engine RUMBLE, Oil pressure fluctuating

Hi Walt -

In 15 year and two aircraft, I have never balanced a small aircraft wheel in any way and never had a problem.

I have however had a nose wheel rotating in flight. Took a while to find the source of the "rumble".  New bearings, couple of nice shims and a "snick" up of the castellated nut, to slight drag tension, cured that mystery - nothing to do with balance.

My last aircraft always had intermittent in flight rotation of the main wheels (rumble) - nothing to do with balance - just learnt to put the brake on on gently to provide just enough drag to stop in flight rotation but not enough to lock the wheels up, if I forgot to take the brake off before landing.

I see this front end shake quite often. In all my year I have never found an aircraft tire in balance. I balance all tires. I use the 1/4 oz. stick on weights. Never had one come off when installed properly. usually tire seem to take 3 - 8 weights. I have seen some tires take up to 20+ weights. Those tires get sent back. I just had an RV12 come in with a server front end shake. Everyone thought it was either bad engine mounts, gearbox or an engine issue. The owner shot a video of the front shake and I knew then it was the front wheel out of balance. I found this  tire was so far out of balance I had to use 18 weights. I advised the owner to get a new tire. The new one took four weights and he's never had a front end shake since. I find main tires can be way out of balance too so they are always balanced. Then all the vibration goes away. I've been on flights with owners in their plane that ask me what I did to their plane after an annual. I ask why. They stated it always shook. Now it doesn't. I balanced the tires.

Moral of the story is balance all aircraft tires. Some are terrible. 

I use a Marc Parnes wheel balancer. It's quick, easy and accurate.

http://www.marcparnes.com/Universal_Motorcycle_Wheel_Balancer.htm

I have the caster friction set at the mfg recomendation so its not spinning on that axis. To test the balance I spin the wheel to 50 mph rotation speed and let it decay, if the nose strut vibrates douring this test I add the appropriate weight until it is smooth. Mains are easy to stop in flight with the brakes.

This vibration / rumble frequency follows the engine RPM not the airspeed, angle of attack,r flight direction, or control surface deflection does not change it. 

You have not told us what the aircraft is. Metal skinned aircraft can develop all sort of resonance noises.

If not wheels rotating in flight - go with Rotax Wizards suggestion.

Sean Griffin wrote:

You have not told us what the aircraft is. Metal skinned aircraft can develop all sort of resonance noises.

If not wheels rotating in flight - go with Rotax Wizards suggestion.

First sentence in the first post, “I have a 912ULS in a RV-12”

I replied to what Rotax Wizard suggested on the gearbox in the very next post.