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Landing gear shimmy
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Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Mon Apr 30, 2012 9:36 pm Post subject: Landing gear shimmy

Hey, I've talked about this before, haven't I?  I've had 3 big changes I wanted to make for a long time:  1)  Fix fuel distribution issue.  2)  Make some short wingtips.  3)  Eliminate the landing gear shimmy.



I installed the flow straightener tube and vanes that Nick sent me about a year ago.  That did an amazing job.  Perfect?  No.  Good enough that I'm not considering installing fuel injection?  Yep.  So #1 is adressed.



I still need to make short wingtips, but my passenger yesterday asked "Why does it shake like that after you land?"   GRRRRR.



So I bought an axe handle and started chatting with neighbors here on the airport about cutting and shaping it.  I figured I'd buy a 1.125" ball cutter and put a radius in it.  Kept putting that off because it didn't sound like fun.



Then it hit me:  I really just need it stiffer in fore/aft flexing.  And whatever stiffener I make needs to glue to the gear leg so it needs to be a very good fit.  Nothing fits better than fiberglass built in-place!



I decided I would tape off the gear and lay up 3 laminations of 7oz cloth directly onto the back of the gear leg.  I did that tonight, and it's curing.  Tomorrow I'll sand the back of that and use some flock and 5 minute epoxy to attach a 1/8" thick piece of balsa wood that extends about 1.5" behind the gear.  Once that's cured I'll lay up about 6 layers of cloth on both sides of the balsa, overlapping onto the section that's on the gear.  So the gear shouldn't end up much stiffer since it only has 1/8" thickness between the 6 layer lamination, but it should be SUPER stiff across the 1.5" section.



So I should get that done tomorrow, and let it cure overnight.  Then I'll pop it off the gear, trim it, and glue it onto the landing gear leg.



Any thoughts?  Nick, any suggestions on the best adhesive to attach to the gear?  Should I remove the paint or glue to the paint?



When I glue it I'll put a couple of zip ties around it to hold it while it cures, and leave them on just in case it ever comes loose.



Fingers crossed, I can report in a few days that I have eliminated the dang shimmy once and for all.

dwwilt

Joined: 06 Dec 2010
Posts: 192

Posted: Tue May 01, 2012 4:30 am Post subject: Landing gear shimmy

It would be nice to see some pictures of the stiffener as you built it and as you complete the installation.

 

      

   Dennis

   N616DW (Arion Lightning S/N 132)

 

 

 

   --

_________________
Dennis

Clive J

Joined: 03 Nov 2007
Posts: 340
Location: UK

Posted: Tue May 01, 2012 5:25 am Post subject: Landing gear shimmy

Hi Dennis, I don’t think you could stick anything to a gear leg that wouldn’t come off when the leg flexed. Where wood has been fitted before wasn’t it held on with hose clips?

To hold the wood in place without hose clips  I would have thought you would need to ‘bandage’ it on with fibreglass tape. As you say, good fit and glued but held all along so it has to flex with the leg.

For sticking things to aluminium there is a primer chemical, nasty stuff, I don’t know what it’s called, some kind of chromate maybe, needs painting on and curing with heat. 

Interestingly I have Lighting gear legs on my Esqual but no shimmy, even with the great big RV spats (which I took off, anyone want to buy them?). Different wheels and brake discs though.

Just about to fit 6 ply tyres so maybe that will change 

 

Good luck, regards, Clive

 

From: owner-lightning-list-server(at)matronics.com [mailto:owner-lightning-list-server(at)matronics.com] On Behalf Of Dennis W. Wilt

Sent: 01 May 2012 13:29

To: lightning-list(at)matronics.com

Subject: Re: Landing gear shimmy

 

It would be nice to see some pictures of the stiffener as you built it and as you complete the installation.

 



Dennis



N616DW (Arion Lightning S/N 132)

 

--

dwwilt

Joined: 06 Dec 2010
Posts: 192

Posted: Tue May 01, 2012 5:45 am Post subject: Landing gear shimmy

Clive,

    

   I don't think I am really interested in putting this modification on my Lightning, but the concept was interesting and I would like to see how it is done.  I have the LSA gear on my Lightning and do not have a shimmy.

