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Gun Convergence behavior = buggy???


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The Convergence seem to be buggy. Convergence in planes that have only nose mgs and cannons is always questionable compared to planes with guns in their wings where Convergence is reasonable. If I set Convergence to 500m as example on planes with only nose mgs and cannons and try to hit the bomber from six at 500m without any wind I can't hit him always I need to push the nose up to hit him same as I would aim with 100m Convergence on that bomber that is 500m away. When I now set 550m as convergence I hit that bomber at 500m with nose mgs and cannons, confusing! I used the sights in F-4, G-2 or G-6 for the test because I find them better for the prove.

 

 

Is this a vertical convergence problem in this game or is this a problem that I can't set vertical convergence in this game?

 

https://theairtacticalassaultgroup.com/forum/attachment.php?attachmentid=7203&d=1391101496

 

 

 

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31 minutes ago, Livai said:

 

Is this a vertical convergence problem in this game or is this a problem that I can't set vertical convergence in this game?

 

https://theairtacticalassaultgroup.com/forum/attachment.php?attachmentid=7203&d=1391101496

 

The third figure in your picture is correct, but the convergence is not 150m, but 400+m. The convergence is where your bullets cross the sightline on the way down to the ground, not the first crossing, where the bullets start flying over the sightline.

It might be, that the trajectory only touches the sightline, but the trajectory is much more curved than in your picture. For example the 20mm engine gun of the 109s had a convergence setting of 400m, but crossed the sightline for the first time at about 180m. As the 20mm gun was fix. it was the gunsight, which was moved through the convergence setting.

When you hit the bomber at 500m with a convergence setting of 550m, it is because the bomber is not standing, but moving. In the time, your bullets need to make the 500m, the bomber moves forward. So the distance between you and the bomber is 500m, but the distance between the position, in which you shot and the position in which you hit the bomber, is 550m.

 

1 minute ago, PaladinX said:

Convergence is where the bullets cross (from left and right guns).

Its NOT the bullet drop.

It is both. The machineguns of the 109s for example didn't have horizontal convergence, they were parallel.

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That is what convergence does, taking the arc also into account, so that at a set convergence bullet hits where your pipper points. Haven't tested how it is in the game.

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24 minutes ago, Robli said:

That is what convergence does, taking the arc also into account, so that at a set convergence bullet hits where your pipper points. Haven't tested how it is in the game.

The bullets don't arc on their own. If the barrel angle is set above the line of sight, then the bullet will rise until gravity and momentum takes over to cause bullet drop. The bullets don't rise in their line of flight for any other reason.

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Just now, PaladinX said:

 

But convergence setting does IMHO not affect the vertical adjustment.

 

This is incorrect - the convergence setting in the game does affect the vertical trajectory -  and very easy to check. Take a 109 F4 and fly on auto level - or on the ground with engine running. Set minimum convergence - fire cannon only, note highest point the tracers reach in the sight picture. Now restart with the convergence set to maximum and repeat. The tracers now reach much further above the pipper.

 

How accurately the vertical setting makes the shots cross the sight line at the convergence distance, and at what speed this is supposed to be calibrated, I do not know. But the difference is significant.  

Edited by unreasonable
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3 hours ago, Rjel said:

The bullets don't arc on their own. If the barrel angle is set above the line of sight, then the bullet will rise until gravity and momentum takes over to cause bullet drop. The bullets don't rise in their line of flight for any other reason.

 

Great to know. Similarly, I guess, bullets from left and right wing guns don't just meet up at a convergence distance, but do that because the barrel angle is set like that? 😉

Anyway, did not mean to sound sarcastic, is good that you pointed that out, in case someone thought that there was some other magic affecting the shape of the arc besides muzzle velocity and gravity.

P.S. There is also resistance from air. Haven't checked if the arc at high altitude is different than on sea level, in the game.

