20/20 Hindsight - different armament?

[wmaxt]

In WWII the problem with 20mm was ammo count and and the slower rate of fire. I agree with Del that I want at least a few .5s. The mix gives good time of fire as well as a great punch. As the 20mm rate of fire increased and the capacity to hold enough ammo for more than 15-20 seconds of fire they became the weapon of choice.

I think that applies to both ground attack as well as Ariel attack. Whatever gun you use has to get enough in the air to hit then destroy the target. If your spread is to wide due to a slow rate of fire, have to make repeated passes or run out of ammo prior to destruction of the target you've failed.

wmaxt

 

[Tony Williams]

Succesful is relative, by the time of their arrival they would have been totally outdated by first gen. jets, at least over europe.

We have been talking here about whether a given weight of armament would have been practical in a piston fighter, or whether it would have had unacceptably detrimental effects on the plane. How piston fighters compare with jets in combat is an entirely different issue.

Do You have an idea about hi alt decremental effects of recoil?
Lunatic actually studied this before. A long time ago he calculated that each round coming out the pair of Ho155's on the Ki-84-1c would slow the plane approximately 11 mph. Such huge recoil forces are a real issue for effective weaponry in any aircraft.

Actually I do know something about recoil - and the calculations you quote do not make sense. I have suddenly understood why you are so concerned about recoil, if you are basing your opinions on this. Let's take the example you quote - the Ki 84: this weighed much the same as the Hurricane or Spitfire (give or take a bit). So logically, if the recoil from each Hispano shot generates about 70% of the recoil of the Ho-155, then each pair of rounds fired from Hispanos would slow the British planes down by about 11 mph x 70% = 8 mph (16 mph with four-cannon layouts). The RAF did an analysis during WW2 of the use of cannon ammo in combat, and discovered that an average of 17 rounds per gun were fired in each burst of fire, taking 1.7 seconds (five bursts were fired on average, for 85 rpg total). So according to the calculations you quote, the Spitfire would have been slowed down by 8 x 17 = 136 mph after each burst, the Hurricane by 272 mph - which means it would have fallen out of the sky before it finished one burst! This is clearly wrong.

I think I may have posted this already, but it obviously bears repeating (along with a bit more) - from Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45:

"This might be an appropriate moment to dispel one of the favoured myths of big-gun aircraft; that the recoil had a drastic effect on their speed. To take the example of the USAAF's B-25 fitted with a 75 mm M4 gun; the aircraft weighed around 12,000 kg and attacked at perhaps 400 km/h, the gun fired a 6.8 kg projectile at around 2,200 km/h. A simple rule of thumb is to multiply the weight by the speed to achieve a rough "momentum index" (it is actually a bit more complicated than this, as the expanding propellant gasses contribute to the recoil). It will be apparent that the aircraft has at least 200 times the momentum of the projectile, and a single shot will therefore not greatly slow it. In fact, at the end of an attack run in which several shots were fired, the plane would typically be slowed by 10-15 mph. The effect on fighter speed of long bursts of heavy gunfire (especially from automatic cannon) could be noticeable, particularly in a turning battle when the aircraft might be manoeuvring at the extremes of the flight envelope, close to stalling."

Now let's do some calculations about what the effect might have been. I explained the methodology in some detail here: BASIC BALLISTICS Each Hispano round fired a 130 g projectile at about 860 m/s. Each contained around 32 g of propellant, with an escape velocity of about 1,200 m/s (this is a typical average for high-velocity guns). This gives a recoil momentum factor of (130 x 860) + (32 x 1,200) = 150,000 g/m/sec (let's convert the grams to kg to make the calculation easier, giving 150 kg/m/sec). Assuming a typical loaded aircraft weight of 3,600 kg, one shot would generate enough recoil to move the aircraft backwards at 150/3,600 = 0.04 m/s. So a burst of 17 rounds per gun would push the aircraft back at 17 x 2 x 0.04 = 1.35 m/s (2.7 m/s for four-gun layouts). Now if the aircraft is flying at 400 kmh when it fires, that is equivalent to 110 m/s. So after firing their 17-rpg bursts, a two-cannon Spitfire would be slowed by 1.25 %, a four-cannon Hurri by 2.5 % (all figures rounded), which amounts to 5 or 10 kmh respectively.

I have no argument with the proposition that four 20mm Hispanos would have been quite enough for almost any target - that's what the RAF concluded, anyway. But what we are debating is whether the P-47 could have coped with six, and remained an effective fighter. My conclusion to that is yes, certainly.

Tony Williams: Military gun and ammunition website and discussion forum

 

[mosquitoman]

In my opinion, 20mm cannon weren't needed by the USAAF because there weren't any heavy bombers being operated by the Luftwaffe, these would have needed 20mm to take down, just like the Luftwaffe fighter armament went to higher and higher calibre eg 2cm, 3cm etc. The RAF and USAAF mainly had fighters and fighter-bombers to deal with

 

[Twitch]

We must realize that guns are fired for very brief periods. A 3 second burst is a VERY long burst. Recoil of all guns firing does slice a bit of speed off but its effects were alwayd dealt with. I have never met a fighter pilot that said the shaking of his ship was so severe due to recoil that he could hit the enemy once he had a good gun solution. We have inertia at work in its best form too so that diving and zooming crates that lose any speed from breach recoil reactions are extremely negligable. That few MPH is quickly regained.
We are analyzing and over analyzing what was never a problem to begin with.

 

[Jank]

Mosquitoman said, "In my opinion, 20mm cannon weren't needed by the USAAF because there weren't any heavy bombers being operated by the Luftwaffe, these would have needed 20mm to take down, just like the Luftwaffe fighter armament went to higher and higher calibre eg 2cm, 3cm etc."

Yes, and no one disagrees with that position. This thread, like so many others, concerns a question of what if and whether it could have been practically done. This thread is really about whether the P-47, later in the war, in the ground straffing and close support role, would have benefitted from an outfitting of 6 x 20mm cannons and whether their installation would have been practical in terms of degradation of flight performance.

 

[delcyros]

I have no argument with the proposition that four 20mm Hispanos would have been quite enough for almost any target - that's what the RAF concluded, anyway. But what we are debating is whether the P-47 could have coped with six, and remained an effective fighter. My conclusion to that is yes, certainly.

I want to point out first that I enjoi our debate, so if You ever had the impression that my statements are unpolite, be sure I don´t want to be agressive anyway.

According to Your calculations (which make sense to me as did Lunatics one´s), a late war P-47 (M) would be slowed down by a three sec. burst of six 20mm guns by 10 mp/h or ~16 Km/h.
mv: 880 m/s Lunatic's WWII Aircraft Gun Ballistics Page
proj. weight: 0.13 Kg
cartridge load: 32g
powder vel.: 1200 m/s
rof: 600 rpg (3 sec burst equals to 30 rounds per gun)
P-47 M typical loaded weight: 6029 Kg Republic P-47 Thunderbolt
net recoil: 152.9 Kp per gun (917.4 Kp for a six gun burst)
recoil / weight relation: 0.152
brutto deceleration for 3 sec. bursts: 4.56 m/s (16.41 Km/h or 10.19 mp/h)
compare original P-47M:
recoil / weight relation: 0.067 (for eight 0.50 M2)
brutto deceleration for 3 sec. bursts (a 40 rounds): 2.68 m/s (9.64 Km/h or 6 mp/h) That´s typical for ww2 fighter A/C (between 5 and 10 km/h deceleration) but once a single gun is out of calibration (rof is always slightly different for individual guns due to gunwear, condition, etc.), the mid wing layout causes problems in aiming, esspecially for long to medium range deflection shots. I have always taken the comparably low recoil / weight relation USAAF fighter A/C had, as an advantage, overcoming the shortcomings of the mid wing layout. They do imply a very stable gunplatform even at hi alt, so I conclude that the long range gunnery is much easier with them.
Note that the brutto deceleration of 6 gunned P-47 exceeds typical ww2 fighter A/C values according to my database.
This is significantly more than any Fighter A/C of ww2 had, so I conclude this weapon layout would be impractical for hi alt, hi speed engagements. I would like to be more concrete here but I have a lack in cartridge loads for different guns of my database.
This layout could be used for low alt strafing / attacking purposes as Jank pointed out, but I do think this is lot of overkill.

 

[Tony Williams]

In my opinion, 20mm cannon weren't needed by the USAAF because there weren't any heavy bombers being operated by the Luftwaffe, these would have needed 20mm to take down, just like the Luftwaffe fighter armament went to higher and higher calibre eg 2cm, 3cm etc. The RAF and USAAF mainly had fighters and fighter-bombers to deal with

I would put it slightly differently:

.50 = OK against fighters, as long as you have at least six of them.

20mm = OK against fighters, as long as you have two of them; great against fighters and fine against medium bombers/attack aircraft also, if you have four of them.

30mm = optimum against heavy bombers, as both the Germans and the Japanese discovered.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Tony Williams]

I want to point out first that I enjoi our debate, so if You ever had the impression that my statements are unpolite, be sure I don´t want to be agressive anyway.

You are not, and I hope I don't appear to be so either. There is nothing I enjoy more than a well-argued debate on an issue like this!

According to Your calculations (which make sense to me as did Lunatics one´s), a late war P-47 (M) would be slowed down by a three sec. burst of six 20mm guns by 10 mp/h or ~16 Km/h.

I have no argument with that, except that three seconds is a very long burst of fire.

Note that the brutto deceleration of 6 gunned P-47 exceeds typical ww2 fighter A/C values according to my database.
This is significantly more than any Fighter A/C of ww2 had, so I conclude this weapon layout would be impractical for hi alt, hi speed engagements.

That would be true if more lightly armed planes were felt to be on the limit of recoil tolerance. I am not aware of any evidence that they were. In fact, the only mentions I have found of recoil affecting shooting accuracy, so the pilot had to re-aim after a few shots, is for planes carrying very powerful cannon: the Hurricane IID (2x40mm), Il-2M3 (2x37mm), Yak-9T (1x37mm). I have read no such reports concerning the 4x20mm RAF fighters. So I must therefore conclude, unless hard evidence to the contrary can be found, that 6x20mm would not have caused the P-47 significant problems.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

Something worth to spent some thoughts. I would like to hypothezise for a moment.
Hypothesis is: There was a "rule for useful recoil" of ww2 fighter A/C altough it was never credited as a design feature this way. (By the way, the VVS credited the 4 20mm upgunned Hurricane as unsuited for dogfighting and reduced the armement to soviet .50´s) Impossible to prove and to some degree will remain speculation. But perhaps we can make it plausible?
Can You give me the cartridge datas for MG 151/15; -20; Berezin .50; Shvak and MK 108 + Ho-155 and Ho-5?
I would like to compare the recoil values for typical and prooven ww2 fighter A/C but still miss the cartridge loads (probably 20+-% plus) for recoil addings:
(typical weights: clean configuration and 50% fuel)