 

 

     Have a wonderful day,

    

   Dennis

   N616DW (Arion Lightning S/N 132)

 

 

 

   --

_________________
Dennis

checkpoint2(at)comcast.ne
Guest

Posted: Tue May 01, 2012 6:15 am Post subject: Landing gear shimmy

Hi  Dennis, I missed your message re. gear shimmy.  When does it occur,   can you isolate it as far as main or nose

   wheel?    Bob Haas S/N 00071  N330BH.

  From: Dennis W. Wilt (dwwilt(at)AOL.COM) 

 Sent: Tuesday, May 01, 2012 9:45 AM

 To: lightning-list(at)matronics.com (lightning-list(at)matronics.com)  

 Subject: Re: Landing gear shimmy

  Clive,

 I don't think I am really interested in putting this modification on my  Lightning, but the concept was interesting and I would like to see how it is  done.  I have the LSA gear on my Lightning and do not have a  shimmy.

  Have a wonderful day,

 Dennis

 N616DW (Arion Lightning  S/N 132)

 -----Original  Message-----

From: James, Clive R <clive.james(at)uk.bp.com>

To:  lightning-list <lightning-list(at)matronics.com>

Sent: Tue, May 1, 2012  9:25 am

Subject: RE: Landing gear shimmy

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Where wood has been fitted before wasnâ€™t it held on with hose  clips?

 To hold the  wood in place without hose clips  I would have thought you would need to  â€˜bandageâ€™ it on with fibreglass tape. As you say, good fit and glued but held  all along so it has to flex with the leg.

 For sticking  things to aluminium there is a primer chemical, nasty stuff, I donâ€™t know what  itâ€™s called, some kind of chromate maybe, needs painting on and curing with  heat. 

 Interestingly  I have Lighting gear legs on my Esqual but no shimmy, even with the great big RV  spats (which I took off, anyone want to buy them?). Different wheels and brake  discs though.

 Just about to  fit 6 ply tyres so maybe that will change 

 Good luck,  regards, Clive

 From: owner-lightning-list-server(at)matronics.com (owner-lightning-list-server(at)matronics.com) [mailto:owner-lightning-list-server(at)matronics.com? ([email]owner-lightning-list-server(at)matronics.com?[/email])]  On Behalf Of Dennis W. Wilt

Sent: 01 May 2012  13:29

To: lightning-list(at)matronics.com (lightning-list(at)matronics.com)

Subject: Re:  Landing gear shimmy

 It would be nice to see some pictures of  the stiffener as you built it and as you complete the installation.

  Dennis

  N616DW (Arion Lightning S/N  132)

  --

dwwilt

Joined: 06 Dec 2010
Posts: 192

Posted: Tue May 01, 2012 6:18 am Post subject: Landing gear shimmy

Bob,

    

   I have not had a problem with gear shimmy, yet.

 

 

     Have a wonderful day,

    

   Dennis

   N616DW (Arion Lightning S/N 132)

 

 

 

   --

_________________
Dennis

wb2ssj(at)rochester.rr.co
Guest

Posted: Tue May 01, 2012 6:31 am Post subject: Landing gear shimmy

Interesting subject:

  

  I have reduced the amount of shimmy with  better tires, but have not cured it entirely. Maybe Nick can elaborate on the  reasons for the new gear on the Lsa, as it might give us with the older gear  some ideas on things to try.

       Tex,

 [quote]   ---

n45bm(at)yahoo.com
Guest

Posted: Tue May 01, 2012 6:53 am Post subject: Landing gear shimmy

The way I would do it would be to clean thoroughly the bare aluminum gear leg, then wrap two layers of 3" wide glass cloth on the gear, in a spiral manner. After cured, sand the rear of the glass and flox/epoxy a shaped pine wood (or similar wood) shape to the rear. I would then wrap the gear and fairing with another three layers of glass cloth. You could then attach the factory fairings over that or finish the glass cloth. Heck, even better would be to do it to all three gear legs. (Or maybe you may not have any shimmy problems at all with the standard setup).

