Re: Tail Wheel Shimmy - Attempt #2

[Bob Webster]

'Hello all again,

I finally figured out how to get this onto an E-mail without bugging John with it. There was a drawing of his bolt/axle angles that were highly exaggerated to get the point across, but this yahoo system will not allow pictures.

His fix was simply done by bending the spring. We don't have that luxury and the only way I can see to fix it is to have a shorter trailing link to move the wheel forward or weld some new fittings to the fuselage for the old link. Basically, according to this treatise, the top of our oleo strut should be pointing further towards the rear of the bird. I remember one note where somebody said, "Fairchilds shimmy -Wacos don't." I haven't a Waco handy to compare, but maybe the top of theirs is pointed aft more.

I did receive a note from my last attempt at this from John Casner saying that Bill Sprigs has made a casting that cured the problem. I am assuming that it is a casting of a shorter trailing link.

I hope this clarifies the whys and wherefores of our shimmy problem. We need to give credit to Gilbert Pierce and the Cub Club for this article. I hope it at least clarifies our problem. If anybody wants a copy of the drawing mentioned, let me know and I will E-mail it to your address.

Bob
____________________________________________________________________________________________________________

CONSTRUCTION & MAINTENANCE

Tail Wheel Shimmy

Gilbert Pierce Technical Counselor EAA Chapter 182 Gpiercel @midsouth.rr.com

I noticed when flying my Piper Clipper heavily loaded, I experience tail wheel shimmy on my Scott 3200 when landing on a hard surface such as concrete or asphalt. Several years ago I was parked at the landing end of runway 36L at Oshkosh. I always get tail wheel shimmy when landing there. This afforded me the opportunity to observe many landings as I lounged under my wing. It was here that I made the discovery that about 50% of the tail wheel airplanes landing on 36L experienced tail wheel shimmy. I believe the grooved runway exacerbates the problem. Anyway, those tail wheels were not just shaking side to side, they were rotating around their pivot axis 360 degrees and doing so violently. On my recent trip to Alaska I had tail wheel shimmy on almost every landing unless I really greased it on. On my return I vowed to solve the problem.

I started the quest for a solution on the Internet. I was told that if you raised your tail wheel tire air pressure it would assure the tail wheel would shimmy no more. I was told to reduce the tail wheel air pressure. I was told I had too much grease in the tail wheel. I was told that if the tail wheel didn't spit grease at you when you walked by it, it did not have enough grease, hence the shimmy. I was told to loosen my steering springs, I was told to tighten my springs. I was told that the pivot axis must be absolutely vertical so that the surface the tail wheel swivels on is parallel with the ground hence the pivot bolt would be vertical. Mine was. I was told the pivot bolt must face forward at the top, I was told the pivot bolt must face aft at the top. So what did I do? I took the tail wheel apart and made sure it was mechanically in top-notch condition and adjusted to the manufacturer's specifications again. It was. Then I tried each and every remedy listed above except changing the angle of the pivot bolt, no help. The Scott 2000 tail wheel does require some tension on the steering springs to control the unlock tension and hence shimmy. The Scott 3200 installation instructions say that chain tension is not required or recommended. The tail wheel condition and installation was determined to be in accordance with all of the available manufacturer's literature I could find. I still had shimmy on pavement with the aircraft close to gross weight.

Next I got out an old 1950's auto repair manual that explained king pin front wheel suspension systems and steering castor angle. If you have ever pushed a grocery cart through the supermarket with one of the front wheels shaking side to side you have experienced wheel shimmy and improper castor angle. What I learned from the chapter on steering alignment was basic steering geometry. To measure your tailwheel steering geometry, drop a line drawn parallel to and through the pivot axis (pivot bolt) and extend it to the floor and make a mark on the floor where this line hits or use a straight edge parallel to the steering axis shaft (pivot bolt). Next, drop a line vertically from your tailwheel axle to the floor or again use a straight edge and make a mark on the floor. This will also be where your tailwheel contacts the floor. Now move this line or straight edge that passed from the axle to the wheel/floor contact point horizontally until it intersects the pivot axis line at the pivot axis midpoint. The line that is parallel to the steering axis must hit the floor ahead of the line dropped vertically from the wheel axle. The angle formed by these two lines is your castor angle. The larger the castor angle the better as far as tail wheel shimmy is concerned. In other words, the farther ahead of the tail wheel that the steering axis line hits the floor the greater the castor angle and the less likely that will have shimmy. To put it another way, the steering axis pin or bolt must be vertical or tilted with the top pointing behind or to the rear of the airplane when the airplane is fully loaded. Emphasis on fully loaded.

