Today's Message Index:
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1. 02:47 AM - Re: Gear leg shimmy experiments (Malcolm Ferguson)
2. 04:13 PM - Re: Gear leg shimmy experiments (N1BZRich@AOL.COM)
3. 04:52 PM - Gear leg toe-in (Peter and Jan Disher)
4. 06:52 PM - Re: Gear leg shimmy experiments (Johnny Thompson)
5. 09:17 PM - Re: Gear leg toe-in (N1BZRich@aol.com)
6. 09:51 PM - Re: Gear leg shimmy experiments (N1BZRich@AOL.COM)
Message 1
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| Subject: | Gear leg shimmy experiments |
There was extensive discussion on another list that escapes me for the
moment, concerning gear leg shimmy. It was a while ago but if my memory
serves me right, the conclussion went along the following lines.
The cure was slight toe out under all weight conditions, say half a degree
or so. With any toe in the tyre will try to track inwards. As the leg slopes
out the inward motion tries to stand the leg up. As the plane has inertia
and a heavy weight it does not give, so ultimately the tyre is squashed down
till it finally looses adhesion. As it flicks outwards it moves upwards
slightly and travels past its neutral point then begins its cycle over
again. It finds a natural harmonic at a speed peculiar to an individual
aircraft and surface and does not happen on surfaces with reduced adhesion
such as grass,dirt,stones etc.
I have suffered from shimmy sporadically for a long while till I adjusted
the toe in after following that discussion-relatively easy to do on my
earlier design spring steel gear legs.
As an aside I have been trying to decide what aircraft is to be my next
project(if the drought ever breaks here in south OZ) and have narrowed it
down to a Legacy FG or a Lightning with a Rotax 914turbo. In most cases the
Lightning looses nothing to the Legacy except in sheer speed. The Lightning
actually has a better useful cockpit load and half the fuel consumption.
I reckon the 914 should be capable of cruising at VNE at FL150 on
23-24litre/hr which would put it only 40 kts less than the Legacy. One
lovely advantage to the Legacy is its high glass transition temperature
allowing sexy paint schemes with dark colours.
Just dreaming about aviation is almost as much fun as flying.
Buz, I eagerly await your next epistle. They are always enlightning.
Malcolm Ferguson
Early model Esqual 912s 340hrs
Message 2
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| Subject: | Re: Gear leg shimmy experiments |
Hi Malcolm,
Good to hear from you again. I guess I did not realize you have a Rotax
on your Esqual. How do you like it and how long to get your 340 hours. Do
you only use auto fuel? What type and size prop are you using. Speed
results? Too bad we are not closer, I think a race is in order.
Also didn't know that you have the older style spring gear legs. That
would make it easier to adjust toe in/out with shims. Interesting that the
article you mention says that toe out is the key. Can you direct me to that
discussion? What ever (in or out) I know that the "toe" will make a
difference, but with an airplane as light as these there is a big percentage of
weight
change from it being at gross weight to it being fairly light. As a result,
I think the toe would probably need to be different for different weights,
tire pressure, etc., which we can't do. Therefore we need to know what works
best for the range of aircraft weights. That may or may not be 1 degree toe
in that the Esqual and Lightning kits call for. Actually the 1/2 degree of
toe out that you mention makes more since in my mind (especially since these
are not tail draggers). But I am convinced that to completely cure the shimmy,
it will probably take a combination of the correct toe (in or out???) and
some way to stiffen the legs to change the shimmy, harmonic vibration, or
whatever we want to call it. That is why the RV guys use the broom stick method
- it is a big help even if it is not the total real answer.
Blue Skies,
Buz Rich
**************Start the year off right. Easy ways to stay in shape.
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
Message 3
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Hi Buz,
In my manual for toe-in it states 2.25" over 57.5" for each side, 1 in
60 I though was app. 1 Deg. there for the manual would be over 2 Deg.
would it not ?
