Free Power Turbine v Fixed Turbine
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Very interested in which helos (61, Blackhawk, Puma?) have the system as you describe with a clutch system on one engine and how it works
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Sorry Brian. I suspect that I was misreading the tone of some of your earlier posts.
I do not recall the exact details, but essentially it was as follows.
One engine (let’s say Engine 1) would be connected to the main gearbox by a normal freewheel unit. This would permit the engine to drive the gearbox, but would prevent the gearbox from driving the engine.
The other engine (let’s say Engine 2) would be connected through a selectable clutch and freewheel unit. This was essentially two concentric drive shafts. The one through the centre was permanently connected through a freewheel unit to the accessory gear train in the main gearbox. The outer shaft was connected through the selectable clutch and freewheel unit to the main drive train in the main gearbox.
With “Acc Drive” (Accessory Drive) selected Engine 2 could be started to drive the main gearbox accessories such as oil pump and hydraulic pump, without driving the main rotor. In this way it was possible to ensure that the gearbox was lubricated and the flying control hydraulic systems were charged before rotor engagement. In this conditions Engine 2’s gas generator and free power turbine would both be running.
Engine 1 would then be started with the rotor brake still on. In this condition its gas generator would be running but its free power turbine would not.
After post-start checks, the rotor brake would be released and Engine 1 would start to turn the rotors.
In order to engage engine 2 to the rotors the pilot must first ensure that the number 2 freewheel was in a freewheeling condition. To do this he would advance the speed select lever on Engine 1 and retard it on Engine 2. He would then select “Main Drive”. This would cause the selectable clutch to engage with the main drive train. The pilot would then advance the Engine 2speed select lever and match the two torques so that both engines were driving the rotors.
The great risk with this system was that if the pilot selected “Main Drive” with the speed select levers in the wrong positions, there would be a very loud “BANG” when the selectable clutch engaged and immediately took up the load of driving the rotor.
I can recall one occasion when a pilot was asked to do a single engine run. He started Engine 2 in ACC Drive and the maintainers carried out a few checks. The maintainers then asked him to engage the rotors. He selected Main Drive without starting the other engine. The rest as they say, is history.
It is quite possible (indeed highly probable) that I have got the engine numbers the wrong way around, but the process was pretty much as I have described it.
I do not recall the exact details, but essentially it was as follows.
One engine (let’s say Engine 1) would be connected to the main gearbox by a normal freewheel unit. This would permit the engine to drive the gearbox, but would prevent the gearbox from driving the engine.
The other engine (let’s say Engine 2) would be connected through a selectable clutch and freewheel unit. This was essentially two concentric drive shafts. The one through the centre was permanently connected through a freewheel unit to the accessory gear train in the main gearbox. The outer shaft was connected through the selectable clutch and freewheel unit to the main drive train in the main gearbox.
With “Acc Drive” (Accessory Drive) selected Engine 2 could be started to drive the main gearbox accessories such as oil pump and hydraulic pump, without driving the main rotor. In this way it was possible to ensure that the gearbox was lubricated and the flying control hydraulic systems were charged before rotor engagement. In this conditions Engine 2’s gas generator and free power turbine would both be running.
Engine 1 would then be started with the rotor brake still on. In this condition its gas generator would be running but its free power turbine would not.
After post-start checks, the rotor brake would be released and Engine 1 would start to turn the rotors.
In order to engage engine 2 to the rotors the pilot must first ensure that the number 2 freewheel was in a freewheeling condition. To do this he would advance the speed select lever on Engine 1 and retard it on Engine 2. He would then select “Main Drive”. This would cause the selectable clutch to engage with the main drive train. The pilot would then advance the Engine 2speed select lever and match the two torques so that both engines were driving the rotors.
The great risk with this system was that if the pilot selected “Main Drive” with the speed select levers in the wrong positions, there would be a very loud “BANG” when the selectable clutch engaged and immediately took up the load of driving the rotor.
I can recall one occasion when a pilot was asked to do a single engine run. He started Engine 2 in ACC Drive and the maintainers carried out a few checks. The maintainers then asked him to engage the rotors. He selected Main Drive without starting the other engine. The rest as they say, is history.
It is quite possible (indeed highly probable) that I have got the engine numbers the wrong way around, but the process was pretty much as I have described it.
Last edited by Keith.Williams.; 19th Mar 2008 at 09:11.
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There was no clutch system in the venerable old Sea Pig.
As described above you would start the number one engine in 'Accesory Drive' which was a selectable freewheel which enabled the gearbox through shaft to drive the accessories. This allowed one engine to be started to power up the generators and the hydraulic systems thus allowing blade fold/spread. Normally the number one would be run at 104% to prevent chattering when the main rotors were engaged.
Once the aircraft was ready for rotor engagement the number two engine was started against the rotor brake to prevent rotor turning as the free power turbine is directly connected to the input drive shaft. Pre torquing the number two before releasing the rotor brake enabled a smooth acceleration of the rotors. Once the rotors were up to speed the number one engine would be brought back to ground idle to enable the freewheel to be switched over from the through shaft to the main input shaft. Then both engines are brought up to speed to match torques and drive the main rotor.
