B-17G with a 20 mm cannon onboard!

[Erich]

Ho-Hun your last remark was to counter the forward attacks in staffel strength by the Luftwaffe, but that was changed by summer of 44 due to the over increase of flight time through the bomber formation from the front with all US heavies mgs blasting anything that came through the line. The LW switched due that it would be easier to just confront the tail section only of the B-24/B-17 masses

 

[HoHun]

Hi Micdrow,

>Not to get off subject but some may find the bottom picture is of interest also.

Hm, I'd like to point out that for readers using Opera, there is no bottom picture but both are in the same line. This causes posts with multiple pictures to become very hard to read as the post (in fact, all of the posts on the current page of the thread) can easily become several times as wide as the user's screen.

This is (in my opinion) not a bug in Opera, but in the forum template as it separates the images with the   HTML entity which is a "non-breakable space" - it announces "do not insert a line break between the adjacent elements".

MSIE doesn't give a damn and inserts the line break anyway, obviously resulting in the easy-to-read layout the designer of the forum template intended.

If the   entity would be replaced by an ordinary space, or even by a <br> tag, Opera would display the thread the same way as MSIE, and that's probably the way the designer meant it to be displayed.

Since you're a moderator here and have a good technical understanding, I thought maybe if I'd point it out to you, you might know which administrator to contact for a possible fix I once made a post in the suggestions/comments subforum, unfortunately without receiving an answer:

http://www.ww2aircraft.net/forum/co...sts-attachments-simple-improvement-12757.html

I guess my post appeared too technical over there :-/ But it's really an inconvenience that hits me at least once every day, and often in the most interesting threads where people have posted great high-resolution pictures, such as in the current Do 335 thread - or this fascinating thread on B-17 armament, of course!

Regards,

Henning (HoHun)

 

[Micdrow]

Hi Henning,

As to your question on the board I would say you would have to contact evengilder or horse. I know next to nothing about the message board or how they work. In fact I usually have to go to evengilder ever time I have a problem. Most of the stuff you wrote about the message board idea is well like speaking a foreign language to me.

My main roll as a moderator is to allow me to make the technical area data base and to be able to move files around more easily.

On a side note the B-17 is probably one of my most favorate aircraft. After this weekend maybe I will start a technial thread about the aircraft and some of its different configurations. Currently Im trying to orginize all my data since I seem to have a scatter mess right now.

 

[renrich]

I don't think there is any mystery to max effective range. I don't have my ballistic tables handy but there is something called the rule of three. With a 3006, 270, 308 class of cartridge, sighting in to be 3 inches high at 100 yards will mean that the bullet will be within 3 inches of point of aim anywhere out to 300 yards. On a deer size target that means that a shot within 300 yards is essentially point blank and the shooter does not have to worry about range or bullet trajectory. A 50 BMG has a ballistic coefficient much higher than a 308 or 270 bullet and it's muzzle velocity is similar so point blank range on a target such as an airplane would be much further than 300 yards. Some 20 mms are not going to have as much MV as the 50 BMG and the projectile of the 20 mm is going to have a substantially lower BC so the 20mm is not going to be as flat shooting. Of course the more curved trajectory of the 20 mm will be compensated for by the bore sighting and gun sight of the fighter. I believe the bottom line is that the 50 BMG would have somewhat of an advantage in making a hit at longer ranges but that would be off set by the greater destructive power of the 20 mm shell. If both AC were firing at 300 yards or less, the 20 mm would have an advantage.

 

[Glider]

Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.

You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position

 

[drgondog]

Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.

You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position

I'm with you Glider. I have sat in that stupid bicycle seat for both the Cheyenne and the 'original' tail gun position. I have serious doubts that a single 20mm could be mounted as the breech was much longer than the M2.

I am ware of the Nose mount - that was feasible from a dimension perspective.

Seecil - the YB40 flew ~ 10-15 combat missions. It's only redeeming quality was the chin mount that converted to the B-17G. It kept up going in - but every body else lost 5000 pounds at the target and it didn't.

Eric is right about the tactics change - for at least two reasons. In March 1944 with the increase in P-51 Groups more Sweeps were plannned out in front, and secondly - the heavy armament of the Fw 190A8 made more rounds on target available from rear - and it was easier to find and stalk a bomber wing from the rear. Pilot skills requirements to shoot a B-17 at closing speeds of 500+ head on were far higher than 200-250mph from 6 O'clock.

