Free Power Turbine v Fixed turbine

[CARVE111]

Can anyone tell me what the disadvantages and advantages are of a free power turbine aginst a fixed turbine. Just a general query I am interested to know about. Many thanks. Carve

[Shawn Coyle]

Free turbine has a lot better fuel economy, doesn't need a heavy clutch, is quieter (but that may be because the only fixed shaft engine I'm really familiar with - the Astazou on the Gazelle, is pretty shrill).
Fixed shaft has the best response to power changes that was possible pre-FADEC equipped engines, can sit at idle without the rotors turning (but some machines with free turbines can do that with the rotor brake on). Astazou was tricky to start if the micro-pump was out of adjustment, but once running the engines were bulletproof and would keep running until they either ran out of gas or oil or some water in the fuel interrupted the game.
Air-starting an Astazou was not something you'd want to do without a lot of air under you - don't know if anyone ever put an automatic re-light kit on them or not.
Hope that helps....

[[email protected]]

Free power turbines are the way to go for multi engine installations as well because both FPTs run at the same speed and only the gas generator speed needs to be adjusted to maintain Nf and thus NR.

[ShyTorque]

Although there is no turbine lag with a constant speed turbine. :)

Good old mechanical lumps like centrifugal clutches can be checked with a feeler guage and torque wrench; DECUs relying on software can sometimes be wild cards in the equation and it's far more difficult or sometimes just not possible to troubleshoot any glitches afterwards.

I'm just trying to think why two constant speed engines like the Astazous on the Gazelle (cracking little aircraft) couldn't work together, side by side. Certainly no worries about mobile phones there.

[CRAN]

The key reason for the widespread adoption of the free turbine engine in helicopter applications is its torque-speed characteristic.

A free turbine engine has a torque speed characteristic much like a DC Motor, whereby it can 'theoretically' produce its maximum torque to zero power turbine shaft speed. Therefore if rotor rpm decays, the engine naturally has more torque available at the lower power turbine shaft speed to recover it. Real engine do vary from the ideal for a variety of aero-thermodynamic reasons, but the basic principal holds true.

However, in a single shaft engine the maximum torque that the engine can produce generally drops with reducing engine speed, to the point at around 35-45% ERPM that it can't produce any useful torque at all! Therefore, when a disturbance occurs that drops rotor rpm the engine has very little torque available to recover it...clearly this has implications!

There are lots of interesting details about how either type of engine works in helicopter applications (fixed speed variable load), but generally the balance of theoretical arguments and practical arguments mean that the improved torque characteristics of free turbine engines and the elimination of the need for a clutch make this configuration the most suitable choice for helicopter applications.

Hope this helps,

CRAN

[ShyTorque]

But if a pilot allowed Nr to decay to 35-40% he would have already long lost aerodynamic control of the helicopter....

10 % loss of Nr is a typical minimum for OEI operations on a free turbine twin, as little as 5% for AEI.

[MightyGem]

Astazou was tricky to start if the micro-pump...
Yes, my passenger was rather puzzled when I got out and gave it a bang and a waggle to get it started.

[Shawn Coyle]

Shytorque:
The reason why there are no twin fixed shaft installations is that the engines have to run at exactly the same speed - and I doubt there are sensors that are accurate enough to be able to match the two speeds exactly for a fixed shaft system.
I could be wrong, but the characteristics of a free turbine allow the turbines to be matched and allow the compressors to make up the difference much more easily.
There must be a reason why no-one ever made a twin fixed shaft engined helicopter…

[ShyTorque]

Shawn,

Yes, that may be the answer. The critical thing would be to ensure that both engines provide positive drive so the lower engine's freewheel didn't continually slip in and out of engagement.

Having said that, I'm not totally convinced that this might not be possible to arrange. If a freewheel / centrifugal clutch has a pre-determined breakout force, any minor torque splits would still leave both engines in engagement.

Torque sharing and Nr control could presumably still be "conventional", as for turbine equipped aircraft.

[CARVE111]

Thanks for all the feedback, it has provided the answers I was after

[Brian Abraham]

so the lower engine's freewheel didn't continually slip in and out of engagement
As long as the engine is providing some Tq, no matter how little or how much, you would have positive engagement ie one could be at 1% and the other at 100%. Clutch will only disengage on a fixed shaft dual engine set up (just talking theory here, as mentioned previously no one has built one) if one engine RPM is below the other and hence producing 0% Tq.
I doubt there are sensors that are accurate enough to be able to match the two speeds exactly for a fixed shaft system
I don't see the problem as being any different to that of a free turbine. All you need is a system for beeping RPM and Tq matching as with the 76 for example.
Where's Nick when you need him?

