Author Topic: Hispano vs Mg151 (no whine)  (Read 940 times)

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #30 on: November 06, 2002, 02:59:57 PM »
Hi Charge,

>To those with data and ability: How much drop do these two cannons ammo have in 300 meters? Or what is their flight time for this distance?

Here's the flight time table for the MG151/20 mine shell compared to the 20 mm AP M75 round:

d (m) - MG151/20: t (s) - 20 mm AP M75: t (s)
100 - 0.13 - 0.13
200 - 0.29
300 - 0.47 - 0.43
400 - 0.69
500 - 0.94 - 0.77

The MG151/20 data is from the official Geran ballistic table (805 m/s muzzle velocity). The M75 data is based on a 800 m/s muzzle velocity, but the flight times are rough estimates from the speed graph and might be slightly inaccurate.

I'm not sure why the AP projectile's muzzle velocity is below the Hispano-typical 850 m/s, but I think the numbers cast an interesting light on the topic nevertheless :-)

Regards,

Henning (HoHun)

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #31 on: November 06, 2002, 03:06:00 PM »
Hi again,

It just occurred to me that I mistook yards for meters with the M75 AP projectile. Please ignore my results for now :-)

Regards,

Henning (HoHun)

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #32 on: November 06, 2002, 03:18:51 PM »
Hi yet again,

Here's the corrected flight time table for the MG151/20 mine shell compared to the 20 mm AP M75 round:

d (m) - MG151/20: t (s) - 20 mm AP M75: t (s)
100 - 0.13 - 0.13
200 - 0.29
300 - 0.47 - 0.44
400 - 0.69
500 - 0.94 - 0.79

Regards,

Henning (HoHun)

Offline Hooligan

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Hispano vs Mg151 (no whine)
« Reply #33 on: November 06, 2002, 07:09:36 PM »
In the case of a fighter aircraft in a tailchase, ballistically things are worse because the shooter and target are both moving.  If the shooter and target are both travelling at say... 150m/s, then a projectile with a nominal muzzle velocity of 800 m/s would actually be going 950 m/s when fired.  The higher speed means higher drag and the deceleration of the round relative to the still moving target will be more pronounced.  A projectile fired at a target 500m distant might actually have to travel 650m or 700m before it catches the target.  This results in significantly more drop than would be experienced firing at a static target, and slower rounds will suffer greater penalties.

Hooligan

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #34 on: November 06, 2002, 10:59:29 PM »
Hi Hooligan,

>In the case of a fighter aircraft in a tailchase, ballistically things are worse because the shooter and target are both moving.  

Of course, you're right. That's why I provided data for up to 500 m range though Charge specifically asked for 300 m :-)

But actually, drop isn't a big issue since the sightline is depressed to compensate for that. From the trajectory diagram for the Fw 190A-7/A-8, at 650 m was about 1.2 m below the sight line, which is not quite 2 mil. (A typical WW2 gunsight circle would have 70 mil diameter.) From the P-38 boresighting chart which only runs to 500 yards, I'd say its 20 mm cannon was about as far below the sight line as the Fw 190's at 650 m.

(The flatter trajetory meant that the P-38's cannon arched to only 15 cm above the sight line compared to the Fw 190's 80 cm.)

It's interesting to have a look at shooting accuracies the Luftwaffe considered to be realistic against heavy bombers under combat conditions:

d (m) - Ph MG151/20 - Ph MK103 - Ph MK214
500 - 9.1% - 10.0% - 10.5%
1000 - 3.3% - 3.8% - 3.8%
1500 - 0.8% - 1.3% - 1.5%

The 50 mm MK214 as a large-caliber high-velocity weapon of course had a significantly flatter trajectory than the MG151/20, but that only paid off beyond effective range.

I think that reinforces my belief that drop is less interesting than flight time, and in fact, Charge had originally asked for deflection shots :-)

The necessity to account for deflection of course greatly reduces the sensible engagement range, so it certainly makes sense that Charge wanted to look at ranges of up to 500 m. Judging from the typical convergence settings used by the USAAF, they did indeed expect to fire at ranges of 250 to 300 yards only :-)

Regards,

Henning (HoHun)

Offline Tony Williams

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Hispano vs Mg151 (no whine)
« Reply #35 on: November 07, 2002, 01:40:19 AM »
Quote
Originally posted by HoHun

I'm not sure why the AP projectile's muzzle velocity is below the Hispano-typical 850 m/s, but I think the numbers cast an interesting light on the topic nevertheless :-)


It was heavier, weighing in at 165g rather than 130g, and in fact the US stats I have give an MV of only 775 m/s. I'm not sure how much use this saw; I would expect it to have been used against ground targets, as it contained no HE or incendiary material as you would expect for use against aircraft.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Military gun and ammunition discussion forum: http://forums.delphiforums.com/autogun/messages/

Offline Tony Williams

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Hispano vs Mg151 (no whine)
« Reply #36 on: November 07, 2002, 01:45:06 AM »
Quote
Originally posted by Tony Williams
The 117g API shell in the MG 151/20 took 0.898 secs to reach 500m, by which time it slowed from 720 to 430 m/s and dropped 3.37m from the line of sight. Figures for the 92g M-Geschoss are 0.952 secs, 790 to 363 m/s and 3.5m.


