REFLECTOR: Damper

Tue Aug 30 16:08:22 CDT 2011

John, I'm in but need it for an RG.  On my third nose gear now. 

Shimmy is really bad if you have not experienced it.  Much worse than a Cessna.
Almost 600 hrs.
My first shimmy loosened the original bushings and bent the pivot bolt.  I installed the reinforced bushings then cracked a taco weld after a shimmy.  (Too much air in the nose tire).  Recently noted poor taxi behavior and found snapped pivot (grade8) bolt at the end of threads. Maybe my bolt was too short but one would think that size bolt's sheer strength could take anything the nose could dish out on normal landings. I just added the captivator extender/doubler on this RG.   I replaced the pivot with a longer bolt. I've found air pressure in the 30 # range is best for me.  The down side is if I load up heavy getting lift on takeoff is difficult and extends the takeoff roll a lot.  

Rene' Dugas

Sent from Rene's iPad.

On Aug 30, 2011, at 7:03 AM, "John Abraham" <john at velocityaircraft.com> wrote:

> About 99.9% of the time a shimmy is caused by the pilot.  Watching most people land their aircraft and talking to them after about how they landed, they will tell you they held the nose off perfect and are getting the feel for the aircraft.  However, if you watch most people’s first touchdown the nose makes contact with the ground and bounces as they “hold” the nose off.  This may or may not lead to structural fatigue.  Nut tension does have a lot to do with this.  However; even if you have proper tension and strike the nose down crooked, you increase the chances of a shimmy.  The FG’s using a wheel pant become at higher risk for two reasons.  The weight behind the pivot point making the shimmy possibility worse and laziness.  I don’t mean lazy as flying technique, I mean as human nature everyone with wheel pants on while flying across the country on a trip will go out and pre-flight the aircraft before their last leg of the trip and test the nose gear tension and if it is loose will say “it is good until I get home” because they don’t want to remove the wheel pant.
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> Anyone that has experienced a shimmy will tell you that it feels like it takes forever to stop the aircraft and something that you don’t want to go through again.  There is an owner that actually caught a nose gear shimmy with a winglet camera, I will see if I can get them to post it so you can see what the nose wheel on your aircraft does when it happens.
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> As for the damper system, it should considerably reduce the probability of a shimmy with your aircraft.  It works on motorcycles racing at 200MPH.  But there’s always a but.  The weak point in the system is the damper pin.  It is designed to shear if too much load is put onto the system so that it doesn’t destroy the entire thing.  This will only happen if you go past the travel limit of the damper.  In the fork stop limits will also be installed to help prevent this.  For all FG owners that are going to install this system.  It is recommended to use red line tape on the outside of your wheel pant marking the limits so people that don’t know you have it installed don’t try to kick the wheel around and break the stops or damper.
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> I will have a final price for everyone by the end of the week for the systems.  Price driver is how many damper systems I can order as we get a price break around 10 I think.
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> John Abraham
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> Chief Pilot; Executive V.P. Sales and Marketing
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> P: (772) 589-1860                                         200 W. Airport Dr.
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> F: (772) 589-1893                                         Sebastian, FL 32958
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> www.velocityaircraft.com
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> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Craig and Denise Woolston
> Sent: Monday, August 29, 2011 11:37
> To: 'Velocity Aircraft Owners and Builders list'
> Subject: Re: REFLECTOR: Damper
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> I agree with Andy, everything in aircraft design is a compromise.  But, one small disagreement though, some of which is antidotal, some hard engineering.  I’ve had three “shimmy events” on my XL-FG in 4 years (650hours).  The first I wasn’t smart enough to measure nut tension after to “prove” that is was fine and I was a beginner Velocity pilot so you might be able to contribute that to “pilot technique” but I would argue that last two were not pilot technique errors.  The next two the tension was accurate, the problem is each of our planes is built slightly different based on measurement guidelines from the factory but the shimmy equations of motion and whether the system is stable our unstable is highly dependent on small differences in geometry, which is why people have had difference results.  There is also a difference in a limit cycle oscillation shimmy event which is what most people experience with a “loose” nose gear nut and I believe is more prevalent on the RGs because of the geometry difference (go read the papers if you don’t believe us) and is stable and a divergent oscillation shimmy event which is what I’ve had on the last two which result in the nose wheel completing 360 degrees of rotation, obviously unstable.  Andy’s right, I never want to do that again.  I’ve shed nose gear pant parts during these events and got lucky they didn’t take out that expense rotating composite part on the back of the aircraft, not to mention I believe it could lead to nose gear failure.  Which is what is in video footage of this phenomena on youTube in believe or not a canard type aircraft and nose gear fails.  So, I politely disagree with Andy’s statement that the current system works, the current design is not robust enough to variations in aircraft build, aircraft load configurations and real world flight test.
