London Survey

by Peter C Masefield
Technical Editor, The Aeroplane, London, England

Specialization of aircraft armament for specific tasks has made possible the fullest utilization of airplane performance.

Among the great advances in military aviation since this war began, one of the most prominent has been the development of armament — both offensive and defensive — in aircraft of all categories. The number of guns carried, the efficiency of their installation and their hitting power have all been increased many times over. In fact, just as specialized airplanes are being developed for almost every purpose — in place of the old, inefficient "general purpose" compromise — so also specialized armaments are being installed today for specific tasks — dogfighting, anti-bomber fighters, ground strafing, etc.

Spitfires, Hurricanes, Thunderbolts, Focke-Wulf Fw-190s, Wildcats, Flying Fortress bombers and a host of others all boast the power of their armament which has been proved in action. What is not so clear is just how good their armament is in relation to each other for the job that each sets out to do.

In the past no very satisfactory method of assessing the efficiency of the firepower of any individual fighter or bomber has been in use. Weight of fire in pounds of projectile per minute has been quoted and accepted. Yet this method cannot set the comparisons in true perspective for it takes into consideration only one quality. Probably its popularity has grown chiefly from its ease of calculation.

To gain a broader view, the three fundamental qualities of any gun must be considered and assessed. They are:

  1. Rate of fire in rounds per minute (or second).
  2. Mass of each projectile in pounds.
  3. The muzzle velocity of each projectile in ft/sec.

In modern guns the rate of fire varies from 85 rounds per minute for the big 37-mm cannon up to 1,100 rounds per minute for rifle-caliber machine guns. The weight of individual projectiles varies from one-fortieth of a pound for rifle-caliber bullets up to more than one pound for a 37-mm shell and even up to 50 pounds for the shell of a 127-mm (five-inch) gun, which might well be installed in aircraft if the slow rate of fire (12 rounds per minute at present) can be multiplied. Muzzle velocities range from 1,250 ft/sec (850 mph) up to 2,880 ft/sec (1,960 mph). All these qualities must be taken into account.

Ideal armament, obviously, calls for a gun which pumps out the greatest number of the biggest projectiles at the highest muzzle velocity in the shortest possible time. The airplane which carries that gun must be able to fly as fast, as far and as high as possible and have space for a maximum amount of ammunition to feed it. Yet, like everything else, a gun must be a compromise. If the projectile is to be big the rate of fire must be low; if the rate of fire is high the duration of fire will be reduced. A careful study of the various factors involved in different tactical duties will suggest which of these qualities should be given precedence for each particular task.

In any case two points must be considered:

  1. The combined muzzle velocity, rate of fire and size of projectile of the gun, and
  2. The individual "punch" of each projectile.

On the first point the combined qualities can be reckoned as the rate of muzzle energy expended — in other words, the horsepower of the gun. The horsepower or firepower of a gun can be calculated from the formula:

WV2X
               HP  =   ———  =   wv2x / 35,420
1100 g
Where

w= weight of projectile in lb
v = muzzle velocity in ft per sec
x = rate of fire in rounds per sec
g = 32.2

On the second point the individual hitting power or punch of each projectile can be reckoned in foot-pounds. A third point which must not be lost sight of is the necessity for a certain minimum number of strikes per second, particularly in dog-fighting.

In fact any gun can be considered as an engine developing a certain muzzle horsepower. For instance the 45-hp German MG17 rifle-caliber synchronized machine gun is to the 650-hp British Hispano 20-mm cannon as the small four-cylinder Continental engine is to a nine-cylinder Wright Cyclone engine.

The extraordinary development in aircraft armament since 1918, is shown by a glance at the accompanying table of modern armament. (Figure 1) We have progressed from 32 hp in 1918, to some 716 hp today and from a punch of 2,240 ft-lb per projectile up to 127,000 ft-lb for the 37-mm cannon — with undoubtedly bigger ones on the way.

An interesting point is that although the punch per projectile of the 37-mm cannon is great this quality is offset by its low rate of fire which results in only 1.4 strikes per second. Thus, although the big cannon is excellent for shooting up ground targets, in which the objectives are in the sights for some seconds, it is almost useless in a dogfight in which one-fifth of a second is a not unusual length of time for snap shooting. Whereas a .50-caliber gun would score two hits in that time the bigger cannon would not score at all.

All this raises the question of special armament for special purposes — in both the offensive and defensive aspects. With a limited and known range of guns from which to choose the designer has to determine his best combination for any particular purpose within the limits of weight imposed. Let us then consider first, which is the best gun for each specialized task; next, what is wanted in the various categories of aircraft; and finally, how existing types of aircraft meet the general requirements.

First and foremost we can place the single-seater dog-fighter designed to combat enemy fighters and thus gain the command of the air so vital to military operations. In this category rate of fire is important because of the need for snap shooting in the shortest possible interval of time. Provided that both the rate of fire and the punch per projectile are reasonably high the gun with the greatest horsepower will be the best for the dog-fighter because in the expression "horsepower" are included all the other relevant qualities.