[Lt.Fubar]

Than why not use differential ? Just put it between the engines and gearbox,. With it, you don't need to accurately control rpm of both engines. As long as both are producing power - it will be transfered to the main gearbox. The fun part begins when one engine quits, it would need a brake on its shaft, or else, the working engine would rotate the not working one, instead of the rotors.

Anyway that would add weight (the brakes and differential) and would need more maintenance, the free turbine is the easiest and cheapest way to do multi engine platforms.

[ShyTorque]

Brian, :confused: I thought I covered that in the following paragraphs:

Having said that, I'm not totally convinced that this might not be possible to arrange. If a freewheel / centrifugal clutch has a pre-determined breakout force, any minor torque splits would still leave both engines in engagement.

Torque sharing and Nr control could presumably still be "conventional", as for turbine equipped aircraft.

I think it's down to the weight of two mechanical clutches.

[Brian Abraham]

Shy, just saying the same with different words and from a alternative perspective I guess. :ok:

[ShyTorque]

Fairey nuff :ok:

[Shawn Coyle]

Just to add to the confusion on this - it appears that as early as the mid-1950's that Piasecki had two fixed shaft engines in the YH-16A, and worked out a way to make them work together. So it could be done!

[ShyTorque]

4 wheel drive? ;)

[Brian Abraham]

it appears that as early as the mid-1950's that Piasecki had two fixed shaft engines in the YH-16A, and worked out a way to make them work together. So it could be done!
Lets not forget Sikorsky's CH-37 "Mojave" with its twin Pratt & Whitney R-2800 radial engines, 2,100 hp each. Doesn't get more fixed shaft than that, though may be different engineering challenges between piston and fixed shaft turbine. Be interesting to hear from anyone who had the opportunity to fly one.

[relyon]

The Ka-26 has two nine-cylinder radial piston engines, but I've found few details on the power flow. There's more information at: ttp://www.hmfriends.org.uk/restorka26.htm (http://www.hmfriends.org.uk/restorka26.htm) and http://en.wikipedia.org/wiki/Kamov_Ka-26.

Bob

[BristowOffshore]

Used mostly in the Gazelle and the SA 360 single engine Dauphin, the fixed shaft turbine was interesting to fly. In a time when every turbine had to be started by hand manipulating the throttle, the fixed shaft had auto start. (1968) Much like todays EC 135, you just flipped the switch and the engine came automatically up to ground idle. After that you advanced the overhead throttle lever to speed the engine up and engage the centrifugal clutch and start the rotors turning. Once you reached 100 % governed speed, you were ready to go.
The clutch had a "sweet spot" that you had to hit with the throttle to keep fron ruining the clutch. ( Like the other posters said.)
The clutch also meant that all practice autorotations were to the ground. NO power recovery, as the clutch couldn't take the sudden load. As per aircraft flight manual. Kind of limits training.
Another draw back of the fixed turbine, was that it operated at one compressor speed. At low power settings, the compressor air was vented, which made it a screamer. At high power settings the compressor couldn't get enough air to the hot section to produce higher power, because of course, it only turned at the governed power turbine speed.On a cold, dry day, you had lots of engine power. On a hot & humid day, you didn't have so much engine power. There was no spinning the compressor faster to compensate.( Not to be confused with helicopter performance.)
The Gazelle and Sa 360 each had a Red light mounted on the dash within the pilots main view. You could increase power with collective until the Red light blinked, and when it came on solid Red, you couldn' t pull any more power. That was it. To pull above that level would put the fire out because the one speed compressor couldn't get enough air to the hot section to keep the fire going.
The joy of hopping a Gazelle with three oil field personel on, over to the edge of the helideck, and then the final hop over the edge, is something few young helicopter pilots will thrill to. Nor should they. Free turbine is so much better.

[mtoroshanga]

The engine before the Astazou was the Artouste which was fitted to the Alouette series (2 and 3 models). It started of as the APU on the NordAtlas hence autostart which could give endless problems with blocked torch igniters etc. We used to run them on deisel from time to time in the early 60's in Rhodesia. Fuel burn is the primary downside, they are very gutsy but also reliable. Susceptable to compressor erosion as well.