Tom Gardnier wrote:

Folks

I am dealing with a vexing problem which has plagued me throughout my phase 1 testing and that is, I can't get rid of vibration from my engine prop combination.  I am running a Lycoming IO360/Slick Mags (non-counter weighted crank) with a new Hartzell HC-C2YK-1BF/F7666A.

The prop was dynamically balanced on the ground at 23" map and 2400rpm to 0.05ips.  Interesting thing is the vibration increases significantly to 0.25ips at 2500RPM on the ground.  When flying, I can see the ASI needle vibrate a little (+/- 0.5mm) at high power settings >19Inches and
any RPM.    I have not been able to measure the vibration in the air
since it is difficult to mount an accelerometer in the vertical plane under the cowl but I do see the stick vibrate a little from side to side when I let it go.

Some additional background:

1.       The engine has accumulated 44 hours since new

2.       The crank was dynamically balanced at the factory

3.       When the engine was built it was tested with a club and found
to be vibration free

4.       The prop has a 4" extension by Sabre Industries

5.       The balancing weight was mounted to the flywheel in the form of
an AN3 bolt/nut (20 grams)

6.       The spinner is solidly fixed to the prop and back plate and is
centered

7.       The crossover exhaust and tailpipes are fixed to the engine
only

8.       The cylinders are 9:1 compression

9.       All cylinder compressions are even and in the 70s

10.   The engine mount bolts are torqued correctly to 40-45ftlb

11.   The engine does not/cannot touch the cowl

When I tested the aircraft at 17,500 feet, I noted the vibration was significantly less pronounced or non-existent due to the low power output.  I noticed the same thing in cruise decent at 18 inches or lower.  Talking to the prop balancing mechanic, he stated that it is unusual for balance to be ok at 2400 RPM but to be significantly different at 2500RPM+.  

I would be very grateful for any help and assistance from anybody with knowledge or experience in correcting vibration issues such as the one I described above

 
Tom,
Have you checked the prop run out and blade pitch. It's very unlikely that a new prop would have either of these problems, but rather easy to check the run out particularly, and the addition of a prop extender is itself a potential source of run out.

Easy way to do a rather accurate run out check is to remove one sparkplug from each engine, then build a measurement stand from concrete blocks so it extends a couple of inches into the prop arc. Clamp a thin/small piece of wood or similar to the top block and adjust it to just clear the leading edge of the blade. Carefully rotate the prop by hand and measure the clearance between the stick and the leading edge of the other blade(s). You may need to work this a bit to get the farthest forward blade as your reference. The position of the leading edge should not exceed 1/16" from blade to blade.

The angle is rather harder to measure without special tools, but this time mark an equal-radius position on the back of each prop blade as the reference measurement point. Tape a straight edge to the engine cowling or engine so the end extends forward to just behind the prop. Use a protractor and a second straight edge to measure the prop angle WRT the taped straight edge. The angle is not significant as long as they are all within 1 degree. Note that this is not a complete check for a constant speed prop, but a start...

BOB