When my airplane was empty the steering axis bolt was vertical. When I loaded the airplane, the tailwheel spring compressed and the top of the steering axis bolt was pointing to the front of the airplane. This would put the extension of a line drawn through the steering axis behind the tail wheel contact point. Bad news - it will now shimmy. You don't want the castor angle to be too large because it will make steering on the ground more difficult. The large castor angle will tend to lift the rear of the airplane slightly as you turn the aircraft. This is called self-centering effect. Having the steering axis bolt vertical or inclined slightly with the top pointing back when fully loaded should be sufficient.

So how do you correct this angle? There are two easy solutions. If your airplane is like most, the spring is bolted at the front to the airframe with a bolt that passes through the spring leaves. The spring then rests on a pad several inches behind the point through which the through bolt passes. Usually the spring is clamped to the pad at this point. You can add a shim between the pad and the spring to increase your steering angle. Or you can take the route I took. I took the spring off and laid it on a piece of poster board and traced out its arch. Then I took the spring to a spring shop and had them re-bend the spring until the tail wheel end of the spring was about 1-1/2 inches below the original as drawn on the poster board. In other words I increased the arch slightly. Walla-no more shimmy when loaded.

One other point. You should carry sufficient air pressure in your tail wheel to keep the tire firmly attached to the rim when it hits the pavement on landing. Due to the small diameter of the tail wheel, it accelerates very rapidly on contact. If you have insufficient pressure in the tire it will slip on the rim and cut the valve stem. You now have a flat tailwheel tire. I know- it's happened to me twice. I now carry a minimum of 45 pounds of pressure in my Scott tailwheel.. The same thing can happen to your main tires but with more surface contact area around the rim it is less likely to happen unless your plane lands at very high speeds.





[Non-text portions of this message have been removed]'

[Bob & Ileen Waldron]

'Bob Webster,

In your note, you said "> I did receive a note from my last attempt at this
from John Casner saying that Bill Sprigs has made a casting that cured the
problem. I am assuming that it is a casting of a shorter trailing link.

>

I use one of Bill Sprigs Trailing Links, and it works GREAT. The link is
LONGER than the original. I believe that the LONGER link achieves the
geometry that your article describes, i.e.. "Move the tailwheel behind the
steering point. I suspect that a SHORTER trailing link would be a disaster.

Bob Waldron
1939 Fairchild 24 K
Stillwater, MN'

[Jamie Treat]

'All,

The longer link works fine, just makes it harder to get the tail wheel over center when hand moving. Also does not steer as well in motion. Takes a little more lead time. Sometimes require a little brake also.

I also have a source of new Drag links, C&C machined out of Alum stock. They are not FAA/PMA approved. Fellow Fairchild owner makes them for me. Currently we are making only stock lengths. If the tailwheel is clean and within factory tolerance, shimmy will be nil. I maintain 4 F-24's, none of them have a shimmy. I also land my F-24 at around 80mph at 7000ft alt plus density. Fast moving, some times with a rear load and more fuel, I cannot keep the tail off as long as I want, still no shimmy.

