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50 Tail wheel shimmy

 
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viperdoc(at)mindspring.co
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PostPosted: Mon Apr 16, 2007 9:52 am    Post subject: 50 Tail wheel shimmy Reply with quote

Jack the tail up. Check 6 things as follows;
1) lateral movement (play) in the vertical strut. The shim on the mounting
bolt that goes thru it to support the entire assembly attaching the shaft
to the fusalage can be worn.
2) take the fork with the tailwheel locked and try to rotate it from side
to side (left and right). If it will rotate slightly, check the shim tab
that goes into shaft of the strut. this tab is right above the neck of the
fork. It looks something like rectanglar nail head like the kind used to
hold rails in the 1800's togather for lack of a better way to discribe it.
3) Check the locking pin as it slides down into the metal ring on the shaft
of the fork. Does it have lateral play. Does it completely lock?
4) What shape are your tailwheel bearings in? You will have to pull the
wheel off the assembly to check those. Are the races smooth or pitted. How
old is the grease that is in them.
5) Is the wheel collar nut tight (but not so tight as to impeed the free
rotation of the tire.) This has nothing to do with shimmy, but is the
collar nut safety wired?
6) What shape is your rear wheel tire in? Scuffed, worn, out of round. Does
the tailwheel make grinding noises when spun? Is it properly inflated?
This has nothing to do with shimmy but it will make the tailwheel bounce if
it is not properly inflated. That is the aft strut shock absorber. It also
will contribute to fore/aft play in the tailwheel if the bearings in the
shaft and aft cap (where it mounts to the aft bulkhead) are worn. Combine
the worn bearing (aft shock absorber cap/bulkhead mount) with a worn shim
on the main bolt that mounts the vertical tailwheel shaft to the empanage
and you get a tailwheel assembly that will just flop around in any
direction when push, pulled or rotated. That gets your attention too.
Hope this helps. I will send you the excellent article that Tom Johnson did
on the tail wheel assembly. It did a better job than our handbook did on
sorting out tailwheel issues.
Doc
Quote:
[Original Message]
From: flir47 <me262pilot(at)comcast.net>
To: <yak-list(at)matronics.com>
Date: 4/16/2007 11:37:28 AM
Subject: 50 Tail wheel shimmy



Anyone had an issue with TW shimmy on a Yak 50???

If so what is the silver bullet?

--------
You built and fly an RV.......BIG DEAL!!!!


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mark.bitterlich(at)navy.m
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PostPosted: Mon Apr 16, 2007 10:58 am    Post subject: 50 Tail wheel shimmy Reply with quote

These are all good things Doc, and well worth mentioning.

However, what I have found to be the biggest contributor to tail wheel
shimmy is simply that the tail wheel is NOT LOCKED when you touch down.
In a lot of YAK-50's, as soon as you take off and for any reason push
the stick forward, the tail wheel will unlock and then it will COCK off
to one side in flight. When you go to land, the pin is not in the hole
and the tail wheel will start a very violent swing back and forth and
then will suddenly STOP as you get slow enough for the locking pin to
drop into the hole.

None of the steps you have listed below will stop this.

The only solution I found was to re-design the point where the tail
wheel releases with the stick. I wrote an article on exactly how to do
that some years ago, and I think Tom might have published it again from
time to time. He also has done a good job accumulating a lot of
information others have written, along with what he has discovered
himself.

In my 50, I have it now where the control stick has to be pushed ALL the
way forward... I mean to the limit... Before it will unlock the tail
wheel. This means it is kept locked for the entire flight and then
LANDS with it locked. MAJOR shimmy is now gone... Very minor stuff is
still there, but it is caused by the issues you have explained below.
If anyone wants to know how to go about doing this, send me a line and I
will dig up the article and send it again, or I can explain it person to
person.

Mark Bitterlich
N50YK

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PostPosted: Mon Apr 16, 2007 11:00 am    Post subject: 50 Tail wheel shimmy Reply with quote

It depends on what type of shimmy it is. What kind of landings are you
doing? Wheel or 3 point?

