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Gear leg shimmy experiments

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N1BZRich(at)aol.com
Guest

Posted: Thu Jan 03, 2008 5:37 pm Post subject: Gear leg shimmy experiments

Hello all Lightning and Esqual listers,

     Just before Christmas I send out what I called a  "two year report for N31BZ" which was some thoughts on the Jabiru 3300 and my  highly modified Esqual LS (meaning lots of Lightning Stuff) after flying for two  years and some 400+ hours.  One thing I mentioned was that I planned to do  some experimenting on the gear leg shimmy that we sometimes see (for me it is  almost always when landing or taxiing on a hard surface and occurs only when at  about 22 mph).  Following is that paragraph about the shimmy and below that  will be some preliminary results of the shimmy experiments.

  

 I have had no real airframe issues during the past  two years.  The only thing I plan to change, when I get the time, will be  to do some experiments on the main gear legs to try to get rid of the sometime  gear leg shimmy.  I may go the "broom stick" mod that the RV guys do, but  before doing that I want to try an easier fix by using a small "V" angle  aluminum strip (say 1/8" thick by 1/2" on the angles, by what ever length the  gear leg is) that will be clamped to the trailing edge of the gear leg.   All I need is the time.  I already have the parts.  This shimmy occurs  on landing roll out on hard surface as you are slowing down through about 22  mph.  It does not happen on grass.  Tire pressure definitely has an  effect, with lower pressure being better.  I normally air my mains up to 25  PSI and then re-air them when they look low.  That low look seems to be  about 15 to 18 PSI.  I will let you all know the results when I get the  time to do the experiments. 

  

     As I mentioned above, I have now done some  preliminary experiments to find an easy fix to the main gear leg shimmy  problem.  Actually, I should say that Joe and Linda Mathias and I have done  some experiments since we used their Lightning (N59JL) for the  experiments.  They had their gear leg fairings off during the week after  Christmas, so it just seemed like a good idea to try my idea on their  airplane.  We actually tried two different size aluminum stips clamped to  the trailing edge of the gear legs.  The first piece of aluminum "V" angle  was 1/16" thick by 1/2 wide and 13 inches long.  We used three hose clamps  on each gear leg to hold these 1/16" V strips in place on each main gear  leg.  We tried the thinner material first thinking that if it worked it  would weigh less.  Results were not that noticeable on the taxi test -  maybe some improvement, but not near the reduction in shimmy that we  wanted.  Next we put the  13" long strips of 1/8" thick by 3/4"  wide and used the same hose clamps to securely attach the strips to each  gear leg trailing edge.  Results were very promising on the taxi test and  one flight that Linda made that day.  Her exact words were "no gear leg  shimmy on that landing".  

     Obviously more test flights will be made to confirm  that this is a good fix, but we did want to get the word out in case others want  to add to our efforts and try it out on your airplane.  The aluminum V  angle seems to be available at all Lowes, Home Depot and Ace Hardware stores  that I have visited, and it is 1/8" thick by 3/4" wide and I think 42" inches  long .  Only one piece of the aluminum strip is required (about $8.00)  and you will cut the length to fit each of your gear legs.  We used two 13"  pieces on N59JL.  The hose clamps are standard hose clamps like we all have  laying around the hangar or readily available at the hardware stores.  The  long term fix will probably be to weld the aluminum V strips to the  aluminum gear legs, but the clamps allow you to get them on quickly for the  testing.  What this fix is doing is adding stiffness to the gear legs, just  like the RV guys do by fiber-glassing a grooved broom stick to the trailing edge  of the gear legs.  I think out fix is less work.  

     Let us all know if you have any questions and if  you try this and what results you have.  I should have had some photos to  send out with this, but alas, I did not think to take photos.  I am sure  Linda will let us know when she has made a few more landings with this gear leg  mod in place.  

 Blue Skies and Happy New Year,

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

Colin K.

Joined: 04 Sep 2006
Posts: 157
Location: Oklahoma

Posted: Thu Jan 03, 2008 6:32 pm Post subject: Gear leg shimmy experiments

Buz,

  

 What about using epoxy to attach the  aluminum angles?

