Nose Wheel Vibration Solved

[Sven]

Like many other Mooney drivers, we’ve been experiencing some problems with nose wheel instability and understood that this is just the way the Mooney works. Yesterday I found out this is absolutely not true and found out the reason why from the folks who discovered the problem in 1977.

My partners and I own a very rare fixed-gear Mooney M20D, which is basically an M20C. We’ve owned it for a little over two years and have been working closely with our very kind A&P to address 15 years of neglect in a harsh outdoor coastal environment.  Each year we tackle a big new project based on the triage skills of our mechanic. This year we tackled the gear.
It turns out our shock disks had not been changed in 20 years. So we took the gear off and decided to sandblast all the rust and grime and put on a new coat of paint. Yesterday, something happened I place in the “guardian angel” category. The first paint job didn’t work (bad paint). So I stripped the paint off and started over. I reached for the small collar that holds the shock disks in tension at the top of the stack. I was about to rub off a little rust with a brass brush but could not find it. So, I reached for the steel wool. As I rubbed the top rim of the collar a strand of wool got wedged in a very small crack that before this, went completely unnoticed. I grabbed my reading glasses and discovered this crack went through the entire collar. As I held it up I could also see the collar was bent. This part was destined to be placed back on the plane. There’s no way I could do that so I called my friends at LASAR and explained my situation. And here’s where the good stuff begins

I talked with Dan who is the “know everything” guy. During our conversation he asked me if we’ve been having problems with taxiing, take offs and landings. You betcha we have. In fact, a couple months ago the plane shot left and blew a tire on touchdown. I almost went into the rhubarb and took out a runway light. I couldn’t explain what happened. It felt like it was in a rut and no matter what I did the plane was going left. He told me the problem was that collar.

Back in the mid-late 70’s LASAR was getting complaints about nose wheel instability. They discovered that the collar was not tall enough. This created a situation where the centerline of the nose wheel was aft of the datum. Basically, it’s trying to push a backward shopping cart wheel forward. Impossible, right? The nose wheel wants to make a 180° turn to be stable. That’s the shimmy we’ve been experiencing taxiing and the source of the squirrely take offs and landings. The fact that our shock disks were outdated made the situation even worse. The straighter the nose gear stands, the more forward of the datum the nose wheel is. I was carrying 4 adults which placed my CG further aft making it very bad. That’s three strikes against me

LASAR was reporting these findings to Mooney but they weren’t listening. Then he said there were three accidents that got the FAA’s attention. Mooney listened and the FAA issued service bulletin #202. LASAR fixed the problem by putting special washers under the collar which caused the nose wheel to move aft of the centerline where it belonged.

Dan told me he suspects that when I had the blowout the nose wheel was cocked to the left and when it touched, it was so far left that it shot me over. I was even giving it a slight amount of right rudder before touchdown due to a small crosswind. He said if I had not caught that crack on the collar, I was headed for certain disaster. The new collar places the bolt center hole a full 1/8-inch higher. He said that a variance as small as 1/32 can cause the shimmy.

So, thanks to bad paint, steel wool, reading glasses and Dan at LASAR, we will have a safer Mooney. I know this explanation was long but it helped me to know the engineering behind the problem. Pushing a pivoting wheel backward is a recipe for disaster and I narrowly avoided that scenario.The only way you’re going to see that collar is to take off that collar. You can do it without the tool by jacking up the plane and putting the weight on the nose. You’ll need to rent the tool to put it back on, however.

I hope this helps any fellow Money drivers out there with similar issues. I really hope this helps someone else avoid an accident. If our runway wasn’t 150 ft wide, those fixed-gear Moneys would be even more rare.
 
-Sven

[conom06d]

very interesting. Looks like i will have to get a hold of dan to see if this is the same issue i have.

[crxcte]

Sven, so from the picture you posted, B would of been the correct.  Picture B then add a washer or 2 under it.

[Sven]

Thanks, Chris. The collar LASAR is sending me will be longer at the bottom. I've still got to repaint the gear so it willbe a couple weeks before I can be assured this whole thing worked. I guess the big trick is to scrunch those disks tightly to avoid the nose gear standing up straight.

[Alan Fox]

Check my post from two weeks ago to check the collar ...

[Wakeup]

Wow!! Mine is doing all of the above. On take off I have to put a ton of right rudder in to keep it straight. I understand all the reasons for needing more right rudder but I keep telling my FI how excessive it is. Plus... About every other landing I have nose wheel shimmy. I will get this all looked at next week by Cole Aviation. Thanks. Troy

[rbridges]

Wow!! Mine is doing all of the above. On take off I have to put a ton of right rudder in to keep it straight. I understand all the reasons for needing more right rudder but I keep telling my FI how excessive it is. Plus... About every other landing I have nose wheel shimmy. I will get this all looked at next week by Cole Aviation. Thanks. Troy

 

I spoke with Dan at LASAR and just ordered a rebuilt steering horn.  Should be here next week and installed shortly afterwards.  

[chrisk]

Since it seems some folks still haven't reads this:  http://www.donmaxwell.com/publications/MAPA_TEXT/M20-202%20-%20Eight-Second%20Ride/EIGHT_SECOND_RIDE.HTM

 

I also experienced this when I first flew my Mooney.  I landed, with the instructor in the right seat, and thought to myself, he must have had his foot on the rudder pedals.  After a few more landings, I knew what it was.  I remembered this article and knew "darting" the thing to tell Dugosh when I dropped it off.  They had it fixed in under an hour. The plane lands great now.