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.