Nose wheel shimmy

[a4fly]

We've got a Robin DR400:) and a DR500:) both of which have experienced bad nosewheel shimmy (only once on each aircraft), so bad that the spats were damaged on both occasions.Both times were on tarmac. We're landing them at the correct speed.The group that we run has got thousands of hours of experience on all types of aeroplane and none of us have encountered anything this bad before. Anybody else experienced this on other Robin types?

[Rod1]

The Robin has a clever system for centring the nose leg in flight. The POH specifies a procedure during landing to reengage the nose gear steering, if you do not do this you can get shimmy. It is some years since I owned a DR400 so I will not try to remember the full text, but if you are not following it you will have a shimmy problem!

Rod1

[BackPacker]

I don't know if it's actually related to shimmy, but the DR400 indeed does have a nosewheel lock which automatically engages as soon as the pressure on the nosewheel is less than a certain amount. I don't know the exact amount but to give you an idea: if you're pushing an empty DR400 backwards by the prop, on about half the occasions the nosewheel locks. You then need quite some downforce on the prop - I estimate some 30 kilos - to disengage the lock.

It's something you have to be very aware of when landing. Particularly with an aft CofG and a well-flared landing the nosewheel lock may not disengage altogether.

The solution is to positively apply the brakes as soon as the nosewheel is on the ground. Even if you have plenty runway ahead of you with the first turnoff way down the end, it's a good idea to brake heavily for an instant or two, to make sure the lock is not engaged.

[A and C]

This problem is usualy due to three things.

1 Incorrect tyre pressure (unlikely as the single cause but it contributes)

2 Incorrect nose leg oil quantity and air pressure (both must be correct)

3 Incorect adjustment of the steering head bearings.....this is the biggest contributor to the DR400 shimmy problem.

This assumes that there is no play in the torque links, usualy these are OK unless the aircraft is doing lots of landings (glider tugs)

A contributor to this was the very poor english translation of the aircraft maintenance manual that had all the data scattered over a number of chapters. Fortunaly Robin published a much better manual two or three years back that has all the data in one place and is writen in clear english.

Backpacker

The nose wheel centering cam lock should only engage when the nose gear shock strut is (allmost) fully extended, if it was engaging on the ground it indicates that the nose leg had been over inflated. The most likely reason for this is to give the correct nose gear extention without checking the oil level is correct. If the oil level is low this will effect the "air spring rate rise" and result in the nose leg being too soft at high loads (towards full compression) and too hard at low loads (towards full extention). It is pointless to try to inflate the Robin nose leg without first checking the oil level in the leg but this requires jacking up the aircraft and it is far quicker to over pressure the leg with a high pressure rig and send the customer away.