  

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With nose mounted guns I assume the bullets drop point is at 500m when I set the convergence to 500m and when a bomber is at 500m the bullets should hit him. If I had gunpods under the wings and nose mounted guns all guns should criss-cross at 500m. With nose mounted guns I hit that bomber at around 400m with the convergence set to 500m. In my test I fire single rounds from 800m,700m,600m,500m,400m,300m and the bomber is in the aimpoint from my gunsight flying from six and without wind. Doing this with gunpods under the wings again I had no problems to hit him at 500m or above.

 

Looking through the aimpoint in the gunsight I see how the bullets from the nose mounted guns fly but don't hit at 500m the bomber. As example 600m and above set as convergence for nose mounted guns I see clear that the bullets stop to fly straight through the aimpoint in the gunsight. When bullets fly straight through the aimpoint in the gunsight and I can't hit him at 500m but only around 400m having him in the aimpoint with a convergence set to 500m then I see a problem a huge problem.

 

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51 minutes ago, Robli said:

 

Great to know. Similarly, I guess, bullets from left and right wing guns don't just meet up at a convergence distance, but do that because the barrel angle is set like that? 😉

Anyway, did not mean to sound sarcastic, is good that you pointed that out, in case someone thought that there was some other magic affecting the shape of the arc besides muzzle velocity and gravity.

P.S. There is also resistance from air. Haven't checked if the arc at high altitude is different than on sea level, in the game.

  

No problem. Sarcasm bothers me not at all. 

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59 minutes ago, Livai said:

With nose mounted guns I assume the bullets drop point is at 500m when I set the convergence to 500m and when a bomber is at 500m the bullets should hit him. If I had gunpods under the wings and nose mounted guns all guns should criss-cross at 500m. With nose mounted guns I hit that bomber at around 400m with the convergence set to 500m. In my test I fire single rounds from 800m,700m,600m,500m,400m,300m and the bomber is in the aimpoint from my gunsight flying from six and without wind. Doing this with gunpods under the wings again I had no problems to hit him at 500m or above.

 

Looking through the aimpoint in the gunsight I see how the bullets from the nose mounted guns fly but don't hit at 500m the bomber. As example 600m and above set as convergence for nose mounted guns I see clear that the bullets stop to fly straight through the aimpoint in the gunsight. When bullets fly straight through the aimpoint in the gunsight and I can't hit him at 500m but only around 400m having him in the aimpoint with a convergence set to 500m then I see a problem a huge problem.

 

 

If you are flying 500m behind a bomber, and fire your guns, the bullets will have to travel more than 500m to hit it since the bomber is moving forward all the time the bullets are in flight.

 

 

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4 hours ago, unreasonable said:

How accurately the vertical setting makes the shots cross the sight line at the convergence distance, and at what speed this is supposed to be calibrated, I do not know. But the difference is significant.  

It is. I`ve tested 100-500m ranges on all but few planes and both the aim and ballistics of the projectile change.

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4 hours ago, AndyJWest said:

 

If you are flying 500m behind a bomber, and fire your guns, the bullets will have to travel more than 500m to hit it since the bomber is moving forward all the time the bullets are in flight.

 

 

Yes and no.

You have to also take into account relative velocities, if you're travelling the same speed as the bomber technically neither of you are moving in a relative sense.

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Just now, Melonfish said:

Yes and no.

You have to also take into account relative velocities, if you're travelling the same speed as the bomber technically neither of you are moving in a relative sense.

 

In an ideal world, with no drag, yes.  Whether they actually converged at the same point when you are moving would depend on the time in flight for the bullets remaining the same. Which I'd assume it wouldn't given the increased air resistance.

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Convergence (and harmonization) is set in a test stand with a stationary aircraft.

Therefore if your testing takes place at speed, altitude or at any sort of angle it will no longer align with the given numbers.

 

To discuss something different but related...

