nose strut question

[kath]

Hi, all,

I'm a renter with a question about nosewheels. The other day I rented a 172SP which had just come out of an engine overhaul. On the ground, it seemed to sit "tail-low" compared to what I was accostomed to seeing. The nose strut seemed more extended that I thought was normal. Not an obvious thing, just a vague "is this normal?" kind of feeling.

As soon as I started the engine, the nose came down to what I would call a "normal-feeling" posture. Taxiing, landing, and in-flight behavior seemed totally normal. After one landing, I observed some nosewheel shimmy when slowing down.
I made a few separate flights that day--each time, it looked tail-low when sitting on the ground, but jolted itself normal as soon as the engine was started.

Back at the FBO, I asked a CFI about the behavior, and whether the tail-low thing and the shimmy might be related... but he didn't know anything about nosewheels either.

Any clues?

--Kath

 

[Eamon]

The strut is adjustable with air pressure. Most 172's should have about three fingers worth of chrome showing while at rest. This should have nothing to do with a shimmy even if it is too high or too low.

Most 100 series Cessnas will shimmy from time to time. I have found that in the two I owned, If the front tire is too full with air, it will shimmy. If the shimmy dampner is low on fluid, it will shimmy. If the front tire is out of balance, it will shimmy. My C-150 used to do it bad when at normal tire pressure, so I would run it a little low & it would be fine.

 

[Stache]

Just a recommedation, when you preform your preflight grab the prop and pull down. The strut should come down to a normal position of about 2-inches of chrome showing or 3-fingers. This is normal. As and aircraft sits on the ramp and someone pushes down on the tail section the strut will extend due to the pressure in the strut.

Some shops will over over service the strut improperly. To service a strut all the pressure is let out. The strut is surviced up with the proper oil MIL-5606 in most cases. Then air pressure is added to raise the strut to the proper height (chrome showing) or pressure. The maintenance manual will explain the proper about of chrome to show or required pressure to add.

The shimmy is caused by several factors as mentioned above. In addition if the spacers (washers) used to adjust the tolerance in the nose scissor is off it will shimmy regardless what you do. If the tire pressure is normal, tire balanced, and no flat spots
on the tire it should not shimmy. If it does the maintenance manual will explain what to check for.

I would suggest writing it up as (nose tire has shimmy at xx knots) and let the mechanic trouble shoot it and provide a return to service after maintenance.

Stache

 

[wsuffa]

Kath,

Frequently the issue you saw was either a sticking strut or an improperly serviced strut. It can result in shimmy.

Been a long time since I flew 172's, so YMMV....

 

[BillG]

Actually I believe the preferred gas is nitrogen, not air, which isn't as good (can have moisture in it.)

 

[Paul Allen]

Lord Mounts makes a dry shimmy dampener for the 172. I had a 1998 172 that had the original fluid type from the factory. I had trouble with it. I had it rebuilt and/or adjusted a few times. I replaced it with the dry type and never had another problem. I was told (by Cessna) that a few years ago Cessna started using the Lord dampener because of the problems with the fluid type.