REFLECTOR: Shimmy Damper 3.3

[michalk]

Sid,

In my reading of rheological oils, and also what I've gleaned from the 
LORD web site ... the company that probably made your damper, I found 
that the original oil was most likely a rheopectic oil.  This is an oil 
that gets more viscous with time in shear.  I think dilatant would work 
as well.

They have oils that are supposed to act in shear, like between two 
plates.  They have oils that are supposed to be forced through an 
orifice like in the LORD damper.

Andrew mentioned STP was supposed to be dilatant.  What happens if you 
try that one?

If you really want to experiment, try adding cornstarch to your oil. 
That should make it dilatant.  There are all sorts of powders that make 
dilatant mixes.  Maybe start with your first oil you tried.  Add 
cornstarch until it thickens up to an acceptable level.  The faster you 
try to actuate the damper, the (non-linearly) more force should be required.

Sid Knox wrote:
> Notes on the "Drag data" graph...
> 1) Notice the stiction at the beginning of movement. For the original
> Belview setup, once broken loose, the friction does not change much as a
> function of turning rate. The graph is somewhat "idialized" in that for my
> nosewheel the drag was not this smooth (steady) going from say 45 degrees
> one side, thru zero (straight ahead), to 45 degrees the other way. Probably
> because of the dirt in the washers (I purposely did not sanitize the 
> washers
> before making measurements as I wanted them to reflect as close as possible
> to the "real world" conditions).
> 
> 2) As expected, the hydraulic damper gave a "rate-based" drag. Or casting
> the situation into an electrical context, the damper lowered the ac gain 
> but
> left the dc gain high (low angular rate, for easy turning) whereas the
> original Belview setup simply "brute force" lowered the both the ac and dc
> gains. The lower ac gain inhibited the shimmy but the accompanying lowered
> dc gain meant that the pilot had to forceably "power" the wheel around
> (using lots of engine power/thrust and agressive braking) to initiate a
> sharp turn (overcome the stiction/friction) at slow taxi speeds.
> 
> 3) And also as expected, thicker oil in the damper lowered the ac gain even
> more with very little effect on dc gain. And cranking down on the big nut
> lowered both gains (more stability/harder to turn),
> 
> Sid Knox
> 
> 
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