Performances with MG151/20 and Mk108

[Parmigiano]

It is generally accepted that Fw190 had a drop in performances when mk108 was installed in the outboard bay to replace the mg151/20

I stepped again recently in this site
WORLD WAR 2 FIGHTER GUN EFFECTIVENESS
and I noticed the weight of the 2 weapons

Now, there is something that I cannot properly understand:

.....................MG 151/20......MK 108
Weapon.Kg.............42.............60
Round...Kg.............0.2.........0.48
# Rounds .............150.............60
Rounds Weight Kg..30..............28.8

Assuming the other stuff (belts etc.) roughly equal, the difference in overall weight is less than 20 kg/wing, that does not sound as something that could dramatically impact the performances.

Some other random considerations:

- Mk108 muzzle was contoured in the wing, that should be a small aerodynamic improvement vs Mg151 (not substantial, but at least not a disadvantage)

- Increased weight in wings should mainly affect roll rate, but it is generally accepted that the 190 had the best roll rate of the bunch, so a 20 kg increase should not change things dramatically

- The shooting effectiveness was greatly improved: 2x131 + 2xmg151 were more than enough to take down a fighter, and the mk108 were the right 'plus' for the bombers (the pilot could choose to fire weapons separately)

Excluding other factors (like the Sturm machines with their additional armour weight), can we find the reason of the reduced performances of the mk108 installation?

[Erich]

Parm I am not so keen whether or not the Fw 190A-6-A-9 fitted with outboard Mk 108's really lost much performanace, had we or if we discuiss the attributes of an armored R2 or R8 versions that a big yes. Even with the A-8/MK or R2 version in JG 301 and they had the Panzerplatten side armor but did have cowling mg's installed, there was a decrease and it was primarily the heavy steel sheeting on the fusleage for pilot protection. the heavy sturms of the 3 sturmgruppen had at least another 500-600 pound of weight and the pilots were told to avoid US escorts at all costs (if possible)

[Parmigiano]

Thank you Erich, that's exactly my question mark...

I thought that the 'R2' was just the mk108 installation without additional armour, and 'R8' was mk108+extra protection, but reading your post it seems that extra armour was fitted in both.

Was then no variant with just the 108 in place of the mg151?

[Erich]

the R2 versus the R8 is an issue and it appears the official German documentation was to show the R2 to be armorless but that is not the case with the mix of armor added via the factory and esepcially in the field, the removal of the mg 131's in the cowling the troughs left open and real hazard for performance and then finally the troughs with the aerodynamci fairings. This was all then promised in the standard fit for the R8 variant delivered in November of 44 to the Sturm units. JG 301 which had a few of the R2 mod3els stripped off some of the armor and most kept the mg 131's in place over the cowling

yes the Fw 190A-7/MK

no armor fitted, mg 131's installed and outboard MK 108's replaced the MG 151/20's. The a/c used in JG 11 and some in JG 1 for frontal attacks on the US bomber formations. I./JG 11 being quite successful with this a/c.

E

[lesofprimus]

Agree with Erich 100% on all the above.... Some R2's had the armour, some did not, some had 131's, some did not... There doesnt really seem to be a standard except for what the individual Gruppen, Staffel or just pilot preferred...

I have also read that the 108's were not the performance killer, but the extra steel plating was...

[Twitch]

Also agree. Its a moot point since most with the 30 mm also had the extra weight of armor.

[Soren]

To answer your question Parmigiano;

No, there would be no decrease in performance by installing the Mk108's over the MG151/20's - firstly because the aerodynamic change is very much non-existent, and secondly because the difference in weight is so small ~20kg.

In any case weight doesn't really decrease speed that much, not even if the difference is large, its more the climb rate and maneuverability which suffers -however with a difference of just 20 kg, climb rate and maneuverability is very much unaffected.

[Jank]

I am surprised that delcyros isn't weighing in here on the recoil forces proving unmanageable both structurally and affecting flight performance.

[Parmigiano]

Recoil forces impact flight only when the gun is fired, that is 5-10 seconds of a mission! It may create a problem in shooting accuracy, but not in performances (unless the recoil shred the wing structure, in that case performances are impacted AFTER the shooting )

Maybe a point that we have underestimated is: does the mk108 require a strenghtened installation bay because of the recoil, that may translate in another extra weight?

[lesofprimus]

The Mk 108 does not require any additional support structure or bracing...

[Tony Williams]

The MK 108, like the MG-FF, had quite soft recoil characteristics because of the mechanism. They both used the Oerlikon-type "advanced primer ignition blowback" ation, in which the gun fired while the bolt was still travelling forwards at maximum speed, so the initial recoil impulse was used up in stopping the bolt before it could be pushed backwards again. Incidentally, this mechanism also meant that these guns could never be synchronised, because the moment of firing could not be controlled precisely enough.

The MG 151 used a short-recoil action which delivered a sharper recoil blow. I don't know the actual peak recoil forces of the two guns, but I would be surprised if the MK 108 was worse than the MG 151.

Tony Williams: Military gun and ammunition website and discussion forum

[lesofprimus]

And quite accurate information TW, thanks...