As Peter Hebert said and I mentioned in an email a week and a half ago, this seems to me an issue of resonance of the gear leg.

The reason the changes that you'd make to your car don't fix your airplane is that your car likely has no resonant modes near 1/rev frequencies of your tires, so vibes are due to strong out of balance.  Any modes that do exist are also probably strongly damped.  If your airplane has a resonant frequency near the rotational frequency of your tires or some other driver in or near your gear then you hardly require any out-of-balance to excite the mode.  Perfect balance will yield no problems, but any (even small) input will get it going.

As I mentioned earlier, a rap test (hit it with a force sensing hammer and measure the vibration response) could illuminate modal frequencies and damping.  Since you're seeing similar speeds, my guess is that there's a mode at a frequency corresponding to the rotational speed of the tire at that ground speed.  Rap testing would also allow one to tune to a more desirable frequency.  After November I'd be willing to help if someone wants to test their plane.

 Cheers,
- Kyrilian


Jim Scales <joscales98@hotmail.com> wrote:

<snip>

-Slight but definitely noticeable shake at steady taxi speed of 12 knots.  One or two knots either side of that number and the taxi is smooth.

 

-High speed taxi tests were conducted by accelerating to 50-60 knots, reducing power and braking as in a normal landing and rollout.   In every test the shake showed up at around 35 knots and went away at about 30 knots when decelerating.  The degree of the shake was directly related to brake force.  Harder braking, harder shake.  Light braking, light shake.  No braking, very little or no shake.  Shake lasts

<snip>

If this were a car and exhibited these symptoms I would suspect any or all of the following issues:

-Out of round tire, flat spotted or faulty ply.

-Severe out of balance tire/wheel assembly.

-Warped or wavy brake disc.

-Worn or defective suspension part.

<snip>

instances the tires were statically balanced.  In all instances, when the shake occurred,

it was at the about the same speeds.  As I write this, though, I seem to have the impression that when I installed new tires the shake would diminish somewhat.  The speeds would remain the same but the severity of the shake seemed to be less.  I don't really have any specific memories to support this.

 

<snip>

same conditions as every other set.  This makes me think it might be an airframe related problem with harmonics and interaction of components, etc.  I wouldn't even begin to know how to chase that down.

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