P-40N (weight: 3500 Kg, six 0.50cal: 228.6 + gaz effects): 0.065
P-39D (weight 3810 Kg, one 37mm M4 + four 0.50cal: 546.8 + gaz effects): 0.143 -heavy recoil gun is nose mounted, plane not suitable for hi alt
P-38L (weight 6875 Kg, one 20 mm + four 0.50cal: 266.4 + gaz effects): 0.038 -plane suited for hi alt dogfights
P-47D-(weight 6350 Kg, eight 0.50cal:304.8 + gaz effects): 0.048 -plane suited for hi alt dogfights
P-51D (weight 4355Kg, six 0.50 cal: 228 + gaz effects): 0.0525 -plane suited for hi alt dogfights

US fighter planes preferred very stable gunplatforms, recoil (netto)/weight ratios well under 0.06 with the notable exception of the P-39, altough this plane received critizism because of the 37mm gun, which was barely used except for ground attacks or bomber interceptions

Ki-61 Ib (weight 3050 Kg, two 20mm Ho-5 +two 0.50cal: 189.6 + gaz effects): 0.062 -plane suited for hi alt dogfights
Ki-84 Ib (weight 3275 Kg, four 20mm Ho-5: 274.56 + gaz effects): 0.083 -plane with limited fighter use at hi alts
A6M5b: (weight: 2750 Kg, two 20mm type99, one 13mm type 3, one 7.7mm type 97: 238 + gaz effects): 0.086, plane suited for hi alt dogfights
J2M (weight 3500 Kg, four 20mm Ho-5: 274.56 + gaz effects):0.078 -plane suited for hi alt dogfights / engagements
N1K1J (weight 3600Kg, four 20mm type 99, two 7.7mm type 97: 400.7): 0.111 -plane suited for high alt bomber interceptions

Typical mid-late war japanese fighter had a very high recoil (netto) / weight relation which usually was between 0.06 and 0.09 with the notable exception of specialized bomber interceptors such as the Ki-84Ic and N1K1J, which performed worser in the fighter role.

Hurricane IIb (weight 3150 Kg, twelve 0.303cal: 95.4 +gaz effects): 0.030-plane suited for dogfights (hi alt wasn´t tactical envelope of Hurris)
Spitfire Vc (weight 2700 Kg, two 20mm MKII+four 0.303cal: 260.6 + gaz effects): 0.096 -plane suited for medium alt dogfights
Typhoon (weight 4600 Kg, four 20mm MKV: 436.8 + gaz effects):0.095 -plane suited for interceptions
Meteor-III (weight 5500 Kg, four 20mm MK V: 436.8 + gaz effects):0.079 -plane suited for hi alt engagements, all guns nose mounted

RAF fighter A/C seems to have put emphasis on maximum useful gunpower (at least so in the case of Hurricane IId and Spit Vc, altough numerous accounts concerning the recoil of the guns exist in this case) for their mid war fighter A/C, late ware fighter A/C (Tempest, Meteor, Spitfire XIV) had a more balanced recoil-weight relation, indicating a somehow increased importance of recoil issues.

Me-109G10 (weight 2735 Kg, one 30mm MK 108, two 13mm MG131: 219.45 + gaz effects) : 0.080 -plane suited for hi alt dogfights, all guns nose mounted
Fw-190 A8 (weight 4400 Kg, four 20 mm MG151/20, two 13mm MG131:357.3 +gaz effects): 0.081 -plane suited for interceptions
Fw-190D9 (weight 4200 Kg, two 20mm MG 151/20, two 13mm MG131:205 + gaz effects): 0.488 -plane suited for hi alt dogfights
Me-262 (weight 6000Kg, four 30mm MK 108:666.6 +gaz effects): 0.111 -plane suited for high alt interceptions -all guns nose mounted
He-162A (weight: 2700Kg, two 20mm MG 151/20: 152.25 + gaz effects): 0.056 -plane suited for hi alt dogfights

German planes favoured the nose mounted guns and hance enjoied some heavy recoil-weight relations. I suspect that this was a reason why no Bf-109 had a 30mm MK103, this would exceed the "rule" by much! The dogfighters (Fw-190D and He-162) had a significantly lower ratio, implying a more stable gunplatform

Yak-3 (weight 2400 Kg, one 20mm Shvak, two 12,7mm: 160.7 +gaz effects): 0.067 -plane suited for dogfights (hi alt wasn´t tactical envelope of the Yak)
La-7bic (weight 3100 Kg, 3 20mm B20: 213.75 + gaz effects): 0.069 -plane suited for hi alt dogfights

VVS fighter A/C with all nose mounted guns also enjoied a comparably low recoil-weight relation, implying a very stable gun platform with little or no recoil issues. (Note that tank hunters such as the Yak-9T with 37 mm NS 37 exceeded the typical ratio.)

What to conclude? Generally spoken, each nation / air force preferred a typical recoil-weight relation for typical purposes. Fighte A/C rarely exceed 0.1. (THEY DO NEVER EXCEED 0.10 FOR MID WING MOUNTED GUNS!) It seems to me that the nose gunned Fighter A/C had sometimes a higher recoil-weight relation than the mid wing mounted ones.
Note that a six 20mm upgunned P-47 would rather fall in the envelope of ground attackers than dogfighters, esspecially with the emphasis of USAAF in mind to limit the recoil-weight ratios. Keep an eye on the distribution of the values. They are typical and not randomly choosen. The "rule" gives a limit of 0.10 (not counting gaz effects) for wing mounted armement for fighter purposes (interceptor do slightly, ground attackers do heavily exceed this limit) with a nation related typical window below.
best regards,

 

[Jank]

Delcyros said, "P-47D-(weight 6350 Kg,"

A combat loaded P-47D-22 and later series came in at 14,500lbs or 6,591 kg in combat loaded trim. (Full internal fuel and full ammunition load.)