There are no clutch systems within this drive train. There are multiple freewheels but these are only used to allow the rotor system to rotate in the event of an input bearing seizure. Also the freewheel within the acc drive system allows the drive to be split between the through shaft and the input shaft. This system can ONLY be switched when there is no load on it. Otherwise you get the famous '1 leads 2 Oooops!' Sea King clunk!
W2P
As described above you would start the number one engine in 'Accesory Drive' which was a selectable freewheel which enabled the gearbox through shaft to drive the accessories. This allowed one engine to be started to power up the generators and the hydraulic systems thus allowing blade fold/spread. Normally the number one would be run at 104% to prevent chattering when the main rotors were engaged.
Once the aircraft was ready for rotor engagement the number two engine was started against the rotor brake to prevent rotor turning as the free power turbine is directly connected to the input drive shaft. Pre torquing the number two before releasing the rotor brake enabled a smooth acceleration of the rotors. Once the rotors were up to speed the number one engine would be brought back to ground idle to enable the freewheel to be switched over from the through shaft to the main input shaft. Then both engines are brought up to speed to match torques and drive the main rotor.
There are no clutch systems within this drive train. There are multiple freewheels but these are only used to allow the rotor system to rotate in the event of an input bearing seizure. Also the freewheel within the acc drive system allows the drive to be split between the through shaft and the input shaft. This system can ONLY be switched when there is no load on it. Otherwise you get the famous '1 leads 2 Oooops!' Sea King clunk!
W2P
Avoid imitations
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It's twenty nine years since I flew the Wessex 5 but I do remember doing "PERRRFs", which stood for Port Engine Rotors Running Re-Fuel, which was one occasion when the procedure described above was used.
As I recall, this was necessary because of the mechanical complications caused by fitting an airframe and main transmission originally designed for one engine, with two engines. A coupling gearbox joined the two drives together. The best reason I can think of for fitting free turbines - far less complicated!
As I recall, this was necessary because of the mechanical complications caused by fitting an airframe and main transmission originally designed for one engine, with two engines. A coupling gearbox joined the two drives together. The best reason I can think of for fitting free turbines - far less complicated!
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Wobble2plank
"There was no clutch system in the venerable old Sea Pig.
The clutch to which I referred was the unit which permitted you to go from ACC Drive to Main Drive.
But it was not a friction plate type clutch as you would have in your car. It was a sliding gear which could be brought into and taken out of engagement
If you do not wish to call it a clutch then that's OK by me......it just takes us back to post number 10 in which I said that free power turbines do not need clutches.
"There was no clutch system in the venerable old Sea Pig.
The clutch to which I referred was the unit which permitted you to go from ACC Drive to Main Drive.
But it was not a friction plate type clutch as you would have in your car. It was a sliding gear which could be brought into and taken out of engagement
If you do not wish to call it a clutch then that's OK by me......it just takes us back to post number 10 in which I said that free power turbines do not need clutches.
Last edited by Keith.Williams.; 19th Mar 2008 at 15:33.
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Must admit I always thought of a 'clutch' system as a system that allowed smooth progressive uptake of drive.
The accessory drive system allowed only either fully engaged or fully disengaged which would make it more of a 'switch' affair.
Semantics really
The accessory drive system allowed only either fully engaged or fully disengaged which would make it more of a 'switch' affair.
Semantics really
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The accessory drive system allowed only either fully engaged or fully disengaged which would make it more of a 'switch' affair
Some models of the RR Merlin and Wright Cyclone piston engines, for example, have two speed superchargers with a "on, off" clutch mechanism for the change from high to low blower, or vice versa.
That confusion that exists as to what a clutch is can be seen in Kieth's post
I said that free power turbines do not need clutches
So all helicopters contain clutches, of one sort or another, and maybe to perform different functions. All certainly have a freewheel clutch mechanism, though I am given to education.
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Brian,
All that you have demonstrated in your most recent post is the need to be very careful in composing posts to pprune threads.
Your statement that "Free turbines do indeed need clutches, to cater for the engine failure case, if for nothing else." is not true.
I cannot think of a single free power trubine turboprop engine which has a freewheel unit or clutch. It is of course possible that some do, but most do not.
The question of whether all helicopters require clutches depends upon the definintion of a clutch. And as we have seen, different people use different definitions. If you re-read all of the posts in this thread I think that you will also find that most contributors do not use your definition of clutch.
At the end of the day the original question was "what is the main advantage of a free power turbine". I believe that I have already answered this. The greatest single advantage does not require any reference to clutches or freewheel units of any kind.
All that you have demonstrated in your most recent post is the need to be very careful in composing posts to pprune threads.
Your statement that "Free turbines do indeed need clutches, to cater for the engine failure case, if for nothing else." is not true.
