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  #1  
Old 10-21-2021, 09:32 PM
Darinh Darinh is offline
 
Join Date: Oct 2005
Location: Kaysville, Utah
Posts: 130
Default Main Gear Shimmy - F1

I have an F1 that has the typical “Rocket Shimmy” at approximately 10-15mph during taxi. I was told this is normal and you either taxi slower or faster to avoid it. Well, I flew another F1 the other day (an incredibly well built example) that has the “shimmy” but it is exponentially less than mine. Both have the same gear and mount and both have the wood dampers installed.

This has lead me to wonder if the alignment on my F1 could potentially be off? In reviewing the logs, I see an entry where the builder “installed 1.5 degrees of toe-out” on each gear. Toe-out seems counterintuitive to me but I don’t have anything to base that intuition on. I have read the builders manual and it appears to illustrate a minute amount of toe-in for the gear. If I recall correctly, the manual gives 1/32” toe-in over a 13 5/8” block which is practically nothing but still a ever-so-slight toe-in.

Does anyone have any practical experience aligning the gear on an F1 (specifically toe-in vs. tie-out) and do you have any pointers or something that has worked to minimize the shimmy? By the way, the wear on my tires is uniform and the airplane tracks straight for takeoff and landing if that matters for the conversation.
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  #2  
Old 10-22-2021, 05:24 AM
sailvi767 sailvi767 is online now
 
Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,389
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I have zero shimmy in the gear on my Rocket however mine have the mod shortening the gear slightly. You might try different tire pressures as this can effect shimmy. We run 45 PSI. Info on gear mod is on Vince’s Rocket site.

http://vincesrocket.com/Additions%20...2010-27-04.htm
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Last edited by sailvi767 : 10-22-2021 at 05:40 AM.
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  #3  
Old 10-22-2021, 08:31 AM
Darinh Darinh is offline
 
Join Date: Oct 2005
Location: Kaysville, Utah
Posts: 130
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Quote:
Originally Posted by sailvi767 View Post
I have zero shimmy in the gear on my Rocket however mine have the mod shortening the gear slightly. You might try different tire pressures as this can effect shimmy. We run 45 PSI. Info on gear mod is on Vinceís Rocket site.

http://vincesrocket.com/Additions%20...2010-27-04.htm
Iíll look into this. I havenít ever heard of shortening the gear on an F1 but I know HRII gear are a bit shorter. Thanks for the info and link!
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  #4  
Old 10-22-2021, 09:12 AM
swaneymj's Avatar
swaneymj swaneymj is offline
 
Join Date: Mar 2005
Location: Oxnard, CA
Posts: 74
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I built mine with the "Indy" mod, shortening the gear legs 2.5". I bought the gear leg weldment from Jeff Mears, but still installed the wood dampers. I've had no shimmy. Toe-in/toe-out has been discussed extensively. Due to some toe-out and camber when I completed the installation my tires were not wearing correctly. I was almost to the point of replacing the tires after the first 35 hours. It took pulling the weldments, welding up the existing hole and redrilling to get zero toe-in/toe-out, which helped immensely. I still ended up with shims for camber. So I've pasted below a bunch of comments I found online when I was researching this issue. YMMV.
Mark Swaney
N76TR, F1 Kit #31

I found this on line:
f1rocket
04-07-2009, 04:16 AM
If you really want to fix your tire wear problem, you need to re-align your gear. First, a little background. The Rocket guys have been messing with this problem for some time because the heavier weight of the IO-540 makes minor gear alignment issues more acute. Also, our gear legs tend to flex a little more so having the proper alignment is really critical to having a good ground handling machine, and as a result, getting good tire wear. Here's what we've learned:

Your goal is to have ZERO toe with the airplane in the three-point configuration with it fully loaded with your normal flying weight. To get this, you have to put some grease plates under your tires and add weight to simulate fuel, pilot, etc.

Since your gear sockets are already drilled, you can either try some axle shims or you can re-drill your sockets to your gear legs. I'm not sure on your particular model of RV how the axles are attached but if you try for zero toe in this configuration, you will not only fix your tire wear problem, but improve your ground handling and gear leg shake.

If you can't get it to zero, having a little toe OUT is preferable to toe IN. For the Rocket guys, toe in is BAD!!.

I know there are lots of opinions around regarding whether you want toe in or toe out. Well, most of that talk is based upon hangar flying. I've persoanlly flown my Rocket with different toe configurations and I speak from experience, at least from a Rocket perspective. As always, YMMV.

rocketbob
04-08-2009, 04:29 PM
All you need to do is buy shims from Robby Grove, and set your alignment for zero toe, zero camber with full fuel. Its a lot easier to do on the RV-8 than it is on the Rocket. I have a friend with over 300+ hours on a set of tires on his F1 and I helped him set the alignment.