Bernardo



--- On Tue, 5/1/12, Bill Strahan <bill(at)gdsx.com> wrote:

[quote]

From: Bill Strahan <bill(at)gdsx.com>

Subject: Landing gear shimmy

To: lightning-list(at)matronics.com

Date: Tuesday, May 1, 2012, 12:36 AM



--> Lightning-List message posted by: "Bill Strahan" <[url=/mc/compose?to=bill(at)gdsx.com]bill(at)gdsx.com[/url]>



Hey, I've talked about this before, haven't I?  I've had 3 big changes I wanted to make for a long time:  1)  Fix fuel distribution issue.  2)  Make some short wingtips.  3)  Eliminate the landing gear shimmy.



I installed the flow straightener tube and vanes that Nick sent me about a year ago.  That did an amazing job.  Perfect?  No.  Good enough that I'm not considering installing fuel injection?  Yep.  So #1 is adressed.



I still need to make short wingtips, but my passenger yesterday asked "Why does it  shake like that after you land?"   GRRRRR.



So I bought an axe handle and started chatting with neighbors here on the airport about cutting and shaping it.  I figured I'd buy a 1.125" ball cutter and put a radius in it.  Kept putting that off because it didn't sound like fun.



Then it hit me:  I really just need it stiffer in fore/aft flexing.  And whatever stiffener I make needs to glue to the gear leg so it needs to be a very good fit.  Nothing fits better than fiberglass built in-place!



I decided I would tape off the gear and lay up 3 laminations of 7oz cloth directly onto the back of the gear leg.  I did that tonight, and it's curing.  Tomorrow I'll sand the back of that and use some flock and 5 minute epoxy to attach a 1/8" thick piece of balsa wood that extends about 1.5" behind the gear.  Once that's cured I'll lay up about 6 layers of cloth on both sides of the balsa,  overlapping onto the section that's on the gear.  So the gear shouldn't end up much stiffer since it only has 1/8" thickness between the 6 layer lamination, but it should be SUPER stiff across the 1.5" section.



So I should get that done tomorrow, and let it cure overnight.  Then I'll pop it off the gear, trim it, and glue it onto the landing gear leg.



Any thoughts?  Nick, any suggestions on the best adhesive to attach to the gear?  Should I remove the paint or glue to the paint?



When I glue it I'll put a couple of zip ties around it to hold it while it cures, and leave them on just in case it ever comes loose.



Fingers crossed, I can report in a few days that I have eliminated the dang shimmy once and for all.





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=372188#372========================http://www.matronics.cs.matronics.com" bsp;               -Matt Dralle, List Adontribution" target="_blank">http://www.matronics.com/contribution [quote][b]

Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Tue May 01, 2012 11:49 am Post subject: Re: Landing gear shimmy

Here are a few pictures.  



Attachment5, the 3 layers on the landing gear (covered in tape) before I pried it off this morning.



Attachment8, the half-tube of fiberglass once it was pried off.



Attachment7, I went with 1/16" X 1" bass instead of 1/8" balsa, attached with 5 minute.



Attachment6, a shot down the length of the tube.  Once I've laid up several layers on both sides and trimmed it, it should flex okay in one direction, and be super stiff going against the 1" length of bass/fiberglass.

Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Tue May 01, 2012 5:08 pm Post subject: Re: Landing gear shimmy

Okay, the stiffeners are built.  I'm waiting on an adhesive test I did to choose which adhesive I use to bond them to the back of the gear legs.



FWIW, they're stiff.  I can barely flex them in the direction the landing gear SHOULD flex, and they won't budge in the other axis.  Makes sense.  Hopefully they're not TOO stiff.  Somewhere in there is a joke about being 18 again, but I'll skip it.  