 

However, the game blends horizontal convergence (sometimes called harmonization) and vertical convergence (sometimes called elevation) under the one setting. It was not a rule, but many planes used elevations far beyond their convergence point (particularly for canon, and it became more popular late war). This affords you a clear sight picture while shooting in a turn, allows you to adjust point of aim with roll (which is far easier than moving the nose position) and so on. An example would be at the introduction of the Spitfire MkVb you have horizontal convergences in the 250-300yd range and verticals in the 400-450yd range. BF109 G->K had vertical convergences well above the 400m value we're used to, up to 600m. Likewise for the P39, with canon vertical up to 900m. (These are from memory, but the topic has been discussed to death)

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Just now, AndyJWest said:

 

In an ideal world, with no drag, yes.  Whether they actually converged at the same point when you are moving would depend on the time in flight for the bullets remaining the same. Which I'd assume it wouldn't given the increased air resistance.

I'm sure someone intelligent has done the maths on this somewhere but say a 50cal of the time, 1910fps add for arguments sake 300mph (for plane flight) to that so it's now 2349fps, but, we're also likely at altitute so the atmosphere is thinner. It would likely be pretty negligible.

Still a fair bit to calculate.

My point was that both craft are moving, your initial postulation was as if only the bomber was moving but the shooting plane was still. I'm just pointing out if you're both moving 500m is still 500m.

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Yeah, it gets complicated. 

 

My general impression, when reading discussions concerning convergence, is that many people seem to have unrealistic expectations of how consistent it would be under real-world conditions. There are lots of factors coming into play (not just forward motion of the aircraft), and expecting guns calibrated on the ground to converge at exactly the same point in the air is probably over-optimistic.

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How would the forward speed of the A/C add to the velocity of a fired bullet? If I’m running across the ground and throw a ball at a stationary target, all that changes is the distance of flight. 

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8 minutes ago, Rjel said:

How would the forward speed of the A/C add to the velocity of a fired bullet? If I’m running across the ground and throw a ball at a stationary target, all that changes is the distance of flight. 

 

Because of Newtons 1st law.

The bullet is sat in the breech already travelling at the speed of the plane. When fired it is accelerated by its propellant by the normal amount.

Hence if the plane is going 300mph (586fps) so is the bullet.

When fired the bullet is accelerated by (not to) 1910fps 

So the bullet is now travelling at 2496fps after being fired. 

 

Did you ever see that mythbusters ep where they fired a cannon from a moving vehicle and the ball looks like it isn't moving? Same thing just in the opposite direction. Law of inertia.

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1 hour ago, Melonfish said:

 

Because of Newtons 1st law.

The bullet is sat in the breech already travelling at the speed of the plane. When fired it is accelerated by its propellant by the normal amount.

Hence if the plane is going 300mph (586fps) so is the bullet.

When fired the bullet is accelerated by (not to) 1910fps 

So the bullet is now travelling at 2496fps after being fired. 

 

Did you ever see that mythbusters ep where they fired a cannon from a moving vehicle and the ball looks like it isn't moving? Same thing just in the opposite direction. Law of inertia.

I guess I’ll go along with your explanation although it doesn’t seem feasible. Muzzle velocity would seem to me to be constant regardless of forward motion. If what you’ve stated is true then would a bullet fired to the rear of the forward moving aircraft actually leave the muzzle at a slower speed due to the fact it was going 300 mph essentially in reverse when fired? 

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51 minutes ago, Rjel said:

Muzzle velocity would seem to me to be constant regardless of forward motion.

 

It is essentially constant relative to the gun.

 

54 minutes ago, Rjel said:

If what you’ve stated is true then would a bullet fired to the rear of the forward moving aircraft actually leave the muzzle at a slower speed due to the fact it was going 300 mph essentially in reverse when fired? 

 

To a stationary observer (someone standing on the ground), the speed of the bullet would indeed be lower.

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3 hours ago, Melonfish said:

I'm sure someone intelligent has done the maths on this somewhere but say a 50cal of the time, 1910fps add for arguments sake 300mph (for plane flight) to that so it's now 2349fps, but, we're also likely at altitute so the atmosphere is thinner. It would likely be pretty negligible.

Still a fair bit to calculate.

My point was that both craft are moving, your initial postulation was as if only the bomber was moving but the shooting plane was still. I'm just pointing out if you're both moving 500m is still 500m.

 

The MV vector is added to the plane's vector, so 500m is still 500m at the same speed for firer and target, but the deceleration of the ammunition depends on the firing plane's vector (plus air density, etc) and ignores the target's velocity.

 

The equations for drag have a squared velocity: everything else scales linearly.  1910^2 = 3,648,100   2349^2 = 5,517,801  which is 51% greater.  So it really is a significant difference, true at all altitudes, although obviously high altitude will counter the absolute effect. I think 50% reduction in density compared to SL is at about 5,000m

 

This is why a chasing fighter firing at a bomber from six o'clock is at a big disadvantage compared to a rear gunner firing the same gun and ammunition. One is in effect firing into a wind, the other has a wind behind him.   

 

 

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One simple example of this effect that you can "feel" are those people-mover treadmills at the airport. If you walk forward while riding on one you are, relative to the treadmill band, walking your normal speed. Once you get to the end, and step onto the floor, you are suddenly (to you) moving rather quickly, and if you aren't paying attention, might stumble a bit.

 

Same thing happens to a bullet leaving a moving gun, only much faster.

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4 hours ago, unreasonable said:

 

The MV vector is added to the plane's vector, so 500m is still 500m at the same speed for firer and target, but the deceleration of the ammunition depends on the firing plane's vector (plus air density, etc) and ignores the target's velocity.

 

The equations for drag have a squared velocity: everything else scales linearly.  1910^2 = 3,648,100   2349^2 = 5,517,801  which is 51% greater.  So it really is a significant difference, true at all altitudes, although obviously high altitude will counter the absolute effect. I think 50% reduction in density compared to SL is at about 5,000m

 

This is why a chasing fighter firing at a bomber from six o'clock is at a big disadvantage compared to a rear gunner firing the same gun and ammunition. One is in effect firing into a wind, the other has a wind behind him.   

 

 

I knew someone would know more on this, thank you. 👍

Would be good to see some usaaf stuff on this,  I'll have a dig later I'm sure they've done something that'll give some more info on it.

I didn't realise the wind squared, I know from experience firing into a wind or with a wind behind doesn't affect shot to bad as long as it's direct, it's the crosswind that gets you,  considering we're firing at angles in a slipstream there's got to be numbers somewhere on this, hell, it may have been pre calculated for the presets given in manuals?

 

 

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The "US Ballistics Lab report a800394"  (search Google should take you to it), which we have been using for kill probabilities, has in it's final appendix some graphs for time of flight vs range and remaining velocity vs range for some ammo types, including a couple of .50 cal types. These are based on a particular set of conditions and compare firing straight ahead vs straight back.

 

If you have been thinking about firing say a rifle on a range, into or with the wind, (or even at people on operations), it is true that the headwind difference would not be very noticeable, but that is because the wind difference added to or subtracted from the MV is so small compared to the MV - we do not practice firing in wind tunnels!  If you have ever played golf, though, you will know that it can make an enormous difference: the wind speed can be high compared to the initial velocity off the tee (especially for me!) although there are other factors too.

 

The other thing you can play around with is a ballistics calculator.  I like the one at  http://www.shooterscalculator.com which can draw graphs for you.

 

When I am playing flying one of the planes with nose mounted guns I tend to set the convergence to 1000m - at short ranges you fire a bit high but I find that easier to see and adjust for than firing low: use the bottom of the top line in a Revi as the pipper for very short range. With wing mounted guns it all gets a lot harder. A historical convergence adjustment that allowed for a beaten zone at convergence rather than a point would help the wing guns considerably IMHO. 

Edited by unreasonable
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Fab stuff thank you, i'll give it a good read, at my range the older guys generally have gas so technically the indoor range IS a wind tunnel of sorts...