 

[delcyros]

I used to reduce all weight figures by a bit to reflect the fuel weight left for climbing to altitude and so.
For some planes no combat load figures were avaiable the last 20 min. in the net (..or I used to take the wrong sources..), so I used some approximations from empty and max weight figures...
best regards,

 

[Tony Williams]

There is one general comment I would make about your theory: that as far as I'm aware, the limitations on the number and size of fighter guns were concerned primarily with weight, and to a lesser extent with space and difficulty of access, but not with recoil. In particular, weight out in the wings affected the handling of the plane (it slowed the roll-rate) which could make it more vulnerable in combat. Weight anywhere in the plane restricted the agility, the rate of climb, the rate of acceleration (except in a dive) and the maximum altitude. So plane makers usually restricted the armament to the minimum they thought would be effective in doing the job.

The most interesting examples of this were the heavily armed and armoured late-war Bf 109 and Fw 190 variants intended to attack USAAF bomber formations, while their more lightly armed and armoured comrades tried to hold off the bomber escorts. The extra armament (and armour) affected the performance and handling of these planes, which is why those engaging the fighters didn't want the extra gunpods - recoil had nothing to do with it.

Similarly, most Russian planes were lightly armed, with the guns usually in the fuselage. This had nothing to do with recoil problems, everything to do with the serious performance limitations caused by the low-powered Russian engines.

Do you have any contemporary sources which raise recoil as a significant issue in limiting the armament of fighter planes? I did a lot of research into original British documents on WW2 aircraft armaments held in the National Archives, when collecting information for the Flying Guns books, and did not find any such concerns.

A few more detailed points:

(By the way, the VVS credited the 4 20mm upgunned Hurricane as unsuited for dogfighting and reduced the armement to soviet .50´s)

Not according to my co-author Emmanuel Gustin, who had this to say (from Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45):

"The armament of Browning .303 guns installed in early Hurricanes was considered deficient, as having an effective range of only 100 to 150 meters. It was considered to install four ShVAK cannon, four UBTs, or two ShVAK and two UBT guns; although a preference existed for the second option the last one was chosen, because there were not enough UBT guns. So about a 1000 Hurricanes Mk.IIA and Mk.IIB were modified to have two ShVAK cannon and two UB machine guns, a quite powerful combination by Soviet standards. The Hurricane's armour protection also fell below Russian standards, and it was replaced. The Mk.IIC with its four Hispano Mk.II cannon retained its armament, and was used as a ground attack and anti-shipping aircraft. By 1941 the Hurricane was obsolescent as a fighter, and it was not highly regarded."

Sources I have state that the Soviets had a very low opinion of the Hurricane as a fighter (so did the Finns who fought against them) because its performance was too poor (possibly because high-octane fuel wasn't readily available?) so they did not use it in aerial combat if they could help it.

Can You give me the cartridge datas for MG 151/15; -20; Berezin .50; Shvak and MK 108 + Ho-155 and Ho-5?

I presume you mean the propellant weights (the projectle weights and muzzle velocities are available on my site here: WORLD WAR 2 FIGHTER GUN EFFECTIVENESS

I do not have propellant weights for all of the rounds, but have the following(results are averages, these did vary): MG 151/15 = 25 g; MG 151/20 = 18 g; 12.7 mm Berezin = 18 g; MK 108 = 28 g; Ho-5 = 21 g. The Ho-155 probably weighed around 30 g as its muzzle energy was similar to the Hispano's, but I have no sources for that.

US fighter planes preferred very stable gunplatforms, recoil (netto)/weight ratios well under 0.06 with the notable exception of the P-39, altough this plane received critizism because of the 37mm gun, which was barely used except for ground attacks or bomber interceptions

On the contrary, the P-39 was valued by the Soviets as an air combat fighter, and some of their highest-scoring aces used it. They were not bothered about its poor high-altitude performance because most Eastern-Front combat was at lower altitudes. Although any fighter (in any air force) might be called upon to attack ground targets from time to time, the Soviets generally left that to the well-armoured Il-2.

RAF fighter A/C seems to have put emphasis on maximum useful gunpower (at least so in the case of Hurricane IId and Spit Vc, altough numerous accounts concerning the recoil of the guns exist in this case) for their mid war fighter A/C, late ware fighter A/C (Tempest, Meteor, Spitfire XIV) had a more balanced recoil-weight relation, indicating a somehow increased importance of recoil issues.

No, as planes grew heavier the armament weight did not increase because the RAF felt that its gunpower was adequate. Although having said that, the prototype Meteor did have six Hispanos but two were removed because their installation was dangerous (they could not be worked on or removed unless the armourer was standing directly in front of the muzzle).

German planes favoured the nose mounted guns and hance enjoied some heavy recoil-weight relations. I suspect that this was a reason why no Bf-109 had a 30mm MK103, this would exceed the "rule" by much! The dogfighters (Fw-190D and He-162) had a significantly lower ratio, implying a more stable gunplatform

The MK 103 was too big to be fitted to the little Bf 109. A special version with a revised design for the gas-operated action, the MK 103M, was experimentally fitted to a Bf 109K-10, but proved unsuccessful. There is no doubt that the Luftwaffe wanted to see the MK 103 fitted to the Bf 109, but it could not be made to work.

