914 RPM/ Boost problem – only 1st flight of the day

Hi Marc

I am thinking first start of the day the engine is cold and as such has a bit more power than when warmed up.  (the block and heads are still cold) If the engine is fitted with an overload clutch then do a break away test as shown in the MML manual.  That should confirm if you have an issue or not.

Cheers

Peter Kl. wrote:

Marc,

I can follow your concern but do not find the 800 revs jump in the log. 

The comment on the clutch sounds very plausible to me (sudden lack of load). However, the question remains open why this effect only occurs once a day during the first flight.

If I understand your description correctly your prop setup seems OK to me (below 5800 revs at full noise).

Keep us updated.

Hi Peter and Rotax Wizard - yes, sorry we do have an overload clutch and I had not thought of that, but it's still strange it only happens on the first flight of the day. Attached is a fuller snapshot of the TCU log so you can see where I get the 800rpm from.

Appreciate the help !

Mark

Rotax Wizard wrote:

Hi Marc

I am thinking first start of the day the engine is cold and as such has a bit more power than when warmed up.  (the block and heads are still cold) If the engine is fitted with an overload clutch then do a break away test as shown in the MML manual.  That should confirm if you have an issue or not.

Cheers

Hi RW - I checked the friction torque a few days ago during a 200Hr check and it was 50Nm (which is what it has always been)

Cheers, Mark

Hi Marc

The issue is the millibar pressure.  You are over target (red numbers) The pressure in the system is what makes torque.  Once you exceed the overload clutch threshold it can break free and slip, at least that is what I am looking at.  This can be very fast and sometimes when it is bad it will not catch again with the result as a overspeed that can damage your valves and bend pushrods.  It would be good to do the breakaway test to see where it is at.  The newest versions of the clutch system are set much higher than before.  They increased it a number of years ago from 500NM up to 600NM for this very reason, to control clutch slip.

Cheers

Rotax Wizard wrote:

Hi Marc

The issue is the millibar pressure.  You are over target (red numbers) The pressure in the system is what makes torque.  Once you exceed the overload clutch threshold it can break free and slip, at least that is what I am looking at.  This can be very fast and sometimes when it is bad it will not catch again with the result as a overspeed that can damage your valves and bend pushrods.  It would be good to do the breakaway test to see where it is at.  The newest versions of the clutch system are set much higher than before.  They increased it a number of years ago from 500NM up to 600NM for this very reason, to control clutch slip.

Cheers

I think the pressure is over target because the TCU is reducing the setpoint to try and lower the rpm and there is a lag in the response. Can the breakaway test be done with the gearbox mounted to the engine and prop ? I did the friction torque, but I guess that's not what we're talking about here.

Cheers