Gun sights don't match FG bullets deacceleration

[NikolaiVChr]

This is due to a bug in FG where wrong unit is used in drag computation so they do not slow down as fast as they should.

Will temp. fix this by using wrong units in F-16 gun sights also.

In AIBallistic.cxx it does this calc with speed being in KT, but it should be in FPS:

speed -= (Cdm * 0.5 * rho * speed * speed * _drag_area/_mass) * dt;

rho is in slugs/ft3
_drag_area is in ft2
_mass is in slugs
Cdm is unitless
dt is in s

[NikolaiVChr]

Have committed a workaround for this.