VVS fighter A/C with all nose mounted guns also enjoied a comparably low recoil-weight relation, implying a very stable gun platform with little or no recoil issues. (Note that tank hunters such as the Yak-9T with 37 mm NS 37 exceeded the typical ratio.)

The Yak-9T was not a tank-hunter - it was designed for aerial combat, and proved very successful in skilled hands (the Yak-9K, with the even more powerful NS-45, was given to an elite fighter squadron to test and they claimed one kill for every 10 rounds fired). As I have said, the armament of Russian fighters was limited by their weak engines. It increased later in the war, as their planes became more powerful.

To sum up, it seems to me that you have become very fixed on your theory that gun recoil was a major limitation on aircraft armament, but I have seen no evidence to support this. On the other hand, it is clear that armament weight (especially out in the wings) was a major concern because of its effect on performance and handling, and this caused all air forces to restrict the number and size of guns.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

To sum up, it seems to me that you have become very fixed on your theory that gun recoil was a major limitation on aircraft armament, but I have seen no evidence to support this. On the other hand, it is clear that armament weight (especially out in the wings) was a major concern because of its effect on performance and handling, and this caused all air forces to restrict the number and size of guns.

I may be fixed to the idea, but only because I believe it is not covered enough and because I think in ww2 there was little if even any covering of the problem, altough it undoubtly existed.
Actually, the Yak-9T unsurprisingly suffered from heavy recoil issues:
"Yak-9T - Yak-9 armed with a 37 mm Nudelman-Suranov NS-37 cannon with 30 rounds of ammunition instead of the 20 mm ShVAK cannon, cockpit moved 0.4 m (1 ft 3 in) back to compensate for the heavier nose. Initially poor quality control lead to multiple oil and coolant leaks from cannon recoil. Recoil and limited supply of ammunition required accurate aiming and 2-3 round bursts. Yak-9T was widely used against enemy shipping on the Black Sea and against tanks, but also successful against aircraft with a single cannon hit usually sufficient to tear apart the target." The designation alone implys a specialized tank hunter variant:
Yak 9 Tyazhelowooruzheny" (Heavily Armed), the design bureau was asked for a -tank destroyer -altough it now is generally assumed to be a fighter plane because of the use in this role by VVS-Garde units. (It actually served in the tunk hunter role as well). Even with this in mind, nobody claims the Yak-9T to be good in the dogfighting role, it´s one and only reputation is incredible devastating firepower.
According to the Hurricane, Yakolev wrote (translated):
"...the GKO accepted the Hurricane, but the armement was considered to weak by Suprun and others, so subsequent changes in weapon layout to two Shvak and two 12.7mm guns became necessary. Later Hurricane with four high velocity 20mm guns suffered from heavier weight and lower firing accuracy and thus were deligated to the ground attack role or refitted with 4 12.7mm guns". (Yakolev, zehl dschisin, p.402)
Note that "lower firing accuracy" may be caused due to heavier recoil as plausible explenation.
According to the P-39 in VVS use, Yakolev wrote (translated):
"The airplane Airacobra was well known to us since the land lease treaties. Initially it made problems because of the weak tailplane, a number of planes were lost without enemy actions due to this weakness. Eventually we removed parts of the armour and sometimes even the 37mm gun in order to improve the dogfighting abilities and the stability of the gunplatform. With more experience, pilots soon learned to use the advantages of this big gun against german bombers, so most Airacobras retained the centreline gun in the end, despite it´s negative aspects" (ebd.p.418 )

This leads us to a certain point: Altough you made clear that other reasons may cause the lower armement, You did not disproved my theory. It is interesting to note that certain recoil-weight relations were never exceeded for fighter A/C and some ratio´s are too close to be explained as random events:
(Yak-3 and La7bis for example but also Bf109G10/G6 and Fw190A8, Ki-84Ib and A6m5b, Spitfire Vc and Typhoon, P47D and P51D)
And there were enough examples to put more firepower on planes, however, none of them reached combat status. Best example for this is the tiny He-162A1 with originally two 30mm MK 108 mounted in the lower fuselage (better than mid wing), but the recoil exceeded 0.1 (actually 0.123 plus gaz effects), hence test pilots were concerned about the comparably large recoil issues for the pretended fighter role. Subsequent changes (He-162A2) to a twin 20mm armement eased the problem, the later A-3 variant again with 30mm had substantial structural reinforcements and a heavier weight to match 0.100 (plus gaz effects). All random?

Question still remains wther or not the recoil of a six mid wing mounted 20mm high velocity guns would detrementally affect the stability of the fighter gunplatform rating. I do believe it does, You disagree.
the brutto recoil is around 152 Kp per one single shot. Depends on the geometry of the placement of the gun. assume the gun is placed in the middle of the wing, ~4.5 ft. away from the centreline and ~1 ft. below the central, longitudinal energy axe of the plane. Any recoil impulse generated there will cause some rapidly or slowly applied force (depending on gunmount) to the airframe, which may slew the plane out of course by a tiny fraction. (Note that fractions are important here) Rof for one gun is as low as 600 rpm, but maybe as high as 1000 rpm for a tweaked gun. This implys there will be never absolute, contemporary balance in firing. (this is also true because of different barrelwears=slightly different mv, different gunstate, different ammo type mixings and so on) A single, uncoupled shot will aplly 152Kp to the right wing 4.5ft. from the energy axe, if the right gun is fired. For the energy axe, this results to a course difference of 1.8712 deg (assuming some stabilizing factors by speed [~300 mp/h] and air density [500 hectopascal or~18000ft alt]) or in other words, a 20ft. wide, circular target originally aimed at 306 ft. distance will be thrown completely out of aim due to the recoil of a single shot fired from a mid wing mounted Hispano 20mm MK II in our example. That´s pretty much recoil for fighter purposes, if You ask me. Now, we know that firing 6 guns simultaneously will positively affect the gunstability but only to some degree (debatable), there is no question that eight 0.50cal do perform much better in this respect than would do six 20mm.

