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.

> 
> ..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.
 
> ..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? 

Regards

Vivian




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