Constantly setDir the chute to its original direction. You might also try a constant setVectorUp but I have had strange experiences using that on chutes, so maybe on the car? Could also use Kronzky's drop script.
// ammo_in_chute.sqf
// res = [my_ammo_box1, getMarkerPos "mk_1"]execVM"ammo_in_chute.sqf";
_box = _this select 0;
_pos = _this select 1;
_para = "ParachuteWest" createVehicle [0,0,0];
_para setPos [_pos select 0, _pos select 1, 100];
_dir = getDir _para;
while {(getPos _para select 2) > 2.5} do
{
_box setPos (_para modelToWorld [0,0,-2]);
_para setDir _dir;
_box setDir _dir;
Sleep 0.002;
};
_box setDir _dir;
_box setPos [getPos _para select 0, getPos _para select 1, 0];
// ammo_in_chute.sqf
// res = [my_ammo_box1, getMarkerPos "mk_1"]execVM"ammo_in_chute.sqf";
_box = _this select 0;
_pos = _this select 1;
_para = "ParachuteWest" createVehicle [0,0,0];
_para setPos [_pos select 0, _pos select 1, 100];
while {(getPos _para select 2) > 2.5} do
{
_box setPos (_para modelToWorld [0,0,-2]);
_box setVectorDir (vectorDir _para);
_box setVectorUp ([ 0, 0, 1]);
Sleep 0.002;
};