About helicopter clutches

[SGAS]

I have a few question about helicopter clutches;

I read somewhere that the Schweizer piston helis uses belts as clutch, is that true , how does it works?

What clutch is fitted to the Robinson?

Why dont use disc-clutches like de ones installed in cars?

The Gazelle is powered by a single spool engine and uses a centrifugal clutch, I asume that helicopters equiped with twin spool engines like the Allison/Rolls-Royce 250 dont need clutches, is that right?

Would'nt centrifugal clutches be suitable for piston helis?

Thanks for your help:ok:

SGAS

[Hilico]

There are people on PPRuNe far, far more qualified to answer this than I, but they haven't yet, so I'll have a go.

The Schweizer doesn't use belts as a clutch; it uses the belts to transfer the power from the engine to the rotor system (one shaft, connected to both main and TR). These belts run round a third pulley which can be moved by an electric motor. When the aircraft is stopped and at rest, the belts are slack. Once you've started the engine, you then use a switch on the panel to run the electric motor, pull the pulley and tighten the belts. You do this very sparingly, click on-click off, and as the belts slowly tighten, the rotor starts to move and speeds up. You watch the engine and rotor rpm on the gauge, and when the needles line up ('join'), you leave the switch on and cover it with the cage so it can't be knocked off accidentally.

Thus the clutch is really a combination of the belts, the pulley and the electric motor.

Quite separately to this, there's a freewheel, so if the engine stops, the rotor doesn't; very useful for autorotation.

The Robinson also uses a pair of belts; I believe the electric motor moves much more slowly and you can just switch it and forget it, once you've wound the rpm to 55%. After the Schweizer, I couldn't believe it when the instructor flicked the switch and then just carried on with the briefing.

Most gas turbine helis with Allison 250 engines don't have a mechanical clutch. Essentially the power turbine, consisting of four wheels, is in two parts: the first two wheels after the burner drive the compressor and the latter two, on a different shaft, drive the rotor. Because they're on a different shaft, there's no direct mechanical link. What gets it going is simply the gas flow through the engine.

Piston helis have had centrifugal clutches, notably the Bell 47.

As to the rest - I don't know. And I may have got some of the above wrong.

[Helinut]

SGAS,

On the sorts of (piston-engine) helicopters you seem to be talking about, there are two sorts of clutch that do two entirely different jobs, and are usually physically separate:

- a freewheel clutch
- a clutch that is used to connect the engine to the rotor system, after the engine has been started.

For those not familiar, the freewheel clutch is what automatically disconnects the rotor system from the engine in the event of an engine failure. As a general rule, any helicopter has to have one of these - to cater for the eventuality of engine failure, to enable the rotors to turn and the aircraft to autorotate. It works like the freewheel arrangement on the back wheel of a bicycle. Most/many helicopters use a sprag type of clutch as their freewheel.

Piston engines are usually started unconnected to the rotor system. Without such an arrangement, it would be necessary to use considerable additional force to turn over the engine at sufficient speed to start it. There are various different systems around - many of them use a belt drive type - the belt is slack while the engine is started and then some tightening system is used after start to conect the engine to the rotor. It is popular because it is light, simple and cheap and does the particular job - a car clutch has a different and more difficult job to do. Turbine powered helicopters don't need such a clutch - the free-turbine gas turbines used in helicopters effectively have an-built clutch because the power turbine can "power-up" before it begins to turn the free turbine.

[RDRickster]

Another example of a piston is the Brantly B2B. The kevlar centrifugal clutch is automatic, enabling you to be up and flying quickly. For those times you want to change passenger without shutting down, a quick throttle chop to idle, followed shortly thereafter by application of the effective rotor brake, will stop the rotors with the engine running very quickly. Then, when ready, release the rotor brake, roll on throttle and the heavy rotor system is up to full speed and ready for flight in seconds.

[Lu Zuckerman]

Bell piston powered helicopters have centrifugal clutches (or if NickLappos is reading this centripetal clutches). Early Sikorsky helicopters also had centrifugal clutches (S-51, S-52 and S-55 (A & C models). The S-55 B & D models were fitted with a more powerful engine and they had hydromechanical clutches as did the S-56. All of the above had Sprag over running clutches (freewheeling clutches).

I believe the H-21 had a centrifugal clutch but instead of having a Sprag freewheeling unit they had a spring loaded jaw clutch that sounded like a machine gun when the helicopter was put into autoratation.

:ok: