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jng62433(at)aol.com
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 Posted: Mon Jan 28, 2008 5:32 pm Post subject: Gear leg shimmy experiments |
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Hi Buz
I enjoyed reading your email regarding the wheel shimmy. My lightning was finished in Shelbyville in September and had shimmy on some of the taxi and landing. After I flew back to Florida and have logged 40 hours on the airplane the right gear seems to be the one that shimmy. As of last week it had become almost violent. I hose clamped a hardwood dowel on the right gear leg and there was no apparent improvement. At this point I removed the wheel point as I felt the weight of the wheel pant might be contributing to the oscillation/shimmy. However, this had no effect on the problem.
I spoke to Nick last Thursday and he said that some lightning owners had replaced the Air null tires with a better tire. Jacking up and taking the weight off the tire I found the right tire had a obvious hump and was out of round. I checked with the aircraft maintenance facility at my airport and he had some tires distributed by Michelin which he said that he had real good results. The Air null tires are 4 ply and the new tires are 6 ply with a higher load rating. I bought two and replaced both main tires and I did a lot of high speed taxis and lifted the nose and ran on the mains only. I could aggravate it and cause it to shimmy at that point I was running 32 lbs pressure, I reduced the tire pressure to 28 lbs and went back out for multiple runs and the shimmy has stopped. However,&nb sp;the disc and the brake assembly are not a precision unit, therefore you can assume it is going to grab on the rotation differently as the wheel turns and heavy braking pressure is applied. It will and can cause the gear leg to shimmy occasionally. After 40 hours on the Air null tires most of the wear on the tread was on only 25 percent of the outer side of the tire. Most of the times these airplanes are flown at less than gross weight and I think the fact that tire foot print on the ground is only 25 percent more tire area on the ground would be better.. I would think that some of the camber could be taken out when the axle brackets are welded and this would give us a bigger foot print more toward the middle of the tire. As we all know many aircraft have similar problems. I certainly enjoyed seeing you at the open house last year.
Jim Goad
N166JG
jng62433(at)aol.com
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pequeajim
Joined: 03 Sep 2006
Posts: 708
Location: New Holland, PA
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| Posted: Mon Jan 28, 2008 6:04 pm Post subject: Gear leg shimmy experiments |
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Jim & Buz:
I wonder if the aircraft sitting in the shop in one place for an extended period of time, (while in the build process), would cause one or both of the tires to go out of round?
Jim!
From: owner-lightning-list-server(at)matronics.com [mailto:owner-lightning-list-server(at)matronics.com] On Behalf Of jng62433(at)aol.com
Sent: Monday, January 28, 2008 8:29 PM
To: lightning-list(at)matronics.com; PLClients(at)PacificLife.com
Subject: Re: Gear leg shimmy experiments
Hi Buz
I enjoyed reading your email regarding the wheel shimmy. My lightning was finished in Shelbyville in September and had shimmy on some of the taxi and landing. After I flew back to Florida and have logged 40 hours on the airplane the right gear seems to be the one that shimmy. As of last week it had become almost violent. I hose clamped a hardwood dowel on the right gear leg and there was no apparent improvement. At this point I removed the wheel point as I felt the weight of the wheel pant might be contributing to the oscillation/shimmy. However, this had no effect on the problem.
I spoke to Nick last Thursday and he said that some lightning owners had replaced the Air null tires with a better tire. Jacking up and taking the weight off the tire I found the right tire had a obvious hump and was out of round. I checked with the aircraft maintenance facility at my airport and he had some tires distributed by Michelin which he said that he had real good results. The Air null tires are 4 ply and the new tires are 6 ply with a higher load rating. I bought two and replaced both main tires and I did a lot of high speed taxis and lifted the nose and ran on the mains only. I could aggravate it and cause it to shimmy at that point I was running 32 lbs pressure, I reduced the tire pressure to 28 lbs and went back out for multiple runs and the shimmy has stopped. However,&nb sp;the disc and the brake assembly are not a precision unit, therefore you can assume it is going to grab on the rotation differently as the wheel turns and heavy braking pressure is applied. It will and can cause the gear leg to shimmy occasionally. After 40 hours on the Air null tires most of the wear on the tread was on only 25 percent of the outer side of the tire. Most of the times these airplanes are flown at less than gross weight and I think the fact that tire foot print on the ground is only 25 percent more tire area on the ground would be better.. I would think that some of the camber could be taken out when the axle brackets are welded and this would give us a bigger foot print more toward the middle of the tire. As we all know many aircraft have similar problems. I certainly enjoyed seeing you at the open house last year.
Jim Goad
N166JG
jng62433(at)aol.com
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N1BZRich(at)aol.com
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| Posted: Mon Jan 28, 2008 6:30 pm Post subject: Gear leg shimmy experiments |
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Hi Jim,
Good to hear from you. I think one of the high points of the first Lightning open house what seeing your airplane make the initial flight. You did a fantastic job on your beautiful airplane and I really like your paint scheme. I wonder why?
I think your analysis of the gear leg shimmy on your airplane is right on target. Of all the things that effect shimmy, I think I mentioned all of them in previous messages with the exception of camber. I totally forgot to mention that aspect of the "equation", but as you said, camber certainly has an effect on how much tire tread is in contact with the runway/ramp. I am glad to hear that changing to a "better" quality tire which probably has a better "grip" helped your situation. Nick and I had previously discussed that, and the success he had with the demo Lightning by going to better tires. I will likely go to Michelins or Goodyears when I change tires next time. My current Air Tracks have 417.1 hours on them and I just reversed the wheels to balance out the tire wear, so I should get another 100 to 200 more hours on them.
The shimmy experiments we did on Linda's Lightning using the aluminum "V" angle clamped to the trailing edge of the gear legs certainly seemed to help (she is still testing them out), but in reality what we probably did was to change the stiffness of the gear leg and therefore only change the speed at which it might occur. Hopefully the change will work for the speeds that the Lightning sees on normal landings. However, with the clamps holding on the aluminum "stiffner" it will be very hard to get the gear leg fairings on, so this is probably not the ideal answer. The wooden dowels like you tried seem to work only if they are "robust" enough to stiffen the gear legs the proper amount, and of course the same fairing problem arises if you clamp them on.
So bottom line, if your success with changing to "better" tires works well on your airplane it will likely work for other Lightnings as well. Therefore, you have come up with a great course of action in my book. I am sure all others on this group appreciate you sharing this. It is good to know that someone else other than the demo has had success with using better quality tires. You might want to let the group know the specific Michelin you used. I know they are 500x5, 6 ply, but is there a specific model number or name?
Blue Skies,
Buz
Start the year off right. Easy ways to stay in shape in the new year.
[quote][b]
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N1BZRich(at)aol.com
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| Posted: Mon Jan 28, 2008 7:02 pm Post subject: Gear leg shimmy experiments |
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In a message dated 1/28/2008 9:06:04 P.M. Eastern Standard Time, pequeajim(at)gmail.com writes:
| Quote: |
I wonder if the aircraft sitting in the shop in one place for an extended period of time, (while in the build process), would cause one or both of the tires to go out of round? |
Hi Jim,
I am sure you are right on, but it does not take that much time for an aviation tire to go out of round. Just a day or two will do it, especially when it is colder. Since the aircraft tires that we use are generally not "radial", they develop a flat spot pretty quickly, but, by the time you taxi to the runway, the heat generated will have them back to round. I think I am still right that the aircraft tires we use are bias tires (not radial) because the cost do develop radial tires for smaller general aviation would not be cost effective for the big tire makers. Probably commercial aircraft, airliners etc, have radials, but they also have different wheels. As far as I know you are not supposed to put radial tires on aircraft wheels designed for bias tires, and vise versa.
While we are talking tires and mounting them, I hope everyone knows that if your tube has a yellow stripe on it, you should line up the yellow strip on the tube with the red dot on the tire when you mount it. If the tube does not have a yellow stripe, then line of the red dot on the tire with the inflation valve on the tube. If you don't do the above, your assembly will not be balanced.
Blue Skies,
Buz
Start the year off right. Easy ways to stay in shape in the new year.
[quote][b]
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