Prop Balancing issue

[VH-XXX]

Hi everyone,

I'm trying to resolve an issue with prob balancing (presumably) causing vibration in the engine.

The combination is an MT 2 blade fixed pitch with fibreglass coating and metal leading edge on a Continental IO-F-240-B (Fadec).

The prop was statically balanced and admittedly one blade is noticeably heavier than the other, but with some weight added to the bulkhead, it's perfectly balanced statically.

With the dynamic balancer (Vitratex) and a around 15 grams strategically placed we get near perfect results, .02 on the display (for those unsure, anything up to .25 is the norm on this unit) so good results there. That is at 2,300rpm.

My assumption is therefore that the engine / crank is A-OK.

However at 2,450 and above a vibration starts and up to 2,850 redline, it's quite bad, the higher the RPM the more vibrations present.

I'm trying to work out why?

My thoughts are that under high load, one prop blade is either twisting or bending differently to the other, thus causing an inbalance. It's the only thing that I can think of.

Any suggestions?

[stevef]

Presumably nothing else has been disturbed, such as the engine being removed/refitted or shock mounts being replaced?

[NutLoose]

I take it the prop was fitted in the correct position, as it is designed to align correctly with the throws on the crank to balance the lot.

[VH-XXX]

The engine has not been touched, no changes.

The aircraft was recently purchased but it's believed that this prop did not belong to this aircraft.

The prop was fitted with #1 piston as top dead centre as per the maintenance manual.

[NutLoose]

Without looking at the book is it with no1 or anticlockwise one hole as per Lycomings......... may be worth as a long shot pulling the prop of and putting it back on 180 degrees round so blade 1 is where 2 is, if that makes sense, also make sure nothing is touching, eng mounts have not sagged, pipe rubbing cowl etc or even the exhaust where it cones through because they will transmit vibration.

I also take it its not a plug breaking down, timing is spot on etc..... we used to have a rough Lycoming at high RPM and it was sucking fuel past the float bowl gasket, primer leak or induction could too.

[virgo]

Try substituting another prop............It'll confirm whether you've got a prop or engine problem.

[DHW]

I agree with NuttLoose turn the prop 180 deg. but remove any weights first.
Also the balancer you are using may be sensing the prop only. You may have to use a unit such as the Microvibe to sense the prop / engine combination. The engine may have an vibration coming in at higher RPMCould be a piton / conrod / Crankshaft etc.

Good luck
DHW

[enginair]

Have you tried balancing it at a higher RPM , we had a scimitar on a Cessna 210 that was fine at 2500 ( crz ) to 2850 ( max t/o ) but below 2450 it was rough , we lost nothing by running at a higher rpm and the aircraft had factory engines go to life at this rpm

Good luck let us know what you find in the end.

Eng

[VH-XXX]

Have a bit of an update. Did a dynamic balance as discussed. Unfortuately due to an "engineering fault" (aka human error) on the subsequent flight, a spinner screw came out and damaged the prop.

Replaced prop with a Sensinich 2 blade, still got vibrations (not looking pretty at this time). Decided to turn clockwise (anti-clockwise rotation) one bolt hole, many of the vibrations went away. Am thinking might rotate another hole and check for vibration reduction or alternatively dynamic balance in current position. Bit of a pain though moving it as it won't stop in the horizontal position now.

[NutLoose]

Err the prop is specifically aligned as per the manual for a reason, that is the prop is in effect the actual flywheel and is aligned with the crank to "throw" for want of a better word the crank over onto the next cycle, you cannot simply alter it. If I remember I will look at the manual tomorrow for you to see the actual position it should be in, but if it is the same as the Lycoming it is TDC no1 and one hole clockwise looking at it if memory serves me correctly. they never normally sit horizontal, again if memory serves me correctly when stopped so you probably have it in the wrong position, which is why you have vibration as it isn't in balance with the crank...........

[Engineer_aus]

Never be afraid to ask around either. You may make a mistake.

[virgo]

Have I got this right ? You've substituted a different propeller (Sensinich two-bladed) and have STILL got vibration ?

Doesn't this point to a problem with the engine ???????????

[blackhand]

However at 2,450 and above a vibration starts and up to 2,850 redline, it's quite bad, the higher the RPM the more vibrations present.
mmm, perhaps a jammed crankshaft counter weight.
I would suggest put another accelerometer(velocometer) at the rear of the engine and check vibe level as you lower the forward vibe level.

Have had same issue with PT6A in Twin Otter ( I know is turbine) and was an engine vibe problem - found with the rear accelerometer.

Cheers
BH

[VH-XXX]

Final update. In summary, switched to Sensenich as previously stated (as that's all I could get at short notice and that's all that's legal on the aircraft) vibrations still present, rotated 1 hole, much better, rotated one hole further, not as good so back to the previous hole which is one from TDC and pretty happy with it. Will do another dynamic balance and will be happy with that. .15 on the balancer is acceptable and with the dodgey prop got it to .03, so shouldn't be a drama this time.

Seems that the MT may have been ok however didn't get a chance to rotate it.

Good call Blackhand, will try the accelerometer at the rear of the engine this time too.

Nutloose, yes, it normally goes on #1TDC, however you rotate a hold at a time to the point of least vibration.

[virgo]

VH-XXXX
Just out of curiosity, why are you SO convinced that your vibration problem is with out-of-balance propellers and NOT with the engine ?

[VH-XXX]

Because changing the prop position in accordance with the aircraft manual resolved the issue.

Also, the prop was not off this aircraft originally and there was no record of vibration in the maintenance release so an assumption was made about the prop and or mounting thereof.

[virgo]

Ah..................we didn't know that.