Is the shimmy very violent and then suddenly stops as you slow down?

Or is it bad to start and then decreases evenly with speed.

If it is the latter, then follow Doc's recommendations. If it is the
former, we need to talk.

Mark Bitterlich


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viperdoc(at)mindspring.co
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PostPosted: Mon Apr 16, 2007 11:05 am    Post subject: 50 Tail wheel shimmy Reply with quote

Thanks but not really! Just had to deal with a case of it from the aft
shock absorber bushing, the strut shim, wheel bearings, and later having
to replace the aft shock absorber after the aluminum end cap (the aft
bearing mount) elongated. Am still looking for a replacement shock absorber
after a friend gave me his extra new one.
Doc
Quote:
[Original Message]
From: flir47 <me262pilot(at)comcast.net>
To: <yak-list(at)matronics.com>
Date: 4/16/2007 1:42:44 PM
Subject: Re: 50 Tail wheel shimmy



Thanks DOC!!!!!! You da man!

--------
You built and fly an RV.......BIG DEAL!!!!


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jan.mevis(at)informavia.b
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PostPosted: Mon Apr 16, 2007 11:31 am    Post subject: 50 Tail wheel shimmy Reply with quote

Very interesting, Mark! I certainly would also prefer having the tailwheel
locked when landing. But does this mean that your tailwheel is also locked
during taxi ? This could be nasty if you have to make a relatively short
turn. Or am I (again) missing something? If you've got to unlock the
tailwheel by pushing the stick completely forward, then a taxi with tail
wind might also be rather awkward. I'd liked to get a copy of that article!

All the best,

Jan Mevis
RA2005K

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PostPosted: Mon Apr 16, 2007 2:01 pm    Post subject: 50 Tail wheel shimmy Reply with quote

It is more than likely that your tail wheel is coming unlocked in flight, and then is "canting" off to one side. Take a look at Doc's picture to see a very vivid example of that very thing.

When you wheel land the aircraft... Which to many is much easier than 3 pointing it .. You are going a lot faster. As you lower the tail to the ground gently, the tail wheel is thrown violently to the other extreme, and it is more than likely that the tail wheel locking pin will not have time to drop through the hole. This means that the tail wheel will be slamming back and forth with great speed and will tear the shit out of the tail wheel.

There are some things you can do as a pilot that will help. When you let the tail wheel down, do so gently and then lift it back up again. If you are lucky, it will lock and then you can lower it again without shimmy. Gently is the key here, force is not needed.

The other alternative... And a better one as far as I am concerned, is to adjust the linkages on the cable that control the locking pin, so that it stays in the hole longer as you push in on the stick. This adjustment is NOT at the tail wheel, but instead is underneath of the right horizontal stab's fiberglass fairing. If you let the pin down too far, you can cause it to come right out of the housing, and that is just as bad if not worse than what you have now.

The answer is to have the pin come down through the whole correctly, and then STOP.. But to only do that when the stick is all the way forward. To accomplish that, you will need to move the cable end to the next hole back in the bolt pattern to where it attaches to the elevator tube that connects the left horizontal stab to the right horizontal stab. You then attach a wire SNUB to the cable so that it will not go down too far. This snub will grip the cable itself and come to a stop at the cable housing.

Pictures would help here.. But....

Mark Bitterlich
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PostPosted: Mon Apr 16, 2007 2:11 pm    Post subject: 50 Tail wheel shimmy Reply with quote

Jan, I have simply changed the point at where the tail wheel unlocks and
where the control stick is when that happens.

In most 50's when you push slightly forward of neutral, the tail wheel
will unlock. Thus, if you wheel land it and push forward on the stick,
you just made SURE that the tailwheel was unlocked.

On mine, I changed this to where you have to push the control stick
every inch of the way FORWARD in order to unlock the tailwheel. So. You
can still unlock it for taxi as you mentioned, but unless you are doing
heavy inverted stuff like tumbles and so forth, it is doubtful that you
will ever push the stick ALL the way forward in flight ... That in turn
causes the tailwheel to unlock. The only time I actually UNLOCK the
tailwheel now is when I have to make a sharp turn.

