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Gtblu
Joined: 18 Jul 2006
Posts: 37
Location: Australia
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 Posted: Fri Jun 13, 2008 2:14 am Post subject: Engine Vibration |
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Hi all,
Just wanted to close issue of an 912 uls engine that was vibrating/resonating at 5000 revs and above. I think I have finally resolved the issue by exchanging props. Vibration disappeared instantly prompting me to really focus on the original prop. It seems the bolts attaching prop to engine were fractionally too long so that torquing them up wasn't tightening them properly. We are talking microns here but enough to allow a resonance on certain days given certain conditions. A little wear on the washers reduced the tension causing the sudden change. Temperature may have been expanding or contracting various bits such as hub and bolts to make more or less tension as well. A little wear on the hub and "smoke" on the back of the hub suggested that things were moving.
Thanks to all for ideas and thoughts, and especially Roger Lee and "Buffalo Thom" for great advice and patience.
It's been fascinating trying a few props as they are all so different in their nature prompting me to try more. I'd like to hear other users experience on this ie what revs do they vibrate in, etc.
The wooden 2 blade prop creates a kind of rumble in the air and feels softer on the engine. Its swish out of time with the engine was quite disconcerting to start with. It has a vibration/resonance zone around 4000 revs
The old three blade vibrated at around 4800, but man does it rip off the strip!
Fascinating
Thanks again
regards
Geoff Bell
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Fri Jun 13, 2008 6:37 am Post subject: Re: Engine Vibration |
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Hi Geoff,
Congratulations.
I know chasing a ghost can be frustrating to say the least. Wood props are usually formed by hand after the roughing out. The prop bolts on a wood prop should be re torqued every couple of months due to wood's expanding and contracting properties. This can be a little more pronounced in different parts of the country (i.e. dry climates v.s. humid climates).
Out of curiosity did you ever balance your old prop and check blade tracking?
I just checked a guys prop that had flown in from N.Y. and traveling around the country. He complained of very slow rate of climb with his 912ULS in a Flight Design CT (100' a min). He was only getting
5150-5200rpm WOT straight/level. When I checked the three blade prop one blade was out from the other two 3/4 degree. I re pitched all of them to achieve 5500rpm. It was like a whole new engine and he stated that the vibration he had been feeling for a while was gone.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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Gtblu
Joined: 18 Jul 2006
Posts: 37
Location: Australia
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| Posted: Fri Jun 13, 2008 3:09 pm Post subject: Re: Engine Vibration |
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Hi Roger,
I did check the tracking, and one blade was out. I voiced my concerns to the manufacturer, and they said that they were within tolerances. I also voiced concern that the pitch varied down the blade on one blade compared to the other two. That is to say that if I pitched the blades at say 200 mm from the tip, they were not equal on the tip by half a degree. Manufacturer also said that was within tolerances, but did say send them back for balance check. I inspected the prop very carefully with magnifying glasses and found some damage right up near the hub on two blades (not the one that was different?). The blade root has come in contact with the corner of the radiator before my time.
I have ordered a new prop!
Such a relief.
Thanks Roger
regards
Geoff Bell
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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Gtblu
Joined: 18 Jul 2006
Posts: 37
Location: Australia
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| Posted: Fri Jun 13, 2008 5:00 pm Post subject: Re: Engine Vibration |
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Hi Roger,
really I'm just very glad it didn't break in flight. It must have been on the cards!
regards
Geoff Bell
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Guy Buchanan
Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA
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| Posted: Sun Jun 15, 2008 12:05 pm Post subject: Engine Vibration |
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At 03:14 AM 6/13/2008, you wrote:
| Quote: | It seems the bolts attaching prop to engine were fractionally too
long so that torquing them up wasn't tightening them properly. We
are talking microns here but enough to allow a resonance on certain
days given certain conditions. A little wear on the washers reduced
the tension causing the sudden change. Temperature may have been
expanding or contracting various bits such as hub and bolts to make
more or less tension as well. A little wear on the hub and "smoke"
on the back of the hub suggested that things were moving.
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Geoff,
Thanks for posting this. I believe I have the same problem,
but couldn't figure out the "smoke".
Guy Buchanan
San Diego, CA
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.
Do not archive
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Guy Buchanan
Deceased K-IV 1200
A glider pilot too. |
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sun Jun 15, 2008 12:48 pm Post subject: Re: Engine Vibration |
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Hi Guys,
If you have smokey bolts, rivets or whatever it usually means something is lose and rubbing. It may not look or feel lose to the touch, but it is under stress or vibration.
We just had a Lightening aircraft crash here in the Tucson area. The Lightening is a low wing ELSA homebuilt. It was fairly new with about
30-40 hours or so. It lost the prop in low level flight and from what I have been told second hand was that he tried to turn back toward a landing site with a 180 degree turn. He was way too low and went in hard. It was a fatality. He would have been better off staying straight and stalling into the low ground cover bushes. He may have turned over, but he would have walked away. I won't make a 180 turn less than 400' in my FDCT and you better be doing things right at that altitude. Sometimes a messed up plane is imminent, but picking the lesser of the evils for a landing site and saving your skin is a better choice.
It's sad when anyone dies enjoying what they love.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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Gtblu
Joined: 18 Jul 2006
Posts: 37
Location: Australia
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| Posted: Tue Jun 17, 2008 11:59 pm Post subject: Engine Vibration |
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Great Stuff Guy,
Were the bolts too long? The other clue I should have twigged to was the torque difference between loose and too tight was too few turns, indicating I was tightening on something with no give., ie the end of bolt threads.
Live and learn.
Thanks for letting me know! You made my day.
Regards
Geoff Bell
[quote][b]
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