Sprag Clutches
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Sprag Clutches
I was interested to read on the thread 'Bell misleads 407 owners' Lu Zuckerman's account of the 205 incident where the engine oversped the rotor and Bell claimed the clutch had been damaged by compressor stalls.
On one occasion whilst waiting for the cool down time to elapse on a Huey I became aware that the rotor was turning at a lower than normal speed (read could count the blades). The tacho confirmed that the engine was overspeeding the rotor and an instant shut down was made. Have always wondered what may have been the result if the clutch suddenly decided to reengage. Maintenance was informed and they released it for ops without any maintenance being performed and the aircraft soldiered on without any further problem. Was told by Bell it was an uncommon event but not unknown.
On one occasion whilst waiting for the cool down time to elapse on a Huey I became aware that the rotor was turning at a lower than normal speed (read could count the blades). The tacho confirmed that the engine was overspeeding the rotor and an instant shut down was made. Have always wondered what may have been the result if the clutch suddenly decided to reengage. Maintenance was informed and they released it for ops without any maintenance being performed and the aircraft soldiered on without any further problem. Was told by Bell it was an uncommon event but not unknown.
Ask the Puma boys what happens when your freewheel suddenly engages!
Bit of a pain when you don't have an N2 indication, and its all under-speeding! Loud bang and varying amounts of damage/component replacement!
Bit of a pain when you don't have an N2 indication, and its all under-speeding! Loud bang and varying amounts of damage/component replacement!
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Freewheeling unit
To: Brian Abraham
Actually, the Bell rep covered up the fact that the engine had experienced three compressor stalls. I found out about it when we pulled the unit out of the transmission. The rollers and the cams were destroyed. It was not until the accident report had been submitted and Bell signed off on pilot error that the Bell rep came forward and told me about the three compressor stalls.
What made matters worse there was no mention about the three compressor stalls in the maintenance records and the normal protocols initiated after a compressor stall had not been implemented.
Many years ago the US Army took delivery of a brand new H-34. They were checking out a pilot who had never flown in a helicopter with a fuel priming system. On helicopters that this pilot had flown the twist grip was rotated to pump fuel through the carburetor. The pilot was twisting the throttle but on this helicopter the throttle canceller had been misrigged and it allowed the helicopter to start with the throttle in the open position. The pilot cranked the engine and it caught on the first two turns and with the throttle wide open the engine spun up to 2800 RPM. This caused the blocker plate in the hydromechanical clutch to overstress to the point that it failed and the engine engaged into the rotor system at 2800 RPM. The helicopter, which had only about 16 hours on it, was written off and turned over to the training school to be used as a training aid.
I was interested to read on the thread 'Bell misleads 407 owners' Lu Zuckerman's account of the 205 incident where the engine oversped the rotor and Bell claimed the clutch had been damaged by compressor stalls.
What made matters worse there was no mention about the three compressor stalls in the maintenance records and the normal protocols initiated after a compressor stall had not been implemented.
Many years ago the US Army took delivery of a brand new H-34. They were checking out a pilot who had never flown in a helicopter with a fuel priming system. On helicopters that this pilot had flown the twist grip was rotated to pump fuel through the carburetor. The pilot was twisting the throttle but on this helicopter the throttle canceller had been misrigged and it allowed the helicopter to start with the throttle in the open position. The pilot cranked the engine and it caught on the first two turns and with the throttle wide open the engine spun up to 2800 RPM. This caused the blocker plate in the hydromechanical clutch to overstress to the point that it failed and the engine engaged into the rotor system at 2800 RPM. The helicopter, which had only about 16 hours on it, was written off and turned over to the training school to be used as a training aid.
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Spragg clutches have failed and then worked perfectly afterwards. I know of at least two incidents of this on the Bell 212 / UH-1N.
Does anyone know of any statistics?
I also note it's not in any flight manuals as an emergency or abnormal procedure. Hmmmm.
You would think that someone would be educating pilots that if this happens they must write it up and submit it as an SDR, so that we can at least track statistics.
Does anyone know of any statistics?
I also note it's not in any flight manuals as an emergency or abnormal procedure. Hmmmm.
You would think that someone would be educating pilots that if this happens they must write it up and submit it as an SDR, so that we can at least track statistics.
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There was a failure on an Enstrom in the late 1970s which failed in both modes due to unauthorised maintenance. (assorted sizes of rollers used): Clutch failed allowing major overspeed and then locked when the engine failed preventing autorotation. 
Aircraft and crew lost.
Report is still on NTSB database I think.
Aircraft and crew lost.
Report is still on NTSB database I think.
Gatvol
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Had one fail to disengage during a checkride in a Cobra. IP cut power and things started to get slow....fast. Appled power all was OK. Tried it again as the IP thought I was slow on collective reduction.......not even. Brought back in power did a precautionary landing on a Golf Course went in called Maintenance and had a Coke...........
Never did find out what was wrong. probably didnt want to know..........
Never did find out what was wrong. probably didnt want to know..........