By the way, thanks for the cartridge infos!
best regards,

 

[Tony Williams]

I may be fixed to the idea, but only because I believe it is not covered enough and because I think in ww2 there was little if even any covering of the problem, altough it undoubtly existed.

I have already agreed that it existed with certain heavily-armed planes, and provided examples of this. I don't agree that it was a significant problem with 20mm fighter armament; 'standard weight' fighters could cope with four 20mm without suffering any significant disadvantage, which means that the big and heavy P-47 could have coped with six.

Actually, the Yak-9T unsurprisingly suffered from heavy recoil issues:

Yes, I'd already mentioned that one in my post No.88.

The designation alone implys a specialized tank hunter variant:
Yak 9 Tyazhelowooruzheny" (Heavily Armed), the design bureau was asked for a -tank destroyer -altough it now is generally assumed to be a fighter plane because of the use in this role by VVS-Garde units. (It actually served in the tunk hunter role as well).

"Heavily Armed" is an accurate translation and is certainly correct, but does not imply use against tanks, or any other specific targets. As I have already pointed out, any fighters could be tasked with ground attack missions when needed, which may have involved shooting at tanks, but that didn't make them anti-tank planes. The fact that, as you say, it served with fighter units (rather than ground attack ones), is a clear indication of its primary purpose.

According to the Hurricane, Yakolev wrote (translated):
"...the GKO accepted the Hurricane, but the armement was considered to weak by Suprun and others, so subsequent changes in weapon layout to two Shvak and two 12.7mm guns became necessary. Later Hurricane with four high velocity 20mm guns suffered from heavier weight and lower firing accuracy and thus were deligated to the ground attack role or refitted with 4 12.7mm guns". (Yakolev, zehl dschisin, p.402)
Note that "lower firing accuracy" may be caused due to heavier recoil as plausible explenation.

Speculation - not proof.

According to the P-39 in VVS use, Yakolev wrote (translated):
"The airplane Airacobra was well known to us since the land lease treaties. Initially it made problems because of the weak tailplane, a number of planes were lost without enemy actions due to this weakness. Eventually we removed parts of the armour and sometimes even the 37mm gun in order to improve the dogfighting abilities and the stability of the gunplatform. With more experience, pilots soon learned to use the advantages of this big gun against german bombers, so most Airacobras retained the centreline gun in the end, despite it´s negative aspects" (ebd.p.418 )

The P-39 was notorious for stability problems, which could cause it to go into a dangerous spin. These were not caused by gun firing.

This leads us to a certain point: Altough you made clear that other reasons may cause the lower armement, You did not disproved my theory. It is interesting to note that certain recoil-weight relations were never exceeded for fighter A/C and some ratio´s are too close to be explained as random events:

If you propose a new theory, it's your job to provide convincing evidence to prove it, not my job to disprove it. Other things being equal, the heavier the weight of armament, the more recoil it is likely to produce. As I have pointed out, it is very clear that armament weight affected aircraft performance, so (generally speaking) no more armament was fitted than was thought necessary to do the job. That this also restricted the recoil was a side-effect, not the main priority.

Best example for this is the tiny He-162A1 with originally two 30mm MK 108 mounted in the lower fuselage (better than mid wing), but the recoil exceeded 0.1 (actually 0.123 plus gaz effects), hence test pilots were concerned about the comparably large recoil issues for the pretended fighter role.

According to my information, the main problem with the MK 108 installation was the nose was too lightly built and was damaged by the gufire, so MG 151s were fitted while the nose was strengthened. I've not read anything to suggest that the effect of recoil on the flight performance caused aiming problems.

Question still remains wther or not the recoil of a six mid wing mounted 20mm high velocity guns would detrementally affect the stability of the fighter gunplatform rating. I do believe it does, You disagree.

Indeed. It looks as if we will have to agree to disagree

Any recoil impulse generated there will cause some rapidly or slowly applied force (depending on gunmount) to the airframe, which may slew the plane out of course by a tiny fraction.

The more guns you have, the less significant is a single firing impulse from any one gun - so a six-cannon layout would have had a smoother total recoil push than a four-gun layout. If your theory was correct, the shooting accuracy of most Spitfires - with just two, wing-mounted cannon - should have been terrible, with the plane being shaken from side to side. I do not ever recall reading such a complaint, and the fact that most Spitfires were built with this armament indicated that it did not cause the problems you suggest.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

The P-39 was notorious for stability problems, which could cause it to go into a dangerous spin. These were not caused by gun firing.


According to my information, the main problem with the MK 108 installation was the nose was too lightly built and was damaged by the gufire, so MG 151s were fitted while the nose was strengthened. I've not read anything to suggest that the effect of recoil on the flight performance caused aiming problems.


The more guns you have, the less significant is a single firing impulse from any one gun - so a six-cannon layout would have had a smoother total recoil push than a four-gun layout. If your theory was correct, the shooting accuracy of most Spitfires - with just two, wing-mounted cannon - should have been terrible, with the plane being shaken from side to side. I do not ever recall reading such a complaint, and the fact that most Spitfires were built with this armament indicated that it did not cause the problems you suggest.