Here are two pictures.

stephenbehn(at)bigpond.co
Guest

Posted: Tue May 01, 2012 11:44 pm Post subject: Landing gear shimmy

This is just a thought, run thin rigging wire secured at the axle weldment 

square plate up through the bottom of the fuselage on the line of the gear 

leg and so that they would lie inside the fairings. Inside the fuslage they 

could be tensioned against the floor or spar box using compression springs 

This would stiffen up the entire system not only the leg and you could apply 

the minium force required to aliviate the shimmy. Rigging wire would have to 

have fastenings crimped on all available at any good marine chandlery

regards

Steve Behn

Lightning 69

still a long way to go.

---

Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Mon May 07, 2012 5:09 pm Post subject: Re: Landing gear shimmy

Well, some possible egg on my face.    First, I finished my stiffeners and instead of adhesive, clamped them to the legs with 3 hose clamps.



I grabbed the gear leg prior, and yanked it back and forth on the ground.  Then clamped the stiffener on and did it again.  It made an incredible difference.  I'll try to get some pictures up tomorrow.



I was excited to try them out with a high speed taxi, so I jumped in fired up.  From my hanger to the main taxiway at F69 is grass, and in that short section I could immediately tell a difference.  The plane just felt much more stable on the grass without some of the wiggling, and back and forth springing of the gear that occurs when you're slow taxiing over clumps of grass.  Encouraging.



Did my high speed run and as I slowed to 20 or so, there was the same old shimmy.  Risking being told that the word doesn't mean, what I think it means...inconceivable!



A bit dejected, I taxied back and pulled in the hanger.  Again I noted a much better feel on the grass taxiway.  My wife came out from the house and asked if they worked, and as I was explaining what happened, I realized it was very unlikely that the stiffeners would make NO difference at all!  Hmmm.  I had heard the nose gear was pretty immune to shimmy, but maybe...



First I lifted the nose up and the noticed the nose wheel was free to flop back and forth.  Certainly no damping action there.  And upon closer inspection I could see dents in the aluminum at the edges of the slot that provides the limits to rotation via the bolt and spacers.  I don't slam the thing around, yet the dents in the aluminum were substantial where the washers on the bolt were digging into the aluminum at both extremes of rotation.  I could easily envision multiple stop to stop impacts creating the dents in the aluminum.



So perhaps I have the best grass-taxiing Lightning in existence, but that wasn't really my goal.  I'm going to rig up a video camera and do what I should have done in the first place, video the gear and find out exactly what is causing the shimmy.  Then I'll fix it.



If it's not the main gear I may leave the stiffeners on anyway.  Total weight of stiffeners and clamps is 9 oz.  I'll fly it up to a grass strip I frequent and decide after I have the shimmy gone.

Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Mon May 07, 2012 5:15 pm Post subject: Re: Landing gear shimmy

Here are the pics:  One pic of the stiffeners, and a strip of rubber that I decided to clamp between the stiffener and gear leg.



Next is a pic of the stiffener clamped in place

Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Tue May 08, 2012 1:36 pm Post subject: Re: Landing gear shimmy

Okay, no egg on my face I suppose.  Finally got a video camera to check things out.  It's clearly the main gear.  



It should be on youtube here:  http://youtu.be/rzb_eeyhLJU  in a few minutes.



This clip makes it pretty visible starting at 0:28 on the landing roll.  I don't use the brakes much to try to discount that influence, and I let the nose drop the moment I land so I can see if it's the nose or mains.



On taxiing back, I decided to accelerate and test it again, and the second shimmy can be seen at 1:28.



It may not look like much on a video camera, but that's a serious bit of wiggling in the cockpit.



I'm astounded that the stiffeners don't make an impact on it.  I can't flex them by hand at all in that direction.



This is a big deal for me because I fly into some pretty busy places and it occurs right at the speed I'd be doing on a long runway to hurry up and make a turnoff.  That, and I don't like passengers looking a little freaked out and asking what the heck that was.  And it can't be good for the whole plane to violently shake like that.



