REFLECTOR: Nose Gear Shimmy

[Jones Nick]

Is there something about bad brakes in Texas? I will double check my brakes if I ever go to Texas!

Best regards,
Nick

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Robert Wood
Sent: Saturday, March 24, 2012 6:10 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Nose Gear Shimmy

I had a similar problem with brakes in TX several years ago. The pads from a 172 fit, but when they were originally installed the guy drove the rivets too tight, splitting the pads which then departed the aircraft. Luckily landing at Odessa was straight into a 25kn wind so when I realized I had no right brake I was able to let it slow with speed brake and rudders out. At about 20 Kn I went off the runway and slid the left wheel making a full 270 degree turn through short mesquite. The FBO there was great and I had no damage except a flat from a thorn and a green prop from the mesquite.

Next day new pads installed there after advice from Duane Swing. So the advice is, do not split the pads by driving the rivets too tight! Next advice---always try to land into a 25 Kn headwind.
Third advice, always carry a spare set of underwear!

Bob Wood
Looking forward to seeing the V-twin next week!

Sent from my iPad

On Mar 24, 2012, at 5:54 PM, Mel Bina <pusherpilot1 at gmail.com<mailto:pusherpilot1 at gmail.com>> wrote:
Hi Don,
I'm really sorry to hear about your incident.  I hope that you will get the airplane repaired start to enjoy the flying.
Mel
On Sat, Mar 24, 2012 at 12:23 PM, Donald Bates <aeroopt at yahoo.com<mailto:aeroopt at yahoo.com>> wrote:
Hi Mel,

Yes, I joined the Reflector last year. I guess my article was a little too technical for most as no one has commented on it.

We are in Texas now and the airplane is in Placerville. Won't be back until June. Still need to repair the main gear doors and fix the radio problem with the headsets and mic.

Last year February on my way to Texas I landed at Casa Grande to refuel. and got yanked off the runway by bad brakes and hit a ditch braking the nose gear and mangling the mains. The local mechanic pulled me out and rented me hangar space for four months. I spent a  couple weeks there ordering parts and getting started on the repairs, then went to Texas for a month and came back to finish the job. Took two and a half months to get the plane back to basic flying condition to get to Placerville. Steve M. had just signed it off before the trip but was too lazy to pull the wheels and told me to do it and signed it off and left. I thought the brake pads looked thick enough and so went to fly to TX. The mechanic pulled the wheels and found the left one with grooves well into the disk and the right rivet heads riding on the disks (the left had locked and the right did nothing so I couldn't bring it back on the runway).

Was a big job. A friend from L.A. came there for a couple of days to help get the mains remounted and aligned. The mechanic there didn't know anything about composites and didn't have much time to help anyway. He was busy keeping other planes flying.

So, I still have only about 15 or so flying hours and maybe 30 landings. I was just getting comfortable with it now that the shimmy problem is gone. Hope to be getting some good flying time this summer.

Don & Dianne


________________________________
From: Mel Bina <pusherpilot1 at gmail.com<mailto:pusherpilot1 at gmail.com>>
To: Donald Bates <aeroopt at yahoo.com<mailto:aeroopt at yahoo.com>>; Velocity Aircraft Owners and Builders list <reflector at tvbf.org<mailto:reflector at tvbf.org>>
Sent: Friday, March 23, 2012 4:48 PM
Subject: Re: REFLECTOR: Nose Gear Shimmy

Hi Don,
Are you on the Reflector now?  And, how are things in general, that is, are you flying?

Mel
On Mon, Mar 19, 2012 at 4:36 PM, Donald Bates <aeroopt at yahoo.com<mailto:aeroopt at yahoo.com>> wrote:
Velocity Nose Wheel Shimmy Explained

Recent comments have focused on wheel balance. A well balanced wheel can prevent varying side loads while rolling and turning, which may initiate or contribute to shimmy. Here is what else I learned while fixing my shimmy problem.

Poor shimmy axis alignment can also initiate or contribute to shimmy. The shimmy axis must be truly vertical in the sidewise plane. With sidewise lean the weight of the airplane will force the wheel to turn out of the vertical fore-and-aft longitudinal plane, while the caster arm will tend to center the wheel thus initiating a shimmy.

If the shimmy axis is raked forward at the bottom in the fore-and-aft vertical plane, the tire-ground contact point will be aft and under the shimmy axis extension. Again the weight of the airplane will tend to force the wheel to turn outward while caster tends to center the wheel which can initiate a shimmy.

If the shimmy axis is raked aft, the tire contact point will be aft but above the axis extension and the weight will now tend to force the wheel to center giving us two centering forces. This is a stable or convergent geometry rather than divergent. How much aft rake is needed? On my SERG the shock in the retractable nose gear actuator linkage allows about a 4-5 degree forward rake when compressed. The Diamond trainer aircraft has a similar nose gear setup and I measured about 8 degrees on their shimmy axis so anything in the 5-8 degree range should work well.

Getting the nose gear leg adjusted for enough aft-rake may require more room at the top of the gear leg. I had to cut a two inch diameter hole in my gear-canard bulkhead and pivot channel to allow space for the clevis at the top of the gear leg. However, even with a balanced wheel and good shimmy axis alignment, a shimmy damper is still needed in case the wheel isn’t centered at touch-down.

The viscous shimmy damper is preferable. The friction dampers have a problem generally called sticktion or static coefficient of friction. It is quite a bit higher than the sliding coefficient of friction. If the wheel is turned at touch-down the initial resistance to centering is high. As soon as it begins to slide to the center the friction drops suddenly and there is then an excess over-force that gives the wheel a kick and accelerates it across to the other side, and this kick repeats every time the shimmy reverses direction. This can result in a very hard shimmy especially in a touch-down hard enough to force the wheel to turn outward, if there is forward rake, before it starts to center. It can be very destructive. Don’t ask how I know.

The viscous damper eliminates the sticktion factor of a friction type damper and can be designed with a higher shimmy resistance to help stop the shimmy quicker. I designed and built my own viscous damper before I knew of the new Velocity viscous damper which is a bargain compared to my machine shop costs. It may be all you need and you will find it takes less braking to steer the airplane. However, if your damper leaks, having a balanced wheel and good shimmy axis alignment provides an extra margin of safety.

Don Bates
N14B
SERG

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