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resonance

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tc1917(at)bellsouth.net
Guest

Posted: Sun Sep 07, 2008 3:07 am Post subject: resonance

Will try to answer each as I can.



Does it do it when tied down, doing a static runup?

'does it anything over an idle -- audible tied up, flying, all rpms.



Did you close the rear windows?

' rear windows are open'



Is the fabric tight on top of the wings, inboard near the engine?

'fabric vibrates but have applied tapes, foam, pressure on fabric and does 

not stop the noise - engine producing noise'



Did you check gear box to see if it was shimmed correctly?

'gear box shmming seems to be the last hope.  - just gotta get someone to 

take it down and be able to reshim it - seems I may have to fly to Ronny 

Smiths to get it tried.'



Has anyone else had this problem with a SS and 912?

'have heard of several on Mk models.  One on another plane that just plain 

drove the pilot crazy, sold the plane and they took the 912 off, solved the 

problem'



Has anyone else had this problem with SS and other engine combinations?

'Robert Been sent email stating he has some problem similar but was using 3 

cyclinder (hurth?) and 3 blade. tore up gear box bearings'



Has anyone else had this problem with other aircraft and engine 

combinations?



Dan Horton's 3/4 scale Jenny with 3 cyl Suzuki engine did this.  Never did 

figure out why it did it.

'seems like this is the answer so far'



Ronnie Collins SS did it.  Fabric had been rib stitched by builder. Stitches 

were failing on top of wings inboard near the engine.  Caused by drumming 

from prop.

'I believe Ronny had 582 rotax on it at the time.  I have tried no spacer, 

4" spacer and now have 2" spacer that seems to work best.  Have padded up 

down and all around everything back there I can.  Helped.  Bought an ANR set 

for my military helmet.  Best thing yet.  Still hear the noise but at least 

I can use the radio crystal clear.  Best two hundred bucks I ever spent. 

Oh, yeah, I never have to worry about the ANR Bat going dead.  It will go 

dead as soon as I am about one mile from a busy airport during an airshow. 

Getting good at changing the bat with one hand. Many thanks to the one who 

put me on to it on the list.'



Okay, put your thinking caps on tight.  Hope a qualified rotax gear box man 

is at the homecoming.  That is my last hope Obeywan. Ted Cowan, Alabama, 

Slingshot.

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jbhart(at)onlyinternet.ne
Guest

Posted: Sun Sep 07, 2008 3:48 am Post subject: resonance

Ted,



What propeller are you using?



Jack B. Hart FF004

Winchester, IN

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Ralph B

Joined: 14 Apr 2007
Posts: 367
Location: Mound Minnesota

Posted: Sun Sep 07, 2008 4:11 am Post subject: Re: resonance

I haven't noticed any resonance on the Kolbra 912. It has a tapered 3-blade Warp-Drive prop pitched high for cruise. You might add more pitch and see if the resonance goes away.



Ralph

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Ralph B

Kolb Kolbra 912uls
N20386
550 hours

Last edited by Ralph B on Sun Sep 07, 2008 6:01 am; edited 1 time in total

slyck(at)frontiernet.net
Guest

Posted: Sun Sep 07, 2008 5:28 am Post subject: resonance

 	  Quote: 	
		  

some notes to clarify:
	




 	  Quote: 	
		   Has anyone else had this problem with SS and other engine  

 combinations?

 'Robert Been sent email stating he has some problem similar but was  

 using 3 cyclinder (hurth?) and 3 blade. tore up gear box bearings'



3 cylinder is also a suzuki.  I'm not using a gearbox but I do  
	


suspect the 3 blade was having a high freq problem

with the engine in the 5000 rpm range and damaging the shaft bearings  

in the raven cog belt drive.  Couldn't

hear it or feel it but three times in a row the carefully set up,  

greased and torqued bearing stack lost their torque

load and began to spin the stack.  -not good.

Now with the warp 2 blade I have run all summer with an audible rum  

rum and the redrive has performed perfectly.

This is as much a confidence issue as an annoyance.

My next experiment will be a 2 blade with less inertia.



 	  Quote: 	
		  



 Dan Horton's 3/4 scale Jenny with 3 cyl Suzuki engine did this.   

 Never did figure out why it did it.

 'seems like this is the answer so far'



I wonder if he found a solution?
	




Bob Bean

 	  Quote:

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jbhart(at)onlyinternet.ne
Guest

Posted: Sun Sep 07, 2008 10:43 am Post subject: resonance

Ted,



Are your wing struts streamlined?



Jack B. Hart FF004

Winchester, IN

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aslsa.rng(at)gmail.com
Guest

Posted: Sun Sep 14, 2008 10:47 pm Post subject: Resonance

Ted, I went back through all the posts about your resonance problem and the one thing I can't find mention of under the things you've tried is balancing the carbs. Before you spend money on tearing your gearbox apart or a new prop you might want want to consider spending less than $100 on a set of vacuum gauges. You can buy them all set up and ready to use from suppliers like Lockwood or buy the gauges and damper valves from someplace like McMaster Carr. I did the latter and didn't save a dime over Lockwood's price. :-{  The entire procedure is documented in the Rotax manual ( I'll look it up in the morning and get you chapter and verse ) and takes less than an hour. First you do the  mechanical synching then the pneumatic with the gauges. I've documented my experience with my HKS and despite the fact that I couldn't hear or feel anything wrong, the carbs were 3" vacuum different at 3000 RPM. When I pneumatically balanced them engine RPM increased to 3500. In one of your posts you commented that the rum rum was like a twin with the engines out of synch. Not having your carbs synched is basically that. Two cylinders are carrying more load than the other two. I don't know that this is the be all end all of your problem but if you haven't done it recently I'd recommend you do it or get it done if for no other reason than it's the cheapest of all the other options you've proposed doing.

 



Rick

PS. I also stick to my other recommendation that I sent to you off line. If you can't kill the resonance directly softer engine mounts will dampen it. If the good folks at TNK don't carry mounts of varying hardness they might be able to point you to their supplier so you can contact them and see if they're available.

 



  [quote][b]

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icrashrc

Joined: 16 Mar 2006
Posts: 247
Location: Mishawaka, In

Posted: Sun Sep 14, 2008 11:19 pm Post subject: Re: resonance

If anyone wants to use a set of mercury gauges to check their carb sync I'll be happy to bring them along to the Homecoming. Please let me know off list (at) icrashrc at aol dot com.

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Scott

www.ill-EagleAviation.com

do not archive

Dave Bigelow

Joined: 29 Jan 2006
Posts: 103
Location: Kamuela, Hawaii

Posted: Mon Sep 15, 2008 1:59 am Post subject: Re: resonance

Ted,



This is probably a longshot, but I had some noise issues somewhat like yours with my Firestar.  I thought it was related to the Rotax 503, but when I changed to the HKS engine, I still had the noise.  Turned out to be the cables at the tail.  The airflow of either the propwash (on the ground) or the airstream in flight caused them to vibrate (strum).  I tightened them some, and the strumming went away.



Hope this is the problem, since the solution is simple and cheap.

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Dave Bigelow
Kamuela, Hawaii
FS2, HKS 700E

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