 

[seesul]

On a side note the B-17 is probably one of my most favorate aircraft. After this weekend maybe I will start a technial thread about the aircraft and some of its different configurations. Currently Im trying to orginize all my data since I seem to have a scatter mess right now.

WOW, CAN´T WAIT TO SEE IT!!!

 

[seesul]

Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.

You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position

There´s really no space for the tail installation but I´ve found few mentions on Internet about it (see above) and also friend of mine told me there´s a pic of it in Freeman´s book...he promised to scan it so as soon as I have it I´ll post it...

 

[HoHun]

Hi Micdrow,

>As to your question on the board I would say you would have to contact evengilder or horse.

Thanks, I'll do that!

Regards,

Henning (HoHun)

 

[HoHun]

Hi Renrich,

>I don't think there is any mystery to max effective range. I don't have my ballistic tables handy but there is something called the rule of three.

Hm, I see what you mean, but it's my impression such a kind of well-defined rule is not the basis for the usually quoted maximum errective range figures for aircraft armament. Or Tony Williams simply didn't know about it

>On a deer size target that means that a shot within 300 yards is essentially point blank and the shooter does not have to worry about range or bullet trajectory.

That's a good point - I don't know if you remember the generic patterns "as viewed through the gunsight of a fighter" I posted a while back, but I based these on just this idea of point blank shooting: http://www.ww2aircraft.net/forum/weapons-systems-tech/centerline-guns-configurations-11037.html

(For wing-mounted fighter guns, the lateral deviation from the aim point usually has a larger impact than the vertical one, by the way.)

If we're talking about rapid fire weapon, it might even be necessary to include the pattern size into the consideration as fully automatic fire from airframe-mounted batteries tends to be far less accurate than aimed single shots from a handgun. For the tail turret of the B-17, the dispersion circle was given as 25 mils, meaning that the pattern would spread over a 10-m-wingspan fighter from wingtip to wingtip at 400 m range. Of course, the fighter in a head-on aspect will only fill a fraction of the pattern "disk" so that your hit chances are down to a fraction, too - even if the centre of the pattern perfectly coincedes with your aim point.

Accordingly, I'd say a big part of the problem for the B-17 gunner would be that the fighter with its fixed and considerably more rigid gun mounts would have a considerably smaller dispersion (a 4 to 6 mil circle for the MG 151/20) then the bomber's defensive guns.

For example, at 800 m range, a fighter's pattern (for example from the wing root guns - the outer wing guns would not be worth much beyond convergence range) would have less than 5 m diameter. Even with just two 20 mm cannon firing, bringing that pattern to bear on the bomber would do very serious damage.

The bomber gunner on the other hand would have a pattern with 20 m diameter, meaning that the fighter would only fill a small fraction of the pattern - and that most shots would miss even if the gunner aims perfectly. If we assume that the frontal area of the fighter fills something like 2% of the pattern, we might get 3% hits for the sake of the example as the pattern is denser towards the centre, and that means firing 100 rounds he'd get just 3 hits randomly distributed over the Focke-Wulf - which in the same time could 184 rounds at the bomber, of which (depending on the placement of the pattern) several dozen could hit if the aim is right.

So the gunner would have reason to feel out-ranged, even though the trajectory of his 12.7 mm machine gun is flatter than that of the 20 mm MG 151/20 of his opponent ...

Regards,

Henning (HoHun)

 

[Soren]

The MG151/20 fires a higher SD round at 770 m/s, and therefore it should have a comparable trajectory to the .50 BMG.

 

[renrich]

Henning, I understand that some or maybe all of US wing mounted guns in fighters were mounted in a manner where only one point on the gun was solidly fixed to the airplane. I think that means that the barrel was free to move about with some amount of motion. Can you explain that a little more clearly and how that would affect bullet dispersion. A high SD does not necessarily translate to a high BC.

 

[Glider]

The MG151/20 fires a higher SD round at 770 m/s, and therefore it should have a comparable trajectory to the .50 BMG.

