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CAUTION: SHIMMY CAN FAIL NOSE GEAR FORK
(From CP30, Page 4, October, 1981)
Many of us operating EZs are lax
on checking the friction of the shimmy damper during preflight. This is EZ to do
since we do not see the nose gear when parked nose down. Always check for the 2 to 4
lb. damper friction on preflight. If the damper is free, the gear can shimmy at high
speed and fail the fork within 1/4 second. Further, the failed wheel can strike and
destroy your prop.
Nose wheel rigged at the proper angle and having at least 2-lbs. friction damping cannot shimmy. Some airplanes have had a bent NGI7 that bends under load. Then, the owner backs off on the friction adjustment to allow good taxi qualities. Then with little or no load (rebound) at high speed it can and shimmy. If your gear pivot binds, making taxi turns difficult, check your NG 17 for evidence of bending, or ovalizing. Some time ago we increased the wall thickness on the Long-EZ NG 17 parts sold by Brock to handle the heavier loads. If your NG 17 is not perfectly straight, replace it with a steel tube of at least .125" wall.
One of the reasons that the shimmy damper can easily get out of adjustment is that to get the proper force, the spring is coil-bound or nearly coil-bound. Thus, If a little wear or a slight bolt back-off occurs, the damping action is lost. To solve this, Brock is now having made a supply of springs with a heavier (.083 dia.) wire. New orders filled after October will have the heavy spring. Also, as soon as they are received (mid November), Brock will be sending the heavy spring to all who have bought the nose gear assembly. We have tested the heavy spring on N26MS and have confirmed that the adjustment bolt can be backed off a full half turn before losing adequate damping friction. With the old spring, a 1/8 turn would result in inadequate friction.
The shimmy failures have resulted in the rumor that the nose gear fork is not strong enough. This is not true. The failures were due to high-speed shimmy, not overload. Our nose-gears have been extensively tested to in excess of design ultimate loads (CP #18, page 4) and during punishing development tests of the rough-field capability (CP #25 page 3). Also, the exact same assembly is currently operating at higher weights (2100 lbs) and speeds (90 Kts) in two jet aircraft, the NASA AD-i and Model 73 NGT, without problems of any kind. Of course, the friction is checked during each preflight.
We strongly recommend that each Long-EZ and VariEze use CP #25 spring shock in the nose gear system. This greatly relieves the shock loads experienced when encountering ruts, chuck holes etc.