X Post "How Can That Happen"

Frank Browne

Frank Browne

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This is an excerpt from a 1981 Dept of the Navy publication. It's rather interesting so I thought I'd share......

"21 September 1956-An F11F-1 Tiger, piloted by Grumman test pilot Tom Attridge, shot itself down while conducting test firings off eastern Long Island by running into 20mm projectiles it had fired only seconds before."


That could ruin your whole day!
 
Not difficult to understand. The munitions were fired either level or slightly up and the plane went into a power on descent. It reached the same spot as the falling projectiles at the same time.
 
doesnt the A10 warthog have the same issues with its 50 cal. gatling gun?
 
Frank Browne said:
No. The A-10 fires the GAU-8A 30mm gun. It fires a very high speed ammunition. The A-10's top speed is only around 430 mph.
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At the risk of starting another endless "conveyer belt" thread....

I have some trouble envisioning the mode of what happens here.

Even if it's a SR-71 doing Mach 3+, the ammo IN THE GUN in the jet is also doing the same speed as the jet BEFORE it's fired.

When fired the ammo has to leave at aircraft speed plus projectile speed to an outside observer, and the would still depart the jet at the projectile speed to an observer in the jet.

The only way I can see this collision happening is with ammo that has a high air resistance that slows rapidly, or with a missile that can't sustain the same speed as the jet. I guess a Mach 3 jet could carry bullets that can't fly at Mach 3?
 
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So, back to my earlier question.... a plane that can out fly its own ammunition? Shoot a projectile then fly into it? I can't buy that one. I can follow the idea of shooting up then flying under the projectile.

But, apparently it is possible to fly directly into your own ammunition that has slowed down but you haven't... several versions within....

http://www.f-14association.com/stories-05.htm

My later post about A-6E Intruders... when they dropped bombs, they would pull vertical to give the weapon as little lateral movement as possible as it was released and fell to its target area. This maneuver was from Vietnam where precision bombing not very precise and fell all on pilot skill.
 
Ammo of all kinds can be less than perfect on occasion and an imperfect round imparting substantially subnormal velocity to its projectile would be a problem in virtually any aircraft.
 
mikea said:
The only way I can see this collision happening is with ammo that has a high air resistance that slows rapidly,
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20mm ammunition from that period fired projectiles with fairly low ballistic coefficient. Combined with relatively low initial muzzle velocity, and the projectiles have a fairly steep trajectory. I can easily see how an F-11 could fire a burst then initiate a shallow dive and run into it's own outgoing rounds.
 
Bullets have no lift and if the aircraft is flying level, a bullet fired from the gun will hit the ground at exactly the same time as a bullet dropped from the bomb bay. The amount of drag that a bullet has is substantial and they slow down quick. The faster it goes (being pushed by a fast airplane), the harder it hits the big wall of air and slows down. So point the airplane up and fire a burst. The bullets will slow down and you will not. You bet you can fly through your bullets.

The best example of drag I can think of is a fighter flying supersonic and pulling the throttle(s) out of afterburner. It'll feel like you hit a brick wall. But the same fighter at 200 knots pulling the throttles back to idle is not a big decellerator.
 
JohnAJohnson said:
Bullets have no lift and if the aircraft is flying level, a bullet fired from the gun will hit the ground at exactly the same time as a bullet dropped from the bomb bay.
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Uhh no.

Let's take a high powered rifle and shoot a bullet level from 1 foot AGL. At the same time you pull that trigger I'll drop the bullet onto the ground--we'll see who hits the ground first.
 
jesse said:
Uhh no.

Let's take a high powered rifle and shoot a bullet level from 1 foot AGL. At the same time you pull that trigger I'll drop the bullet onto the ground--we'll see who hits the ground first.
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They will hit the ground at precisely the same time if you discount air resistance. From 1 foot off the ground, any air resistance issue will be minimal, so they will hit the ground at virtually the same instant.

Remember, the bullet from the gun is being accelerated downwards at 32 ft/s/s, just like the bullet you drop. The fact that it's also moving forward is irrelevant.

Question: A monkey is hanging from a branch and you're trying to shoot it. If the monkey lets go of the branch at the exact instant you pull the trigger, where should you aim?

Answer: Aim at the monkey. The monkey and the bullet will fall at the same rate.

Unintuitive, but true.

Chris
 
jesse said:
Uhh no.

Let's take a high powered rifle and shoot a bullet level from 1 foot AGL. At the same time you pull that trigger I'll drop the bullet onto the ground--we'll see who hits the ground first.
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Assuming you point those bullets at each other, I expect that you (the one dropping the bullet) will hit the ground first (having been knocked down by the bullet fired from the gun).

