Author Topic: harmonic between 120 -135 knots  (Read 6220 times)

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Offline Dave in Eugene

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harmonic between 120 -135 knots
« on: March 19, 2009, 07:34:50 PM »
I have been trying to identify the cause of a harmonic that occurs between 120 & 135 knots in my Long EZ.... it is a "Waaaaaba  waaaaba waaaaaba" sound that comes and goes with change in airspeed.

I noticed it more when I went from a 3" extension to a 8" extension. Yesterday I flew it with the 3" extension and it was much less pronounced...went back to the 8" today and it is much louder and the audible range is greater..

It is not in the controls...nothing shaking on the stick..

I think that it is the sound of the prop as it goes through the disturbed airflow of the a/c.

My goal today was to fly to 5,000 to 7,000'  over the airport and stop the prop and see if I could recreate the sound...the deck was a 2,200 today so no luck on that.

Will report back...would appreciate your thoughts or similar experiences.

Thanks,

Dave
« Last Edit: March 19, 2009, 08:53:43 PM by Dave in Eugene »
408 EZ Long EZ 0235-L2C / Great American 62X62 / IFR / GU canard

Offline Tom

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Re: harmonic between 120 -135 knots
« Reply #1 on: March 20, 2009, 09:12:46 AM »
Hi Dave,

My thoughts are you are having some sort of harmonic in your prop.  Do you have another prop you can try? also, note the RPM of the engine when the harmonic starts, write it down, and then cut the rpm exactly in half at the same airspeed, (pull the power and dive), see if there is still a harmonic (it might be very small).   Whatever you do DO NOT continue to run in the harmonic range unless you are over a large dry lake, or an airport!


-Tom

Offline Joe Person

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Re: harmonic between 120 -135 knots
« Reply #2 on: March 20, 2009, 01:58:43 PM »
If you have the "per-plans" mechanical landing brake (with the parallel springs running along the aft left corner of the front seat bulkhead), loop a tywrap figure-8 style around the springs, mid-length, and cinch them together.  See if this makes this harmonic noise go away.  Not saying this is your source, but if so-equipped, is a possible source.

Joe Person
VariEze N79JN
Cozy #879 Under Construction
EAA Tech Counselor 4418
Bothell, WA (KPAE)

Offline Drew

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Re: harmonic between 120 -135 knots
« Reply #3 on: March 25, 2009, 07:58:34 PM »
Could not exactly tell what sound was being made by your spelling----but I guess much better than I could do. 

There are a couple of other items to check:
1.  See if you have anything vibrating in the cockpit.  Things like the canopy safety piece of metal can make a horendous noise at some rpms as it vibrates on other metal pieces---just bend it a little out of the way---but not so that the canopy can open.

2.  Do you have any tape on the plane that is coming loose?  This can create a lot of racket too.
Drew Swenson
Cozy N171ML

Offline ezeguy440ez

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Re: harmonic between 120 -135 knots
« Reply #4 on: May 26, 2009, 06:42:23 AM »
try deflecting both rudders slightly when you notice the harmonic. if it goes away you have trailing rudders.

Offline Dave in Eugene

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Re: harmonic between 120 -135 knots
« Reply #5 on: May 27, 2009, 09:04:56 PM »

Thanks ~ it was worth a try...no cigar however...I don't have another prop but would like to see if that would change the equation...a about certain that it is the culprit.

Dave
408 EZ Long EZ 0235-L2C / Great American 62X62 / IFR / GU canard

Offline dorr

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Re: harmonic between 120 -135 knots
« Reply #6 on: May 27, 2009, 09:34:39 PM »
Soft springs in Rudder can cause vibration there.

Check engine mount for cracks, loose engine mount bolts.
David A.C.Orr
"Beagle"
www.canardfinder.com

Offline rglos

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Re: harmonic between 120 -135 knots
« Reply #7 on: May 29, 2009, 09:57:47 AM »
Dave

Take off the prop and rotate it 90 degrees & remount. See if the harmonic changes.
Process of elimination. The harmonic is rprobably related to rotation. It changed when you went from a 3" to 8" extension. These are rotating elements.
Long EZ, 0-235L2C, 1986

Offline Bill James

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Re: harmonic between 120 -135 knots
« Reply #8 on: June 03, 2009, 09:13:11 PM »
Besides the motor mount, pull on the engine and check for broken extrusions inside the firewall.
Bill James, Fort Worth VariEze N95BJ
Downdraft Plenums, QuickCowls
There was supposed to be anhedral?
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