On reflection, these drop figures I quoted have to be wrong, since gravity causes objects to drop nearly 10m in the first second. I took them from an original German data table but I probably misinterpreted the symbol at the top of the column; these figures may refer to the maximum trajectory height above the sight line.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Military gun and ammunition discussion forum: http://forums.delphiforums.com/autogun/messages/

Offline fats

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Hispano vs Mg151 (no whine)
« Reply #37 on: November 07, 2002, 10:05:04 PM »
Tony,

drop 10 meters in the first second? The final velocity might be ~10m/s ( 9.81m/s ) but the distance travelled ain't. Unless I am completely off the mark I get ~3.96 m drop for the first MG 151 round using those values.


// fats

Offline Tony Williams

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Hispano vs Mg151 (no whine)
« Reply #38 on: November 07, 2002, 11:04:40 PM »
Quote
Originally posted by fats
Tony,

drop 10 meters in the first second? The final velocity might be ~10m/s ( 9.81m/s ) but the distance travelled ain't. Unless I am completely off the mark I get ~3.96 m drop for the first MG 151 round using those values.


You could well be right; physics was never my strong point :)

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #39 on: November 08, 2002, 01:36:56 AM »
Hi Tony,

>You could well be right; physics was never my strong point :)

s = 1/2 * a * t^2

t = 1 s, a = 9.81 m/s^2 => s = 9.81/2 m/s = 4.905 m

If the projectile dropped less, it must have been due to its aerodynamic properties. The MG151/20 mine shell table I have indicates an tranjectory apex point at 287 m, 1.1 m which seems to match your drop value (I'm shooting from the hip here ;-)

Regards,

Henning (HoHun)

Offline hitech

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Hispano vs Mg151 (no whine)
« Reply #40 on: November 08, 2002, 11:30:48 AM »
Tony also realize when you see balistic tables refering to line of sight that most guns are not shot paralle to the line of sight, but first travel up threw it then back down. Any way 1 sec = 16.1 foot drop if the bullet is fired level with the ground.

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #41 on: November 08, 2002, 01:42:48 PM »
Hi Tony,

>It was heavier, weighing in at 165g rather than 130g, and in fact the US stats I have give an MV of only 775 m/s. I'm not sure how much use this saw; I would expect it to have been used against ground targets, as it contained no HE or incendiary material as you would expect for use against aircraft.

So the M75 was a special round not normally used in air combat?

Anyway, here's the comparison again with 180 fps added on top of the AP round's speed at any distance.

d (m) - MG151/20: t (s) - 20 mm AP (2800 fps): t (s)
100 - 0.13 - 0.12
200 - 0.29
300 - 0.47 - 0.42
400 - 0.69
500 - 0.94 - 0.73

As of course a lighter, faster bullet will lose speed more quickly than a heavier, slower one, this is a rather generous estimate for the upper limit of the Hispano's advantage over the MG151/20.

Regards,

Henning (HoHun)

Offline HoHun

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Hispano vs Mg151 (no whine)
« Reply #42 on: November 10, 2002, 05:02:33 PM »
Hi Charge,

>The destruction power has not yet concerned me (YET) in AH but the ability to hit targets within 100 - 500 meters in various G-loadings.

Here's a comparison of what target speeds are the limit for each type of gun for a 90° crossing shot, based on the assumption that the sight from each cockpit is 100 mil below the sight line (as in the P-51D).

d (m) - MG151/20: v (km/h) - 20 mm AP M75: v (km/h) - Hispano advantage (%)

100 - 277 - 277 - 0%
300 - 230 - 245 - 7%
500 - 191 - 228 - 19%

According to this, the advantage given by the Hispano's higher speed could be compared to having a slightly better view over the nose when it comes to deflection shooting.

(For perspective: The difference of the forward visibility of the P-47D and the P-51D is about 25 mil, which is slightly larger than the difference between MG151/20 and Hispano lead requirement at 500 m. Of course, the P-47D's sighting advantage is indepenend of range, while the flight time advantage is less pronounced at shorter ranges.)

Regards,

Henning (HoHun)

Offline illo

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Hispano vs Mg151 (no whine)
« Reply #43 on: November 10, 2002, 07:45:15 PM »
Quote
Originally posted by Vermillion
Luftwaffe guys.  Please realize that mine shells were not used exclusively.  Even at the height of the bomber offensive in the West (where they used a larger percentage of mine shells) mine shells were only 1 in 3 or 1 in 4, in a standard ammunition belt.  So to do the calculation correctly, you must account for ammunition mixes.


More than 1/3 or 1/4 actually. But mostly it was about pilot preferences.

The recommended use of ammunition in the MG151/20 was the same as with the MG FF, as follows:

- 2 Minengeschoß m. Zerl.
- 2 Brandsprenggranatpatronen L'spur m. Zerl oder Brandgranatpatronen
- 1 Panzersprenggranatpatrone o. Zerl oder Panzerbrandgranatpatrone (Phospor) .......o. Zerl