But baring issues like the curvature of the earth or elevation changes those two bullets will come to ground at the same moment, separated by considerable distance.
 
lancefisher said:
But baring issues like the curvature of the earth or elevation changes those two bullets will come to ground at the same moment, separated by considerable distance.
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His post was quite wrong indicating that a bullet fired from the gun in the airplane and a bullet dropped out of the bomb bay would impact the ground at the same moment in time. There is no way this would happen if the bullet was fired level or at an upwards angle.

Yes it's true that two objects will hit the ground if dropped at the same time even if their mass varies (equal drag) this does not mean an object propelled out of a gun with a very high forward speed will drop at the same rate. If this were the case--the whole concept of a gun wouldn't work so well.
 
jesse -
welcome to freshman physics, and the problems that everyone struggles with in freshman physics. the gun works because the bullet goes out of the gun so darn fast that it hits the target before it has time to hit the ground. but everything falls down at the same speed, no matter if its moving forward or not.
 
tonycondon said:
jesse -
welcome to freshman physics, and the problems that everyone struggles with in freshman physics. the gun works because the bullet goes out of the gun so darn fast that it hits the target before it has time to hit the ground. but everything falls down at the same speed, no matter if its moving forward or not.
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Sorry, but I ain't buyin' it.

I remember when our sniper got a .50 cal--damn thing would should straight and level for one mile. No way I'll be convinced that dropping a .50 cal bullet from a distance of three feet versus firing it over a flat range that the two bullets will impact the ground at the exact same time.

And bullets CAN and DO rise--ask any experienced sharpshooter familiar with long-distance shooting.

Remember: You gotta add the component of velocity in this mix. Simply dropping a bullet is nothing more than gravity. But when you add velocity to the mix, things change.

Regards.

-JD
 
CowboyPilot said:
Sorry, but I ain't buyin' it.

I remember when our sniper got a .50 cal--damn thing would should straight and level for one mile. No way I'll be convinced that dropping a .50 cal bullet from a distance of three feet versus firing it over a flat range that the two bullets will impact the ground at the exact same time.

And bullets CAN and DO rise--ask any experienced sharpshooter familiar with long-distance shooting.

Remember: You gotta add the component of velocity in this mix. Simply dropping a bullet is nothing more than gravity. But when you add velocity to the mix, things change.

Regards.

-JD
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how fast does the .50 Cal come out of the gun at? Just curious, but it may be so darn fast that the bullet is darn near in orbit until the drag slows it down enough for it to start falling.

The claim that the bullet dropped and fired will hit ground are based on a few important assumptions. Number 1 is that there is no atmosphere, therefore no air friction, lift, drag, whatever. Number 2 is the earth is flat. Even on the scale of miles when dealing with your .50 cal sniper, that is a decent assumption.

We could spend a couple weeks with our slide rules and figure out all of the aerodynamic effects involved in the firing of the bullet, plus factor in the shape of the barrel of the gun and whatever else, crunch the numbers, and probably come up with a number within 5% of making the assumptions.
 
CowboyPilot said:
Sorry, but I ain't buyin' it.

I remember when our sniper got a .50 cal--damn thing would should straight and level for one mile. No way I'll be convinced that dropping a .50 cal bullet from a distance of three feet versus firing it over a flat range that the two bullets will impact the ground at the exact same time.

And bullets CAN and DO rise--ask any experienced sharpshooter familiar with long-distance shooting.

Remember: You gotta add the component of velocity in this mix. Simply dropping a bullet is nothing more than gravity. But when you add velocity to the mix, things change.

Regards.

-JD
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I've been thinking about it some.. I suspect that the rise of the round being fired out of the rifle is do to a slight upward angle in the barrel. I also suspect that the bullet never rises to a point above the axis of the barrel.

I understand the whole theory of them both hitting the ground in the same moment of time. But I feel the other factors that are being applied, massive spin, the bulleting tumbling as it goes through the air ..Versus a bullet being dropped straight down nice and clean would make a very measurable difference in which one hits the ground first.

The oversimplification in some physics classes really frustrates me sometimes.

When I'm not so lazy I'll try to find some long range shooting charts and maybe come up with something more concrete.
 
tonycondon said:
We could spend a couple weeks with our slide rules and figure out all of the aerodynamic effects involved in the firing of the bullet, plus factor in the shape of the barrel of the gun and whatever else, crunch the numbers, and probably come up with a number within 5% of making the assumptions.
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and as we discussed on Skype tonight...5% could easily be a 2 second difference from 10,000 ft
 
yep and it still would be generally insignificant, and the error would be worth the amount of timed saved in making the calculation.

fast, accurate, or cheap. pick two.
 
jesse said:
His post was quite wrong indicating that a bullet fired from the gun in the airplane and a bullet dropped out of the bomb bay would impact the ground at the same moment in time. There is no way this would happen if the bullet was fired level or at an upwards angle.

