A Question about Turbine Helicopters...

[ASWFlyer]

Hey everyone!

I've done a search and can't find any specific information on what i want to know...maybe someone can help me...?

Basically, i know with a piston engine (such as a car) you can select neutral and the car's engine can still remain running with the car's useful output not actually driving the wheels...

But turbine helicopters puzzle me...Is there some sort of ''clutch'' on a turbine helicopter which allows it to keep the engines running but the rotors stopped? My head tells me that with helicopters there's no such like 'clutch' but that it must be a direct input of power from the engines to turn the rotors...?

If that makes any sense to anyone, then i guess my question is, a) Can you actually run the engines without the rotors turning, and b) how?

Thanks in advance for any replies...

Cheers!
ASWFlyer :ok:

[wg13_dummy]

Fixed power turbine will usually have a clutch. Not in the usual sense of a car clutch though but one that acts on engine RPM (although a basic comparision to an automatic box). Below a certain RPM, the clutch plates will be disengaged. Once the engine is accelerated, the clutch pads, through centrifugal force will bite onto the wall of the clutch housing therefore providing drive to the gearbox and blades.

In a free power turbine, the blades may stay stationary below a certain amount of NH (high speed compressor speed) or more usually be held by a rotor brake.

A freewheel can be used in the opposite sense. Ie if the engine fails, the freewheel disengages and prevents the rotor rpm being decayed by the slowing and/or stopped engine.

[NickLappos]

Virtually all modern helicopters use a "free" gas turbine engine, where the gas producer (where the power is actually produced) is free to spin separately from the rotor. The gas producer provides a very fast, very hot and very powerful stream of air that blows against the turbine section of the engine. This air gap inside the engine is exactly like the torque convertor of a car's automatic transmission. As the gas producer blows harder against the turbine, the turbine gets a very big torque that compells it to rotate.

The turbine power section is attached to the rotor thru the transmission. There is a free wheel unit at the entrance to the transmission that does not allow a jammed engine to drag down the rotor. Like a kind of ratchet drive, the free wheel unit allows the rotor to spin faster than the transmission.

[Thomas coupling]

That will never catch on Nick:ugh:

[i4iq]

You might find Chapter 5 useful as a basic introduction...

http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

[Gomer Pylot]

On some helicopters, it is permissible to operate the engine(s) at idle with the rotor brake engaged, thus the rotors not turning. This is not possible on most, because the rotor brake isn't strong enough to insure that it won't be overcome by the engine. The clutch is there to permit the rotors to turn without the engine(s) operating, thus enabling autorotation in the event of engine failure, not to permit the engine(s) to operate without the rotors turning. There is really no provision for allowing the engine(s) to operate without the rotors turning under normal circumstances.

[skadi]

As far as I remember, some french helicopters ( Gazelle/Alouette? ) have a kind of magnetic clutch, which enables the engine to start up without rotors turning.

[wg13_dummy]

On some helicopters, it is permissible to operate the engine(s) at idle with the rotor brake engaged, thus the rotors not turning. This is not possible on most, because the rotor brake isn't strong enough to insure that it won't be overcome by the engine. The clutch is there to permit the rotors to turn without the engine(s) operating, thus enabling autorotation in the event of engine failure, not to permit the engine(s) to operate without the rotors turning. There is really no provision for allowing the engine(s) to operate without the rotors turning under normal circumstances.


Surely the purpose of the freewheel and not the clutch? If the clutch was used for this purpose, the blades would be slowed down by the slowing engine, down to it's engage/disengage speed - usually just above ground idle. Freewheel will disengage as soon as there is a difference in engine/rotor speed- RRPM higher than engine equivalent, similar to 'coasting'.



skadi, the Gazelle has a dry centrifugal clutch. The clutch should start to engage between 27,000 and 34,000Nc. From 27,000, the Schnorr washers compress (unequally to prevent clutch snatch) enabling the clutch pads to come into contact with the clutch housing therefore providing drive.

[biz-buz]

I cannot help thinking that there is a certain confusion here between the operation and job of the clutch and of the freewheel unit. All helicopters have a freewheel of some sort, but not all have a clutch.

