Shimmy and nose wheel offset

[DrZoos]

Just wondering who has nosewheel set dead straight when rudder is straight. Mine has visible nosewheel pointed to the left . Supposedly to reduce shimmy. ..??

 

Problem is its too much so turns crap to the right and requireshuge rudder input on takeoff,

 

So my question is howmuch to take out. I think you need some to offset thrust, but how much.

 

Whats your thoughts

 

 

[Head in the clouds]

Directional offset is a pretty sad way to go about fixing the problem. I presume the intention is to keep a constant load on one side of the tyre i.e. keep it constantly scrubbing.

 

Nosewheel and tailwheel geometry is a poorly understood thing and even the big manufacturers have sometimes used belt and braces approaches to preventing shimmy under some conditions, even Cessna stuck with nosewheel steering dampers because they can shimmy catastrophically when the strut is compressed and a few other factors aren't ideal in combination.

 

I'm assuming that you don't at present have shimmy problems but that the recommended offset is an indication that the aircraft type does have the problem? There is extensive internet discussion on the subject in general, if you PM me your email I can send you some links.

 

In your case it appears that you might like to set your wheel straight(er) and still avoid shimmy problems (note that bad shimmy can rip a noseleg off in seconds and if you're really unlucky it'll go through your prop as well, there are a couple of Youtube videos of that happening). If adjusting the steering setting is your plan the following are things that you might want to check and rectify if necessary, which will help to reduce/delay the onset of shimmy -

 

The main culprit, other than original poor design, is a soft strut (for those that have air/oleo struts), note that shimmy generally gets worse as the steering axis tilts back so as the strut (or spring leg) compresses (or flexes) the steering axis tilts further back and increases the shimmy tendency. Correct strut pressure will keep the strut longer while under load and reduce shimmy tendency. Alternatively if it has a tubular spring noseleg check that it isn't bent and correct if necessary.

 

Have the wheel and tyre dynamically balanced - this is hardly ever done on aircraft wheels and even when it is they go out of balance again quite quickly due to scrubbing and flat spotting, even the nosewheel can get small flat spots from firm landings. Also some people rotate their wheels from mains to nosewheel and the mains can get very out of balance so if you then put that tyre on the nose it can be quite nasty. Unbalanced wheels make a big contribution towards shimmy onset, so even keeping them clean of accumulated dirt inside the rim can help.

 

Check the tyre for roundness, flat spots, bulges and tyres that were originally moulded poorly all add to shimmy onset.

 

As far as your question of how much to take out etc, I can't see why you need any offset to counter thrust since you simply use the pedals as required to keep on the centreline. If it was mine I'd set the rudder and nosewheel straight when the pedals are centred and address any shimmy problems separately. However that's just how I would go about it, with due consideration for anything the manufacturer says about it of course, but I'd give scant regard for offsetting the steering to prevent shimmy ... for one thing it means that you'd be flying around with the rudder straight and the nosewheel turned and the nosewheel acts like a front rudder, so wherever you have the pedals there will be some form of rudder input at all times which would have to be cancelled out by some input of aileron which means you'd always be slipping or skidding, not a very efficient way to get around.

 

I don't think the offset steering really would do anything to prevent shimmy anyway since the worst and most likely situation for nosewheel shimmy would be under braking after landing and at that time you would have the nosewheel pointing straight down the runway (hopefully) regardless of rudder position, so the offset wouldn't be doing anything because the wheel wouldn't be scrubbing. Some minor shimmy can take place when the nosewheel is touching the ground very lightly during take-off acceleration and perhaps that is where a little scrub could prevent it but in that situation it would probably be easier to just lift the nosewheel off the ground.

 

Disclaimer - I'm not saying you should do it how I would go about it.

 

 

[DrZoos]

Hi thanks so much for the detailed responce , i just got home after a few long days so will re read ... But wow great info thanks