Question re Short Duration Overspeed on Takeoff

[eetrojan]

The other day, when I advanced the throttle to WOT for takeoff in an Evektor SportStar with a Rotax 912 and a fixed-pitch prop, the engine briefly went "over-speed" as if the propeller wasn't biting. It lasted no more than a second.

Is this a potential damage issue, or symptom of a current or future problem?

Or, is it just telling me that I'm an idiot and need to advance the throttle slower and smoother?

 

[Capt. Geoffrey Thorpe]

The line service manual recommends checking the torque on the overload clutch if slipping is suspected. Section 5-50-00. The spec is 442 - 590 foot pounds - which is a fair bit of torque. The engine generates 60-70 foot pounds (I assume on the engine side of the gearbox).

Lead and oils with friction modifiers can cause problems with the overload clutch.

 

[eetrojan]

Thank you!

I didn't know it had an "overload clutch." I learned something new and that definitely sounds like a promising lead.

 

[Capt. Geoffrey Thorpe]

In most 912s there are two clutchey things in the gearbox - the overload clutch (multi wet disk clutch) and the torsion damper. The torsion damper uses ramped dogs and springs - that allows the propeller to slip about 30 degrees between the dogs and avoids directly transmitting torque pulses that can beat up the gearbox. The overload clutch is designed to protect your crankshaft in the event of a prop strike.

The clutches and gearbox are the reason that Rotax is fussy about what kind of oils you use.