 

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23 hours ago, Yogiflight said:

It is both. The machineguns of the 109s for example didn't have horizontal convergence, they were parallel.

 

23 hours ago, PaladinX said:

Yes, parallel means NO convergence.

But convergence setting does IMHO not affect the vertical adjustment.

Obviously you have NO idea about the weapons system in a Bf-109.  Machineguns and motor cannon had several points of convergence vertical and horizontal with total convergence setting of 400 m. I.A.W. Bf-109F weapons manual. ...By the way ...it´s same for other BF-109 G, K, etc.

image.thumb.png.a2152ddb18a3d8fbd49b2dac3d6d2574.png
 

Spoiler

image.png.8671b60c6d6b77a8249409b557ff46ce.png


 

Edited by III/JG52_Otto_-I-
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Posted (edited)
19 hours ago, AndyJWest said:

 

If you are flying 500m behind a bomber, and fire your guns, the bullets will have to travel more than 500m to hit it since the bomber is moving forward all the time the bullets are in flight.

 

 

 

This work only when you turn after or climb after that plane to  move your plane into shooting position and open fire instantly from six - Why? - thanks to the turn or climb you lost speed and you are slower than your target at the moment when you start to shoot. The bullets like you said already need to fly more distance to keep up with your target speed. In my test I am faster than my target and all bullets fly the distance how far I am from that plane.

 

The MG151/20  Ammunition fly 1.100 m / s  enough speed to fly straight. What is a 500m distance for a bullet that fly 1.100 m / s and no wind is around to change his course. And when the Game fail in this test with nose mounted guns without wind why should be gunnery more accurate with wind enabled? 

 

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1 hour ago, Livai said:

The MG151/20  Ammunition fly 1.100 m / s  enough speed to fly straight. What is a 500m distance for a bullet that fly 1.100 m / s

Only at the muzzle of your gun. As soon as they leave the muzzle, they get braked down by drag. See here:

 

11 hours ago, unreasonable said:

The MV vector is added to the plane's vector, so 500m is still 500m at the same speed for firer and target, but the deceleration of the ammunition depends on the firing plane's vector (plus air density, etc) and ignores the target's velocity.

 

The equations for drag have a squared velocity: everything else scales linearly.  1910^2 = 3,648,100   2349^2 = 5,517,801  which is 51% greater.  So it really is a significant difference, true at all altitudes, although obviously high altitude will counter the absolute effect. I think 50% reduction in density compared to SL is at about 5,000m

The faster you fly and the higher speed of your bullets when leaving the gun, the more they get braked by the drag. The loss of velocity and the loss of altitude in 500m distance is quite high.

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Fortunately I found this tabella in German forum

 

Um wie viel fällt eine Granate auf 100m, 200m, 300m, 400m und 500m?

 

MG 151 2cm M-Geschoss

 

0m (Mündung) Zeit= 0 sec / Höhe 0m / Geschwindigkeit 775 m/sec

100m Zeit= 0,139 sec / Höhe -0,09m / Geschwindigkeit 671 m/sec

200m Zeit= 0,300 sec / Höhe -0,40m / Geschwindigkeit 574 m/sec

300m Zeit= 0,489 sec / Höhe -1.02m / Geschwindigkeit 487 m/sec

400m Zeit= 0,712 sec / Höhe -2,05m / Geschwindigkeit 415 m/sec

500m Zeit= 0,974 sec / Höhe -3,66m / Geschwindigkeit 355 m/sec

600m Zeit= 1,276 sec / Höhe -6,05m / Geschwindigkeit 309 m/sec

 

Die langsamsten Geschosse für das MG 151/20 haben 705m/s an der Mündung diese fallen auf 300m Entfernung nur um 7cm tiefer als das oben angeführte beispiel.

 

It shows the time after leaving the gun (Zeit), the loss of altitude (Höhe), and the remaining velocity (Geschwindigkeit) of a German 20mm mine projectile in distance to the gun.