On this basis the best gun for the fighter is undoubtedly the 20-mm Mauser MG 151/20 when used as a free-firing gun, not checked by synchronizing gear as on the Fw-190. Although the punch of this gun (29,700 ft-lb per projectile) is not so high as that of the British Hispano cannon (32,200 ft-lb per projectile) the rate of fire is superior and the horsepower is 11 per cent better. The weight of the guns is approximately the same. There is little to choose between the Mauser and the Hispano but on the balance the Mauser wins for the dog-fighter — reckoned gun for gun.

When we come to the question of installation one further factor presents itself — that of weight. Four Hispanos or six .50-caliber Brownings are an approximately equal load. The four Hispanos will develop a combined total of 2,535 hp, score a total of 43 strikes per second and have a punch of 32,200 lb per projectile. The six Brownings will develop 1,776 hp, score 75 strikes per second and have a punch of 13,000 lb per projectile. On the balance, the four Hispano cannon would appear the superior armament. They would score eight hits in one-fifth of a second and in that time deliver a bigger punch than the 15 hits from the .50-caliber bullets.

The second important fighter category is the specialized destroyer of bombers. A really fast two-engined type may well prove best for this purpose. From the armament viewpoint the chief need is a gun with a combination of powerful punch per projectile and high horsepower. Again the Mauser appears to have a slight advantage over the Hispano. Although the punch per projectile is seven per cent less, the horsepower delivered is greater and no bomber is likely to have armor which will resist a blow of 29,700 ft-lb from the Mauser and yield to that of 32,200 ft-lb from the Hispano. Both the Mauser and the Hispano has a clear advantage over any other gun for anti-bomber duties. The big 37-mm cannon has too slow a rate of fire (and consequently too low horsepower) to be effective, although its punch is very great indeed. The .50-caliber Browning, notwithstanding its high rate of fire, has less than half the punch. Thus in both these fighter categories the total horsepower of the guns is a remarkably good criterion.

In a third category, that of the ground-strafer or tank-buster, punch is the predominant requirement. A gun with a small punch per projectile will not penetrate the armor. The drawback to the big gun is its slow rate of fire — 1.4 strikes per second for the otherwise excellent 37-mm gun. The only solution would appear to be to pack as many of the big guns as possible into the aircraft, and this demands a big airplane. Again gun horsepower is the measure of efficiency provided that the punch is high. A big gun with a small horsepower means that the attacking airplane will not have time to get in more than one or two shots while the target is in the sights. As in bombing, concentration of fire is needed to achieve results. Either many tank-busters with one gun each or fewer tank-busters with many guns each provide the answer to this problem.

When we turn to bombers we find that a different set of conditions are required for their defensive armament. Three qualities predominate:

  1. Range of fire so that fighters can be held off to the greatest possible distance by the greatest possible firepower.
  2. Maximum possible gun horsepower concentrated against one target.
  3. A good all-round defensive field with no blind spots.

Range and firepower are bound up with muzzle velocity, the mass of the bullet and rate of fire. Thus they are a function of gun horsepower. The concentration of fire possible against one target is purely a matter of disposition of guns and turrets; as is the elimination of blind spots.

To prescribe an ideal defensive armament for bombers is more difficult than for fighters. What is easier is to say what should not be included. Obviously the very big cannon is out of the question because of difficulty of aim and slow rate of fire. Similarly the rifle-caliber gun should be eliminated today. The rate of fire is great but the range is small and the horsepower developed is puny compared with the bigger guns.

At present the .50-caliber Browning has shown itself to be the best all-around gun for bomber defense. This is because it is big enough to pack a useful punch at long range and yet light enough to mount conveniently in movable positions or in a turret. Nevertheless bigger guns would be an advantage if they can be mounted satisfactorily. Again the Mauser MG 151/20 would appear to be the best proposition with the British Hispano a close second.

Finally let us glance at existing installations and attempt to assess their merits. In Figure 2 some of the better-known fighters are set out in ascending order of the horsepower developed by their guns. A point which should be included in the assessment but for which complete figures are not available is the duration of maximum firepower. About 30 seconds is a fair average for a single-seater fighter today although the Fw-190 shows up very badly on this basis with a duration of maximum intensity of only eight seconds.

Through the examination of the potentialities of armament, horsepower has been shown as the best guide to efficiency provided that the punch per projectile is adequate for the particular job in hand. This has a great bearing in assessing the merit of the armament of the various types of fighter.