Probably one of the best ideas I had, and never implemented, was to
simply move the cable away from the stick and attach it to something
else so that I could lock and unlock the tailwheel by a different device
.. I.E. NOT THE STICK. This is how all the Sukhois do it anyway.

You are perfectly correct about the issue of now having to push the
stick all the way forward to unlock it when you are also dealing with a
tailwind. It IS rather awkward in that situation. But that situation
is not all that common. Landing is. This small issue is still yet
another reason to just put in a stand alone lever to control tail wheel
locking.

Mark

P.s. I'll look for the article Jan but you could also search the
archives and search for "tail wheel shimmy".


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PostPosted: Mon Apr 16, 2007 5:45 pm    Post subject: 50 Tail wheel shimmy Reply with quote

Mark,
If you can send me those pictures when you can. I had not thought of moving
the cable attachment to the lower bolt on the elevator attachment (for the
right and left).
Doc
Quote:
[Original Message]
From: Bitterlich, Mark G CIV Det Cherry Point, MALS-14 64E
<mark.bitterlich(at)navy.mil>

Quote:
To: <yak-list(at)matronics.com>
Date: 4/16/2007 5:14:14 PM
Subject: RE: Re: 50 Tail wheel shimmy


MALS-14 64E" <mark.bitterlich(at)navy.mil>

Quote:

It is more than likely that your tail wheel is coming unlocked in flight,
and then is "canting" off to one side. Take a look at Doc's picture to see

a very vivid example of that very thing.
Quote:

When you wheel land the aircraft... Which to many is much easier than 3
pointing it .. You are going a lot faster. As you lower the tail to the

ground gently, the tail wheel is thrown violently to the other extreme, and
it is more than likely that the tail wheel locking pin will not have time
to drop through the hole. This means that the tail wheel will be slamming
back and forth with great speed and will tear the shit out of the tail
wheel.
Quote:

There are some things you can do as a pilot that will help. When you let
the tail wheel down, do so gently and then lift it back up again. If you

are lucky, it will lock and then you can lower it again without shimmy.
Gently is the key here, force is not needed.
Quote:

The other alternative... And a better one as far as I am concerned, is to
adjust the linkages on the cable that control the locking pin, so that it

stays in the hole longer as you push in on the stick. This adjustment is
NOT at the tail wheel, but instead is underneath of the right horizontal
stab's fiberglass fairing. If you let the pin down too far, you can cause
it to come right out of the housing, and that is just as bad if not worse
than what you have now.
Quote:

The answer is to have the pin come down through the whole correctly, and
then STOP.. But to only do that when the stick is all the way forward. To

accomplish that, you will need to move the cable end to the next hole back
in the bolt pattern to where it attaches to the elevator tube that connects
the left horizontal stab to the right horizontal stab. You then attach a
wire SNUB to the cable so that it will not go down too far. This snub will
grip the cable itself and come to a stop at the cable housing.
[quote]
Pictures would help here.. But....

Mark Bitterlich
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PostPosted: Mon Apr 16, 2007 8:21 pm    Post subject: 50 Tail wheel shimmy Reply with quote

Thanks, Mark! I'm going to implement your modification too.

As for tailwheel issues, the biggest problem I once had was the tailwheel
construction getting disconnected from the oleo due to improper
safety-wiring. The connecting rod could rotate almost freely in the oleo.
This happened after taxi and take-off from a rough gras field. Fortunately I
could make a very short emergency landing on another grass field.
Jan
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PostPosted: Tue Apr 17, 2007 1:25 pm    Post subject: 50 Tail wheel shimmy Reply with quote

The problem is HOW you go about adjusting it so that you CAN move the
unlock position all the way forward.

If you take the fairing at the right horizontal stab root off, you will
be able to see the mechanism that controls the up and down movement of
the tail wheel locking pin. The adjustment that you see at this
location is there to control the DEPTH of the pins movement into the
locking ring hole located just above the tail wheel tire. Adjusting this
does not really solve the problem very well, because it is possible to
adjust the pin down too far, and then... You simply run out of pin and
it drops down through the locking pin hole with cable hanging on behind
it.

