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Propeller? Gearbox? "Wow-wow-wow" resonance

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 390
Location: Aberdeen area

Posted: Thu Jun 18, 2020 6:25 am Post subject: Propeller? Gearbox? "Wow-wow-wow" resonance

The title more or less says it all! The engine is a 912ULS with 600 hours total and the propeller is an Airmaster which is serviced/greased every 100 hours as specified and is well balanced. The carburettors are regularly checked with a CarbMate electronic synchroniser and maintained as perfectly matched as possible.



The aircraft is delightfully smooth to fly, yet there is a "fly in the ointment". During the last 50 hours or so of flying, a constant "wow-wow-wow" sound, akin to that of a piston twin aircraft with unmatched rpm, has gradually increased. For most flights since the sound first appeared I could level off in the cruise, select "Manual" on the propeller control and reduce engine speed to about 4700 rpm, which reduced the sound oscillation to almost nil.



But recently it has become more intrusive, to the extent that further reduction of engine rpm doesn't eliminate the problem. So I'm thinking of removing the gearbox and getting it checked over professionally, in case the dogs or their springs could be the culprits. By the way whenever I do the friction torque check, it always results in a reading of around 40 to 45 ft/lbs.



Within the last approx. 20 flying hours I've replaced 2 exhaust springs in different locations, which might be indicative of undesirable vibrations



Any advice please before I resort to gearbox removal. I've done it before and know what's involved, but if you believe that it might be a wasted exercise please tell me soon, thanks

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Last edited by JonathanMilbank on Sat Jun 20, 2020 3:30 pm; edited 3 times in total

Remi Guerner

Joined: 14 Dec 2010
Posts: 284

Posted: Thu Jun 18, 2020 11:28 pm Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Jonathan,

I do not think your gearbox could cause your problem. The cause may be a bug in the propeller controller. I would ask Airmaster for advice.

Good luck

Remi

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SPURPURA

Joined: 04 Apr 2015
Posts: 68
Location: KAPV

Posted: Fri Jun 19, 2020 4:12 am Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Doubt that it’s propeller related. Try a stethoscope. If you have 600 hours on it, you may be ready for some gear box service. I had 700 hours on mine before getting rebuilt.Don’t let it go too long as it is full of expensive parts.

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N951EU - Tri-gear & 912ULS, N77EU- Mono & 914
I'D RATHER HAVE A BOTTLE IN FRONT OF ME THAN A FRONTAL LABOTAMY.

clivesutton

Joined: 23 Jan 2013
Posts: 187
Location: KENILWORTH

Posted: Fri Jun 19, 2020 5:58 am Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Hi Jonathan, what you describe is 'Heterodyning' or 'Beat-frequency' created by the difference between two noise or vibration frequencies very close together.  



Do you have a recording on e.g. a mobile phone that i could listen to - along with the engine speeds in play at the time?  And what about:-



1) How does the frequency of 'wow-wow' change if you speed up or slow down the engine a bit?

2) How does the 'wow-wow' effect change with Airspeed?

3) Remi's CS controller idea is viable too - does wow-wow occur when that is turned off/the prop is not trying to hunt for optimum speed vs pitch?



It's impossible to say from just the description given, but assuming none of the above items make a significant difference, my initial suspicions would be one prop blade with a slightly different prop pitch to the others and/or blade tip tracking differences between blades.  Both of those parameters should be quite easy to check on the ground (no engine running needed) and reporting any variation you find?



Clive



PS i did just retire from 40 years doing such stuff

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 390
Location: Aberdeen area

Posted: Sat Jun 20, 2020 4:02 am Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Hi Everyone,



Thanks for all your replies with suggestions, several of which I'm now following up. After a thorough in depth investigation with engine cowls off, I've got a couple more things to try like unclamping the exhaust header tubes and moving the silencer slightly before tightening again. This is intended to move it a little further away from the air duct against which it might be vibrating in flight, when the engine leans over due to torque.



Clive, my mobile 'phone is a Tesco £20 special, which only does calls and texts. The frequency of the 'wow-wow' doesn't seem to change when I vary rpm in the cruise, but then I'm only varying it by about 6%. I also can't say that I notice any change with airspeed. I switch the controller to manual during the cruise to reduce rpm from 5000 to 4700, so the CS function probably isn't a factor.



Thanks for the pointer regarding one propeller blade tip tracking differently from the others. Although the wonderful Airmaster propeller which I've had for nigh on 20 years is virtually 'bomb proof', I'll definitely investigate.



Best wishes,

Jonathan.

