Free Power Turbine v Fixed Turbine
 

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", thanks in advance for your answers.

Twin spool Vs. single spool?
 

AFAIK, one advantage of the twin spool (if this is what you mean) engine is that you dont need a clutch system on helicopters fitted with this kind of engine.

The advantage of the free power turbine is that it is simpler than a fixed turbine as there is no mechanical link between the gas turbine and the gearbox. The downside is that it has to 'windmill' in the exhaust flow thus losing a little bit of available power in the process.

The fixed turbine does as Silvio says need a clutch system which complicates things slightly.

Most free power turbine helicopters that I have flown have been twin engine helos and therefore needed an Accesory drive system to allow one engine to power the ancillaries whilst the second engine brings the rotors up to speed.

As to a direct 'engineering' advantage I'm afraid I couldn't tell you factually. Ask Nick Lappos who could give you the techie answer. :)

In flight we want the rotors to turn at an approximately constant RPM.

But we want the power output of the engine to increase when we pull the collective up and to decrease when we push the collective down.

Power output of an engine is proportional to RPM. So we want the engine RPM to be vary with changes in collective position.

With a fixed turbine we cannot easily vary engine RPM without varying rotor RPM.

A free power turbine is connected to the rotor but not to the engine spools (the gas generator). So we can have a governor keeping rotor RPM consant by varying fuel flow to the gas generator.

If we up pole the rotors tend to slow down. The governor senses this and increases fuel flow to the gas generator. Gas generator RPM then increases to increase power output to restore the rotor RPM.

There are of course other advantages but that is the main one.

I taught turboshaft engines for (censored) years - all the above answers are on the money.

A further advantage is that a rotor brake can be fitted to stop the main rotor for pax loading/unloading while keeping the core engine running. This is known as "locked rotor" ops.

the only advantage of a free turbine vs fix turbine by michelin gourmet book, is you can cook your eggs and coffee from the exaust gaz without having to turn the blades.

ad salt or mayo...

Thanks
 

All repies most appreciated, cheers carve.

Whether it be a Free Power Turbine or Fixed Turbine, all helicopters require a clutch mechanism. The fixed turbine just needs a somewhat more complex arrangement than the simple sprag clutch on a free turbine. Engineering wise the free turbine has the advantage that the N1 and N2 shaft turbines can be optimised in design to perform the individual task required, in the same way that some RR turbo fans have three shafts.

If above answer is correct how does a tpe 331 turbo prop operate at 100% n1 from idle tq to 100% tq????
Simple use the propellor to use up excess tq to maintain n1 at 100% ie increase blade angle. Do the same with rotor blades.
Read a little on the constanst speed gas turbine theory.

If we wish to vary the power output of an engine in flight we have three options.

1. Vary the mass flow rate of the air passing through it (this is usually done by varying the RPM).

2. Vary the amount of energy that we give to the airflow by varying the amount of fuel we burn (this results in variations in gas temperature).

3. Vary the pressure rise within the engine (this is usually done by varying the RPM, but can also be done to some extent by increasing fuel flow rate at constant RPM).

To achiveve maximum operational flexibility we must vary all three factors simultaneously.

The first tuboprop engines were produced simply by extending the compressor drive shaft forward and connecting it through a reduction gearbox to the propellor.

This meant that if we wanted constant propellor RPM then we would have to accept constant engine RPM. Although this worked (and still does in some engines) it made the engines more prone to problems such as compressor surge, particularly at high power settings.

By disconnecting the gas generator from the output shaft, a free power turbine permits us to vary all three of the above factors simultaneouly while keeping our output shaft RPM constant. This gives us maximum operational felxibility.

If for example we select a power increase, the fuel flow is increased. This increases the gas generator RPM, which increases air mass flow rate and also increases the pressure rise and temperature rise. At the increased RPM the compressor is more able to cope with the increased pressure ratio that is being demanded of it.