Jamie S. Treat

----- Original Message -----
From: Bob & Ileen Waldron
To: fairchildclub@yahoogroups.com
Sent: Tuesday, June 10, 2003 1:41 PM
Subject: Re: [fairchildclub] Tail Wheel Shimmy - Attempt #2


Bob Webster,

In your note, you said "> I did receive a note from my last attempt at this
from John Casner saying that Bill Sprigs has made a casting that cured the
problem. I am assuming that it is a casting of a shorter trailing link.
>
I use one of Bill Sprigs Trailing Links, and it works GREAT. The link is
LONGER than the original. I believe that the LONGER link achieves the
geometry that your article describes, i.e.. "Move the tailwheel behind the
steering point. I suspect that a SHORTER trailing link would be a disaster.

Bob Waldron
1939 Fairchild 24 K
Stillwater, MN


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[Non-text portions of this message have been removed]

'

[Buell Powell]

'Jamie,
I am trying to find enough parts for a tailwheel. Walter Albert
has the strut and was missing a fork. I have found the fork but Walter has
had the strut checked and says it's in too bad shape to rebuild. Any ideas?
I also need a drag link so could you could give information on that too.
Also, I have a good source for the E80 starters if you need one.
Thanks,
Buell

----- Original Message -----
From: "Jamie Treat"
To:
Sent: Tuesday, June 10, 2003 7:59 PM
Subject: Re: [fairchildclub] Tail Wheel Shimmy - Attempt #2


> All,
>
> The longer link works fine, just makes it harder to get the tail wheel
over center when hand moving. Also does not steer as well in motion. Takes a
little more lead time. Sometimes require a little brake also.
>
> I also have a source of new Drag links, C&C machined out of Alum stock.
They are not FAA/PMA approved. Fellow Fairchild owner makes them for me.
Currently we are making only stock lengths. If the tailwheel is clean and
within factory tolerance, shimmy will be nil. I maintain 4 F-24's, none of
them have a shimmy. I also land my F-24 at around 80mph at 7000ft alt plus
density. Fast moving, some times with a rear load and more fuel, I cannot
keep the tail off as long as I want, still no shimmy.
>
> Jamie S. Treat
> ----- Original Message -----
> From: Bob & Ileen Waldron
> To: fairchildclub@yahoogroups.com
> Sent: Tuesday, June 10, 2003 1:41 PM
> Subject: Re: [fairchildclub] Tail Wheel Shimmy - Attempt #2
>
>
> Bob Webster,
>
> In your note, you said "> I did receive a note from my last attempt at
this
> from John Casner saying that Bill Sprigs has made a casting that cured
the
> problem. I am assuming that it is a casting of a shorter trailing link.
> >
> I use one of Bill Sprigs Trailing Links, and it works GREAT. The link
is
> LONGER than the original. I believe that the LONGER link achieves the
> geometry that your article describes, i.e.. "Move the tailwheel behind
the
> steering point. I suspect that a SHORTER trailing link would be a
disaster.
>
> Bob Waldron
> 1939 Fairchild 24 K
> Stillwater, MN
>
>
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>
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> To unsubscribe from this group, send an email to:
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>
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>
>
>
> [Non-text portions of this message have been removed]
>
>
>
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>
>
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>
>
>

'

[John Berendt]

'Bob I agree with Bob Waldron the longer link works well. Bill spriggs has
the mould for the casting you have to have the bushings installed in it but
it is worth the cost. Bill told me he was out of them but if there is a
demand he could have some more made. The link is about 5/8ths of and inch
longer than the original.
JohnBerendt

----- Original Message -----
From: "Bob & Ileen Waldron"
To:
Sent: Tuesday, June 10, 2003 2:41 PM
Subject: Re: [fairchildclub] Tail Wheel Shimmy - Attempt #2


> Bob Webster,
>
> In your note, you said "> I did receive a note from my last attempt at
this
> from John Casner saying that Bill Sprigs has made a casting that cured the
> problem. I am assuming that it is a casting of a shorter trailing link.
> >
> I use one of Bill Sprigs Trailing Links, and it works GREAT. The link is
> LONGER than the original. I believe that the LONGER link achieves the
> geometry that your article describes, i.e.. "Move the tailwheel behind the
> steering point. I suspect that a SHORTER trailing link would be a
disaster.
>
> Bob Waldron
> 1939 Fairchild 24 K
> Stillwater, MN
>
>
>
> To unsubscribe from this group, send an email to:
> fairchildclub-unsubscribe@yahoogroups.com
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>

'

[Airy-Hugues Millet]

'. If the tailwheel is clean and within factory tolerance, shimmy will be
nil. I maintain 4 F-24's, none of them have a shimmy. I also land my F-24 at
around 80mph at 7000ft alt plus density. Fast moving, some times with a rear
load and more fuel, I cannot keep the tail off as long as I want, still no
shimmy.

