Vibration after lift off

[Mike Howard]

I have developed a strange engine vibration that occurs approximately 3 to 5 seconds after I lift off while at full throttle. The vibration started just after I replaced my nose gear leg and fork. I do not believe that it is related to an out of balance nose wheel because it does not happen until more that 5 seconds after the nose wheel lifts off.
Here is the strange part the vibration goes away completely if I pull the throttle back to about 50% for a second or two and then push it back in for full throttle. The vibration does not occur for the remainder of the flight for any throttle position.
I have checked the lower engine mounts where the new gear leg clamps to the firewall. I thought that the nuts might have bottomed on the bolt so I added a washer under the nut. The problem seems a little less but it is still there.
Anny ideas?

[BlndRvtr]

Could it be a harmonic set up by the new NG leg, fork and wheel assembly? More specifically, is it related to nose wheel tire rotation. As the tire stops spinning, the harmonic subsides? A camera aimed at the nose gear could be revealing.

George

[kritsher]

I'd check the breakaway force on the nose wheel.

[bobg56]

You probably do this put I'll suggest it anyway, my RV12 has vibration after lift off but its the main wheels spinning, I apply the brakes and it stops.

[cajunwings]

Sounds like the tire/wheel balance and it’s worse when the weight is off and it’s spinning down. Make a takeoff, don’t touch anything and see if it stops in the same time frame. Tire balancing might be needed.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

[Dgamble]

I get this too but it seems to occur more often after a touch & go than a normal takeoff. The only difference between the two is that the nosewheel is probably turning faster after the touch & go than it is after a normal takeoff.

My normal takeoff is stick full back until the nosewheel lifts, then just hold that attitude until it flies itself off the runway. That keeps the speed down for the nosewheel.

I bought the motorcycle wheel balancer from Harbor Freight and did find the nosewheel to be out of balance. I just live with it now because I don’t trust modern wheel weights that are held on (barely) with double-sided tape, and they don’t seem to be made out of lead anymore, so you have to use more of them.

[Mike Howard]

Follow up information on strange vibration.
I performed a test take off today with the stick full back. The nose lifted off very quickly with little or no nose wheel spin up. [b]No vibration, problem solved? maybe?[b]
This still doesn't explain why the vibration stops immediately after I reduce the throttle with a normal take off technique. Throttle position shouldn't have any impact on vibration caused by an unbalanced tire.
Also why did the vibration only start after I installed the new nose gear leg and fork? I didn't change the tire or wheel and it wasn't a problem before I installed the new pars.

[Mike S]

Originally Posted by Mike Howard

Also why did the vibration only start after I installed the new nose gear leg and fork? I didn't change the tire or wheel and it wasn't a problem before I installed the new pars.

This is a new "system" and probably has different vibration frequency.

Professor Horton could explain it most likely.

[rk2436]

I bit the bullet and bought some Michelin Pilot tires. I'm done with those cheap POS Aero Classics. No more vibrations and they make the nicest chirp too!!

[RFSchaller]

I bought a motorcycle tire balance rig from Harbor Freight. It’s a rod with two conical pieces that when pushed together clamp the wheel on either side at the bore for the axle. The rod then acts like an axle and rests on two bearings so the heavy spot rotates to the bottom, and you can add weights until the wheel doesn’t rotate due to a heavy spot. I’ve found that you can balance really bad tires even on the aircraft axles and get a marked improvement.