All of this occurs while the free power turbine and output shaft RPM remain constant. The overall effect is that the gas generator is permitted to operate far more efficiently that it would have done, had we demanded a power increase at a constant gas generator RPM.

Brian Abraham's statement that "Whether it be a Free Power Turbine or Fixed Turbine, all helicopters require a clutch mechanism." is untrue.

A single engine helicopter with a free power turbine does not require a clutch. But all helicopters require a free-wheel unit to permit autorotation following a turbine seizure.

Twin engine helicopters usually have a clutch on one engine. This permits that engine to be started first to drive accessories such as hydraulics, with the main rotors stationary. After the second engine has been started and the rotors run up to normal RPM, the clutch is then used to connect the first engine to the rotating rotors. Both engine swil then be driving the rotors.

Each engine will be connected to the main transmission through a separate free-wheel unit. But only the engine that is to be started first needs to have a clutch.

Kieth, I think you are confusing terminology. The free wheel unit on every helicopter I've ever flown, single or twin engine, was provided by a sprag clutch.
What type of helicopter uses this type of set up? None that I've flown I'm afraid (76, 212, 412).

We are certainly using different definitons for the term clutch.

I take it to mean a component whereby (either manually or automatically) the engine can be disconnected from and reconnected to the output shaft to permit the engine to rotate without rotating the output shaft.

A freewheel unit is a component which permits the output shaft (and rotors) to turn while the engine is not.

Most bicycle rear wheels contain a freewheel unit, but most people would never call it a clutch.

If you look at the context in which the term clutch has been used by others in this thread, I think that you will find that it is more in keeping with my defibnition, than with what appears to be yours.

Would a freewheel unit permit a fixed turbine engine to be started without also starting the rotors?

In some cases a freewheel unit and a selectable clutch are combined into a single unit. This is the system which I described in my previous post. This sytem is (was) used in Wessex Mk 5, Lynx and SeaKing, and probably may more helicopter types.

These "disconnect clutches" units are not commonly used elsewhere; I can't think of any modern designs using one. The Wessex one was "vulnerable" to mishandling (big bang and a new gearbox, please). Sikorsky has moved away from this design and prefers free turbines and a big rotor brake.

The Gazelle, which is equipped with a coupled turbine, has a ramp and roller freewheel and a centrifugal clutch. The engine can be run at idle (25,500 rpm) with the clutch naturally disengaged. As the throttle is advanced the shoes move outwards and engage the transmission, turning the rotors. The pilot had to monitor the speed at which engagement took place, it it was too reluctant it meant that new clutch shoes were needed.

The military Puma used to have a system whereby one free turbine could be disconnected so it's associated engine could be run without turning the rotors, but it wasn't selectable from in side the cockpit. It proved unreliable and if it slipped (it did), it caused the main rotor gearbox oil pump to stop so they were removed in the early 1970s, at least from the RAF ones.

It doesn't surprise me at all that selectable clutches are not being used in modern designs. I have known of several cases of pilots making the wrong selections and shock loading the gearboxes. In some cases it was a matter of them having learned the routine without learning why it was required. In others it was simply a case of loss of concentration. But the result was pretty much the same in all cases.

Getting back to the assertion that all helicopters require clutches. This is not true.

And getting back to the original question of what is the main advantage of free power trubines. It is as I described.

Another disadvantage of a fixed turbine engine speed is the possibility of reduced engine stall margins.

Kieth, are you saying that a sprag clutch is not a clutch? Could you also list the helicopters that have clutches (by your definition).

Brian, if we are to have any meaningful discussion we need to ensure that we are clear about what we mean by the words that we use. It also helps if we take into account the context of that discussion.

Earlier in this thread Silvo said

"AFAIK, one advantage of the twin spool (if this is what you mean) engine is that you dont need a clutch system on helicopters fitted with this kind of engine."

Wobble2plank then said

"The fixed turbine does as Silvio says need a clutch system which complicates things slightly."

