RE: [Jsbsim-devel] Spins, wrecks, and wind

[Jon B. <js...@ha...>]

> The long answer is that the technique we use to reproduce the
> characteristics of the airplane doesn't really allow it.  There are ways
> to build a spin simulator with the coefficient technique though I'm not
> sure if the resulting sim would be good for normal flight and, if not,
> whether or not there's a good way to combine both techniques into one
> simulation.


I had a discussion recently with someone about modeling spins using aero
coefficients and our aero modeling approach.  He relayed that he believed it
was possible, but that a change in coordinate system might be advantageous.
I agree that we could model spins with our approach.  It might require some
modifications to our aero code, but I think we could approximate spin
behaviour fairly well.  I have seen a paper on spin modeling and they used a
different approach - one that was meant solely for spins. However, this gets
into the philosophical issue, do we want to approximate - or possibly badly
model - spin behaviour and stand the chance of providing someone with the
wrong idea?


> > * During abnormal contact with the ground, I've seen some bizarre
> > behavior. Hitting hard on gear seems to cause the ship to recoil off the
> > ground at high speeds, and scraping hard can either result in recoil, a
> > scrape warning, or a crash detection. In my mind this means JSBSim aims
> > more to flag these events and rely on the parent application to
> > handle to proper effect on the aircraft. Is this a correct assessment?
>
> I would say, yes.  The trouble here is that once you contact the ground
> with anything other than the gear, the particular details of the
> aircraft structure become important and, landing gear excepted, we don't
> try to model structural dynamics.

We don't model structural dynamics, but I don't believe that is what is
being asked. We do aim to model ground reactions with a high enough degree
of fidelity to be absolutely believable. As I stated before, part of the
problem may be bad spring/damping coefficients, and also that we should be
breaking things and/or changing coefficients when loads are exceeded. The
thing is: what do we expect to happen when we land real hard? Should we
freeze the simulation rather than proceed with a false reaction? Should we -
in a gamelike approach - portray a canned crash scenario? Should we attempt
to model the dynamics of a crashed and deforming aircraft?  There are
several ways to look at it.

> >
> > * One of the things we're planning on is a cellular wind
> system, allowing
> > aircraft to experience fast changes in wind speed and direction
> depending
> > on location. The wind system in JSBSim seems to model changes only
> > globablly - did I miss something in the code? Secondly, JSBSim doesn't
> > seem to handle downdrafts or updrafts; again, is this a correct
> > assessment?
>
> JSBSim only knows what the wind is at the current position.  It's up to
> the parent app to know how wind varies as the aircraft position changes.

The parent app - in this case FlightGear - can provide wind direction and
magnitude at any time - it knows about the terrain.  However, JSBSim does
have the hooks, as I mentioned before, for providing tubulence modeling.
Adding code to do additional kinds of wind modeling is not terribly
difficult. There are papers written on this topic.

Jon