  

 Do you think there is a risk of failure and  subsequent damage from flying metal?

  

 While a good weld is very strong, could  the heat be enough to change the properties of the gear leg  material?

  

 Colin K.

 OK

 [quote]   

   --

_________________
Colin K.
OK

pequeajim

Joined: 03 Sep 2006
Posts: 708
Location: New Holland, PA

Posted: Thu Jan 03, 2008 7:58 pm Post subject: Gear leg shimmy experiments

I wonder how the RV guys attach theirs?  

         

  

From: owner-lightning-list-server(at)matronics.com [mailto:owner-lightning-list-server(at)matronics.com] On Behalf Of Colin J. Kennedy

 Sent: Thursday, January 03, 2008 9:31 PM

 To: lightning-list(at)matronics.com

 Subject: RE: Gear leg shimmy experiments  

  

     

Buz,  

    

   

    

What about using epoxy to attach the aluminum angles?  

    

   

    

Do you think there is a risk of failure and subsequent damage from flying metal?  

    

   

    

While a good weld is very strong, could the heat be enough to change the properties of the gear leg material?  

    

   

    

Colin K.  

    

OK  

  

       [quote][b]

N1BZRich(at)AOL.COM
Guest

Posted: Thu Jan 03, 2008 8:34 pm Post subject: Gear leg shimmy experiments

In a message dated 1/3/2008 9:33:55 P.M. Eastern Standard Time,  cjk129(at)cox.net writes:

  	  Quote: 	
		     What about using epoxy to attach the    aluminum angles?

    

   Do you think there is a risk of failure    and subsequent damage from flying metal?

    

   While a good weld is very strong,    could the heat be enough to change the properties of the gear leg    material?

	


 

 Hi Colin,

     I guess we need to do some sort of study on the  strength of epoxy when used on aluminum.  Probably the best way would be to  epoxy it in place then wrap it with one or two fibergass cloth wraps.  That  is what the RV guys do - they fiberglass the wood to the gear leg.  I  really don't think that an epoxy failure would be a major problem, but you  certainly would get the shimmy back.  The un-epoxyed aluminum  part should stay inside the gear leg fairing (certainly with the  fiberglass).  Probably my only concern would be to immediately check your  brake line down that gear leg.  

     As to welding aluminum, I don't have the answer as  to whether the heat would alter the temper of the gear leg We need to ask Nick  that.  I seem to remember seeing some aluminum welding rods at Oshkosh that  had a working temperature of around 700 dgrees and could be used with a propane  flame.  Other than that method, I guess TIG would be the way to go.  I  only weld steel (4130) with oxygen accelyne.  

 Blue Skies,

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

N1BZRich(at)AOL.COM
Guest

Posted: Thu Jan 03, 2008 8:35 pm Post subject: Gear leg shimmy experiments

In a message dated 1/3/2008 10:59:52 P.M. Eastern Standard Time,  pequeajim(at)gmail.com writes:

  	  Quote: 	
		     

I wonder how the RV    guys attach theirs? 	


 

 They fiberglass the wood dowel to the gear leg.

 Blue Skies,

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

malannx(at)bigpond.com
Guest

Posted: Fri Jan 04, 2008 2:47 am Post subject: Gear leg shimmy experiments

There was extensive discussion on another list that escapes me for the 

moment, concerning gear leg shimmy. It was a while ago but if my memory 

serves me right, the conclussion went along the following lines.



The cure was slight toe out under all weight conditions, say half a degree 

or so. With any toe in the tyre will try to track inwards. As the leg slopes 

out the inward motion tries to stand the leg up. As the plane has inertia 

and a heavy weight it does not give, so ultimately the tyre is squashed down 

till it finally looses adhesion.  As it flicks outwards it moves upwards 

slightly and travels past its neutral point then begins its cycle over 

again. It finds a natural harmonic at a speed peculiar to an individual 

aircraft and surface and does not happen on surfaces with reduced adhesion 

such as grass,dirt,stones etc.



I have suffered from shimmy sporadically for a long while till I adjusted 

the toe in  after following that discussion-relatively easy to do on my 

earlier design spring steel gear legs.



