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Landing Gear Shake

Not even a hint on takeoff for me, and that's an interesting point. My plane accelerates fairly rapidly though so I think I'm probably moving through the speed zone fairly quickly.

I changed to better tires (Michelin Airs) which I checked and have virtually no runout, and it really didn't help. I failed to balanced the tires/wheels though which I plan to to at annual this year. I think there's some chance that'll have a positive impact for me but then again maybe it's wishful thinking. One way to find out, test it!
 
go figure

2006 my neighbor and I received our RV7 kits together. Upon completion his could not be taxied as the gear bounced badly. The wood fix worked. I had no problems what so ever. Go figure!!!!!
 
Aeronut,

My landing gear DID shake on both take off and on landing. After adding the wood stiffeners it seems to have solved the shake both in Take off and Landing
 
Yup I have the wood stiffeners in my gear also. No shake at about 25 psi. but this is hard on tires. Maybe an Exorcist is needed for the true fix ( might as well laugh as cry).:rolleyes:
 
Shimmy eliminated

Just wanted to close the loop on this issue. I explained my situation here, and that I was waiting for an opportunity to balance my wheels/tires. Well, they got balanced today and all shimmy is GONE! Each wheel took 1 oz of weight and that did it.

So, the moral to the story is balance your wheels. Our EAA Chapter bought one of those balancers from Deser Tire... good investment.
 
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Tire psi.

Randy,
Question, Still running at lower psi? Would you think of test running at a higher psi now for different control on landing and taxi? Or just leave it the same since that is what you are used to?
Peter
 
Three components of shimmy?

I have some experience looking at failures of some structures used in rail vehicles. I am not an expert but listened to some and read a lot and will offer some food for thought.

In the structures we worked on there were three components to the issue.

1) excitation source (stay cool now) - in our case it was steel wheels with a flat spot. They are 36" diameter wheels and at around 45 mph the frequency of the flat spot hitting the rail was right about at the harmonic frequency of the structure. Trains often run at 45 mph and when they did this structure would vibrate and fatigue the structure eventually causing some cracks in some equipment mounted to the car.

2) structural harmonic frequency - structures have a frequency that they will most easily vibrate. Most structures are so stiff that the frequency is very high and there are few if any excitation sources at the same frequency. Sometimes structures are designed and built where mass, stiffness and inherent dampening provide a structure that is easily excited at a frequency commonly found.

3) dampening - Sometimes the material or construction methods will quickly absorb or dampen any vibration.

In the case of this problem I suspect:

1) there are many possible excitation sources and variables including wheel geometry (toe in and camber), wheel/tire balance, tire pressure, frictional coefficient between tire and surface, and probably others

2) structural harmonic frequency is an issue with some gears and not others because of structural stiffness and the way the mass of the landing gear, wheel/tire/axle/brake system and wheel pant is distributed. These gears are basically a long beam with a mass at the end. If you change the mass or it's position you will change the harmonic frequency. This would include adding stiffeners or maybe changing tires or wheels/brake discs. I would guess Grove gears and Vans gears (on 8's) have different natural frequencies due to slightly different shapes (therefore stiffness) and distribution of mass. Longer gears would be different than short, tapered tube different than a constant cross section gear etc. Even clamping force in the gear mount could effect the harmonics.

3) dampening - I don't see where there is any real dampening in this system. Everything is steel except the tires and I doubt they provide any dampening, only a poor connection to the ground.

Think of this landing gear as a tuning fork. If the fork does not get excited (no source) or the excitation frequency is a lot different than it's natural frequency, or if you lightly touch the vibrating tuning fork with your fingers the vibration will either not occur or will dampen out quickly.

The moral of the story is if you eliminate the source of the exitation source, or change the natural frequency of the structure (stiffeners) or add dampening you will probably eliminate or reduce the vibration, or shimmy in this case. That's why some people get it bad, some get it in one gear leg only and some people (airplanes) never exhibit it with enough energy for the pilot to notice it.
 
Dead on, John.

I suspect low inflation pressure provides some damping. Cranking up the torque on a nose wheel swivel nut should increase damping for its lateral oscillation, but that's another issue.

Fine point; changing the natural frequency (change in stiffness or mass) would not necessarily eliminate a resonant oscillation. Assuming the excitation is rotational, it would move it to a new frequency, ie a higher or lower rolling speed. Seems unlikely we could change natural frequency enough to move the resonant range out of the operating range. Best success is probably in finding and eliminating the excitation source.....which is what Randy did, see above
 
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DanH - yes, you might change the frequency enough to avoid problems. If the natural freq is reached at 70 mph and you never exceed 65 then it becomes a non issue.

I think that the system is unstable enough that once it starts vibrating it keeps vibrating untill speed goes way down. The system needs dampening so the oscillations decay rapidly.

In the railroad product we changed mtl thickness from 1/4" to 5/16" and the natural freq went way up to around 70 mph because the structure got much more stiff. The added thickness also dropped stress low enough that fatigue problems were eliminated.Interesting project.
 
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Randy,
Question, Still running at lower psi? Would you think of test running at a higher psi now for different control on landing and taxi? Or just leave it the same since that is what you are used to?
Peter
I will start increasing the pressure in 2 mph increments to see what happens, but overall I like the way the plane feels at 20 psi. That wouldn't work with the larger RVs, but with the light RV-3 it seems to work just fine.

To the point made above, indeed the tires are the only thing providing any damping, might as well let them do their thing -- the more pressure you run the less damping you get.
 
