Nosewheel shimmy -- revisited..
If you let the breakout force get low the nose gear is very prone to oscillation that is so violent you will think the nose tire has blown out and your thoughts turn to the next level in the landing scenario - limiting damage. The oscillations do not stop until the airplane completely stops moving. The washers are not common flat washers and stacking unknown shim washers in nosegear assembly to line up the hole in the nose strut with the exact notch in the nut that will give you the precise breakout force as measured with an uncalibrated fish scale, pulling at an angle that you hope is normal to the strut tire plane at an approximate distance from the pivot point is just silly. Going over the target breakout force is not an operational problem beyond the possible increase in brake wear when you need to turn and start moving from a standing still situation as mentioned by Scott McDaniels. I have had mine up well over 23 lbs specified for my RV-6A and it is not a problem. As scott says it gets lower with use in the early life of operations. I set mine as close as I could get it in the initial build and the initial flights were fine. The first time it went into the oscillation I had no knowledge about the "settling in" breakout force reduction that is characteristic of this assembly. When I went through rechecking everything I found the and found the breakout force was down in the mid teens. I checked it every so often after that and at every annual condition inspection. A couple more times during first 100 hours or so I had to tighten the nut. Since then as Scott says, the breakout force has held steady.
I have an RV6A. On my very first landing in phase one; I got the shimmy as I let the nose wheel down too soon. Since that time, I've kept it light on the nose during the rollout process. Except for some rare (just a few) minor shimmy at certain taxi speeds after landing.............everything has been fine for 100 hrs.
In that last month, I noticed that our airport runway has been getting bumpier. Probably due to so many hot days. The RV6A sit's tail low compared to the newer 7s & 9s. This puts it closer to a flying attitude as it rolls down the runway.
By keeping the nose light on landing, and hitting the bumps, it wants to make the nosewheel pop a few inches into the air. This is causing a problem. It's now turning into an oscillation shimmy that won't stop until the airplane is completely slowed down. This has happend twice; while touch and goes at another airport in between were with no problem at all. A couple of guys in a Cessna reported that it was flipping from stop to stop.
The breakout force (not the nut torque) is still at 24 lbs, which is slightly higher than the recommended 22 lbs., using the fish scale. It's stayed at that for quite a while, as more hours were put on the plane.
I see that Bob keeps his above that recommendation. I think that's what I'll do; but would like to know what others have done. BTW --- I immediatly went for another landing on this bumpy runway; but planted the nose wheel as soon as it touched down. And all was fine. I'll just have to gauge the runway, and apply stick pressures as needed............ I suppose.
I still prefer to keep the nosewheel light.
As of now, the gear leg is the updated style. The fork is the old style with the Cleveland axle, which has a nice easy spin with no play. Air is at 36 lbs.
L.Adamson --- RV6A