As an aside I have been trying to decide what aircraft is to be my next 

project(if the drought ever breaks here in  south OZ) and have narrowed it 

down to a Legacy FG or a Lightning with a Rotax 914turbo. In most cases the 

Lightning looses nothing to the Legacy except in sheer speed. The Lightning 

actually has a better useful cockpit load and half the fuel consumption.

I reckon the 914 should be capable of cruising at VNE at FL150 on 

23-24litre/hr which would put it only 40 kts less than the Legacy. One 

lovely advantage to the Legacy is its high glass transition temperature 

allowing sexy paint schemes with dark colours.



Just dreaming about aviation is almost as much fun as flying.

Buz, I eagerly await your next epistle. They are always enlightning.



Malcolm Ferguson

Early model Esqual 912s 340hrs

N1BZRich(at)AOL.COM
Guest

Posted: Fri Jan 04, 2008 4:13 pm Post subject: Gear leg shimmy experiments

Hi Malcolm,

     Good to hear from you again.  I guess I did  not realize you have a Rotax on your Esqual.  How do you like it and how  long to get your 340 hours.  Do you only use auto fuel?  What type and  size prop are you using.  Speed results?  Too bad we are not closer, I  think a race is in order.  

     Also didn't know that you have the older style  spring gear legs.  That would make it easier to adjust toe in/out with  shims.  Interesting that the article you mention says that toe out is the  key.  Can you direct me to that discussion?  What ever (in or out) I  know that the "toe" will make a difference, but with an airplane as light  as these there is a big percentage of weight change from it being at gross  weight to it being fairly light.  As a result, I think the toe would  probably need to be different for different weights, tire pressure, etc., which  we can't do.  Therefore we need to know what works best for the range of  aircraft weights.  That may or may not be 1 degree toe in that the Esqual  and Lightning kits call for.  Actually the 1/2 degree of toe out that you  mention makes more since in my mind (especially since these are not tail  draggers).  But I am convinced that to completely cure the shimmy,  it will probably take a combination of the correct toe (in or out???) and  some way to stiffen the legs to change the shimmy, harmonic vibration, or  whatever we want to call it.   That is why the RV guys use the broom stick  method - it is a big help even if it is not the total real answer.

 Blue Skies,

 Buz Rich



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

14az(at)mysprocketmail.co
Guest

Posted: Fri Jan 04, 2008 6:52 pm Post subject: Gear leg shimmy experiments

Hi All

 I am just days  away from taxing my lighting so have no knowledge of  this type of gear leg. I have been a pilot, test pilot and maintenance tech  for about 39 years. The only vibration I have ever felt in the landing gear  was caused by tires out of balance, bearings or bullets. Again I have never had  this type of gear legs. I have installed the tires in the position recommended  by the manufacture and have balanced the tires. This always helps and I  would think these legs need all the help they can get.  My questions:

 How many have or had a problem with leg  shimmy?

 Has anyone spin balanced the tires and did you have a shimmy problem before  or after the balance?

 What is the weight (heavy/light) of the aircraft when it is most prone to  shimmy?

 Thanks

         Johnny  Thompson

  

   [quote][b]

N1BZRich(at)AOL.COM
Guest

Posted: Fri Jan 04, 2008 9:51 pm Post subject: Gear leg shimmy experiments

Hi Johnny,

     Certainly mounting aircraft quality tires on  the rims in the correct way is important for balance.  Some people  don't seem to know what the paint spot on the tire is for.  But  you will have to wait for Nick to answer your question about how many Lightnings  have had shimmy.  I think he is still away from SYI and may even be in  Arizona to make the initial flight on several Lightnings out that way.  The  shimmy situation has to do with the round rod gear leg (RVs I think initially  identified the problem)  and it varies from airplane to airplane, and seems  to have something to do with toe, tire pressure, aircraft weight, speed,  landing/taxi surface, and how  you are holding your tongue.  It does  not occur on grass, and only happens at the slower speeds, say around 20 to 22  mph.  And it does not happen on every landing (could be the tongue holding  thing).  On my airplane it happens on most every landing roll out (22 mph)  and is worse when the airplane is light weight with higher tire pressure and on  black top.  Not quite as bad on one concrete runway I often land on.   You will know it when it happens and if you have someone watching you they will  see it as well.  Don't confuse this with the possible nose gear shimmy that  is caused by the castering nose wheel being too loose in the bracket that holds  it on.   Just tighten the 8 nuts/bolts a little so there is some "feel  of tightness" when you have the weight off the nose and turn the nose wheel from  side to side (the nose wheel is in the air for this test with someone pushing  down on the tail of the airplane).