 

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Posted (edited)
1 hour ago, Yogiflight said:

Only at the muzzle of your gun. As soon as they leave the muzzle, they get braked down by drag. See here:

 

The faster you fly and the higher speed of your bullets when leaving the gun, the more they get braked by the drag. The loss of velocity and the loss of altitude in 500m distance is quite high.

 

we also have gravity continuously pulling downwards on the bullet., but we also have atmospheric resistance ( air around us = drag = slowing down force ) continuously slow down the bullet. The drag is less resistance if the bullet move through it quickly enough, the drag itself provides a very substantial resistance. A flat surface offers more resistance than an oval. Bullets have a aerodynamic shape. Since bullets are so aerodynamic, the force of drag doesn’t have that much of an effect, but there is a measurable slowing. Reduce the Bullet spin rate another point to archive the lowest drag. If bullets were shaped like cubes instead, this effect from the drag would be much greater. While the drag force does definitely slow down the bullet, gravity has a much larger effect on the bullet’s flight. The pull of gravity actually adds speed to the bullet, since it gives the bullet downwards speed, while leaving the forward speed untouched, which means the diagonal speed is higher. This additional contribution by gravity means the bullet can be going faster when it hits the ground than it was when it left the muzzle of the gun.

 

BTW The air resistance (drag) depends on the speed of the bullet. Theoretically it would be expected that with higher speed the air resistance would be almost increases quadratically. However, with a smooth sphere, there is a critical speed of approx. 75 km / h, from where the air resistance decreases again. Bullets are not a sphere in WW1 or WW2 there critical speed needed from where air resistance decreases again is far far less. A rough sphere behaves drag different already above 30 km / h speed, their air resistance is lower than in the case of a smooth sphere but their air resistance increases with increasing Speed slowly but continuously. In short a rough bullet air resistance increases with increasing Speed slowly but continuously compared to a smooth bullet where air resistance decreases with increasing Speed slowly but continuously

 

I know that air resistance already has an effect when there is no wind but not that much as with wind or gravity.

Edited by Livai
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14 hours ago, Livai said:

This additional contribution by gravity means the bullet can be going faster when it hits the ground than it was when it left the muzzle of the gun.

😮

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16 hours ago, Yogiflight said:

Fortunately I found this tabella in German forum

 

Um wie viel fällt eine Granate auf 100m, 200m, 300m, 400m und 500m?

 

MG 151 2cm M-Geschoss

 

0m (Mündung) Zeit= 0 sec / Höhe 0m / Geschwindigkeit 775 m/sec

100m Zeit= 0,139 sec / Höhe -0,09m / Geschwindigkeit 671 m/sec

200m Zeit= 0,300 sec / Höhe -0,40m / Geschwindigkeit 574 m/sec

300m Zeit= 0,489 sec / Höhe -1.02m / Geschwindigkeit 487 m/sec

400m Zeit= 0,712 sec / Höhe -2,05m / Geschwindigkeit 415 m/sec

500m Zeit= 0,974 sec / Höhe -3,66m / Geschwindigkeit 355 m/sec

600m Zeit= 1,276 sec / Höhe -6,05m / Geschwindigkeit 309 m/sec

 

Die langsamsten Geschosse für das MG 151/20 haben 705m/s an der Mündung diese fallen auf 300m Entfernung nur um 7cm tiefer als das oben angeführte beispiel.

 

It shows the time after leaving the gun (Zeit), the loss of altitude (Höhe), and the remaining velocity (Geschwindigkeit) of a German 20mm mine projectile in distance to the gun.

 

 

Does the source say what altitude this was done at and at what speed? Obviously it is only easy to check this experimentally while stationary at close to SL, but the numbers could have been adjusted.

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16 hours ago, Livai said:

This additional contribution by gravity means the bullet can be going faster when it hits the ground than it was when it left the muzzle of the gun.

 

That is definitely not true. Terminal velocity (max speed of a free falling object, where force from gravity equals force from drag) for a bullet is about 90 m/s. There is no firearm with so small muzzle velocity. Even BB-guns have a lot higher muzzle velocity.

