WW2 Air Combat Footage: Squiggly Gunnery Footage

[BreakingBad]

Any idea why the old footage from WW2 aerial gun footage has a 'squiggly' image to it?  I've noticed is the plane camera gun footage shows tracer rounds with a 'looping effect'  (as if holding a rope, raising up and down forming a wave).  What is this optical phenomenon?

[Gman]

I believe it was Pyro here who once answered this, having studied ww2 a2a footage for graphics in the game.  I bet HT would remember this question as well.  From what I recall from what HTC said, in real life, the tracers aren't squiggly at all, it's just from the vibration of the camera in the planes from both flight and the firing of the guns.  If you talk to pilots who have fired guns they'll say the same thing I'll bet. 

A high school friend that flies the F18 told me the tracers with the 20mm Vulcan fly in straight line much like a rifle tracer, there is just more of them, and they're harder to see under certain conditions  in the F18 due to the gun being right in the line of sight (large muzzle flash in the dark/dusk/etc). 

I'm sure one of the old hands here will remember that question, as it's been a decade since it came up, at least the one I"m remembering Pyro answering.

[colmbo]

Classic effect of camera movement.

[MrRiplEy[H]]

I think most gun cam films are also in slow motion. It would be fun to see sometimes what the pilot actually had time to do when he was shooting.

[Scherf]

There's also the spiral effect of the spinning tracer material.

[BreakingBad]

it's just from the vibration of the camera in the planes from both flight and the firing of the guns.

I thought that might be the case, but the problem is that the image of the target doesn't have that corresponding bounce to it.  I've also seen still photos that show that whipsaw arc of the tracer rounds too.

It must be some sort of optical illusion, like how on TV a cars wheels can appear to be moving backwards at certain speeds.

Either that, or they just shot squiggly tracer's in WW2.

[Maverick]

Control inputs to try to bring the fire onto target will toss rounds in that manner. Think spraying water out of a hose and then moving it around trying to spray a moving object.

[rabbidrabbit]

There's also the spiral effect of the spinning tracer material.

How so?

[Scherf]

There's some kind of incendiary material at the base of the shell which produces the smoke. As the shell spins after being fired, the material spins too, which results in spiral smoke trails. The early LW gun gams seem to show this the best.

[rabbidrabbit]

What's the rotational speed of the shot?  I'd assume it would be in the order of many thousands of RPM's.  If they are rotating that speed, how does it produce a corkscrew instead of a averaged stream of smoke?

[morfiend]

What's the rotational speed of the shot?  I'd assume it would be in the order of many thousands of RPM's.  If they are rotating that speed, how does it produce a corkscrew instead of a averaged stream of smoke?

  The rotational speed was measured in inches per twist, something like 16inches per twist as an example.


     

[rabbidrabbit]

agreed,

Here is the formula:

Bullet RPM Formula
Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

[MrRiplEy[H]]

agreed,

Here is the formula:

Bullet RPM Formula
Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

RPM has no meaning, one revolution per foot means that you can see the spin easy in the smoke trail if the round is going to leave one instead of emitting a trail from the full width of the round. The spinning motion of the round probably ejects the smoke also outwards if the exit of the tracer material is not in the dead center of the round.

[rabbidrabbit]

RPM has no meaning, one revolution per foot means that you can see the spin easy in the smoke trail if the round is going to leave one instead of emitting a trail from the full width of the round. The spinning motion of the round probably ejects the smoke also outwards if the exit of the tracer material is not in the dead center of the round.

I'm not sure I'm following you.  One revolution per foot would be true if it were traveling at one foot per second.  It's not.  It's traveling at 3000 ft per second.  Thus, is rotating 3000 times per second.  There will be a variance based on twist rate and muzzle velocity but even taking half the 3000 turns per second it is still spinning 1500 times every second.  How do human eyes see the revolutions as a corkscrew going through the air?  It would be spinning way too fast to see without reviewing via a high speed camera.   

I'm all for the effect, it looks cool but I'm not certain your explanation works.

What am I missing?

[Motherland]

I'm not sure I'm following you.  One revolution per foot would be true if it were traveling at one foot per second.  It's not.  It's traveling at 3000 ft per second.  Thus, is rotating 3000 times per second.  There will be a variance based on twist rate and muzzle velocity but even taking half the 3000 turns per second it is still spinning 1500 times every second.  How do human eyes see the revolutions as a corkscrew going through the air?  It would be spinning way too fast to see without reviewing via a high speed camera.   

I'm all for the effect, it looks cool but I'm not certain your explanation works.

What am I missing?

https://www.youtube.com/watch?v=Qhm7-LEBznk


3000 (R/S) / 3000 (F/S)=
3000 (R/S) * 1/3000 (S/F)=
3000/3000 [(R*S)/(F*S)]=
3000/3000 (R/F)*(S/S)=
1 (R/F)*1=
1 (R/F)


Not only is this a real effect, it's easily observable.

https://www.youtube.com/watch?v=-brcvjI_ZVE