Yes it's true that two objects will hit the ground if dropped at the same time even if their mass varies (equal drag) this does not mean an object propelled out of a gun with a very high forward speed will drop at the same rate. If this were the case--the whole concept of a gun wouldn't work so well.
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Jesse, stop and look at the situation closer. Now if you fire upwards, then the one fired from the gun will hit later, but if fired level over level terrain, they will hit at the same moment. There is one accelleration happening on the dropped bullet, gravity straight down. There are two accellerations on the fired bullet, forward and down. They are seperate issues. Since the gravitational accelleration is a constant, it will effect both bullets equally, therefore they will hit the ground at virtually the same time (the difference will be a small fraction of a second due to other forces involved which are small on a bullet.)
 
Henning said:
Jesse, stop and look at the situation closer. Now if you fire upwards, then the one fired from the gun will hit later, but if fired level over level terrain, they will hit at the same moment. There is one accelleration happening on the dropped bullet, gravity straight down. There are two accellerations on the fired bullet, forward and down. They are seperate issues. Since the gravitational accelleration is a constant, it will effect both bullets equally, therefore they will hit the ground at virtually the same time (the difference will be a small fraction of a second due to other forces involved which are small on a bullet.)
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I am looking at the situation from the perspective of a bullet being fired level in the real world environment combined with the jump the bullet out of the bomb bay will have on it as it accelerates towards the ground with very little tumbling. I understand the physics behind why the bullets would hit at the same time without outside forces acting upon it. Aerodynamic drag, etc.

The thing is our above discussion was talking about a real world situation. We were not talking about how a bullet would drop on our pieces of paper in physics class.

The fact of the matter is there are so many variables that would separate the bullets impact from the ground by a fair amount. A crosswind for example can cause rise on a bullet (some say because of the bullet being tipped up by the wind upon exit of the barrel) while others say it's lift being generated by the high speed rotation on the bullet in relation to the crosswind.

As the bullet slows down you can expect that it's going to start to tumble. It's common from what I know that this will result in almost a horizontal spiral which will have moments of rise and moments of drop.

The chance of the bullet being fired completely level in a combat situation --umm--not likely. They may go up, they may go down.
 
jesse said:
I am looking at the situation from the perspective of a bullet being fired level in the real world environment combined with the jump the bullet out of the bomb bay will have on it as it accelerates towards the ground with very little tumbling. I understand the physics behind why the bullets would hit at the same time without outside forces acting upon it. Aerodynamic drag, etc.

The thing is our above discussion was talking about a real world situation. We were not talking about how a bullet would drop on our pieces of paper in physics class.

The fact of the matter is there are so many variables that would separate the bullets impact from the ground by a fair amount. A crosswind for example can cause rise on a bullet (some say because of the bullet being tipped up by the wind upon exit of the barrel) while others say it's lift being generated by the high speed rotation on the bullet in relation to the crosswind.

As the bullet slows down you can expect that it's going to start to tumble. It's common from what I know that this will result in almost a horizontal spiral which will have moments of rise and moments of drop.

The chance of the bullet being fired completely level in a combat situation --umm--not likely. They may go up, they may go down.
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Very cool, I'd never considered the effect of lift due to the bullet's rotation because I'd never thought about crosswinds or tumbling. Thanks for that.

If a right crosswind causes the bullet to lift up, a left crosswind will cause it to accelerate downwards even faster than with gravity alone. As you say, when it starts to tumble, it'll have a lift component at 90 degrees to the direction of the apparent crosswind the bullet sees as it yaws, creating erratic forces.

If we neglect the effect of such lifting forces, the aerodynamic drag will have a very minimal effect on the time to fall to the ground. It will have some effect because drag is non-linear, but the effect will only be measurable when the bullet's forward speed is similar to its downward speed (ie. only when shooting very, very, long distances). Since in our case we're only considering a horizontal initial trajectory, the bullet will always be moving very quickly forward when it hits the ground and aerodynamic drag can be completely ignored with no measurable difference.

Your point about aerodynamic lift is an excellent one and something I've never considered before. However, due to the erratic flight of a tumbling bullet, and due to the random nature of crosswinds (left or right), sometimes the bullet fired from the gun will beat the dropped bullet and sometimes it wouldn't. Therefore, I think we can safely say that:

On average, a bullet horizontally fired from a gun over level terrain will hit the ground at the same time as a bullet dropped at the same instant the trigger is pulled.

Chris
 
CowboyPilot said:
Sorry, but I ain't buyin' it.
Remember: You gotta add the component of velocity in this mix. Simply dropping a bullet is nothing more than gravity. But when you add velocity to the mix, things change.
Click to expand...

Only if some of that velocity is vertical. And that's the tricky part, getting the bullet to come out of the barrel exactly parallel to the ground (and finding a perfectly flat piece of ground big enough probably ain't easy either).