Iconoclast
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Total failure of a Sprag clutch.
The Italian Navy suffered a catastrophic failure on the left-hand freewheeling unit on an AS-61 (Sea king). The pilot put the helicopter into autorotation and landed in the water 22 seconds later. Upon removing the unit you could see where the unit fractured and it was totally destroyed by friction generated heat. The parts got so hot they actually melted.
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a non licenced engineer i new long ago did some moonlighting for a company out west and put the freewheel unit in upside down, twice.
a pilot friend in the territory had had a freeweel let go and he likened the noise to the sounds of a gut shot donkey, so if you know what that sounds like??
a pilot friend in the territory had had a freeweel let go and he likened the noise to the sounds of a gut shot donkey, so if you know what that sounds like??
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I experienced a seized sprag clutch/freewheeling unit quite a few years ago in a 212. If my memory serves me well my first indication was when one of the engines was rolled back to idle the rotor rpm bled down more than usual, then on the same test flight I found the auto rpm were low.
The final clue was when I carried out the next start and cranking #2 engine first, #1 engine showed n1 rpm and oil pressure (this anomaly would not have been apparent if #1 had been started first)
All other c box indications were normal and no metal was located on the chip indicators or filters. The c box was removed and sent out of country to P and W, I never had a chance to read an engineering report so never found out why it seized.
I had'nt flown the aircraft in question for a month or so and no one else had noticed anything unusual so the sprag may have been US for quite a few hours. Things could have got a bit exciting thinking back on it
The final clue was when I carried out the next start and cranking #2 engine first, #1 engine showed n1 rpm and oil pressure (this anomaly would not have been apparent if #1 had been started first)
All other c box indications were normal and no metal was located on the chip indicators or filters. The c box was removed and sent out of country to P and W, I never had a chance to read an engineering report so never found out why it seized.
I had'nt flown the aircraft in question for a month or so and no one else had noticed anything unusual so the sprag may have been US for quite a few hours. Things could have got a bit exciting thinking back on it
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How can you have n1 and oil pressure indicating when even with a stuffed clutch there's no connection between n1 and n2?
You might have had an indication on the triple tacho (n2) but not engine oil px as well.
Taff
You might have had an indication on the triple tacho (n2) but not engine oil px as well.
Taff
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Taff Missed wrote:
If the N2 turbine was being directly driven via the FWU, then it might pull enough air through the engine to turn the compressor turbine.
The N1 has very little load and is very easy to turn.
How can you have n1 and oil pressure indicating when even with a stuffed clutch there's no connection between n1 and n2?
The N1 has very little load and is very easy to turn.
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TAFF
Believe me we had an indication of n1, I demonstrated it several times to convince the engineer on site ( and other pilots ) that there was a problem.
He was finally satisfied when one oil cooler fan could be free wheeled by hand in one direction and the other side couldn't.
By the way, with ground power plugged in, the #2 starter didnt seem to be noticeably affected turning over more equipment than it should have been.
Believe me we had an indication of n1, I demonstrated it several times to convince the engineer on site ( and other pilots ) that there was a problem.
He was finally satisfied when one oil cooler fan could be free wheeled by hand in one direction and the other side couldn't.
By the way, with ground power plugged in, the #2 starter didnt seem to be noticeably affected turning over more equipment than it should have been.
I seem to recall a problem some years ago with R22 sprag clutches - some of the ears were breaking off and stopping it working properly.
That's why, if I don't get a satisfactory needle split during the run-up checks, I do it again.
That's why, if I don't get a satisfactory needle split during the run-up checks, I do it again.
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Sprag Clutch Operation?
Can someone help me with a technical query ref the sprag clutch?
Is my thinking on how it operates correct? The clutch as we know transfers power from the engine to the transmission then the rotor shaft, & rotates one way. My question is, when practising an Auto or the engine actually does cease to work, the freewheel within the clutch wants to rotate the opposite way and the sprags within the outer & inner race apply friction to stop this and this enables the seperation of the low engine RPM from the rotorshaft allowing us to maintain RRPM!
Thanks.
Is my thinking on how it operates correct? The clutch as we know transfers power from the engine to the transmission then the rotor shaft, & rotates one way. My question is, when practising an Auto or the engine actually does cease to work, the freewheel within the clutch wants to rotate the opposite way and the sprags within the outer & inner race apply friction to stop this and this enables the seperation of the low engine RPM from the rotorshaft allowing us to maintain RRPM!
Thanks.
The graphic is a little misleading. In a helicopter the engine would be driving the outer part and the inner part would be the input to the MGB (On some designs shared with the TRDS.)
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What is one of those?
That's why, if I don't get a satisfactory needle split during the run-up checks, I do it again.
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What is an unsatisfactory needle split? and its ok the next time?
Sometimes it works best to roll the engine revs up from about 50% to 75% then immediately cut the throttle back, as the rotors are gaining momentum and it seems easier to split.
I'd say a minimum 5% split would be my go-decision. If I couldn't achieve that, I'd want to know why.
Lafite