     As to spin balancing an aircraft tire, I have never  done that for any of the 6 airplanes I have owned but it could not  hurt.  My experience in other airplanes that weigh about what a Lightning  or Esqual does (my single seat Pitts) was that I never got any shimmy (no  round rod gear leg), but tire pressure certainly made a difference as to how  hard it was to keep straight on landing roll out- lower pressure was  easier. Of course the Pitts touched down much faster than a Lightning or  Esqual.  Bottom line,  all the things we mentioned above will help or  hinder the tendency to shimmy.  The key seems to be get the toe right (not  sure what right is at this point), make sure you have the tires mounted properly  on the rims, start with no more than about 25 psi in the mains, stiffen the gear  leg by some method as discussed before, and hold your tongue just right.   

     Good luck on that first flight.  Let us know  how it goes.

 Blue Skies and check six,

 Buz

  

 My questions:

  How many have or had a problem with leg  shimmy?

 Has anyone spin balanced the tires and did you have a shimmy problem before  or after the balance?

 What is the weight (heavy/light) of the aircraft when it is most prone to  shimmy?

 Thanks





Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

malannx(at)bigpond.com
Guest

Posted: Sat Jan 05, 2008 4:32 am Post subject: Gear leg shimmy experiments

Hi Buz and all,

The major dislike of my Esqual is the small fuel capacity 70 litres and was 

the main reason the early Lightning held no appeal to me till they made the 

bigger tanks available. Can't help you with the source of info as I monitor 

a number of lists and it was 18 months ago or so. We put on about 100 hours 

a year on average. The prop is a Rospeller electric C/S with Helix carbon 

fibre scimitar blades. The performance is very good but I don't think I 

would buy another one next time. There has been 2 props shed blades here in 

OZ due maybe to a design that requires very accurate torqueing of the prop 

blade hubs. Also has a 600 hour TBO. Would maybe plump for a 

Kremin(Woodcomp). This prop can be ordered with reversing blades. Now 

wouldn't that make for a short landing. Have had a lot of trouble with the 

electric servo motor/gearbox(made in Italy). Have modified it and no more 

trouble since. Prop was dynamically balanced in flight and is very smooth.



I generally cruise at 137-140 kts at 5200 revs and full throttle. Get the 

better speed at low altitude and cool airtemps. Will do 145 kts at 5500 

which is max continuous cruise revs but the noise level goes up appreciably. 

Interestingly taking the rear wheel fairings off only has a small reduction 

in speed so they are not drag efficient.



As regards wheel shimmy, it was a saga. The front wheel rotates in flight 

and required very accurate balancing to eliminate vibration. I put it up 

against a bench grinder spinning at 2800 rpm, holding the axle in my 

fingers. Its possible to get no out of balance vibration by trial and error.

The rear wheels I balance on a purpose built hub with the seals and grease 

removed from the bearings then a tad of diesel fuel in the bearings. After 

this I realized that wheel shimmy had nothing to do with wheel balance in my 

case.



I think you are on the right track with toe adjustment and strengthening the 

legs although the later only masks the problem.

Eric Jones has a good article on his website on nose wheel shimmy. 

http://www.periheliondesign.com/downloads/nosewheelshimmy.pdf



Having problems with home built aircraft generates good list traffic eg 

Europa, so I suppose it says somehing about the Lightning that the list is a 

bit quiet.



