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Good afternoon again.
I just remembered another trick about zero-speed
rolling models. Below a threshold speed (say 1 m/s) you make the force
proportional to the velocity. That way you'll get zero force at zero
speed. The other thing that can happen if you don't is that you'll
oscillate about the zero speed point. This will stop that
oscillation.
Hope it helps
Nickolas Hein
Morgantown WV
----- Original Message -----
Sent: Saturday, November 15, 2003 1:14
PM
Subject: Re: [Flightgear-devel] Airport
vehicle (driving) sim
Good afternoon,
This sort of modelling is standard practice for
training simulator companies. They identify a "zero-speed" model for
landing gear reactions. Now you'd think that if it's standard practice
someone would have it written down so newcomers wouldn't have to figure it out
all over again every time, but I don't know where it's written if it is.
You may be able to search the net for zero-speed rolling model and find
something. I've done some of this type of modeling in my own
professional past but I'm not certain that I'm an authority. I do
know that you can identify a separate break-out coeff. of friction (separate
from the skidding one) so you get the right release in a severe
wind.
I've done some other ground handling modeling
including severe turns (where the inside tire introduces alot of resistance
because it is twisting about a point) and tire carcass burn through when
you land with the parking brake set (this happens more often than you'd think
- training pilots for it has helped reduce the frequency)
If any of those things are of interest let me
know and I'll pitch in to help.
Nickolas Hein
Morgantown WV
----- Original Message -----
Sent: Saturday, November 15, 2003 12:25
PM
Subject: Re: [Flightgear-devel] Airport
vehicle (driving) sim
David Megginson wrote:
> Paul Surgeon writes:
>
> I don't know about everyone else's experience but I haven't
found
> > one aircraft in FG that wants to sit still on the ground
even with
> > the engine off.
>
> It might be that the
problem is not ground reactions but
> aerodynamics.
It's the
ground reaction code. :)
JSBSim and YASim do things pretty much the
same way, using a
coefficient of friction for gear as they slide over the
ground. This
integration works fine for a moving aircraft, but it's
really not
right for a stopped one. An aircraft with exactly zero
speed would
produce exactly zero force, and thus be "moved" by a wind
gust, and
then feel a very strong force in the opposite
direction.
An ideal mechanism would keep track of how much force each
wheel
"could" apply in the ground plane, and then calculate the right
amount
to apply to keep the aircraft from moving. This basically
comes down
to solving a bunch of simultaneous equations for each FDM
iteration.
It's a big mess; I'd be really scared of making this
work.
What I did play with at one point is a model where you
basically
switch gear models at slow speeds. If the brakes are on
and the gear
is moving below some threshold speed, then you "remember"
the "nominal
location" of the gear and use a damped spring kind of force
model. If
the spring force goes above the static coefficient limit,
then you
fall back to the sliding force model. This wouldn't be
hard to
implement, but tuning to avoid the inevitable weird behavior
would
probably be a mess.
:)
Andy
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