You will notice that the cable itself is connected to a BOLT on the
shaft that connects the left and right horizontal stabs together. If
you move the cable attachment to another bolt you can make it to where
the pin pulls out (upwards) ONLY when the stick is moved ALL the way
forward.

However, when you do this... You will notice that when you start to pull
the stick all the way AFT.... There will be so much extra slack in the
cable that the pin will drop DOWN and then OUT of the locking ring hole

To prevent THIS from happening, you have to build a cable snubber. This
can be something so simple as a bolt and a nut with a vertical groove
cut down the middle of the threaded portion of the bolt itself. Picture
that in your mind. You slide the cable through the vertical cut in the
bolt, and then when you screw on a nut.. The nut will PINCH the cable to
the top of the cut slot as you tighten it. You then position this
device on the cable so that you control how far DOWN the LOCKING PIN can
go as you pull the stick AFT.

A picture would really help here.

However, if you pull the fairing off, and then look at it as you read
this message, I am betting that it will become obvious.

I invented this method a few years ago, and have been using this lashup
ever since... It really stopped the tail wheel shimmy cold... At least
the VIOLENT type that just eats your tail wheels ALIVE. I used to get
just months out of the tail wheels, now I get years.

Mark Bitterlich
N50YK


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PostPosted: Wed Apr 18, 2007 3:23 pm    Post subject: 50 Tail wheel shimmy Reply with quote

Using a stand alone control lever for tailwheel lock:

In many ways, I think that is the VERY BEST SOLUTION TO THE PROBLEM!

Simple, completely pilot controlled, and not tied in with controls that
might conflict with tail wheel locking needs.

I wrote an article about tailwheel shimmy in my YAK 50 around 6 or 7
years ago. It discussed how to adjust/move the cable attach point and
install the snubber so as to allow the 50's tailwheel to remain locked
through-out the complete flight. The way I verified what was causing
the mess to begin with was by having a friend flying close form on
landing filming the tailwheel with a VHS recorder/camera.

Since we have re-opened this conversation, a point came up that Tom
Johnson has educated me about concerning YAK 50 tailwheel locking pins.
The pin on a lot of 50's has a SHOULDER on it that prevents it from
coming down too far and allowing the pin to fall completely right out of
the hole it slides in to. Tom Johnson has looked at three YAK-50's that
have this shoulder on the pin. I own TWO 50's (one is a parts only
aircraft) and neither of them have a shoulder that prevent the pin from
coming right on out the bottom. Now, either the pins shoulder wore off
of what I have, or the hole it goes in is bigger, or it is somehow a
different part. I need to do a close examination and take pictures to
figure out why his three are different from my two.

However the real issue being discussed is whether it is ok to rely on
the shoulder to keep the pin from sliding down too far, or is it better
to add the snubber, so that the cable ITSELF keeps the pin from going
down too far. In my case, it HAD to be a snubber (or some like device
such as a cable swage). In Tom's case, his pin "shoulder" allowed him
not to use a snubber and instead rely on the shoulder.

If you follow the Russian instructions for setting the pin depth, the
cable and not the shoulder prevent pin over-travel and the shoulder is
there apparently as a backup. That is an ASSUMPTION on my part and is
not verified. Thus, I lean towards still using a snubber on the cable
and not relying on a shoulder. Keep in mind though that with my
airplane I never had the option.

It ends up being the call of every individual how to go about doing it.
Tom Johnson's method does not require you to move the cable attach
point. My method does require that you move it, because I want that
stick ALL the way forward to unlock the tailwheel, not 80%, but ALL the
way. Either method ends up with the cable going slack when you then
pull the stick all the way back. I add a snubber on the cable to
prevent the cable from ever going slack and thus also keeping the pin
from sliding down too far in that condition. Tom is allowing he cable
to go slack and is relying on the shoulder on the pin which so far he
has seen on every 50 he has looked at, but that mine do not have.
Either method works.

