# Re: [Flightgear-devel] Jsbsim trim

```On Mon, 26 Apr 2004 12:14:49 +0200, Mathias wrote in message
<[EMAIL PROTECTED]>:```
```
> On Montag, 26. April 2004 11:49, Arnt Karlsen wrote:
> > > What you describe here will most likely not happen with someting
> > > only velocity dependent.
> >
> > ..no?  ;-)  No velocity, plane is parked.  Wee pushes, to exite the
> > resonance.  IME, the wee pushes straight aft gets the the plane
> > dancing quite a bit, try it.  ;-)
> Sorry, I do not have an aircraft ...
> :(
> But I can well imagine.
>
> > > Keep it in your head and try to move your aircraft around its nose
> >
> > ..not around the nose, around the nose wheel, usually around a point
> > somewhere between the nose wheel and the mains.
>
> Sorry, this is what I meant.
> By doing what you explained the vehicle will twist around its
> longitudinal axis.

..the wee pushes are ideally in paralell to the logitudinal axis, but
"takes advantage" of the moment arm of the wing to make up torque
pulses around a vertical axis, which sets up a yaw oscillation around
possibly the same vertical axis.

> This will mostly not change the weight force on the nose wheel.

..correct.  However it sees sidewall flexing forces.

> So this one will stay fixed. What changes is the weight force on the
> other two wheels which will loose static friction for a short time.

..wrong, except when you move the plane around on wet ice.  This
oscillation is driven by torque pulses, the resonance frequency is a
function of the mass distribution and the tire sidewall flexibility, and
the tires are held by the plane's weight and static friction.

> And when you apply a little side force during this time the aircraft
> will move. The nose wheel is fixed by static friction, thus the ac
> moves around the nosewheel ...

..depends on how you push, it is possible to get the plane dancing
around a point forward of the nose wheel, flexing the sidewalls the
same way, swinging around the nosewheel, "not" flexing the nose
wheel sidewalls, and around a point between the wheels, flexing
the nose wheel sidewalls and mains sidewalls, the oposite ways.

> The more I think about that, the more I can imagine that even that is
> in the Pacejka formula. Not with the real physical reason, but I can
> well imagine that it will just behave like that ...
> I am not shure how big the influence of the gear strut dynamics is in
> this case...
>
> Let's see ...
> I have to push this stuff I have now to Jon first ...

..amen!.  ;-)

--
..med vennlig hilsen = with Kind Regards from Arnt... ;-)
...with a number of polar bear hunters in his ancestry...
Scenarios always come in sets of three:
best case, worst case, and just in case.

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