I admit that my understanding was that the .50 had a MV of around 890m/s and had a better trajectory. The 20mm having far more impact at longer ranges due to the explosive content of the shell, but not due to the trajectory.

 

[Erich]

yes the 2cm was effective IF the LW pilot could get into range before being hammered by .50's of the bombers first, time and again in books and through personal interviews the Fw 190/Bf 109G crews mention getting assailed by .50 rounds before they could get their two to four 2cm to bear during rear attacks. of course the slow firing short range 3cm was even a worse case scenario

 

[HoHun]

Hi Renrich,

>Can you explain that a little more clearly and how that would affect bullet dispersion.

Hm, my point about lateral distance from the aim point was one of simple geometry - both short of and beyond convergence distance, even the idealized trajectory that is not subject to dispersion does not coincede with the aim point.

Bullet dispersion in a wing mount is affected by vibration of the structure it's mounted to, and wings tend to be more flexible than structural members of the fuselage.

Of course, it depends on the actual aircraft in question as well as on the gun mount, but as a rule of thumb wing guns that are mounted outside the propeller disk can be considered to have roughly 50% more dispersion than fuselage mounted guns.

Barrel vibration is also a major source of dispersion - the Luftwaffe found the low-velocity MK 108 to be more capable than the high-velocity MK 103 due to its combination of high rate of fire and small dispersion. High-velocity weapons pay for their flatter trajectory with an increased dispersion, which was one of the reasons the 50 mm aircraft cannon didn't work out for the Luftwaffe.

Regards,

Henning (HoHun)

 

[Erich]

the MK 108 was used due to the aerodynamics and non dampening effect/vibration of the wings in the Fw 190, the longer barrel was not suitable of the MK 103 except for ground attack work on twin engines where it proved itself

 

[Soren]

I admit that my understanding was that the .50 had a MV of around 890m/s and had a better trajectory. The 20mm having far more impact at longer ranges due to the explosive content of the shell, but not due to the trajectory.

That's also true. However firing AP rounds (Not exactly the std.) the MG151/20 should have a comparable trajectory to the .50 BMG. With the lighter and less aerodynamic HE(M) shells the trajectory was worse than the .50 BMG, that's true.


Another thing is that a bomber is a larger target to be shooting at, and the fighter is a more stable gun platform than firing from a mount on the bomber. So for that reason the effective range of the fire from the fighters was longer. Now that doesn't mean that their guns fired at a flatter trajectory, just that they could land their bullet closer together, and since their target was larger they could expect to hit at longer ranges. A small, fast and nimble fighter is a tricky target for a gunner to hit, eventhough he can shoot accurately out to 1,000y at a stationary target the same size.

Still as Erich notes the US gunners were firing at the LW fighters at ranges where they werent even in what theyd call accurate firing range themselves, but neither were the US gunners, they were just throwing bullets at the LW a/c hoping to get some hits. Now despite what many think a single .50cal hit can do a whole lot of damage to an a/c. At 1,000y its still got enough power to tear through an engine block, which wouldn't be so nice if your a/c was fitted with a watercooled inline engine!

 

[drgondog]

Another thing is that a bomber is a larger target to be shooting at, and the fighter is a more stable gun platform than firing from a mount on the bomber.

Soren - how do we know this to be true? A twin 50 mount in the tail of a B-17 is not only 'pretty stable' but also flexible to the extent that the Gunner can compensate for a/c motion.

A second question to be asked, particularly for wing mounts on an Fw 190, is why do we belive that the wings are 'stiffer' under high aero loads, to the point that the convergences are close to ground calibration?

Last, how do we compare the stability of the fighter's approach to the B-17 in flight?

I'm not really challenging the statements - just wondering how they could be proved

 

[seesul]

Still waitin´ for my friends scans of the 20 mm cannon instalation on tail of B-17.
Does anyone have these pics?

 

[renrich]

It is my understanding based on comments made by Linnekin in "80 Knots tp Mach 2" that the guns in the wings of WW2 US fighters jumped around a bit because the mountings were not rigid. He stated that the vibration when firing 6-50s at 600 rpm was very noticeable. It would seem to me that if the muzzle is moving perhaps one half inch or more the dispersion would be pretty major down range. This would be in contrast to bomber mountings which would be more akin to a tripod mount used by infantry or pedestal mount on tanks.