully forward the heli will slow down getting into a vertical autorotation.
I see that at low speeds (say 10 knots) RPM of 70% down to 65% are reachable before main rotor stall sets in. Up to that point coning remains reasonable (below 10°) but of course we run out of steam very rapidly beyond that point and even before the stall will hit the ground with significant vertical speed.
m2c
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Rotor stall
Teathering versus Star flex
My simulator is R44, so fully teathering and coning hinges.
But as per Shawns remarks (he referred to the 407 correction: MD520 if I recall correctly) and my personal remarks, I do not thing this makes a big difference before main rotor stall. The only possible problem with teathering is that when retreating blade stall sets in brutally, tail cone contact will be made easier than with the star flex or other more rigid systems.
d3
My simulator is R44, so fully teathering and coning hinges.
But as per Shawns remarks (he referred to the 407 correction: MD520 if I recall correctly) and my personal remarks, I do not thing this makes a big difference before main rotor stall. The only possible problem with teathering is that when retreating blade stall sets in brutally, tail cone contact will be made easier than with the star flex or other more rigid systems.
d3
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Thanks Crab, I think something like what you describe was happening.
When I introduced cyclic first before collective it obviously flared the disc with low angles of attack on the blades but low disc loading giving rise to the shuddering and loss of lift.
By introducing some collective first loaded the disc a bit more allowing the severe flare of the disc to take place without causing any problems.
It was sort of like the entire disc was stalling rather than the blades singularly.
When I introduced cyclic first before collective it obviously flared the disc with low angles of attack on the blades but low disc loading giving rise to the shuddering and loss of lift.
By introducing some collective first loaded the disc a bit more allowing the severe flare of the disc to take place without causing any problems.
It was sort of like the entire disc was stalling rather than the blades singularly.
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During fast and furious mustering operations in an R22 it is possible to experience a flight condition that I am not really sure how to explain, I guess it is Blade stalling but there is no loss of RPM, just an onset of shuddering and severe loss of lift. It is easily fixed by reducing the amount of control input but if allowed to continue will end up shuddering onto the ground with no loss of RPM although I have only heard of it happening in severe operations close to the ground so it’s all over pretty quickly, back off the control input and the problem is gone. To encounter it proceed at 60kts+ straight and level or in a dive, introduce the cyclic input first quite strongly so that the airframe is still travelling in the same plane but with the disc in a flared attitude obviously intending to throttle off and control RRPM with collective and if you are able to get a severe enough flare and you are fast enough (60+) it will happen. If more collective is input after the shuddering has started it gets worse. I learned to fly around it by introducing a slight amount of collective first before the cyclic and no problem, basically same manoeuvre, same result. I guess I am probably just a rough pilot but I have 18,000 hrs mustering in R22’s so plenty of time to practice. So the main requirements are
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment
ITS TIPS STALL
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment
ITS TIPS STALL
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The rotor can stall.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.
I completely agree that rotor can experience retreating blade stall at any IAS under the right conditions .I was commenting on full rotor stall as a result of low rotor RPM not tip stall. The main difference to me is the disk is stalled in other areas accept the tip of the retreating side in this condition. large portions of the disk are stalled including the root not as a result of reverse flow ,with high coning angles on all blades and rapidly increasing descent rate which is not recoverable to my understanding and control over the disk is gone . This fatal scenerio is much different then tip stall which is recoverable in most cases I would think. Or after experiencing RBS you can prevent by flying at lower altitudes and airspeeds or not loading the disk hard from rough handling at high speed.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.
I completely agree that rotor can experience retreating blade stall at any IAS under the right conditions .I was commenting on full rotor stall as a result of low rotor RPM not tip stall. The main difference to me is the disk is stalled in other areas accept the tip of the retreating side in this condition. large portions of the disk are stalled including the root not as a result of reverse flow ,with high coning angles on all blades and rapidly increasing descent rate which is not recoverable to my understanding and control over the disk is gone . This fatal scenerio is much different then tip stall which is recoverable in most cases I would think. Or after experiencing RBS you can prevent by flying at lower altitudes and airspeeds or not loading the disk hard from rough handling at high speed.
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Reading this makes me shudder to think how naive we were back in the day. My boss and I were up doing patterns in the 269A-1 (TH-55,) talking about Robinsons and wondering how low you could let the rpm decay before recovering. We rolled throttle off, pitched for 60kts and counted to 5 before we got too nervous and bottomed collective and rolled throttle on. RPM was waaay down past bottom of the green. In retrospect it was a stupid thing to do. Proves to me just how forgiving the 269 really is. In the Robbi we'd probably be dead. 