To set the toe, clamp two four foot angles to the brake disc's, parallel to the floor. Drop a plumb line from the CL of the firewall and another one at the tailwheel, and pull a string underneath both points. Measure from the front and back of the angles to each other and also make sure they are even with the center line.

Ed_Wischmeyer
04-09-2009, 05:47 AM
It's really counter-intuitive but on an airplane, you really want a small amount of toe OUT, not toe in like you'd have on a car. Somebody else will have a better explanation, but the reason is that if the plane is banked on the ground, toe in works against you for recovery. There was an article about this in Sport Aviation many moons ago.

When I bought my RV-4, the spec for toe out, measured across one axle, was 0 to .050". The builder had gotten a little enthusiastic, though, and had put in nearly 5/16" of toe out. After I fixed that, and it was an epic project, the RV-4 was so much easier to land... and the tires didn't need to be rotated in / out every 15 hours..
vfrazier
04-09-2009, 10:23 AM
Ed isn't wrong, but he isn't completely correct either. Toe out might help if you are losing control with a wing up, but toe in will help self correct with wings level. So, there isn't really a clear answer here other than zero camber and zero toe.

Here's what a Ladislao Pazmany, author of Landing Gear Design for Light Aircraft and a man who is (was) much smarter than anyone on this forum, including me, says on the subject:

Dear Vince:

Unfortunately your question doesn't have a straight answer. There is a big controversy on the subject. Please review the attached document in order to understand it better. (the attached document was scanned from my book Landing Gear Design For Light Aircraft )
Regards Paz

http://www.vincesrocket.com/toe-in.jpg

This is also posted at this URL if my photo insertion didn't work correctly.
http://www.vincesrocket.com/landing%...ed%20stuff.htm

Enjoy the mental wrestling while you debate this in your head. RocketBob and I agree that the optimum configuration is zero toe/zero camber at whatever weight you typically fly at. Full fuel works for most of us.

For you tapered spring gear leg RVdrivers, lotsa luck changing that alignment!

Mel
07-05-2012, 08:42 PM
Mike has it right. I discovered this while designing the landing gear for my Moni Motorglider.

Got most of my design information from Pazmany's book on landing gear design.

Think of it this way; if you have toe-in and you begin to swerve left, centrifugal force places more weight on the right wheel. If this wheel is toed-in, it will cause the aircraft to swerve MORE to the left. On the other hand if the wheel is toed-out slightly, it will cause the aircraft to be pulled straight.

I know there is a lot of controversy on this subject, but when I built my gear originally, I had a small amount of toe-in. The airplane was quite unstable on the ground. After research, I shimmed for a small amount of toe-out and the airplane became a "pussy-cat"!

Also if you look at the standard RV gear attached to the engine mount, the standard set-up is for straight ahead without weight on the gear. Now, because of the gear design, when you apply weight to the gear, the wheels will actually toe-out slightly as the gear spreads.

BTW, before you "A" drivers jump in, a tricycle gear configuration is entirely a different matter.

redhawk
07-06-2012, 06:14 AM
Hi Adam,
After many tries at getting consistent measurement results, Ive found the best way to measure toe in or out is to first roll your plane on to greased metal pans. By this I mean get 2 metal drip pans, pour a 1/4 cup of oil in their centers, then place a smaller square of flat " cookie sheet" on the area of oil. When you roll the plane on to these cookie sheets grab your prop and just pull down a few times and you'll see the wheels slide to their natural resting position. Make sure both tires are inflated identically, then simply take the longest level (straight edge) you can find (3 ft is good), place one end over the top of the axle nut holding it flat against the sidewalls, and let the other end rest on the ground with a piece of tape under it. Make sure the level is held flat against the tire sidewalls, and make a mark on the tape where the level touches the ground. Repeat this forward and aft for each tire, then measure the distance between the rear 2 marks and the front 2 marks and compare the difference !
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Mark Swaney - N76TR
Team Rocket F1 #31
KOXR, Oxnard, CA
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  #5  
Old 10-22-2021, 05:22 PM
Darinh Darinh is offline
 
Join Date: Oct 2005
Location: Kaysville, Utah
Posts: 130
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Thanks Mark! Great info. Sounds like I have a job ahead of me to get to 0-0. Done this several times on a 180 so I know the basic drill... the issue will be getting the correct shims or going Vince's route and cutting, bending and welding the axles.
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