Malcolm Ferguson

N1BZRich(at)aol.com
Guest

Posted: Sat Jan 05, 2008 6:20 am Post subject: Gear leg shimmy experiments

In a message dated 1/5/2008 7:33:17 A.M. Eastern Standard Time,  malannx(at)bigpond.com writes:

  	  Quote: 	
		  The    major dislike of my Esqual is the small fuel capacity 70 litres  	


 

 Am I reading this right, only 70 litres or about 18.5 gallons?  My  Esqual (on a cold day) will hold a little over 29 gallons or something like 110  litres.  You must have the very early Esqual wings.  I can see why  that would be an operational problem on cross country flights.  As to  props, I am currently running a fixed pitch Sensenich (64ZK56) and it is still  not enough prop as I can over speed the 3300 Jabiru by over 100 RPM when running  full throttle at 5,000', so another prop is on order.  But this prop allows  me to cruise at altitude (say 8 to 10K feet) at a little over 150 knots true  burning slightly less than 6 gallons per hour.  Down on the deck (I live at  sea level) when I have smooth air I can see 180 knots and still pulling the  throttle back to keep below red line rpm.  This prop still gives me over  1000 feet per minute climb rate at gross and short TO rolls even on a hot day.  The purpose of the new prop is to optimize cruise efficiency (speed vs. fuel  burn).

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

EAFerguson(at)aol.com
Guest

Posted: Sat Jan 05, 2008 11:35 am Post subject: Gear leg shimmy experiments

I've been reading this thread with interest. As I have told Buz, I'm   get hardly any gear shimmy with my Light Sport Version Lightning (no spats  or strut fairings). Since it only happens occasionally, and is very brief,  I haven't thought about a cure. For the record, how are the other LSA  Lightnings doing?  Is this something that only affects birds with full gear  clean up hardware? While the answer may be obvious, more weight  brings on  more shimmy, I think that point is worth documenting. Would the other Light  Sporters chime in please.

  

 Earl Ferguson

 N17EF



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

lbmathias(at)verizon.net
Guest

Posted: Sat Jan 05, 2008 2:35 pm Post subject: Gear leg shimmy experiments

[quote]   Earl,

    

       Although my Lightning isn't a LSA, I have checked    the shimmy both with and without gear fairings and wheel pants.   I    currently have the fairings and wheel pants off and there seems to be no    noticeable difference if it's wearing them or not; the vibration doesn't    seem any different.   Initially it started about 17 MPH; after    decreasing tire pressure to 23-25 it starts about 22 MPH.   Now    with the gear leg stiffeners, it takes about 25 MPH to notice the    vibration.

    

                                                                                                      Linda 

    

    

   ---

N1BZRich(at)aol.com
Guest

Posted: Sat Jan 05, 2008 10:06 pm Post subject: Gear leg shimmy experiments

In a message dated 1/5/2008 2:36:37 P.M. Eastern Standard Time,  EAFerguson(at)aol.com writes:

  	  Quote: 	
		  While    the answer may be obvious, more weight  brings on more shimmy, I think    that point is worth documenting. 	


 

 Earl,

     Your above statement is not what I have seen on  31BZ.  For me it is slightly worse at lighter weights.  But as in  anything having to do with aviation, your mileage may vary.

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

N1BZRich(at)AOL.COM
Guest

Posted: Sat Jan 05, 2008 10:08 pm Post subject: Gear leg shimmy experiments

In a message dated 1/5/2008 5:37:05 P.M. Eastern Standard Time,  lbmathias(at)verizon.net writes:

  	  Quote: 	
		  Now with    the gear leg stiffeners, it takes about 25 MPH to notice the  vibration. 	


 

 Linda,

     So you did see shimmy on your next flight.   Bummer.  Was there any improvement with the stiffeners, meaning was the  shimmy any less than it is without stiffeners?  

 Buz



Start the year off right.  Easy ways to stay in shape in the new year. 

  [quote][b]

lbmathias(at)verizon.net
Guest

Posted: Sun Jan 06, 2008 3:54 am Post subject: Gear leg shimmy experiments

[quote]   Buz,

    

       I didn't see it on the landing but it doesn't always    happen on landing roll out; the shimmy occurred during the taxi testing prior    to takeoff.

    

                                                                               Linda 

    

   ---

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