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clivesutton

Joined: 23 Jan 2013
Posts: 187
Location: KENILWORTH

Posted: Sat Jun 20, 2020 5:12 am Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Jonathan, a further investigation suggestion that you can do from the comfort of your home:



1) Download this free sound tool onto your PC:  https://www.nch.com.au/tonegen/index.html



2) Create two sine tones and adjust one to be 1hz higher than the first (i used 440Hz + 441Hz).  Play it and you might get the effect you are hearing in the aircraft.  The Wow-Wow you hear is effectively the difference between the two frequencies so a 'Wow' repeats every 1 second in this case.  Play with different frequencies to get the effect you experience . . . [Caveat - audio speakers and headphones are not that good at reproducing low frequencies - so the quality of what you hear may be affected by that factor]



3) For an engine running at 4700 RPM and a gearbox reduction of 2.43, the gearbox shaft is turning at 32.23 rotations per second at that RPM.  That shaft has three prop blades bolted to it - so the frequency of pulsation from any one blade is 32.23 Hz and the frequency of pulsation from all three blades passing in front of you at 4700 RPM is 96.69 Hz.  



4) based on this logic, if any one of those pulsations is higher or lower than the other 2 (as a result of e.g. tracking or different pitch setting of one blade) i suspect that might be your Wow-Wow source.  



I'm not sure this rules out your gearbox - but it's something to check

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budyerly@msn.com

Joined: 05 Oct 2019
Posts: 288
Location: Florida USA

Posted: Sat Jun 20, 2020 9:55 am Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Jonathan,



By putting the Airmaster in MANUAL mode the prop is fixed, so hunting of the prop is unlikely if the warble persists at any RPM.  As for blade tracking, if the blades are still firm and not twisting, that rules out a tracking issue normally.  



I left some super thin painters tape on all three blades one time after a touch-up.  I know, a good preflight and proper workmanship would have prevented this...  I couldn't figure it and quickly took the brand new plane down to the dealer.  After hauling the plane down to have the gearbox checked, Lockwood could find nothing.  When picking up the plane, I found the thin painters tape, pulled it off and behold, a good sounding engine again and I had a much lighter pocket book.  



Another "brain fart" I'm not proud of, an owner came in with a blade that was beginning to show signs of leading edge wear.  The thin metal leading edge guard was slightly dented and pitted on his Warp Drive.  I didn't notice it was becoming de-laminated (slight bubble).  I should have caught it on a close inspection but didn't.  This was not a warble sound from the outside, it was a sharp hum or buzz, but inside it seemed to have a warble.  Rebuilt the blades (fairly cheap) and all was good.  If your leading edges are unevenly worn or pitted, and or the trailing edge of your leading edge tape even slightly loose, refinish the blades.



Folks are right, the book says for most gearboxes, an overhaul at 600 hours is required for continuing maintenance.  Gearboxes are normally sent off for the overhaul (which takes 30 minutes at Lockwood).  I pull the prop, the gearbox and drive down and have lunch with the boys, then come back.  Nice visit and about $3-400 if no main gear parts are needed.  The time is one hour and the new seals and bearings are quite cost effective.



Exhaust resonance is tough to find.  Exhaust leak soot and spring failures generally point the way that something is amiss.  One must be very careful trying to troubleshoot this during ground runups.   



A problem with the sprag clutch can cause noise, but that is clearly heard around an idling engine, so I doubt that is an issue.



Vibration noise from the airframe is tough to track.  Loose gear components, speed kits, and anything that vibrates in flight will pick up a power plant frequency and resonate along with the other vibrations.  A slight difference in frequency will cause a distinct increase and decrease (like that propeller sync example) giving you fits troubleshooting those type things.



Keep us all posted.

Bud Yerly

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 390
Location: Aberdeen area

Posted: Wed Jun 24, 2020 5:12 pm Post subject: Re: Propeller? Gearbox? "Wow-wow-wow" resonance

Bud and friends,



Today I found what I believe to be the "smoking gun" i.e. the possible cause of the strange "wow-wow-wow" resonance. While standing in front of the aircraft I saw that the propeller spinner is now displaced about 5mm to the left and is sagging about 4 mm. On standing closer it was easy to see that the 2 screws which secure the upper cowl nearest the spinner, are no longer equally spaced either side of the spinner. Voila!



Off came the cowls and a couple of hours were spent closely investigating both the engine bearer frame ( a la Neville Eyre ) which is built to battleship standards and the undercarriage frame which was found to be crack free and as strongly secured to the fuselage tunnel as ever. A friend assisted by repeatedly lifting the engine up and down and side to side, while I used a strong light and felt all over for evidence of movement due to cracking. Then we swapped roles, before reaching the conclusion that both frames are sound and solid.



So all this suggests to me that it could be time to replace the Lord engine suspension rubbers, which haven't been replaced in living memory, or 12+ years! But I seem to recall that there was a bit of indignation in this forum about the excessive hardness of rubbers supplied by Europa. Please refresh my memory and let me know if the hardness problem was ever resolved.



I've tried a search and trawling through various topics, to little avail. All I could find was someone's query about MT04 and Lord J 3608-1.

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