Front wheel balance

[tecnamman]

I know that most Tecnams in Aus are high wing models and therefore this may only be something peculiar to the low wing Sierra-please let me know if it occurs with your high winger.

 

When approaching 40 knots or thereabouts on the t/o run a fairly violent wheel shimmy may develop, particularly in circumstances where a 10 knot or greater quartering crosswind from the left is blowing ( more right rudder than usual required).

 

I wanted to know what was causing it - and see if something could be done to either fix it or reduce it.

 

I won't bore you with all the details - suffice to say that I discovered the front wheel was way out of balance....in fact I took the front end off and spun up the Sava tyre by holding it to the spinning rear wheel of my upside turned bike!

 

Suffice to say it behaved like a drunken gyro....'all over the shop'.

 

I then did a static balance just like you would if balancing a motor bike wheel etc.

 

It was way out.....

 

So I balanced it best I could with mag wheel weights ( couldnt find an electronic balance machine with a spigot small enough).

 

It now seems a lot better, although I still get a little from time to time ..but not as severe.

 

Has anyone tried this fix and if so, did it have any effect?

 

I know some may say it needs a shimmy dampener.......that may be true, but not possible.

 

I would be interested to know if anyone has a different theory about this issue.

 

Regards

 

 

[Guest pelorus32]

G'day Dave,

 

the high wing Tecnams get a bit of a shimmy - but it is most noticeable after takeoff, during inital climb. I think what probably happens is that there is a certain speed at which it is bad and as it slows through that speed you feel it throughout the a/c.

 

This is eased a bit by getting the nosewheel just off the deck fairly early and letting the a/c fly off. Course in a xwind you may not want to do that!

 

Hope you are well.

 

Regards

 

Mike

 

 

[facthunter]

Shimmy and balance.

 

I think that a static balance would be OK on a small nosewheel, but of course dynamic is better. I would be surprised that you would NOT get Shimmy with a castoring nosewheel and no damping. It would be a good idea to ensure that there was some friction in the pivot at least, and it might be susceptible to low tyre pressures. To me it's a deficient design. Nev..

 

 

[tecnamman]

Hi facthunter

 

The Tecnam has a steerable nose wheel via direct rod linkeages to the rudder pedals...it's not castoring like Grumman American etc...so when you get it,you really feel it in the feet!

 

I also tried increasing tire pressure from the recommended 11 psi to 14 psi ( I didn't mention that originally)....this may have helped although I didn't know low pressure could cause or exacerbate nose wheel shimmy.

 

The recommended pressure of 11psi for the nosewheel and 14 psi for the mains in the POH does seem low to me, but thats what Tecnam say!

 

Cheers

 

 

[Guest bateo]

I agree with Pelorus with the high wings, the echo gets a light shimmer when taking off. I have noticed this on some take-off's at initially the same speed, and same climbing angle.

 

 

[facthunter]

Shimmy.

 

Are the linkages quite direct, because I would have thought that would stop it. If there is any lost motion the possibility increases. I got violent shimmy on landing in the Champion once (with a Scott tailwheel which is supposed to be the best). Landing on tarmac. I thought the tyre was flat, thats what it felt like. It removed most of the tread in about 500 metres. It is a cause for concern as it puts big loads on things. Regards Nev..

 

 

[facthunter]

Another look.

 

Tecnamman, I had another look yesterday. There appears to be a lot of trail in the nosewheel set up. This would give a strong castering action. and fairly high feed-back forces through the pedals when taxying or rolling in crosswinds. About all you can do is make sure that the tyre is running true. (that the bead has come out evenly when first inflated) and that the balance is good. and that there is no excessive play in the linkage. ..Nev...

 

 

[tecnamman]

Hi Facthunter

 

Follow this link www.cessna.org/public/sampletech.pdf

 

It says the first thing to check is wheel balance when you have nose wheel shimmy.....although design geometry etc as you suggest probably has a contributing input as well.

 

I've located a motorcycle shop with a dynamic balancer...I described the set up and they seem to think it's possible to mount it up.....will let you know the outcome next week if it can be done.

 

There is no free play in the steering linkeages or pivot points, but some side 'flex' is noticeable with application of alternate side loading.

 

At the end of the day it could be the design geometry and nothing can be done...hopefully an aeronautical engineering guru [who designs landing gears] will stumble into the forum and support my 'theory' or shoot me down in flames!!

 

Cheers for now