If the rifle is pointed up ever so slightly, then it starts out with some vertical velocity which will keep it above the ground longer than a bullet dropped by hand. Conversely if you point it down it will hit the (flat) earth sooner.

Tumbling might have some potential to retard the descent, but you could match that by "tumbling" the bullet you dropped. BTW this concept also assumes that the bullet dropped starts in the same attitude as the bullet fired if there's any air friction involved. That way it presents the same drag profile to vertical movement. As to a bullet generating lift, I think that's only possible if it's unsymmetrical or "flying" with a positive AOA, but since AFaIK a bullet that's not tumbling will stay aligned with the relative wind and therefore produce no lift.
 
Snip a bunch of very correct stuff about initial angle of flight. Note that the bullet's moving very fast so a very small upward angle will give a very large upward velocity.
lancefisher said:
...this concept also assumes that the bullet dropped starts in the same attitude as the bullet fired if there's any air friction involved. That way it presents the same drag profile to vertical movement.
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Actually, you can't treat the horizontal and vertical drag components independently. The only way to calculate the components correctly is to find the size and direction of the total velocity, compute the drag, then split the drag into its two components. The drag on a bluff body such as this will be dominated by the wake drag. There's only one large wake at a slight angle to the horizontal, not one horizontal and one vertical wake. Since the drag varies as the square of the velocity, you can't treat the components independantly. The errors incurred by doing so can be large.

lancefisher said:
...
As to a bullet generating lift, I think that's only possible if it's unsymmetrical or "flying" with a positive AOA, but since AFaIK a bullet that's not tumbling will stay aligned with the relative wind and therefore produce no lift.
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A spinning cylinder perpendicular to the flow will produce lift. This is how a spinning ball can curve in flight. Because a bullet is spinning, any flow perpendicular to its axis of rotation will cause lift.

Chris
 
cwyckham said:
Actually, you can't treat the horizontal and vertical drag components independently. The only way to calculate the components correctly is to find the size and direction of the total velocity, compute the drag, then split the drag into its two components. The drag on a bluff body such as this will be dominated by the wake drag. There's only one large wake at a slight angle to the horizontal, not one horizontal and one vertical wake. Since the drag varies as the square of the velocity, you can't treat the components independantly. The errors incurred by doing so can be large.
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OK, but I wouldn't know how to begin with that. Obviously not an issue in a vacuum, how much effect would you expect in air with a slightly subsonic or worse yet a supersonic TAS?

A spinning cylinder perpendicular to the flow will produce lift. This is how a spinning ball can curve in flight. Because a bullet is spinning, any flow perpendicular to its axis of rotation will cause lift.
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I understand the curving and lofting of a spinning ball, but in both those cases the axis of rotation is not parallel to the direction of flight as it would be with a spinning bullet. I can't see how lift would be created by velocity parallel to the spin axis.
 
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lancefisher said:
OK, but I wouldn't know how to begin with that. Obviously not an issue in a vacuum, how much effect would you expect in air with a slightly subsonic or worse yet a supersonic TAS?
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I suspect that the non-linear effect of the drag will be virtually nill when computing the time to ground impact (though obviously very important when computing things like trajectory shape, etc.)


lancefisher said:
I understand the curving and lofting of a spinning ball, but in both those cases the axis of rotation is not parallel to the direction of flight as it would be with a spinning bullet. I can't see how lift would be created by velocity parallel to the spin axis.
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You are exactly correct. You only get lift when the axis of rotation and direction of travel aren't parallel. The two examples of this that Jesse gave was when there is a crosswind or when the bullet starts to tumble. In either case there will be a component of air velocity perpendicular to the bullet's axis of rotation.

Chris
 
I am late to this discussion, but I have a question. Wouldn't the component of lift related to the crosswind or tumble be dependant on the direction of the rotation relative to the crosswind and the direction of the tumble relative to the direction of travel? If the bullet were turning clockwise as seen from the rear (gun) and the crosswind were from the left, wouldn't the "lift" be negative?

Also, insignificant, but if you are getting onto precise calculations and a bullet with some range, you would need to account for the curvature of the earth and the fact that the earth is falling away from the bullet so that the bullet fired has slightly farther to fall than the one dropped - over a perfect sphere.
 
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Dwight B. Van Zanen said:
I am late to this discussion, but I have a question. Wouldn't the component of lift related to the crosswind or tumble be dependant on the direction of the rotation relative to the crosswind and the direction of the tumble relative to the direction of travel? If the bullet were turning clockwise as seen from the rear (gun) and the crosswind were from the left, wouldn't the "lift" be negative?
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Yes, it depends which way the wind is going, though I think in you example the lift would be up. A right crosswind would give negative lift for a clockwise rotation.

Chris
 
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