projectile angle of attack

[Abstractness]

Question: When the projectile leaves the barrel it is spinning in order to maintain its orientation. if a battle ship fired at 20 degrees gun elevation and the projectile then descended at -20 degrees onto the target, does this mean that the projectile landed on the target like glider with an 'angle of attack' of 20+20=40 degrees? Didn't this misalignment reduce the armor piercing capablities of the projectile?

[Steve Crandell]

I think that if there was no air resistance, that is what would happen. Bullets don't do that though; they tend to orient themselves to coincide with the path, and I think that is because of the sharp nose.

Hopefully someone with a better ballistics background can follow up on this.

[marcelo_malara]

Question: When the projectile leaves the barrel it is spinning in order to maintain its orientation. if a battle ship fired at 20 degrees gun elevation and the projectile then descended at -20 degrees onto the target, does this mean that the projectile landed on the target like glider with an 'angle of attack' of 20+20=40 degrees? Didn't this misalignment reduce the armor piercing capablities of the projectile?

Two versions of the phenomena.

1-I have seen graphs explaining the movement of a rifle bullet in flight and showing its axis not exactly aligned with the line of flight, but this misalignment been much less than the elevation at firing

2-For my part I would say that any angle of attack would produce a lift force that would partially cancel the force of gravity, and so this would eextend the range of the gun. So I would say that the projectile would tend to align exactly with the line of flight, and that the maintenance of an angle of attack would need the presence of an aerodynamic surface.

[wadinga]

Hello Abstractness,

This article may help:

http://www.navweaps.com/index_tech/tech-073.php

Which includes

A good shell design will combine the aerodynamics, the mass properties, and the spin of the shell to permit the projectile to be pointy end forward for the entire flight.

However this article was last updated in 2000, and I believe there have been useful recent articles by Messrs Santarini and the most excellent William Jurens in Warship International, a journal to which all here might consider subscribing and currently is in deed of more enthusiast support to continue its vital work. (Usual disclaimer).

All the best

wadinga

[Abstractness]

This article may help:
http://www.navweaps.com/index_tech/tech-073.php
Which includes

A good shell design will combine the aerodynamics, the mass properties, and the spin of the shell to permit the projectile to be pointy end forward for the entire flight.

wadinga

Thank you. The article continues:

Unfortunately, the very shape of a shell contributes to instability. A WWII era shell generally had a windscreen on the front, and most of the weight concentrated towards the rear of the body. If no spin was imparted by rifling, the shell would tumble or stabilize at an angle of attack near 90. This is undesirable, since it means that the shell would be flying sideways (not good for range or penetration).

Therefore the Bismarck's shells must have been impacting at weird angles when shooting far.

[Abstractness]

I figured something out. If you would shoot it with exactly the right amount of spin, it could impact at the correct angle. This is because once the projectile is in horizontal flight, the air tries to pull up the nose of the projectile which has the center of mass further back. However, due to the gyroscopic effect, the pulling up on the nose results in the nose starting to move to the left or right. The projectile now gets into a gyroscopic precession around the horizontal axis of air stream just like a spinning top precesses around a vertical axis of gravity. If the parameters are right the projectile will have precessed by 180 degrees exactly when the horizonal flight ends end turns into the descend phase. In this case, the nose will now be pointing dowards by roughly the right amount of angle. This means that the projectile will first have have been flying straight out of the cannon, then it will have generated lift, then it will have curved its path to the left or right, then it will have generated a bit of downforce, and finally it flies straight towards its target.

[OpanaPointer]

And that's why guns are rifled.

[Thorsten Wahl]

Therefore the Bismarck's shells must have been impacting at weird angles when shooting far.

No
Alignment of the axis of rotation follows trajectory.

[OpanaPointer]

Therefore the Bismarck's shells must have been impacting at weird angles when shooting far.

No
Alignment of the axis of rotation follows trajectory.

Yep. The whole point of rotation was to avoid deviation of trajectory to the limits of the physics involved.

[Abstractness]

Therefore the Bismarck's shells must have been impacting at weird angles when shooting far.

No
Alignment of the axis of rotation follows trajectory.

Sort of, but it only works if you have exactly the right amount of spin, as I figured out afterwards:

I figured something out. If you would shoot it with exactly the right amount of spin, it could impact at the correct angle. This is because once the projectile is in horizontal flight, the air tries to pull up the nose of the projectile which has the center of mass further back. However, due to the gyroscopic effect, the pulling up on the nose results in the nose starting to move to the left or right. The projectile now gets into a gyroscopic precession around the horizontal axis of air stream just like a spinning top precesses around a vertical axis of gravity. If the parameters are right the projectile will have precessed by 180 degrees exactly when the horizonal flight ends end turns into the descend phase. In this case, the nose will now be pointing dowards by roughly the right amount of angle. This means that the projectile will first have have been flying straight out of the cannon, then it will have generated lift, then it will have curved its path to the left or right, then it will have generated a bit of downforce, and finally it flies straight towards its target.

Too little spin as well as too much spin is gonna make it impact at the wrong angle.

[Steve Crandell]

Well, since they do follow the trajectory fairly closely, they must all have nearly the "correct" spin.

[OpanaPointer]

Well, since they do follow the trajectory fairly closely, they must all have nearly the "correct" spin.

Fudge factors allowed?

[dale3242]

Unfortunately, the very shape of a shell contributes to instability. A WWII era shell generally had a windscreen on the front, and most of the weight concentrated towards the rear of the body.

Most of the mass of solid shot would be towards the rear of the projectile. However, naval shells contain a cavity in the rear of the projectile for a bursting charge. The explosive charge in cavity is considerably less dense than the steel in which the shell is manufactured.