harmonic between 120 -135 knots

[Dave in Eugene]

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

[Tom]

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

[Joe Person]

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.

[Drew]

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.

[ezeguy440ez]

try deflecting both rudders slightly when you notice the harmonic. if it goes away you have trailing rudders.

[Dave in Eugene]

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

[dorr]

Soft springs in Rudder can cause vibration there.

Check engine mount for cracks, loose engine mount bolts.

[rglos]

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.

[Bill James]

Besides the motor mount, pull on the engine and check for broken extrusions inside the firewall.