>
>Jamie S. Treat

Hy Jamie,

can precise if the 4 F24 you are running are SC10 tires and wheels equiped
or not?

I guess shimmy is a commitment of SC10 + working plays (becoming gap!!)+ an
original geometry nearby to enhance those effects!

For example, on our F24 with 10 inches tire, but from an old new stock, with
small ribs on its profile it didn't had shimmy, but as soon as ribs let the
place to a flat surface (perfectly smooth contours), we begin to met shimmy
phenomenom, and as well as the tire begin to be overworn, the shimmy
increase at each landing on hard surface!!

...will be interested in any experience...before choosing between rrruss
aircraft adaptor kit or finding a brand new tire ( does anybody know an
other company than desser, able to get STOMIL tires (oval ribs), as it is a
polish company from the michelin group!!)

Airy-H. MILLET

_________________________________________________________________
Trouvez l'âme soeur sur MSN Rencontres ! http://g.msn.fr/FR1000/9551'

[ranger440c5]

'Ahh.. The Fairchild shimmy. Always a source of discussion. Jamie is
correct and this concurs with our experiences. The 24 installation
will work and provide good service,IF, held close and tight,
unabused, and serviced well. I had made the mention that Wacos don't
seem to have this problem. The geometry is very similar but unlike
the 24 it is stout. That long, feeble, narrow 24 unit has a low
tolerance for slop. There are alot of parts in the installation to
add up slop. The drag link forward pivots,the wretched shoulder
screws, the split trunnion, the keys and their mating slots, the
rivets holding the keys, the top pivot/camplate and the top fuselage
thru holes. All these parts with 60 years of abuse, neglect and wear
add up to a sloppy mess that short of a 45 degree caster angle is
difficult to control. Yes adjusting the caster will affect the
tolerance for this slop. But the core problem is still there..control
the oscillation at its lowest amplitude. Just like a truck tarp, tie
it down snug and it won't flap...it doesn't get the chance. Arrest
the shimmy before it exists, this is done by closing up the
tolerances on the aforementioned parts. I believe the fellow in OR
with the OSH award winnig 24W afew years back ( Beck?) stated in
Sport Aviation that the keys are the secret, very precise. This is a
good place to start. Only so much can be done with the lengthening of
the drag link. The vee formed by the longerons gets mighty close and
remember, when you thump that 24 down tail first and compress the
oleo the drag link forces the lower end of the oleo aft as the
draglink swings toward the horizontal plane. Yes, square profile
tires tend to help. The other item often overlooked is the springs.
Coil springs take a set over the years and the more cycles the
sooner/worse it is. Stamping die people know all about spring
fatigue! Look at some pics from the 40's and alot of those ships look
to set a little higher out of the weeds. By the way if you are
wearing the tires on your mains on the inside (looks like some sort
of Messerschmitt)these springs are a little shorter too.
Unfortunately, unless you have a machine shop in your back pocket
this is a most trying task. The sure fire solution is to stay away
from hard surface!