From these comments it should be clear to us all that they are not talking about a freewheel unit. What they are talking about is a device whereby the fixed turbine engine can be disconnected from the rotors in order to allow the engine to be started without starting the rotors. Then either manually or automatically the two are connecetd together.

In this context a freewheel unit, be it Sprag type or not, is not the type of clutch that is being discussed.

As for your challenge to me to name a a helicopter that does have the type of clutch to which I refer, I do not need to do this because Shytorque has already done so when he said "

The Gazelle, which is equipped with a coupled turbine, has a ramp and roller freewheel and a centrifugal clutch. The engine can be run at idle (25,500 rpm) with the clutch naturally disengaged. As the throttle is advanced the shoes move outwards and engage the transmission, turning the rotors. The pilot had to monitor the speed at which engagement took place, it it was too reluctant it meant that new clutch shoes were needed. "

It is worth noting that this helicopter has both a "ramp and roller freewheel" and also a centrifugal clutch.

Keith, no challenge involved, only education. You said
I'm just enquiring what multi engined helos have this sort of set up. On the 76 you can start any engine first (with rotor brake on if you don't wish the rotor to turn, no clutch required, the N1 spins and the N2 doesn't), electrics run off the engine and hydraulics from the main gear box. You could also, given certain stipulations, stop the rotor with the rotor brake and leave an engine running On the 212 and 412 you may start either engine first and once again electrics run off the engine and hydraulics from the main gear box, but you can not start an engine and have the rotor stopped. All three aircraft only have sprag cluches (free wheel units) and no other clutch system. Only one I've flown with a centrifugal clutch was the good old 47. The H-34 (going back 41 years so memory is rather dim) had some sort of fluid coupling run off the hydraulics to spin the rotor up following engine start where upon positive mechanical engagement was made through a sprag clutch set up (I think I have it right). With reference to words we use, every ground school I've under taken has used the words "sprag clutch" interchangeably when referring to the free wheel unit (47 aside). 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. Thanks in advance.

As I stated a few posts ago,

"In some cases a freewheel unit and a selectable clutch are combined into a single unit. This is the system which I described in my previous post. This sytem is (was) used in Wessex Mk 5, Lynx and SeaKing, and probably many more helicopter types.

Brian, the Blackhawk doesn't have a sprag clutch to allow the transmission to be disconnected. It has an APU and so doesn't need one.

It can be fitted with a transmission lock or a rotor brake (the ones I flew had no rotor brake; I found this a disadvantage on such a big aircraft). The transmission lock was only ever engaged to prevent the rotors turning during non-fuelled compressor washes. It required the ground crew to turn the rotors until a dog engaged. The engines were then spun up for washing using the APU.

The Puma no longer has an engine de-clutch facility either, as I explained earlier; only a rotor brake.

BTW, on the S-76 the hydraulics don't work with the rotor brake holding the transmission.

Don't forget easier starting. less weight for the battery=more payload

Thanks for the reply Kieth, but I'm none the wiser as to the high lighted portion of my inquiry.

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.

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

Free turbine is less noisy.

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!

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.

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 :8

A clutch is defined as "Any of various devices for engaging and disengaging two working parts of a shaft or of a shaft and a driving mechanism."

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
Free turbines do indeed need clutches, to cater for the engine failure case, if for nothing else. It is automatic in operation and disengages the engine drive from the rotor should the engine RPM ever fall below that of the rotor. The mechanism is often referred to as a "freewheel unit" and in all cases of which I am aware, is provided by a mechanism described as a "sprag clutch". A sprag clutch is defined as "A one-directional positive clutch design that connects two shafts when motion causes rotating elements located within races to wedge together. Rotation in the opposite direction disconnects the input and output shaft."

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. :8 :ok:

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.

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. :ok:

PS Never did get a reply to my personal email :{

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.

don't have much experience on the whirlybirds, but wrt engine reliabilty, give me a pratt over the garrett grenade any day.