KR> Nose wheel shimmy

[laser...@juno.com]

 Dan said:

. . . . there is one sure fire cure for nose wheel shimmy.

I'd agree with that!

Mike

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KR> RE: KR nose wheel shimmy

[jscott.pi...@juno.com]

On Sat, 25 Apr 2009 16:16:47 -0400 "Dan Heath" 
writes:
> I have to be a "smart A" here and say that there is one sure fire 
> cure for nose wheel shimmy.
> 
> Daniel R. Heath - Lexington, SC

Yep.  Then you get a tail wheel shimmy. :o)

FWIW, caster, tire pressure, and damping all apply whether we're talking
about nosewheel or tailwheel shimmy.  I get an occassional shimmy in the
tailwheel on my KR.  Since I can't adjust the tire pressure in my hard
tire, and can't adjust the caster unless I want to bend the tailwheel
spring, it is always addressed with damping by snugging down the nut on
the tailwheel shaft where it goes vertically through the tailwheel frame.
 I usually need to snug it down every 75 - 100 hours to prevent shimmy.

Jeff Scott
Los Alamos, NM

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KR> RE: KR nose wheel shimmy

[Dan Heath]

I have to be a "smart A" here and say that there is one sure fire cure for
nose wheel shimmy.

See N64KR at http://KRBuilder.org - Then click on the pics 
See you at the 2009 - KR Gathering in Mt. Vernon, Ill
There is a time for building and a time for FLYING and the time for Flying
has begun.
Daniel R. Heath - Lexington, SC

-Original Message-

If that were my plane I'd have that nose gear 

KR> nose wheel shimmy

[J M]

In the late fifties  some Ercoupes would experience a shudder during  
take off or landing (I don't remember which) and the quick cure was  
to reduce the nose wheel tire pressure about five pounds which set  
the pressure to about 20 psi. I don't think that they ever shuddered  
like the one in the video though.

John Milland

KR> RE: KR nose wheel shimmy

[Mark Wegmet]

Mark L's response to my comment, and my expansion on my earlier remarks.

>Mark W wrote:

>> If the caster angle and wheel & tire balance are right, you shouldn't
have
>> this problem... think GM/Ford/Chrysler front end alignment... not
shopping
>> cart. I've yet to see a Corvette experience 'shimmy' at speeds exceeding
>> anything a KR would land at.


>Mark L wrote:
>I would have to disagree with that.  You can have a perfectly balanced 
>wheel/tire, but external forces such as which way the wheel is pointed at 
>touchdown, or even which way the wind is blowing, can get the shimmy 
>started.  That's a side to side phenomenon, ..
>Mark Langford
>

Maybe I should have emphasized caster angle a little more... caster angle
(the angle from the vertical pivot point to the contact patch of the tire on
the pavement, similar to the trail on a motorcycle) can improve the
stability and natural tendency of the tire to track true. I agree with all
of the points that Mark L has made, but the video I saw showed the shimmy
continuing all the way down the runway... if the caster angle and balance
had been properly done, the shimmy would tend to cancel out as the plane
went down the runway. Granted, I know more about motorcycles and cars than I
do about planes, but once the bird is on the ground, I would think similar
rules would apply. Yes damping is important, but if the geometry of the
set-up is off, it won't help the shimmy unless the gear is "locked in" (too
stiff) which kind of defeats the purpose of a castering nose wheel doesn't
it?

JMHO

Mark W
N952MW

KR> RE: KR nose wheel shimmy

[Mark Langford]

Mark W wrote:

> If the caster angle and wheel & tire balance are right, you shouldn't have
> this problem... think GM/Ford/Chrysler front end alignment... not shopping
> cart. I've yet to see a Corvette experience 'shimmy' at speeds exceeding
> anything a KR would land at.

I would have to disagree with that.  You can have a perfectly balanced 
wheel/tire, but external forces such as which way the wheel is pointed at 
touchdown, or even which way the wind is blowing, can get the shimmy 
started.  That's a side to side phenomenon, not a "perpendicular to the 
axis" thing.  But it's really a matter of insufficient damping, which is the 
real cure for a shimmy like that, regardless of what got it started.

There's an aerodynamic effect (it has a name but it's been 10 years since I 
read about it) which describes how this oscillation happens to aero 
controls, or even stop signs in a high wind...similar to flutter.  Damping 
is the key to fixing it, not balance.  Stop signs have no damping, just a 
metal post which acts a spring to help perpetuate it.  Diehl accounts for 
damping through washers or something, and as mentioned, there's an 
adjustment/cure.  If that were my plane I'd have that nose gear off and be 
checking for cracks and wallowed out attachment points though


Mark Langford
N56ML "at" hiwaay.net
website at http://www.N56ML.com