For the record, I've balanced tires, changed to 6 ply tires, balanced those, and now the fiberglass stiffeners.  No relief yet.  Not sure what comes next.

wampam

Joined: 20 Oct 2008
Posts: 49

Posted: Tue May 08, 2012 2:18 pm Post subject: Landing gear shimmy

Bill,



I have been plagued with similar problems.  I tried most of what you have done, except for stiffeners, with little or no effect.  I did notice an improvement when I remove the landing gear fairings.  I did a lot of research on the subject at one time.  There are number of papers on the subject on the NASA web site. These papers go back to the 1940's up to the 80's and 90's.  There are just to many variables for a simple fix.  Things like tire pressure, runway surface material, the kind of rubber used on the tire, flexing of the airframe, shape of the gear, weight on the tire, toe in and toe out and the list keeps going on.  Even modern transports have problems at times and I been on some where the shimmy was very noticeable.



A FULL-SCALE INVESTIGATION OF THE EFFECT OF

SEVERAL FACTORS On THE SHIMMY OF CASTING WHEELS

April 1940, Langley Aeronautical Laboratory.



TECHNICAL NOTE 3632

CORRELATION, EVALUATION, AND EXTENSION OF LINEARIZED

THEORIES FOR TIRE MOTION AND WHEEL SHIMMY

Langley Aeronautical Laboratory, Langley Field, Va.

Washington

June 1956



TECHNICAL NOTE 4001

SOME CONSIDERATIONS OF HYSTERESIS EFFECTS

ON TIRE MOTION AND WHEEL SHIMMY

Langley Aeronautical Laboratory, Langley Field, Va.

Washington

June 1957



REPORT No. 686

STABILITY OF CASTERING WHEELS FOR AIRCRAFT LANDING GEARS



REPORT 1299

CORRELATION, EVALUATION, AND EXTENSION OF LINEARIZED THEORIES FOR TIRE

MOTION AND WHEEL SHIMMY



NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

TECHNICAL MEMORANDUM 1378

DETERMINATION QF THE ELASTICC CONSTANTS

OF AIRPLANE TIRES

Washington

November 1954



NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

TECHNICAL MEMORANDUM 1337

Analytical Study of Shimmy of Airplane Wheels

Washington

September 1952



NASA/TM- 1999-209143

ARL-TR- 1976

An Overview of Landing Gear Dynamics

Langley Research Center, Hampton, Virginia

May 1999



Shimmy of Aircraft

Main Landing Gears

September 2000

ISBN 90-9014104-9

Copyright 2000 by I.J.M. Besselink

--

mark(at)flylightning.net
Guest

Posted: Tue May 08, 2012 2:32 pm Post subject: Landing gear shimmy

Hi Bill,



I'm sure you've already looked at this but what is your tire pressure?



We've also just experienced a case where the gear shimmied with no pants on

and then the shimmy was almost next to nothing with the wheel pants back on.

Go figure.



Also, I don't remember what setting we used when we set the toe in for your

gear. Double check that as well.



Best regards,



Mark



--

JamesL(at)Lugoj.com
Guest

Posted: Tue May 08, 2012 2:46 pm Post subject: Landing gear shimmy

On 5/8/2012 2:36 PM, Bill Strahan wrote:

 	  Quote: 	
		   It should be on youtube here:  http://youtu.be/rzb_eeyhLJU  in a few minutes.


	


Looks like a bit of resonance kicking in. Not sure, but it looks like maybe each 

leg alternates in vibration; that is as one vibrates forward the other is going 

backward.



I'm not a mechanical engineer and don't play one on TV, but my uninformed guess 

is that when one of the legs flexes, the leg geometry may be such that it can't 

help but cause the wheel to change its toe-in or toe-out angle. In this case as 

one of the legs starts to bend it may be causing a toe-out condition in the 

wheel, which causes even more bending and more toe-out. This would likely cause 

the plane to pull to one side, causing the other wheel to experience a relative 

toe-out condition, setting up the same thing there. Energy starts swapping sides.