I cannot think of a single free power trubine turboprop engine which has a freewheel unit or clutch. It is of course possible that some do, but most do not.
The question of whether all helicopters require clutches depends upon the definintion of a clutch. And as we have seen, different people use different definitions. If you re-read all of the posts in this thread I think that you will also find that most contributors do not use your definition of clutch.
At the end of the day the original question was "what is the main advantage of a free power turbine". I believe that I have already answered this. The greatest single advantage does not require any reference to clutches or freewheel units of any kind.
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Kieth, It would help very much if you read the posts and understand what is being said. As you say
CARVE111 in his opening post asked (bolding mine)
Strike 1
You are quite right, but when did the conversation turn to turbo prop. CARVE111 is talking helicopters (see above).
Strike 2
If the contributors do not agree with the definition of a clutch as defined in my previous post they either need to get an education or explain in engineering turns what their definition is. I repeat the definition so that there is no misunderstanding. “Any of various devices for engaging and disengaging two working parts of a shaft or of a shaft and a driving mechanism.”
My reference to piston engine supercharge drive clutches was in response to wobble2plank’s post and his idea that a clutch was progressive in nature. In the supercharger drive applications mentioned there is no control over the engagement and so dramatic was the response in the P-51 that it was known to unnerved inexperienced pilots.
Most people don't because they don't probably bother to understand the underlying principles of operation, but I can assure you they do indeed contain a clutch (talking coaster hubs here. Made in Birmingham by Perry).
From
http://en.wikipedia.org/wiki/Bicycle_brake_systems
When such a hub is pedalled forwards, the sprocket drives a screw which forces a clutch to move along the axle, driving the hub shell or gear assembly. When pedalling is reversed, the screw drives the clutch in the opposite direction, forcing it either between two brake pads and pressing them against the shell, or into a split collar and expanding it against the shell.
Kieth – For the entertainment we are providing might I suggest we split the royalties 50/50. Will negotiate you a larger share should you desire.
PS Never did get a reply to my personal email
you have demonstrated in your most recent post is the need to be very careful in composing posts to pprune threads
From an engineering point of view," what are the advantages of a free power turbine over a fixed turbine engine when fitted to a helicopter"
I cannot think of a single free power trubine (sic) turboprop engine which has a freewheel unit or clutch
Strike 2
If you re-read all of the posts in this thread I think that you will also find that most contributors do not use your definition of clutch
My reference to piston engine supercharge drive clutches was in response to wobble2plank’s post and his idea that a clutch was progressive in nature. In the supercharger drive applications mentioned there is no control over the engagement and so dramatic was the response in the P-51 that it was known to unnerved inexperienced pilots.
Most bicycle rear wheels contain a freewheel unit, but most people would never call it a clutch.
From
http://en.wikipedia.org/wiki/Bicycle_brake_systems
When such a hub is pedalled forwards, the sprocket drives a screw which forces a clutch to move along the axle, driving the hub shell or gear assembly. When pedalling is reversed, the screw drives the clutch in the opposite direction, forcing it either between two brake pads and pressing them against the shell, or into a split collar and expanding it against the shell.
Kieth – For the entertainment we are providing might I suggest we split the royalties 50/50. Will negotiate you a larger share should you desire.
PS Never did get a reply to my personal email
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Fixed Turbine = Alouette iii and free turbine = Bell 206 Jetranger.
Fuel Consumption, ALO 3 = horrendous as engine always at 33 500 rpm, Jetranger = much better as power output pretty much equal to whats required.
Power demand Response time ALO 3 = Awesome, as in pull on lever hand it to engineer and tell him to keep it there, not even a beat from the engine.
Jetranger you have to think a little ahead as in the gas producer has to produce before you can use. Dont autorotate which allows the engine to roll back to idle and then expect to be able to pull power without first pulling a little just to warn the engine...however the new electronically governed engines do it almost as well.
Both fun and do their jobs as designed, mountain flying/ rescue work = fixed turbine. Taking the boss`s wife to lunch, traffic patrol etc free turbine
Ruggedness, fixed turbine any day for the milspec guys and girls. Only way to stop it is to disconnect the fuel line.
Fuel Consumption, ALO 3 = horrendous as engine always at 33 500 rpm, Jetranger = much better as power output pretty much equal to whats required.
Power demand Response time ALO 3 = Awesome, as in pull on lever hand it to engineer and tell him to keep it there, not even a beat from the engine.
Jetranger you have to think a little ahead as in the gas producer has to produce before you can use. Dont autorotate which allows the engine to roll back to idle and then expect to be able to pull power without first pulling a little just to warn the engine...however the new electronically governed engines do it almost as well.
Both fun and do their jobs as designed, mountain flying/ rescue work = fixed turbine. Taking the boss`s wife to lunch, traffic patrol etc free turbine
Ruggedness, fixed turbine any day for the milspec guys and girls. Only way to stop it is to disconnect the fuel line.
Last edited by groundfloor; 21st Mar 2008 at 13:11. Reason: spelling
