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Discussion Starter #1 (Edited by Moderator)
I'm trying to understand the basic mechanics of a bullet firing down from an aircraft.

I know that gravity pulls the bullet towards the earth and so I assume that the more mass a projectile has the harder it is to push off course. But in the real world is that actually true or would it work in some other way?

Also I think that atmospheric forces must play a part but generally speaking would they make much difference to a shell fired straight down? For example a 20mm round traveling at 1000 m/s must have a lot of force. Would a 10 mile an hour wind or even a 100 mile an hour wind make any difference to something traveling with that much power?

For arguments sake lets say the 20mm had a range of 3Km. Does it matter if the aircraft is at 5Km and it is firing straight down? Does range even matter when the bullet has no choice but to fly all the way to the earth? Would it get wobbly or fly off course after 3Km?

Would the direction of the aircraft make much difference? For example an AC-130 would be in level flight and shoot down towards the ground from its fuselage. An A-10 would be flying with its nose towards the ground when it shot.

To keep it simple I'm using a bullet that fires straight down. I know in real life it would be at some sort of angle down.



Thanks so much for any input! :)

 

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Welcome to the shooters forum.
You are expecting an answer thats way above my pay grade! :D
I'm not sure of your question to begin with. If you are asking if the bullet will continue to accelerate on the way down? Its my opinion it would reach a point where drag would exceed the thrust or gravitational pull.
Does wind have an effect on the projectile? Definately!
 

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I'm trying to understand the basic mechanics of a bullet firing down from an aircraft.

I know that gravity pulls the bullet towards the earth and so I assume that the more mass a projectile has the harder it is to push off course. But in the real world is that actually true or would it work in some other way?

Also I think that atmospheric forces must play a part but generally speaking would they make much difference to a shell fired straight down? For example a 20mm round traveling at 1000 m/s must have a lot of force. Would a 10 mile an hour wind or even a 100 mile an hour wind make any difference to something traveling with that much power?

For arguments sake lets say the 20mm had a range of 3Km. Does it matter if the aircraft is at 5Km and it is firing straight down? Does range even matter when the bullet has no choice but to fly all the way to the earth? Would it get wobbly or fly off course after 3Km?

Would the direction of the aircraft make much difference? For example an AC-130 would be in level flight and shoot down towards the ground from its fuselage. An A-10 would be flying with its nose towards the ground when it shot.

To keep it simple I'm using a bullet that fires straight down. I know in real life it would be at some sort of angle down.

Here is a link to an image that might help. For some reason I can't get the image to insert directly into the post.
https://imgur.com/a/y58ke

Thanks so much for any input! :)
Welcome to Shooters Forum, Droid. :)

Wind deflection is wind deflection, irrespective of the force of gravity (or lack thereof) on trajectory. The same is true of ballistic coefficient and the loss of velocity due to friction. All you have done by firing straight down (or straight up) is eliminate (in theory) trajectory. If your question is whether or not wind still changes the flight path of a projectile fired straight down, the answer is...of course it does, and it is pursuant to all of the same mass/BC factors that apply to bullets fired (more or less) on the horizontal.

How could it be otherwise?
 

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Discussion Starter #4 (Edited)
Thanks broom_jm!

Sometimes things that seem obvious work in a different way than expected. Glad to know that doesn't.

A followup: if a projectile has a flat range of 3 Km and it's fired down from an altitude of say 6 Km will it begin to go wobbly or spin out of control after 3 Km or is the flat range not really meaningful in this situation? By that I mean minus atmospheric forces will the shell continue straight on its path all the way to the ground or go out of control after 3 Km? Is there a basic, mechanical reason why a bullet will continue straight or "go crazy" at a certain distance?
 

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Definitely --- Welcome..

count me out on this one, I just gave up on "harmonics" and am going back to one of my old favorites.. "How long is a string ?"
 

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Look up a topic called "aerodynamic jump".
 