--- In fairchildclub@yahoogroups.com, "Airy-Hugues Millet"
wrote:

>
> . If the tailwheel is clean and within factory tolerance, shimmy

will be

> nil. I maintain 4 F-24's, none of them have a shimmy. I also land

my F-24 at

> around 80mph at 7000ft alt plus density. Fast moving, some times

with a rear

> load and more fuel, I cannot keep the tail off as long as I want,

still no

> shimmy.
> >
> >Jamie S. Treat
>
> Hy Jamie,
>
> can precise if the 4 F24 you are running are SC10 tires and wheels

equiped

> or not?
>
> I guess shimmy is a commitment of SC10 + working plays (becoming

gap!!)+ an

> original geometry nearby to enhance those effects!
>
> For example, on our F24 with 10 inches tire, but from an old new

stock, with

> small ribs on its profile it didn't had shimmy, but as soon as ribs

let the

> place to a flat surface (perfectly smooth contours), we begin to

met shimmy

> phenomenom, and as well as the tire begin to be overworn, the

shimmy

> increase at each landing on hard surface!!
>
> ...will be interested in any experience...before choosing between

rrruss

> aircraft adaptor kit or finding a brand new tire ( does anybody

know an

> other company than desser, able to get STOMIL tires (oval ribs), as

it is a

> polish company from the michelin group!!)
>
> Airy-H. MILLET
>
> _________________________________________________________________
> Trouvez l'âme soeur sur MSN Rencontres ! http

://g.msn.fr/FR1000/9551'

[Jamie Treat]

'Airy-Hugues,

Still using Ribbed and Smooth Contour. Mine was Ribbed, now worn smooth. I find no difference between the two as for shimmy. The only source I have been using is Desser. Also pick up a old tire that is serviceable now and then.

Talked to Charlie Webb at Sun-N-Fun, he just converted to the Russ Adapter rings and modern tire. He still has a little shimmy now and then, but better. Also before you purchase the rings, Call Russ and give him your dimensions of the wheel, minor difference between P/Ns.

Jamie

----- Original Message -----
From: Airy-Hugues Millet
To: fairchildclub@yahoogroups.com
Sent: Wednesday, June 11, 2003 12:31 PM
Subject: Re: [fairchildclub] Tail Wheel Shimmy - Attempt #2



. If the tailwheel is clean and within factory tolerance, shimmy will be
nil. I maintain 4 F-24's, none of them have a shimmy. I also land my F-24 at
around 80mph at 7000ft alt plus density. Fast moving, some times with a rear
load and more fuel, I cannot keep the tail off as long as I want, still no
shimmy.
>
>Jamie S. Treat

Hy Jamie,

can precise if the 4 F24 you are running are SC10 tires and wheels equiped
or not?

I guess shimmy is a commitment of SC10 + working plays (becoming gap!!)+ an
original geometry nearby to enhance those effects!

For example, on our F24 with 10 inches tire, but from an old new stock, with
small ribs on its profile it didn't had shimmy, but as soon as ribs let the
place to a flat surface (perfectly smooth contours), we begin to met shimmy
phenomenom, and as well as the tire begin to be overworn, the shimmy
increase at each landing on hard surface!!

...will be interested in any experience...before choosing between rrruss
aircraft adaptor kit or finding a brand new tire ( does anybody know an
other company than desser, able to get STOMIL tires (oval ribs), as it is a
polish company from the michelin group!!)

Airy-H. MILLET

_________________________________________________________________
Trouvez l'âme soeur sur MSN Rencontres ! http://g.msn.fr/FR1000/9551


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[Non-text portions of this message have been removed]

'

[james chybicki]

'Hi Buell-
I have 2 drag links, and possiblly an extra Strut. I
will need to check that. I am off onto my Honeymoon
for the next 2 weeks, so if you can wait till early
July, please do.
Jim
P.S. did you ever find an instrument Bulkhead???
I am still looking. I have the flat bottom version,
but I need the type that has a drop down in the
middle.




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[Sierragroup1@aol.com]

'I can attest to "that fellow named Beck" (Larry Beck) in Oregon's statement
about the setup of the tailwheel. I now own that airplane, NC81369. I am
running a round profile smooth tire. No shimmy whatsoever. I owned another F24W46 a
few years ago, and we could never deal with the shimmy problem. Larry told me
that he spent hours and hours making sure that all of the tailwheel components
were per the original. There are no mods whatsoever. I have probably put 150
hours on the airplane and no shimmy problems.
Marshall Friedman'