Swept-back gear legs _might_ have an inherent geometry problem due to how they 

rotate the axis of the wheels when they flex due to loads. So the toe-angle 

would also change with changes in how much you load the airplane.



Perhaps gear legs straight out (right angle to the average direction of motion) 

would flex mostly only up or down, changing the camber angle of the wheels but 

not the toe-angle.



But as I said, I am not an engineer and the above might not be valid observations.

wb2ssj(at)rochester.rr.co
Guest

Posted: Tue May 08, 2012 4:09 pm Post subject: Landing gear shimmy

Bill, great video, thank you for your effort.  Tex



---

vettin74(at)yahoo.com
Guest

Posted: Tue May 08, 2012 4:29 pm Post subject: Landing gear shimmy

What about a different materal for the leg all together?



We dont have the problem on the lsa gear that often if at all. But the overlay angle or sweep of the gear is not as pronounced.



Have thought about the use of titanium gear legs. That material is said to be "dead" material due to make up. It does flex but does not spring back with as much pep. Sonex uses this for their main legs, it is a bit more expensive but may be a solution. Let me look into it.

Nick



Sent from my pocket 



On May 8, 2012, at 7:06 PM, "Tex Mantell" <wb2ssj(at)rochester.rr.com> wrote:



[quote] 

 

 Bill, great video, thank you for your effort.  Tex

 

 

 

 

 

 

 

 ---

barnett6088(at)msn.com
Guest

Posted: Tue May 08, 2012 7:19 pm Post subject: Landing gear shimmy

Bill

There is a natural resonance in my landing gear around 8 to 10 miles per hour.  If you are slower or faster there is no vibration but when you hit that speed the wheels bounce pretty good.  By adding or subtracting weight (Wheel pants etc.) you just change the resonant frequency.  With a resonance problem in your landing gear the wheels can be perfectly balanced but the slightest excitement from a bump in the runway or an out of balance tire, at the right speed, will get it going 

 

Try this, jack a wheel off the ground and using an air gun with a lot of pressure, in the area of the tread spin the wheel up to see if you have a balance problem or out of round tire.  Make sure you warm up the tire first by a few landings and then before it cools off jack it up and let it sit for an hour to stabilize (reach it's natural shape).  Then test it.  You may need to put a padded support under the area of the fuselage so the gear can flex freely during the test.  If the tires are allowed to cool after a flight while the plane is sitting on them they will develope a flat spot that will lead to wheel bounce no matter what you do.  

 Gary Barnett N335AL Sn 133.



 	  Quote: 	
		   Subject: Re: Landing gear shimmy

 From: bill(at)gdsx.com

 Date: Tue, 8 May 2012 14:36:50 -0700

 To: lightning-list(at)matronics.com

 

 --> Lightning-List message posted by: "Bill Strahan" <bill(at)gdsx.com>

 

 Okay, no egg on my face I suppose.  Finally got a video camera to check things out.  It's clearly the main gear.  

 

 It should be on youtube here:  http://youtu.be/rzb_eeyhLJU  in a few minutes.

 

 This clip makes it pretty visible starting at 0:28 on the landing roll.  I don't use the brakes much to try to discount that influence, and I let the nose drop the moment I land so I can see if it's the nose or mains.

 

 On taxiing back, I decided to accelerate and test it again, and the second shimmy can be seen at 1:28.

 

 It may not look like much on a video camera, but that's a serious bit of wiggling in the cockpit.

 

 I'm astounded that the stiffeners don't make an impact on it.  I can't flex them by hand at all in that direction.

 

 This is a big deal for me because I fly into some pretty busy places and it occurs right at the speed I'd be doing on a long runway to hurry up and make a turnoff.  That, and I don't like passengers looking a little freaked out and asking what the heck that was.  And it can't be good for the whole plane to violently shake like that.

 

 For the record, I've balanced tires, changed to 6 ply tires, balanced those, and now the fiberglass stiffeners.  No relief yet.  Not sure what comes next.

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=372654#372654

 

 

 

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