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resonance 912
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Steven Green

Joined: 05 Feb 2006
Posts: 118

Posted: Sun Sep 14, 2008 9:26 am Post subject: resonance 912

Ted,



Is the noise cyclicle in amplitude?  If so there may be two fundamental 

frequencies creating a "beat".  Is the wing fabric noticable tighter on one 

wing than on the other, in the prop area?



If I remember correctly you said that the noise was independant of engine 

speed.  What about air speed, does it change with air speed?  Does it do it 

in a high speed decent with the engine at idle.



Did you say it would do it during a static run up?



Steven Green

Hope to see ya'll in London soon

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_________________
Steven Green

tc1917(at)bellsouth.net
Guest

Posted: Mon Sep 15, 2008 2:08 am Post subject: resonance 912

Okay, the easy answer first.  Yes, first time out the gate, at Miss Lite, a 

year ago October, I suspected that was it.  Was told by Ronny Smith that it 

was not the carbs, it needed the expensive four inch spacer (which I had but 

not installed).  I installed it.  Same-O.  Later, purchased vacuum gauges 

and went after the carbs.  Set them and checked them -- four times in so 

many months.  No joy.  Took off the four inch spacer and bought another two 

inch spacer (needed different bolts of course) still no joy.  balanced 

better for flying though and left it on.  Thought maybe it was noise from 

the prop hub and bought another expensive play pretty spinner.  Looks good. 

No workie.

Now, it makes the noise on run up from about 3500 on, on the ground!!!! 

Makes no difference how fast I am flying, it is there.  I have it on decent 

for landing, feathered back, just not as loud, same beat though.  Taking 

off, full throttle, I dont notice it cause of all the extra noise but as 

soon as I pull back -- it says "I'm baaaack!".

On to the fabric.  Although it maybe coming from there, why would it not 

change in beat?  I have taped up the wing area approximately where it should 

be on both wings, no change in sound.  I padded the back, sides, top inside 

and around, no change, just maybe not as loud to me in the cabin.  It is not 

a thumping sound.  It is a metallic sound to me as gears meshing or washers 

running.  I am so convinced it is in the gear box, I will PAY someone to 

check it (must be a qualified rotax mech) and then if it is in there, I can 

get it on warrantee.  Just wait and hear it yourself next week.  Going to be 

an expensive run from here to there but I just hope I will be able to get 

gas along the way.  See ya there.  Ted Cowan, Alabama, Slingshot 912UL

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slyck(at)frontiernet.net
Guest

Posted: Mon Sep 15, 2008 6:01 am Post subject: resonance 912

Ted, this is a long shot, but are your cylinder compressions equal?

BB

On 15, Sep 2008, at 6:07 AM, Ted Cowan wrote:



 	  Quote: 	
		   



 Okay, the easy answer first.  Yes, first time out the gate, at Miss  

 Lite, a year ago October, I suspected that was it.  Was told by  

 Ronny Smith that it was not the carbs, it needed the expensive four  

 inch spacer (which I had but not installed).  I installed it.  Same- 

 O.  Later, purchased vacuum gauges and went after the carbs.  Set  

 them and checked them -- four times in so many months.  No joy.   

 Took off the four inch spacer and bought another two inch spacer  

 (needed different bolts of course) still no joy.  balanced better  

 for flying though and left it on.  Thought maybe it was noise from  

 the prop hub and bought another expensive play pretty spinner.   

 Looks good. No workie.

 Now, it makes the noise on run up from about 3500 on, on the  

 ground!!!! Makes no difference how fast I am flying, it is there.   

 I have it on decent for landing, feathered back, just not as loud,  

 same beat though.  Taking off, full throttle, I dont notice it  

 cause of all the extra noise but as soon as I pull back -- it says  

 "I'm baaaack!".

 On to the fabric.  Although it maybe coming from there, why would  

 it not change in beat?  I have taped up the wing area approximately  

 where it should be on both wings, no change in sound.  I padded the  

 back, sides, top inside and around, no change, just maybe not as  

 loud to me in the cabin.  It is not a thumping sound.  It is a  

 metallic sound to me as gears meshing or washers running.  I am so  

 convinced it is in the gear box, I will PAY someone to check it  

 (must be a qualified rotax mech) and then if it is in there, I can  

 get it on warrantee.  Just wait and hear it yourself next week.   

 Going to be an expensive run from here to there but I just hope I  

 will be able to get gas along the way.  See ya there.  Ted Cowan,  

 Alabama, Slingshot 912UL

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Dana

Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA

Posted: Mon Sep 15, 2008 8:38 am Post subject: resonance 912

One bit of useful data to solve the resonance problem is to know the 

frequencies.  When you hear a "beat frequency" resonance, like a twin 

engine airplane with the engines out of synch, the beat frequency is equal 

to the difference in the primary frequencies.  For example, if one engine 

is turning 3000 rpm and the other is at 3060 rpm, that's a 60 rpm 

difference or 1 cycle per second, i.e. the beat frequency would have a 1 

second period.



Timing the beat period and comparing it to the other various speeds (engine 

rpm, exhaust pulse frequency, reduction ratio, etc.) may lead to a match 

which points to the source.



-Dana

--

  He's dead, Jim.  Grab his tricorder and his wallet.

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