The clutch is designed to engage the transmission with the turbine drive, once the latter reaches a designated speed. Its job is to ensure the smooth and stress free acceleration of the drive train.

The freewheel is designed to let the rotors continue turning when the turbine/engine stops or slows down, either as a result of an engine failure of as part of a planned "engine failure" autorotation practice.

In real terms, in a Bell 206 almost as soon as the engine starts the rotors start to turn. In a Gazelle, you can start the engine and run it up to the first governed range, before the clutch engages. Only once you let the brake off and speed the engine up further does the drive shaft spin quickly enough to engage the centrifugal clutch.

Multi engine helicopters have a complex drive system whereby you can disengage the engine from the rotor drive and only drive the accessories, in order to run your generator instruments radios etc. Larger helicopters, like airliners, have their own dedicated generator unit.

Hope this helps.

[ShyTorque]

And just to be different, some early examples of the RAF version of the Puma AS330J were supplied with a manually operated declutch unit on the (port?) engine to enable the engine to be run on the ground as a (very thirsty) APU.

They were de-modded because they had a habit of disconnecting in flight, so the engine wouldn't drive, which took the edge right off the pilot's day. :ooh:

[wg13_dummy]

Same on the Lynx. It has an accessory drive actuator on the No1 (port engine).

In Acc Drive, No1 can be fired up and this allows AC and Hyd systems to be run up and tested without the blades going round (again, a thirsty APU). Its drive goes through the front acc drive plate (2x Alts, 2x Hyd pumps) and not to the gearbox. Only when accessory drive is selected to main drive, is the No1 actually providing drive to the gearbox.

The usual setting for the No1 is 95% to allow for the Alts and Hyds to be run (100% is fine but the engine is then deemed to be auto governed and a runaway up would be hard to catch if your heads down doing AFCS checks and the like). To go from acc drive to main, the No1 needs to be brought back down to ground idle otherwise its a bit like trying to change gear in you car with your foot fully down on the accelerator! Back in the old days, it was possible (and done several times!) to select 'drive' with the ECL up at 95% or at least on the way down to ground idle. This usually caused bits of gearbox and acc drive actuator to disappear across the dispersal at a high rate of knots followed by a one way conversation with the big man. :sad:

We now have a microswitch to prevent this but it is still possible to fcuk the box if youre not careful.

[skadi]

An Accessory Drive System like in the Lynx is also installed in the Seaking, so the No 1 could be used as APU without rotors turning.

[ShyTorque]

Further to my post about the Puma declutch unit, I recall that if the unit suffered slippage in flight, the drive to the (single) main rotor gearbox oil pump slipped too.

The ability to have turbine engines running with the rotor stopped is type specific. For example, in general, the S-76 marque can have one or both engines started and running at idle against the rotor brake and the turning rotor can even be brought to a halt with engines at idle (mighty fine rotor brake on that aircraft, Capt. Lappos). The slight exception is the B model where one can start both engines against the brake but only stop the rotor provided on engine is stopped and the other at idle, due to the larger residual thrust at idle of the P&W engines. This ability is a huge benefit when loading/unloading and when starting in high winds.

On the other hand, the Agusta 109 cannot be started with the rotor brake on, not even one engine. Not so good. This is something that S-76 pilots need to be very aware of when moving from one type to another...... :oh:

[diginagain]

The mantra for going from Acc to Main on the Lynx is something I think I'll take to the grave...

[NickLappos]

The H-3 (military S61) has an internal electric clutch mechanism, as skadi said. It must also be energized only while the #1 engine is at idel, else bad things happen inside the transmission.

Rumor has it the VH-3A with President Johnson on board has that clutch actuated while at 100% rotor/Nf. The result was a scrubbed Presidential mission, and a pilot with 1 year in Vietnam to think about that little red warning light next to the switch!

[skadi]

The mantra for going from Acc to Main on the Lynx is something I think I'll take to the grave...

Yes, the same on the good old Seaking. A wide playground for the instructorpilots, especially in the Simulator....

skadi