Torsional vibration

Hi John,

How far from the prop center did you measure? 26", 28" or 30"? This is important to multiply your 13 lbs. by. If it was 30" then that's only 390 in/lbs and that's on the low side and I'd have that gearbox checked especially after 9 years. If it was only 26" out on the blade that would be even worse. What I normally see is 440 in/lbs to 490 in/lbs. 460 - 485 is very normal. I would run the engine any more till you get the gearbox checked as it could cause other internal damage.

p.s.

What rpm was this problem at? Not too low I hope.

p.s.s.

Sometimes spending a little money up front even though we don't want to saves big bucks later from other damage.

Roger Lee wrote:

Hi John,

How far from the prop center did you measure? 26", 28" or 30"? This is important to multiply your 13 lbs. by. If it was 30" then that's only 390 in/lbs and that's on the low side and I'd have that gearbox checked especially after 9 years. If it was only 26" out on the blade that would be even worse. What I normally see is 440 in/lbs to 490 in/lbs. 460 - 485 is very normal. I would run the engine any more till you get the gearbox checked as it could cause other internal damage.

p.s.

What rpm was this problem at? Not too low I hope.

p.s.s.

Sometimes spending a little money up front even though we don't want to saves big bucks later from other damage.

I measured it out about 1 inch from the prop tip as I didn't have a measuring tape handy.  So that should be around 32 inches.  Also, I didn't fully lock the crankshaft per the SB.  I moved the prop forward so it engaged with the engine, then moved it back about 10 degrees before measuring. Not sure if this invalidates my test.  I had expected the tension to be low which would explain the vibration.

It was at fairly low RPM.  around 1000 prop RPM which should be about 2272 engine RPM. 

My plan is to overhaul the gearbox anyway, but just wanted to double check that this was most likely the cause.  I did check both ignition circuits L,R, and Both which didn't change the behavior.

thanks  John

Hi John,

Yep the test wasn't any good. The engine rpm at 2200 is okay, but the prop test wasn't good on a few fronts. This is explained in the Line Maint. manual on how to do it at the end of the manual.

Gotcha.  Thanks!  I'll read up on it and perform the test again according to instructions.  Do you know what size allen key is required to unscrew the crankcase locking port?  I had imperial wrenches, but didn't get an exact fit.  Also, is it best to heat up the bolt first?

That bolt should come out pretty easily – it's just a plug.  But do you have the crankshaft locking pin (240 880)?  You'll need that; it costs about $7.

It's also a little tricky finding the notch it goes into on the counterweight if you haven't done it before...