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First off. We don't shoot straight down or straight up so it's like asking how many bottle caps it takes to pave a parking lot. ;)

Air drag eventually makes the bullet obey its shape and center of gravity. Some fly straight ahead like a spear or arrow and some wander about the center of rotation and finally tips out of aerodynamic equilibrium and tumbles almost straight down no matter where it was headed to start with. Hold your hand out the window at about 70 mph and see how 'point first' is the best way and once a certain point is passed, the wind takes over.

There are numerous ballistics soft ware programs on the web for free. To use them in your hypothetical 20mm cannon round, you'll need velocity and Ballistic Coefficients to calculate windage movements and trajectories on the level. Windage will be the same no matter the orientation of the bullet exit, but is entirely dependant on the angle of the wind's force on the bullet. 'Full Value' windage is 90 degrees to travel.
 
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Discussion Starter #9
Aerodynamic jump is interesting! Reading that makes me feel a little more confident that the kinds of things that alter the flight of the bullet have to do with the atmosphere. The bullet itself doesn't necessarily "go nuts" because of some internal force. Something from the outside has to mess it up.
 

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@JBelk Thanks! I tried one of those ballistic programs but I don't think I put the numbers in correctly. Also I tried to test a 105mm (the AC-130 shoots one of those) but the shell size was too large for the program. DOH!

Is there a program that you like?
 

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Actually, I haven't used one in many years. I have a range and measure the holes and don't have to guess. ;)

You can do your own calculations by using various tables and formulas. "The Bullet's Flight" by E.B. Mann is one of the books on the subject used for many references.

The math involved with the AC-130 is staggering using computers to make the vectors connect. All I know is it is very tricky to shoot crows while standing through the sunroof of a VW Bug... ;)
 

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My theory on this question:
Shooting straight down form a moving aircraft will NOT have the bullet hit the earth directly under the aircraft, if fired from a reasonable height, say over 1000 ft AGL.
If fired from an AC130, it will have a Side wind of well over 100 Knots to deflect it behind the aircraft.
If fired From a sufficient height, the bullet wil slow to 'terminal velocity dependent on its mass And aerodynamic drag but will retain rotational speed for much Longer than it does 'Forward' velocity.

As an aside: bullets or bombs released from Moving aircraft Do not hit the earth either at the point directly under them at time of release nor at the point directly under the aircraft at the time of Impact.
the WW2 Norden Bomb Sight did a creditable job in estimating where a Bomb dropped at a given height from an aircraft moving at a given velocity would hit the earth. Variations in the release still scattered the bomb hits something like a shot gun pattern.

Chev. William
 

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All true. There is one additional effect due to the bullet emerging from the barrel into the side wind - the jump.
 

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I think he's thinking of 'air-borne artillery' like the AC-130 that has a side mounted, aim-able, rapid fire 105 gun in the waist. I trained on 105 Howitzers (marked 1918) at Ft. Sill. Even with the realitivly short range of that 40 lb projo, the FDC solution included earth rotation during the time of flight. In Korea with an 8 inch battalion it became much more important. Our targets were NNW at 12 to 16 miles distance. We practiced in all directions, but the guns were laid to greet Joe should he 'jump'.....with nukes.

I watch a LOT of WW-II gun camera footage. I knew a couple of S. Pacific fighter pilots growing up and they told me of the 'ride the smoke' dive angle that had the plane and the 50BMG ammo on the same trajectory. You can see it easily in the strafing footage on D-Day. Some planes are 'markers' that hose a stream of all tracers at hidden gun emplacements and vehicles, but most are depending on the sparkle of API hits to 'aim the plane'.
 

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An M118 (one I have coefficients for) has a jump of 9.84 MOA when fired sideways from a plane at 300 mph.
 

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Fired straight down, the bullet still has the forward velocity of the aircraft. That should give it a curve.

Looking at films of WWII aircraft waist gunners shows a curved trajectory. They had to aim ahead quite a bit to hit a moving target.
 
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