On Fri, 10 Mar 2017 10:52:30 +0800, David R Brooks
wrote:

Yes, I know this isn't strictly metalworking, but there are plenty of
gun people here.
A howitzer fires, with elevation 45 degs. So the shell exits, pointing
upward at 45 degs., spinning. When it hits the target, it is (to 1st
approximation) falling at 45 degs., but the spin should have kept it
pointing upward. This means it would be flying sideways-on.
This sounds wrong to me, but where is the error?

The short answer is that the entire shell is effected by gravity. The
spin serves only to stabilize it just like a top. As the shell is
symmetrical there is effectively no lift developed by the spinning
shell, i.e., if one side is going up the other side with exactly the
same shape, size and velocity is going down :-)

The spin serves to stabilize the shell and is a complex subject. See
http://bisonballistics.com/articles/...y-robert-mccoy
for a review of a book giving a rather complete explanation of
exterior ballistics.

It might also be noted that a cross wind does effect the
bullet/shell's flight and a cross wind will generally move the
bullet/shell. A wind from the right tends to move the bullet in the
direction of 10 o'clock while from the left to toward the 4 o'clock.
Note that this movement depends on the direction of spin of the
bullet/shell as well as wind direction.

There has been rather extensive studies of exterior ballistics since
at least the 1800's. Siacci's theorem in dynamics is the resolution of
the acceleration vector of a particle into radial and tangential
components, which are generally not perpendicular to one another.
Siacci formulated this decomposition in two papers which were
published in 1879.
--
Cheers,

John B.