Freeturbine

[REVENUEMAN]

Can anyone tell me the advantages/disadvantages of a freeturbine system.

[Keith.Williams.]

The term "free power turbine" means an extra trubine that is not connected in any way to the main gas generating part of the engine. In effect we have a complete gas turbine engine with an extra turbine immediately behind it.

The purpose of the free power turbine is to extract power from the hot gas flow in order to drive the output shaft. This shaft is then connected to the propeller, helicopter main gearbox, or whatever else it is being used to drive.

The main advantage of this system is that the RPM of the free turbine (and hence output shaft) is independent of that of the gas generator. So we can have a constant speed propeller or helicopter rotor system, while allowing the engine to accelerate or decelerate as required to match the power required at any given stage of flight. This makes the system much more flexible in terms of the range of power output values available.

To understand what this means just consider how early turboprops worked. The turbines, compressor and propeller shaft were all connected directly together. To have a constant speed propeller meant having a constant speed engine. So opening the throttle to increase power simply increased the pressure and temperature in the engine, but left RPM constant. There is clearly a limit to how high such pressures could become before the egine started to surge. In reality the propeller RPM was usually allowed to vary within a specified range, then held constant only at very high power settings.

Other benefits include the ability to start engines with the propeller or helicopter rotors stopped (using a prop brake or rotor brake). So in a free power turbine engine the starter motor is required to spin up only the gas generator. This is obviously easier than spinning up the entire propeller or rotor system. So free power turbine engines are easier to start and are able to use lighter starter systems.

The principle disadvantge is that the system is a bit more complicated and slightly heavier, than it would be if the output shaft were connected directly to the gas generator.

[ShyTorque]

Keith,

Just for your info, the Aerospatiale / Westland Gazelle series has no free turbine. Instead it has a centrifugal clutch and an overrun freewheel device. The engine is normally started with the rotor brake on. The brake is then released and the throttle is advanced to the "Fly" position. Once the rotor is brought up to speed the engine is effectively "constant speed" and very well it works too. :ok:

[Torquelink]

Another - alleged - disadvantage of the free turbine is slightly slower response to power inputs compared to fixed shaft but, as far as I know, these aren't dramatic.

[Keith.Williams.]

Skytorque,

Yes I am aware that some light (and usually older) helicopters use a centrifugal clutch for starting, then hold constant engine RPM with changing power demands. But this constant RPM regime is less flexible than the free turbine system. This is why most heavy (and modern) types use free turbines.


Torquelink,

In some cases free power turbines can be slower to respond. But consider how a fixed shaft responds if the rate of collective increase is really excessive. A stalled/surged gas generator responds very slowly indeed (if at all). Even free turbines engines can stall/surge of course, but at least they are not tied directly to the rotors.

[ShyTorque]

Keith,

OLD? the Gazelle? It's not even 30 yrs old yet, that's young for a helicopter!

I agree about the bigger helis. Things do get more complicated when you have two engines driving the same rotor.

BTW, it's Shy, not Sky!

[Blacksheep]

I worked on Gazelles back in 1973 ShyTorque and by my calculation that was more than thirty years ago.

We also had some proper wobblycopters - good old Westland Whirlwinds powered by the Gnome - which, as all true wobblycopter magicians know, used a free power turbine. The twin-engined Wessex used a pair of Gnomes but wasn't that much more complicated than the single-engined Whirlwind - if one engine stopped, Hawker-Siddeley's wondrous Electronic Fuel Control System - a magical contraption that performed the tricky computations using magnetic amplifiers - sorted it all out and ran the remaining engine up to full power. If you were lucky...

[sycamore]

S-T,
First flew the SA341-01 in Feb `71, when it was all civvie, ie low-back seats, different control set-up, so in jack-stall it would always roll-out, away from the turn.It was a much more rigid a/c than present models. Think I`ve seen about 350 kph on one test session.Then the extra doors got put in, tail-boom cranked down a bit, etc so it`s character changed, but it`s still a bit of a Ferrari, even if it has Citroen door handles !!

Regarding fixed shaft- vs free-turbine; a lot of f/s turbines have a centrifugal stage after the axial compressor and if you do manage to surge one , the centrifugal bit usually keeps pulling the air in and a momentary pause will clear it; with an axial f/t, it usually just goes, and the JPT/TIT/ is gone around the clock, leaving little silver bits in the jet-pipe; however, you usually have to be really trying, and an HP dump switch is advisable, a la Wessex/ S-K...

[ShyTorque]

Yes, I'm sure you're correct about the earlier date into service of the type. I was thinking more of the red and white ones I used to instruct on, I think they entered service in 1976.

Blacksheep,
We're probably from the same background. I trained on the Whirlwind 10 then went on to the Wessex 5, before going on to earn a living flying the "more modern" Plastic Pursuit Ship - equipped with steam age engine control systems!

[Sheep Guts]

The most famous Free Power Turbine Engine ever produced is the PT6. The major benefits of a free power turbine are the following:

1. Nil Negative torque feedback. With direct drive turbines in fixed wing , NTS (negative torque sensing) is required as rapid decelerating can cause what is called prop loading especailly on descent.

2. Less moving parts ease of servicing.

3. More robust less suseptible to F.O.D. ie centrifugal compressors commonly used.


Dissadvantages:

1. There is s delay in using reverse pitch as the turbine needs to accerate before the propellor actuator can become effective. Some aircraft call for high idle prior to landing in reverse required landings.

2. cant think of another but there are more .

Anyone care to elaborate?



Sheep