Pete Disher
OZ
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| Subject: | Re: Gear leg shimmy experiments |
Hi All
I am just days away from taxing my lighting so have no knowledge of
this type of gear leg. I have been a pilot, test pilot and maintenance
tech for about 39 years. The only vibration I have ever felt in the
landing gear was caused by tires out of balance, bearings or bullets.
Again I have never had this type of gear legs. I have installed the
tires in the position recommended by the manufacture and have balanced
the tires. This always helps and I would think these legs need all the
help they can get. My questions:
How many have or had a problem with leg shimmy?
Has anyone spin balanced the tires and did you have a shimmy problem
before or after the balance?
What is the weight (heavy/light) of the aircraft when it is most prone
to shimmy?
Thanks
Johnny Thompson
Message 5
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| Subject: | Re: Gear leg toe-in |
Pete, your math is right. When we recently adjusted Linda's I think I
remember using 1" at 58" for 1 degree. I don't have the manual in front of me,
but the numbers you quoted would give over 2 degrees. Some other important
parts of setting the toe is to have some weight in the cockpit (not sure what
amount the manual recommends) so that you have the gear legs spread to a normal
position. Then make sure the centerline of the aircraft does not move while
you do the measurements for toe. One technique (if you don't have the jig
that they use at the factory) is to drop a plumb bob from the nose and tail
then use a chalk line to connect the two spots. This line establishes the CL
of the aircraft on the hangar floor. Then measure out perpendicular from that
center line to the centerline of each main tire. That then becomes the
start point for setting the 1" at 58". Of course, you probably already knew all
of this.
Buz
**************Start the year off right. Easy ways to stay in shape.
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
Message 6
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| Subject: | Re: Gear leg shimmy experiments |
Hi Johnny,
Certainly mounting aircraft quality tires on the rims in the correct way
is important for balance. Some people don't seem to know what the paint
spot on the tire is for. But you will have to wait for Nick to answer your
question about how many Lightnings have had shimmy. I think he is still away
from SYI and may even be in Arizona to make the initial flight on several
Lightnings out that way. The shimmy situation has to do with the round rod gear
leg (RVs I think initially identified the problem) and it varies from airplane
to airplane, and seems to have something to do with toe, tire pressure,
aircraft weight, speed, landing/taxi surface, and how you are holding your
tongue. It does not occur on grass, and only happens at the slower speeds, say
around 20 to 22 mph. And it does not happen on every landing (could be the
tongue holding thing). On my airplane it happens on most every landing roll out
(22 mph) and is worse when the airplane is light weight with higher tire
pressure and on black top. Not quite as bad on one concrete runway I often land
on. You will know it when it happens and if you have someone watching you
they will see it as well. Don't confuse this with the possible nose gear
shimmy that is caused by the castering nose wheel being too loose in the bracket
that holds it on. Just tighten the 8 nuts/bolts a little so there is some
"feel of tightness" when you have the weight off the nose and turn the nose
wheel from side to side (the nose wheel is in the air for this test with
someone pushing down on the tail of the airplane).
As to spin balancing an aircraft tire, I have never done that for any of
the 6 airplanes I have owned but it could not hurt. My experience in other
airplanes that weigh about what a Lightning or Esqual does (my single seat
Pitts) was that I never got any shimmy (no round rod gear leg), but tire pres
sure certainly made a difference as to how hard it was to keep straight on
landing roll out- lower pressure was easier. Of course the Pitts touched down
much faster than a Lightning or Esqual. Bottom line, all the things we
mentioned above will help or hinder the tendency to shimmy. The key seems to
be get
the toe right (not sure what right is at this point), make sure you have the
tires mounted properly on the rims, start with no more than about 25 psi in
the mains, stiffen the gear leg by some method as discussed before, and hold
your tongue just right.
Good luck on that first flight. Let us know how it goes.
Blue Skies and check six,
Buz
My questions:
How many have or had a problem with leg shimmy?
Has anyone spin balanced the tires and did you have a shimmy problem before
or after the balance?
What is the weight (heavy/light) of the aircraft when it is most prone to
shimmy?
Thanks
**************Start the year off right. Easy ways to stay in shape.
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
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