Bubble,
Yes, I believe you are "right on" with your explaination of our landing gear shake. You have accurately described just how my aircraft acted before we added the wood stiffeners and that is when my shake started the ONLY thing that would stop it was getting to a lower speed.

A knowlegable friend of mine says that stiffness, (of our gear legs) is dependent on outside dimension of the gear leg.
 
So, the moral to the story is balance your wheels. Our EAA Chapter bought one of those balancers from Deser Tire... good investment.

And being in a chapter that most likely wouldn't invest $375 in a tire balancer, I went the Harbor Freight route:

http://www.harborfreight.com/cpi/ct...gle&zmam=33951326&zmas=12&zmac=112&zmap=98488

I've had good luck with some of their stuff, and am hoping this pans out as well. Note that it is an offset shaft balancer, which should be more sensitive than a standard balancer. Won't take but a minute to tell if it's junk.

Tony
 
An hour with Google will turn up balancers by the dozen. The one from Harbor Freight uses a different technology than most. To see what a high dollar one looks like, go here:

http://www.mcmaster.com/#balancing-ways/=1rntm9

then scroll down to static balancing ways. I had to order, as the local Harbor Freight had none in stock. Should someone be able to find them in a store, check that the wheels rotate freely and have a machined edge to support the shaft. If yes on both counts, they should work fine. Otherwise, I'll report back in a couple of weeks.

Tony
 
Gear leg Shake

We had the same problem with the same answers from all of the gear builder and the factory, I think its a metal or heat treat problem, We solved this by taking an al. tube the id of the largest od on the gear, annealing the tube sliding it over the gear for the length you want to stiffen, form into an airfoil shape that will fit under the fairings. Pour epoxy into the tube, rotate it on the gear leg to make glue contact, align it, tape off the bottom and fill with the epoxy. We added an arrow shaft of fiberglass to lessen the epoxy amount and let it set. We were told to balance the fairings and all the other nonsense. This fix cured it and made a decent gear out of it.
 
Jess, I doubt it's a heat treat or metal problem. The stiffness is much more effected by the shape and size than heat treat. In your case you put a much less stiff material (Aluminum and fiberglass arrow shaft) around a stiff material and changed the stiffness enough to get rid of the shakes.

What makes you think it's a heat treat or metal problem?
 
low cost wheel balancer

Got my hands on this

http://www.harborfreight.com/cpi/ct...gle&zmam=33951326&zmas=12&zmac=112&zmap=98488

sooner than expected. It is not Harbor Freight junk. It is one very sensitive balancer. How sensitive? I taped a 1" dia disk that weighed 3 grams perpendicular to the shaft, then rotated the shaft to the 3/9 o'clock position and turned it loose. It swung to the bottom repeatedly. All parts are steel, and the bearings are very smooth. As I said in an earlier post, it's a non-standard balancer, with the main 1/2" shaft riding on four 2" dia steel wheels.

Tony
 
Got my hands on this

http://www.harborfreight.com/cpi/ct...gle&zmam=33951326&zmas=12&zmac=112&zmap=98488

sooner than expected. It is not Harbor Freight junk. It is one very sensitive balancer. How sensitive? I taped a 1" dia disk that weighed 3 grams perpendicular to the shaft, then rotated the shaft to the 3/9 o'clock position and turned it loose. It swung to the bottom repeatedly. All parts are steel, and the bearings are very smooth. As I said in an earlier post, it's a non-standard balancer, with the main 1/2" shaft riding on four 2" dia steel wheels.

Tony

Sweet, do RV wheels fit ?
Tom
RV3 978TM
Edit just checked my plans , they do.
 
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Tom,

The "base sold separately" comment refers to a dial indicator and base. You'll need that only if you want to determine runout to the nearest thou. For a rough idea of runout, just plant your fist and stick a thumb against the outer diameter of the tire, then spin the tire.

Tony
 
Got the wheels balanced today. Will taxi test tomorrow.

http://picasaweb.google.com/tonyboytoo/RV3BMiscellaneous#5334691680719732562

1/4 ounce strip weights free from the local tire dealer. Used two ounces per tire. The red dot on a tire really does mark the light side, but when the dot is next to the valve stem the stem interferes with weight placement. Ended up rotating both tires 180 deg, then split weights to keep access to wheeel bolts. There is some side play in the aluminum spacers that go outside the bearings, so it's best to remove them so the cones on the balancer will seat on the bearing surface. Pumped the tires up to 40 PSI and checked runout with a dial indicator. Average was .050, which isn't enough to cause a problem. Will run the tires at 22 PSI

Tony
 
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Success!!

After fiddling with tire pressures some, the magic number was the same as Randy Lervold found to work- 20PSI. Taxi tests from 5-35 knots and no shake at all. Read previous posts for all the details.

Tony
 
Sounds promising, will be interested to see what happens on landing roll out. My shake is dependant on what type of landing I do. Wheelies = shake, 3 pointers = 50% chance of shake.
Tom
 
A small note

I had a very small window of tire pressures that did not induce the gear shake. Something along the lines of 3-4 PSI. One cold morning would be fine but a warm afternoon and I would get the shakes.

I noticed my brakes were dragging a bit after use so I put a spacer on the return spring and now my window seems to have opened up to around 10 PSI. I air the tires to 25 PSI on an average day and in the past 15 hours have not had a bad shake at all.

After looking at the runout on my Air Hawks, I'm not surprised it doesn't shake all the time. I can freehand draw a more round circle than these tires.
 
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