Well, if the P-39 was that unstable (which is true due to bad stall behavior) it doesn´t make sense to remove the M4 as a mean to improve dogfighting performance (all under the assumption that the recoil issues are neglectable)
and stability. Actually, VVS did so, that´s why I conclude there have been recoil related issues on that plane.

The He-162 is something different. Here we have an example, where the weight of the plane is identic to the variant with 30mm MK 108 and 20mm MG151/20. So any explenation why the MK 108 was dropped is recoil related . The He-162 fuselage was metal structure (except the nose, which was plywood, but the nose has nothing to do with the gunmounts) with an inclined 12mm fireproof St52 bulkhead beeing the structural backup for the gunmounts. Factoring standart production methods in 1944/45, this still is a very strong structural part of the plane. I will dig out something, but I am sure I read somewhere that THK was concerned with the recoil effects on static tests (structural) and test pilots beeing concerned about low lateral stability for firing in flight tests ("Schiessanflugtests").

And regarding the Spitfire, there do exist such concerns in flightreports. One might first collect them all but I have a note somewhere about a specific Spitfire recoil issue at hi alt from a pilot report. Will check it out. (have to wait to return home for my books a week)
best regards,

 

[Jank]

This discussion has broken down into a complex, myriad of equational monstrosities and as such, the forest is being lost through the trees. The fact remains that the Hawker Hurricane handled four six 20mm's just fine. No evidence has been produced that the ensuing recoil of that set up caused stability problems.

Ergo, a 6 x 20mm set up in the Thunderbolt would not present any such concerns either.

It is not disputed that the set up of 6 x 20mm with 175 rounds per gun would have about the same weight as 8 x .50's with full ammunition loads.

It is claimed, but without evidence, that a 6 x 20mm setup in a Thunderbolt would presernt recoil serious problems resulting in stability issues. Why then no such recoil issues with the Hurricane?

The production run of 4 x 20mm armed Hurricanes was large. Any recoil associated problems would be peppered throughout the RAF's records and the numerous pilot accounts that have been memorialized to writing. Can you produce a single one?

On a semi-related note, the XP-72, an evolutionary development of the P-47, was ordered to be produced in two variants. One with six .50 cal guns (I assume for air to air engagements) and the other with four 37mm cannons, two in each wing, which I assume was for air to ground applications. Would this second version have presented difficulties?

Republic XP-72

Bear in mind that if so, that is not constitute proof that a 6 x 20mm armed Thunderbolt would have experienced significant stability related issues as the recoil of four 37mm guns must have been quite jarring indeed. Alternatively, however, if a four 37mm gun set up would have worked, as the USAAF believed it would have, that would tend to support that a 6 x 20mm set up would have worked just fine.

I would just point out that the production of the XP-72 was cancelled not because of any armament related issues but because the USAAF had decided that it was in need of long range escorts instead.

Enter the P-47N.

 

[Glider]

I may well be getting into areas where you know more than me about recoil forces but I would suggest that they were not the be all and end all of the various problems.

The P39 being unstable could be more to do with the weight sensitivity. The P39 was very suseptible to changes in weight to the extent that the ammo (or ballast) needed to be retained before flight. As such, a plane in combat that has fired the ammo and may well have suffered damage may well seem to be unstable. This alone would be an argument for keeping the M4.

He162. 2 x 20 would appear to make more sence as an armament. The 162 was fast and had high closing speeds whilst the 108 had a short range with poor ballistics. This is not a good combination. Space presumably was a concern. The 108 was short but bulky and ammo limited. Another good reason for the 20mm.

The Spit at high altitude may have recoil problems but this could be to do with the limitations of flying and fighting at high altitude where its easier to stall. I admit that I have heard that the Spit wasn't as good a gun platform as a Hurricane, but not that it was a poor platform.

The observation about the later Hurricanes with 4 x 20 being relegated to GA in Russia probably had more to do with the fact that the Russians considered them outclassed by German fighters of the period. Whereas they do pack a significant punch in the GA role and the Hurricane could carry bombs something that most light Russian fighters didn't do that well.
Its also worth remembering that Russian fighters had their guns on the certre line where gun forces had a minimal effect due to it being on the thrust line of the plane. Wing mounted guns (all wing mounted guns) had a tendancy to be less accurate if the plane was turning, as the wings themselves obvoiously flex. When manouvering one wing is normally going faster than the other introducing differing forces. I cannot remember which Allied Ace but one of of the British pilots always checked his turn slip before firing which suprised me at the time, but made sense when I thought it through.

 

[Tony Williams]

Well, if the P-39 was that unstable (which is true due to bad stall behavior) it doesn´t make sense to remove the M4 as a mean to improve dogfighting performance (all under the assumption that the recoil issues are neglectable)
and stability. Actually, VVS did so, that´s why I conclude there have been recoil related issues on that plane.

I have a book on the P-39 which includes many first-hand accounts by pilots who used it in combat. One complained about the cannon because it jammed a lot. Three said that the low velocity meant that it had a curving trajectory, so they had to get close before firing (although a couple commented that once you understood the trajectory, it was very accurate and could be shot with precision). One complained about the small magazine capacity. Not one mentioned recoil at all. If you want to quote the P-39 as an example of a plane with recoil problems, you're going to have to provide some very hard evidence - your case is based on pure supposition.

The He-162 is something different. Here we have an example, where the weight of the plane is identic to the variant with 30mm MK 108 and 20mm MG151/20. So any explenation why the MK 108 was dropped is recoil related .

I am sure I read somewhere that THK was concerned with the recoil effects on static tests (structural) and test pilots beeing concerned about low lateral stability for firing in flight tests ("Schiessanflugtests").