 

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Posted (edited)
On 1/4/2021 at 12:34 AM, AndyJWest said:

Yeah, it gets complicated. 

 

My general impression, when reading discussions concerning convergence, is that many people seem to have unrealistic expectations of how consistent it would be under real-world conditions. There are lots of factors coming into play (not just forward motion of the aircraft), and expecting guns calibrated on the ground to converge at exactly the same point in the air is probably over-optimistic.

 

Yes and no, when on a university subject area physics teach you that a smooth sphere has a critical speed of approx. 21 m / s, from where the air resistance decreases again. If you have a  rough sphere drag behaves different already above 8 m / s  critical speed from where the air resistance is lower than in the case of the smooth sphere but the air resistance increases with increasing Speed slowly but continuously. 

 

23 minutes ago, Robli said:

 

That is definitely not true. Terminal velocity (max speed of a free falling object, where force from gravity equals force from drag) for a bullet is about 90 m/s. There is no firearm with so small muzzle velocity. Even BB-guns have a lot higher muzzle velocity.

 

 

You can go to each etage from a skyscraper and drop a sphere whatever on something from the second floor the damage done is far less than from the last floor. Gravity gives speed. And when you increase the weight from the object the effect is a lot larger. Nobody drops ping pong balls from above :ph34r: A bullet has his weight and speed when left the gun you fire from up to down where gravity adds speed and gravity is stronger than the drag when shoot from above to down. Wind can change the course from the bullet or even gives additional speed. Even the wind can increase the speed from a bullet when you fire the same direction how the wind flows

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Just now, Livai said:

 

Yes and no, when on a university subject area physics teach you that a smooth sphere has a critical speed of approx. 21 m / s, from where the air resistance decreases again. If you have a  rough sphere drag behaves different already above 8 m / s  critical speed from where the air resistance is lower than in the case of the smooth sphere but the air resistance increases with increasing Speed slowly but continuously. 

 

Livai - we are talking about known quantities here. The drop in speed of bullets in air can be and has been measured. From which data ballistics coefficients are calculated. We are talking about speeds in the 500-1,000+ m/s range, ( or ~100m/s for a bullet in free fall): what happens at 21m/s is of no relevance.  I can assure you that years of military ballistics calculations were not made in vain. 

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24 minutes ago, unreasonable said:

 

Livai - we are talking about known quantities here. The drop in speed of bullets in air can be and has been measured. From which data ballistics coefficients are calculated. We are talking about speeds in the 500-1,000+ m/s range, ( or ~100m/s for a bullet in free fall): what happens at 21m/s is of no relevance.  I can assure you that years of military ballistics calculations were not made in vain. 

 

I found a University Powerpoint Präsentation, was about drag, speed, direction change when the wind flows and about deflection. The most interesting part was that a smooth sphere big as a football has a critical speed of approx. 21 m / s where the air resistance decreases again. A bullet is smaller, faster, smooth surface and aerodynamic compared to a football. When a football with a rough surface already above 8 m / s speed has a lower air resistance than in the case of the smooth football but his air resistance increases with increasing Speed slowly but continuously and on the smooth football  approx. 21 m / s  his air resistance decreases with increasing Speed slowly but continuously.

When a football behave this way how a bullet behaves then? = the same way

 

Download Powerpoint Präsentation

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1 hour ago, unreasonable said:

Does the source say what altitude this was done at and at what speed? Obviously it is only easy to check this experimentally while stationary at close to SL, but the numbers could have been adjusted

Unfortunately not. It was a post of Gunsmith86, an absolute expert with lots of sources about weapons, ammunitions and explosives. Unfortunately he is not active in the forum anymore. I really hope he is doing fine.

My guess is, that it was done at ground level at zero speed back at that time, so the numbers would definitely change in combat. But this is just a guess.

He also posted a second tabella for the MG/FF.

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