Bottom line, if you make these adjustments to prevent shimmy, be careful
that you understand all the ramifications of what you are doing.

r/s,

Mark Bitterlich
N50YK

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PostPosted: Wed Apr 18, 2007 7:22 pm    Post subject: 50 Tail wheel shimmy Reply with quote

Mark,
What year is your 50? Mine is a 78 and I do not recall seeing a shoulder on
the locking pin. I will double check tomorrow PM.
I have adjusted my cable all the way out in the manor that is shown in
Tom's article. The cable will not adjust far enough out to allow full
forward stick throw. I go about 2-3 inches past neutral on the stick. I did
that after the picture that you saw was taken when I figured out that was
part of my problem. Those that have flown formation on me since have not
commented on the tailwheel being canted off center. The last pictures taken
after that have not shown it to be canted off center. I have not had
another episode of sudden kicking of the tail with touch down either.
Doc
Quote:
[Original Message]
From: Bitterlich, Mark G CIV Det Cherry Point, MALS-14 64E
<mark.bitterlich(at)navy.mil>

Quote:
To: <yak-list(at)matronics.com>
Date: 4/18/2007 6:28:53 PM
Subject: RE: Re: 50 Tail wheel shimmy


MALS-14 64E" <mark.bitterlich(at)navy.mil>

[quote]
Using a stand alone control lever for tailwheel lock:

In many ways, I think that is the VERY BEST SOLUTION TO THE PROBLEM!

Simple, completely pilot controlled, and not tied in with controls that
might conflict with tail wheel locking needs.

I wrote an article about tailwheel shimmy in my YAK 50 around 6 or 7
years ago. It discussed how to adjust/move the cable attach point and
install the snubber so as to allow the 50's tailwheel to remain locked
through-out the complete flight. The way I verified what was causing
the mess to begin with was by having a friend flying close form on
landing filming the tailwheel with a VHS recorder/camera.

Since we have re-opened this conversation, a point came up that Tom
Johnson has educated me about concerning YAK 50 tailwheel locking pins.
The pin on a lot of 50's has a SHOULDER on it that prevents it from
coming down too far and allowing the pin to fall completely right out of
the hole it slides in to. Tom Johnson has looked at three YAK-50's that
have this shoulder on the pin. I own TWO 50's (one is a parts only
aircraft) and neither of them have a shoulder that prevent the pin from
coming right on out the bottom. Now, either the pins shoulder wore off
of what I have, or the hole it goes in is bigger, or it is somehow a
different part. I need to do a close examination and take pictures to
figure out why his three are different from my two.

However the real issue being discussed is whether it is ok to rely on
the shoulder to keep the pin from sliding down too far, or is it better
to add the snubber, so that the cable ITSELF keeps the pin from going
down too far. In my case, it HAD to be a snubber (or some like device
such as a cable swage). In Tom's case, his pin "shoulder" allowed him
not to use a snubber and instead rely on the shoulder.

If you follow the Russian instructions for setting the pin depth, the
cable and not the shoulder prevent pin over-travel and the shoulder is
there apparently as a backup. That is an ASSUMPTION on my part and is
not verified. Thus, I lean towards still using a snubber on the cable
and not relying on a shoulder. Keep in mind though that with my
airplane I never had the option.

It ends up being the call of every individual how to go about doing it.
Tom Johnson's method does not require you to move the cable attach
point. My method does require that you move it, because I want that
stick ALL the way forward to unlock the tailwheel, not 80%, but ALL the
way. Either method ends up with the cable going slack when you then
pull the stick all the way back. I add a snubber on the cable to
prevent the cable from ever going slack and thus also keeping the pin
from sliding down too far in that condition. Tom is allowing he cable
to go slack and is relying on the shoulder on the pin which so far he
has seen on every 50 he has looked at, but that mine do not have.
Either method works.