The damage was not necessarily caused by recoil. The very short barrel of the MK 108 produced spectacular muzzle blast, and this might have caused damage to the wooden nose cone. Even if it was recoil-related, it doesn't necessarily support your case. It is obvious that the MK 108 produced more recoil than the MG 151, and the He 162 was designed with the lightest possible structure, so might have needed some reinforcement around the gun mounting. I don't see that that is relevant to your point about the recoil affecting the flying or shooting qualities of the plane. There is no evidence that it would not have performed very well with the MK 108 once the strengthening took place. 'Low lateral stability' was an aerodynamic issue, unconnected with gun firing.

In fact, the main reason for the change may have been simpler: Kosin in his study 'The German Fighter' states that "the Russian advance brought a halt to production of the MK 108 in Poznan, which necessitated converting the aircraft to take the MG 151/20..."

And regarding the Spitfire, there do exist such concerns in flightreports. One might first collect them all but I have a note somewhere about a specific Spitfire recoil issue at hi alt from a pilot report.

I do recall reading one account in which a high-altitude interception was attempted. One of the cannon jammed because of the extreme cold, and the pilot was unable to hit the target because the plane immediately slewed to one side when the single cannon fired. Which is exactly what one would expect.

To support your case that a plane with the size, weight and strength of the P-47 would have been adversely affected by the recoil of six cannon, you need hard evidence. So far, you have produced none.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Tony Williams]

I admit that I have heard that the Spit wasn't as good a gun platform as a Hurricane, but not that it was a poor platform.

That's a different issue. The Spitfire had marginal lateral stability, which made it difficult to aim steadily. The upside of that was that it was instantly responsive to the controls, making it very agile. The Hurricane was more stable, which made it easier to keep in the aim, but the handling wasn't as responsive. None of this was affected by the armament they were carrying.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

To support your case that a plane with the size, weight and strength of the P-47 would have been adversely affected by the recoil of six cannon, you need hard evidence. So far, you have produced none.

I showed that the recoil of this layout exceeded typical ww2 Fighter A/C values by much. This may not be a hard evidence but it cannot be neglected either. We should remember that no such weapon layout was tried out and tested so far.
In this light nobody will be able to give hard evidence for an armement layout not tried. You want me to quantify the unquantifyable. I am not going to do so, I am only able to make it plausible to some degree or to throw my concerns into the discussion.
One step out of my possibility (but probably possible for You) would be to ask fighter pilot´s with combat experience about their specific recoil experience. I do not think this was properly done. I suspect this would be more enlighting on this highly interesting subject. Such a study would be worth the expense.

In case of He-162 M2, the damage caused by static firing of the MK 108 was not due to blast effects but due to recoil. The metal structure wasn´t prone to damage caused by blast effects (the metal skin did not differ from that executed in the Me-262 or Me-163B, which also had a plywood nose). Compare the He-162M4 with two MG 151/20, it´s test records do still exist.
The He-162M6, again with MK 108 for ground firing tests confirmed the recoil related problems known from the M2-prototype. (note that the plywood nose wasn´t damaged, the gunmount itself was repeatedly thrown out of calibration).
Neither weight nor space was a major concern and initially the Luftwaffe wanted this plane to carry 30mm guns instead of 20mm ones (the later A-3 subtype again carried 30mm guns). Personally, I regard 20mm guns as better for the specifc purposes in retrospect.
And no, low lateral stability was a concern during firing of the guns according to "schiessanflug" reports, it may be eased by aerodynamic issues (which must be proofed first) of the He-162, but it is not caused by them independent from recoil issues in this case.
Note that Koznan wasn´t the prime production centre for MK 108 and note that MK 108 were well avaiable to wars end from various decentralized production rings around Bernburg, Schwechat, Holstein, Wien and others (to name those avaiable in march 45). Neither the Me-262 production, nor the Ta-152, Bf-109K, Me-163B or even the Ju-388 production suffered from a Mk 108 shortage till wars end.
I am not going to say recoil was the only factor to define the gunplatform charackteristics of a plane, the opposite is the case, the gunplatform charackteristics do allow a certain amount of recoil without detremental effects on firing accuracy. That´s why I studied recoil-weight relations for different planes. So far, I have not found anything to exceed 0.10 for pure fighter A/C, which made it into combat. Interceptors: yes, Ground attackers: yes, pure fighter: no.

According to the Hawker Hurricane IId the fact remains that this plane with this weapon layout wasn´t primarely used for dogfights. Attack planes usually had not to aim long range, wide deflection shots, so recoil related issues naturally were of less importance to them. Nethertheless, I would like to hear a Hurri´s pilot opinion on this matter, esspecially with the differences between 0.303 cal and 20mm armement.
As a sidestep concerning the XP-72: This plane also did not reached combat status, the proposed armement layout was not accepted by official standarts (no firing in flight tests were acomplished), so it totally remains speculation and not fact. With a typical weight of 6575 Kg and a netto recoil of 1578 Kp (pluz ~20% gaz effects= 1894Kp) for four 37mm M10, a 3 sec. burst will slow down the plane by approximately 33.2 Km/h or 20.6 mp/h, a significant deceleration, not to speak of recoil issues due to much fewer pulses!
The P-47 would need significant wing modifications. At first it would need structural (spacial) modifications in order to handle the larger ammo feed belts and cases, then it would need a stronger gunmount with special applications to reduce the peak of recoil (which was new to me), both would increase the weight. At next it would need structural reinforcements of the wing structure to handle the impulses, again increasing weight. Why wasn´t this tried if there were no such obstacles, Jank?