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> Craig
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> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Andy Millin
> Sent: Monday, August 29, 2011 8:28 AM
> To: 'Velocity Aircraft Owners and Builders list'
> Subject: Re: REFLECTOR: Damper
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> The damper is like everything on an airplane; a compromise.
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> My motivation comes in large part from talking with those that have had a shimmy.  Those that have had a shimmy, don’t EVER WANT TO DO THAT AGAIN.
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> Those that haven’t are not as concerned, or may even see this as frivolous and unnecessary.  To each his own.
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> A bad shimmy doesn’t just shake the confidence, it can damage the plane and possibly leave one stranded.
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> Scott Swing was sure that proper landing technique can eliminate most of the shimmy problem.  One must gain the experience first though.  :)
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> Mark Riley suffered a bad shimmy incident with his XL/RG just before coming to Oshkosh.  The difference between shimmy and no shimmy was ¼ turn of the nut.
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> Let me be clear.  I believe the current setup works.  One must check the nut tension as part of the pre-flight.  Forgetting once can be unforgiving.
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> The new damper costs more, weighs more, and adds complexity.  Like most stuff in aviation.  It also provides positive damping and improved ground handling.
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> Andy
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> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Douglas Holub
> Sent: Monday, August 29, 2011 10:06 AM
> To: Velocity Aircraft Owners and Builders list
> Subject: Re: REFLECTOR: Damper
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> Before I started flying I installed a locking pin a la Mishler on my Cozy nose wheel just in case there were any shimmy problems. My design wouldn't let the nose wheel rotate 360 degrees, and it was a nuisance moving the plane. It sounds like this damper is a good idea, but I'm sure you'll want to make a tow bar and find a place for it in the plane for traveling.
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> I never got around to making a tow bar, and then after I started flying I've never had a hint of a shimmy.  My pre-flight does include checking the nose wheel for tightness. (If it is not hard to move with my foot it is too loose), but maybe the more flexible fiberglass strut has something to do with it, too.
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> I removed the locking pin this year, and I love having a full castering nose wheel again. I hope I'm not putting a damper on this promising damper idea.
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> Doug Holub
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> Standard FG with electric nose lift.
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> ----- Original Message -----
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> From: Andy Millin
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> To: 'Velocity Aircraft Owners and Builders list'
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> Sent: Monday, August 29, 2011 9:34 AM
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> Subject: Re: REFLECTOR: Damper
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> Not shown in the picture was the steering limiter.  If you look at the Damper 3 picture, you’ll see the studs sticking out of the top of the gear fork.  They are AN-4 bolts that have the heads cut off.  There is a pawl mounted on the king pin.  The pawl hits the stops before it hits the limits on the damper.
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> So, yes, we will need to have some type of steering/tow bar.  This is the same arrangement that you have on just about all production aircraft.
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> I would ask John to comment on what it was like to push the plane around with the damper on it.
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> I know this solution is not for everyone.  I feel what I am gaining is worth more than what I am giving up.
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> You may feel differently.
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> Andy
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> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of John Dibble
> Sent: Monday, August 29, 2011 8:58 AM
> To: Velocity Aircraft Owners and Builders list
> Subject: Re: REFLECTOR: Damper
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> Andy Millin wrote:
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> Items of Note:
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>                 The damper is hydraulic.  It is an arm that sweeps inside of a pie shaped chamber.  There is enough range to do a single wheel locked turn.  It will not spin 360 degrees.
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> So, pushing the plane backwards won't be easy?
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> John
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