Shore battery guns

[whels]

Pyro or HT,

When shooting SBs, should the shots alway  go inline with the crosshair?  Ive noticed that at
times, when i shoot the shot trail will go left, right of the + by as much as a half inch.
Ive even noticed say i shot and it shows i need longer shot. I raise and shoot, n shot lands same
place or shorter.

does the SB have a built in variance?

[RAM]

displacement to the left could be due to rifling of the cannon and the gyro effect on the spinning round after exiting the muzzle
the other issue I have no idea.

S!

[SmokinLoon]

displacement to the left could be due to rifling of the cannon and the gyro effect on the spinning round after exiting the muzzle
the other issue I have no idea.

S!

Um... no.  If the projectile was a lead ball, then maybe.   

[Murdr]

Ive worked with rifled artilliry and RAM is correct about the effect he described.

[Murdr]

Its air pressure not gyro though.

[Cthulhu]

Its air pressure not gyro though.

Murdr, can you elaborate on that? Are you talking about an uneven pressure distribution across the base of the projectile as it exits? Like having a poor crown on your rifle muzzle?

I'm familiar with this displacement phenomenon with tube-launched spin-stabilized rockets, but in that case it's due to a combination of what we call "tip off" and gyroscopic precession. Gravity makes the nose drop when the forward bore-riders/sabots clear the tube before the rears do, and precession in turn causes a yaw when the nose pitches down. But that means the round would always yaw in the same direction for a given gun design, depending on left or right hand twist, which I don't think is what you're describing.

Just curious...

[Murdr]

I was refering to the Magnus effect which is relatively small, but still needs to be accounted for in the fire direction control procedures for firing large projectiles from a rifled bore.

I piped in because RAMs post reminded me that of the mortars the US Army uses, the M30 is rifled.  The manual fire direction control procedures for smooth and rifle bore mortars are the same except for one thing.  The rifled mortar had an extra step of adjusting the deflection data for drift.  IIRC it was always either 4 or 5 mills for the M30 (1 mil is 0.05625 of a degree).

Sorry for the hijack.  I took a look offline last night, and it looked to me like there is a small built in dispersion with the SB.

[Cthulhu]

I was refering to the Magnus effect which is relatively small, but still needs to be accounted for in the fire direction control procedures for firing large projectiles from a rifled bore.

Makes sense . Reminder that gun rounds seldom actually fly at zero AOA.

[Jerlle]

It doesn't have to do with the angle of attack of the round.  A cylinder has no angle of attack.  No matter what angle you set it at when the air hits it from the side it is the same since it is symetrical.  The Mangus effect comes from a basic aerodynamics problem.  As a cylinder rotates it generates lift if it is in a uniform flow field.  It can be modeled by adding three basic flows together.  Uniform flow + 2D doublet flow (which is a source and sink flow very near each other) + a 2D vortex flow.  According the Kutta-Joukowski theorem which says, in an inviscid, incompressible flow, the resulting force per unit span is equal to density*velocity(incoming flow)*circulation.  This works for any object in fact.  The circulation is what is calculated from the three basicly flows being added together.  If you want a bit more in depth description you can check out this page...

http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html