Bottom line, if you make these adjustments to prevent shimmy, be careful
that you understand all the ramifications of what you are doing.

r/s,

Mark Bitterlich
N50YK

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PostPosted: Thu Apr 19, 2007 9:31 am    Post subject: 50 Tail wheel shimmy Reply with quote

I have an 84 and a 74.


Mark


Roger Kemp <viperdoc(at)mindspring.com> wrote:
[quote]--> Yak-List message posted by: "Roger Kemp"

Mark,
What year is your 50? Mine is a 78 and I do not recall seeing a shoulder on
the locking pin. I will double check tomorrow PM.
I have adjusted my cable all the way out in the manor that is shown in
Tom's article. The cable will not adjust far enough out to allow full
forward stick throw. I go about 2-3 inches past neutral on the stick. I did
that after the picture that you saw was taken when I figured out that was
part of my problem. Those that have flown formation on me since have not
commented on the tailwheel being canted off center. The last pictures taken
after that have not shown it to be canted off center. I have not had
another episode of sudden kicking of the tail with touch down either.
Doc
Quote:
[Original Message]
From: Bitterlich, Mark G CIV Det Cherry Point, MALS-14 64E

Quote:
To:
Date: 4/18/2007 6:28:53 PM
Subject: RE: Re: 50 Tail wheel shimmy

--> Yak-List message posted by: "Bitterlich, Mark G CIV Det Cherry Point,
MALS-14 64E"

[quote]
Using a stand alone control lever for tailwheel lock:

In many ways, I think that is the VERY BEST SOLUTION TO THE PROBLEM!

Simple, completely pilot controlled, and not tied in with controls that
might conflict with tail wheel locking needs.

I wrote an article about tailwheel shimmy in my YAK 50 around 6 or 7
years ago. It discussed how to adjust/move the cable attach point and
install the snubber so as to allow the 50's tailwheel to remain locked
through-out the complete flight. The way I verified what was causing
the mess to begin with was by having a friend flying close form on
landing filming the tailwheel with a VHS recorder/camera.

Since we have re-opened this conversation, a point came up that Tom
Johnson has educated me about concerning YAK 50 tailwheel locking pins.
The pin on a lot of 50's has a SHOULDER on it that prevents it from
coming down too far and allowing the pin to fall completely right out of
the hole it slides in to. Tom Johnson has looked at three YAK-50's that
have this shoulder on the pin. I own TWO 50's (one is a parts only
aircraft) and neither of them have a shoulder that prevent the pin from
coming right on out the bottom. Now, either the pins shoulder wore off
of what I have, or the hole it goes in is bigger, or it is somehow a
different part. I need to do a close examination and take pictures to
figure out why his three are different from my two.

However the real issue being discussed is whether it is ok to rely on
the shoulder to keep the pin from sliding down too far, or is it better
to add the snubber, so that the cable ITSELF keeps the pin from going
down too far. In my case, it HAD to be a snubber (or some like device
such as a cable swage). In Tom's case, his pin "shoulder" allowed him
not to use a snubber and instead rely on the shoulder.

If you follow the Russian instructions for setting the pin depth, the
cable and not the shoulder prevent pin over-travel and the shoulder is
there apparently as a backup. That is an ASSUMPTION on my part and is
not verified. Thus, I lean towards still using a snubber on the cable
and not relying on a shoulder. Keep in mind though that with my
airplane I never had the option.

It ends up being the call of every individual how to go about doing it.
Tom Johnson's method does not require you to move the cable attach
point. My method does require that you move it, because I want that
stick ALL the way forward to unlock the tailwheel, not 80%, but ALL the
way. Either method ends up with the cable going slack when you then
pull the stick all the way back. I add a snubber on the cable to
prevent the cable from ever going slack and thus also keeping the pin
from sliding down too far in that condition. Tom is allowing he cable
to go slack and is relying on the shoulder on the pin which so far he
has seen on every 50 he has looked at, but that mine do not have.
Either method works.

Bottom line, if you make these adjustments to prevent shimmy, be careful
that you understand all the ramifications of what you are doing.

r/s,

Mark Bitterlich
N50YK

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