On Saturday 18 September 2004 23:40, Arnt Karlsen wrote:
> On Sat, 18 Sep 2004 13:21:13 +0100, Vivian wrote in message
>
> <[EMAIL PROTECTED]>:
> > Arnt Karlsen wrote:
> > > Sent: Friday, September 17, 2004 9:47 PM
> > > To: FlightGear developers discussions
> > > Subject: Re: [Flightgear-devel] Problem with ballistic
> > > sub-model
> > >
> > >
> > > On Fri, 17 Sep 2004 16:09:12 +0100, Vivian wrote in
> > > message
> > >
> > > <[EMAIL PROTECTED]>:
> > > > Arnt Karlsen wrote:
> > > > > Sent: Friday, September 17, 2004 10:03 AM
> > > > > To: FlightGear developers discussions
> > > > > Subject: Re: [Flightgear-devel] Problem with ballistic
> > > > > sub-model
> > > > >
> > > > > On Fri, 17 Sep 2004 07:56:42 +0100, Vivian wrote in
> > > > > message
> > > > >
> > > > > <[EMAIL PROTECTED]>:
> > > > > > Ampere K. Hardraade wrote
> > > > > >
> > > > > > > Sent: Thursday, September 16, 2004 7:12 PM
> > > > > > > To: FlightGear developers discussions
> > > > > > > Subject: Re: [Flightgear-devel] Problem with
> > > > > > > ballistic sub-model
> > > > > > >
> > > > > > > On September 16, 2004 01:08 pm, Vivian Meazza 
wrote:
> > > > > > > > There are some other basic shortcomings as well:
> > > > > > > > the submodel doesn't inherit the parent
> > > > > > > > accelerations, or the velocities and
> > > > > > > > accelerations due to roll, pitch and yaw. Only
> > > > > > > > release droptanks when flying straight and level
> > > > >
> > > > > ..uh, in the real world, this is possible if not
> > > > > permissible, with fun consequences like one or more
> > > > > hard points releases jammed for at least a while etc.
> > > > >
> > > > > > > They shouldn't inherit accelerations.
> > > > > >
> > > > > > Quite right - they shouldn't. I was getting over
> > > > > > enthusiastic there, and forgetting my Newtonian
> > > > > > physics.
> > > > >
> > > > > ..don't worry, there is also Murphy law physics.  ;-)
> > > >
> > > > Right, back to Newton :-). I think I've solved the
> > > > problem. Mixing elevation up = positive with  speed down
> > > > = positive  nearly made my brain blow a fuse..  ;-)
> > > >
> > > > I had to reverse a number of signs to get it right. I
> > > > took the opportunity to add roll to the submodel so that
> > > > droptanks will come off with the right orientation. I
> > > > not yet added either the parent rotational speed to the
> > > > submodel, or yaw, so if you release droptanks with
> > > > significant roll rate or yaw angle on the aircraft the
> > > > submodel will not be quite right. Straight and level, or
> > > > nearly so, is fine.
> > >
> > > ..precisely, we will need roll rate, yaw, yaw rate, pitch
> > > rate etc,
> >
> > I can do all of that, providing I can get at the location of
> > the CofG to relate the offsets.
> >
> > > but no accelerations except gravity, to get it right.
> >
> > Not strictly true. We also need to apply aerodynamic forces.
> > Drag is already applied, and wind can be applied, but no
> > other. Wind is that experienced by the parent, not the
> > submodel. This approximation is OK for tracer, less so for
> > bombs.
>
> ..eh, accellerations, no, forces, yes.  Both "bomber" and
> "bomb" sees the same wind etc until release of child.  In a
> bomb bay or in a gun, the wind exposure happens as these
> objects emerge outta these shielded hideouts.
>
> ..If either (plane or bomb etc) object passes thru say wind
> shear, wing tip vortices, then the wind forces are
> _different_, even if they can be approximated as "the same" as
> the bomb drops thru that vortice in a millisecond.
>
> ..and don't forget gun recoil forces.  Gun "childs" also
> experience wind drift.  ;-)
>
> > > ..also, when we get that far in the modelling; some
> > > dive bombers had release rigging that threw some, say
> > > centerline bombs, clear of the propeller, adding to the
> > > fun we dream up here.
> >
> > We can already do that - just apply an appropriate initial
> > velocity, and instantiate at the right offsets.
> >
> > > ..also keep in mind most bombs are hung by more than one
> > > points, so the hardpoint mechanism and the flight
> > > conditions, attitude, rates etc, act together deciding
> > > which points release first, second etc on each bomb.
> >
> > We can probably ignore that.
>
> ..true, but see below.
>
> > > ..this too, has a major impact on the initial ballistics,
> > > think bobbing bombs dropping from B-52's or B-17's, on
> > > dropping out of the bomb
> > > bay, some of this is sudden exposure to the airstream,
> > > some is "un-even" release, asymmetric or whatever.
> >
> > We could probably add some randomness to account for this,
> > if you think it's a significant factor, given all the other
> > approximations, chief amongst which could be that the
> > submodel has no inertia, and so aligns instantly with its
> > trajectory. Again, OK for tracer, but for bombs?
>
> ..this is a design philosophy decision; how close to reality
> _do_ we wanna go?  My point is "do as you like, but don't
> cut off future development by hardcoding stuff, leave open
> hooks as bait for future developers to go berserk on." ;-)

The development philosophy behind FG seems, to me, to be: get 
something working, then refine it:)

I'm still at the get-it-working stage:)

LeeE

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