Schweizer 300 input driveshaft inspection

[Pilot DAR]

Lesson learned the hard way, when starting the Schweizer 300, keep hand firmly on the throttle! Oops!

So now that I've learned my lesson, there's an aspect I don't quite understand, perhaps someone can help...

Thou shall not exceed 1600 engine RPM with the clutch not engaged, and if you exceed 2000 RPM, it's an MPI inspection of the input driveshaft.

The input driveshaft is a small shaft, splined on both ends, which connects the engine crankshaft to the input drive pulley. Presumably the spines allow the shaft to float, and allow for motion of the engine relative to the airframe (and input drive pulley). I understand that its is this freedom to move which creates the opportunity for unwanted motion at high RPM, and possible cracking of the shaft at the splines. As long as it is under the load of driving the rotors, this unwanted, potentially damaging, free motion does not occur.

The input drive pulley (as the output pulley) is supported by two ball bearings. It is not supported by the input driveshaft. Thus it is not the mass of input drive pulley, or tension load of the belts, preventing damage to the input driveshaft, only the drag of the rotor drive train. When the clutch is not engaged, the only load on the input drive pulley is that of it slipping under the loose drive belts.

So with no rotor drive train load on the input driveshaft, do not exceed 2000 engine RPM. On the ground, this is easy to visualize.

In flight, however, you're cruising along at 3100 RPM and enter autorotation. When you lower the collective, the freewheeling clutch disengages, and at that moment, the engine (now slowing down from 3100 RPM, but still well over 2000 RPM) is no longer connected to the rotor drive train. It's only turning the two pulleys (and idler) and belts.

How is this any less harmful to the input driveshaft? It's turning a lot faster, and for a much longer duration than the one second overspeed I caused, and all it's driving is the belts.

Any informative thoughts to complete this lesson would be appreciated. Oh, and the input driveshaft was fine when inspected.

Thanks, Pilot DAR

[SEL]

One explanation I was given (and I'm no mechanic!), is that it is to do with the shaft whipping about in the bearings. At high engine RPM, the shaft will oscillate about it's axis. When the clutch is engaged, the shaft is braced thereby preventing it 'whipping' about.

In autorotation, the freewheel unit disengages the drive to the rotor transmission but the clutch remains engaged, the belts still tight and holding the engine output shaft.

This could all be wrong and those in the know will surely descend shortly. I don't believe everything I'm told although this sounded credible!

SEL

[mountjoy]

Pilot Dar, not sure which model 300 your flying, but 3100rpm, is a bit high considering upper red line is 2700, at 3100 rpm, rrpm would be well above engine on limits to mate !

[Whirlygig]

Sounds like an old (ish) C model rather than CBi. 3100 rpm is slap-bang in the middle for a C!

Cheers

Whirls





Oh dear - I've come over all peculiar having actually made a technical contribution to Rotorheads.

[Phil77]

I want to second SEL...
I asked a guy who flies and maintains schweizers for 15+ years; he told me he was wondering about Pilot DAR's issue in the past and asked the factory. They in turn told him that the tension of the belts is enough to not overspeed the engine (or the driveshaft for that matter).

Its only hearsay but from "reliable sources".

Philip

[Heli-Ice]

Whirls

What next? Teaching rotor rigging? This helicopter stuff is really getting in your blood I see. :ok:

You are spot on, 3100 Rpm is in the middle of the green on the S300C.

When are you coming over and take my C for a run?

Tell me, shouldn't you be at Heli Expo leading old SAS around the place and keeping him out of trouble?

My mechanic told me that it is ok if the engine coughs up to 2000 for a few seconds when it kicks in if you just ease the throttle back to 1600 Rpm asap after it fires up.

[rotorrookie]

If you follow the check list in right manner and open the trottle 1/8 of an inc and not more(even better to give it little less than that), your engine will mostly rev up to around 1800rpm.

[RVDT]

Ah! If it was only that simple!!:ugh:

http://en.wikipedia.org/wiki/Torsional_vibration

With the clutch engaged there is enough mass to provide some torsional damping, without there obviously isn't!

This engine or other derivatives also has an issue when it lives its life with a propeller connected to it as well - see Piper Arrow, Cessna 177 for examples.

On overspeed - more than 3350, this engine will crack the valve collets which can also rip your nightie!