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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