Without attempting to assess the fighting order of merit of the various types (in which speed, climb, ceiling and maneuver would have to be considered) the following would appear to be the order of the aggregate of the desirable qualities on a basis of armament alone:

SINGLE-SEATER, SINGLE-ENGINED DOG-FIGHTER

  1. Supermarine Spitfire IX (four British Hispano cannon), 2,535 gun hp.
  2. Republic Thunderbolt (eight Browning .50-caliber), 2,368 gun hp.
  3. Messerschmitt Me-109G (three Mauser cannon, two MG17 rifle-ca1iber), 2,238 gun hp.
ANTI-BOMBER FIGHTER (DESTROYER)
  1. Bristol Beaufighter (four British Hispano cannon, six Browning rifle-caliber), 2,950 gun hp.
  2. deHavilland Mosquito II (four cannon, four rifle-ca1iber), 2,836 gun hp.
  3. Westland Whirlwind (four British Hispano cannon), 2,535 gun hp.
GROUND STRAFER AND TANK-BUSTER
  1. Hawker Hurricane IID.
  2. Bell P-39 Airacobra.
  3. Henschel Hs-129.

An interesting point is the poor showing of the Fw-190. This is because the Mauser cannon are mounted to fire through the propeller and consequently are slowed up in rate of fire whereas the outboard cannon are the relatively inefficient 201-hp Oerlikons. The Germans have made surprisingly poor use of their high quality gun.

Unfortunately the Typhoon cannot be quoted in the table nor can any figures be given at this writing for the Hurricane tank-buster except to say that it has heavy-caliber guns [two 40-mm cannon. —Ed] and is a marked advance over any previous airplane in this category. The Hurricane led the way in fighter armament from its early days when it was the first airplane to appear with eight machine guns, then with 12 machine guns and was the first fighter to mount four 20-mm cannon. The 12-machine-gun and the four-cannon Hurricanes provide an interesting contrast. The 12-gun Hurricane IIB packed 220 strikes per minute for 922 hp compared with 43 strikes per minute and 2,535 hp for the four-cannon Hurricane IIC.

Looking back to the earlier fighter armaments we see one reason why the Hurricanes and Spitfires of 1940 triumphed in the Battle of Britain. As a dog-fighter the Messerschmitt Me-109E opposed to them had not only a poor armament horsepower (491 hp against the British 616) but also a miserable rate of fire of 35 strikes per second compared with the RAF’s 147.2 per machine. In those days of little armor the punch per projectile was not of the importance it is today so that the 2,300 ft-lb of the British machines was adequate. The total weight of armament of the Hurricane I in 1940, was 461 pounds. Today, with four cannon, that has been increased to 876 pounds.

All through, the German fighters show to disadvantage. As a dog-fighter the rate of strike of the Me-109F is poor and even the new Me-210 compares poorly with its counterpart the deHavilland Mosquito.

In defensive armament the Allied types show the same superiority. The superiority of the Fortress, closely followed by the Liberator, shows up well. Both can focus as a maximum more firepower on one target than any other bomber can bring to bear in all. This firepower is also more than that of any enemy fighter except the Me-109G and decidedly better than the Fw-190. How superior the bigger guns are to the .303s of the British bombers is shown by the Lancaster’s maximum of 616 hp against one target compared with the 2,072 hp of the Fortress. Yet the Lancaster has a maximum of 147.2 strikes per second on one target against the maximum of 87.5 strikes per second for the Fortress.

Again, examination of the data indicates one reason for the poor showing of the Luftwaffe against the British fighters over England in 1940. The Heinkel and Dornier bombers which formed the bulk of the raiding forces could bring only one gun and 81 hp to bear on any single target — less than one-eighth of the firepower which a single British fighter could direct against them.

All the evidence suggests that the best measurement of the efficiency of an airplane gun is on a basis of horsepower just as it is for an aero engine. If punch per projectile is taken into account as well, a full measure of efficiency is presented in the same way as the combination of horsepower and rated weight forms the best standard on which an engine can be judged.

There is no doubt that in the Mauser MG 151/20 the enemy have a fine gun, but the British Hispano 20-mm cannon is so near it in quality that there can be next to no difference in practical result. In the 50-inch Browning machine gun the United States has produced the finest bomber armament for present-day mountings.

Certainly both British and United States designers have made far better use of the available material than have the Germans. The result, combined with high performance in the aircraft on which they are installed, is qualitative superiority in the air.

This article was originally published in the July, 1943, issue of Flying including Industrial Aviation magazine, vol 33, no 1, pp 66, 86, 89-90, 93.
The original article includes a thumbnail portrait of the author and embeds the 3 tables below.
Photo is not credited.
Figure 1 — Armament Data
(Guns arranged in ascending order of horsepower)
CaliberRate of FireRate of Muzzle EnergyPunch (KE) per ProjectileWeight of FireStrikes per second
Type of GunInchesmmRounds per minHorsepowerft-lblb/min
Vickers .303 (1918) Synchronized0.3037.676400322,740106.7
Lewis (1918) Free-firing0.3037.676550372,24013.79.1
German RMB MG 17 Synchronized0.31187.92600452,45015.610
Browning .300 free-firing0.3007.621,100752,24026.518.4
Browning .303 free-firing0.3037.6761,100772,30027.518.4
German RMB MG 17 free-firing0.31187.921,100812,45028.618.4
German RMB MG 131 free-firing0.5118139001947,20067.515
German Oerlikon FF0.7872045020114,700112.57.5
German MK 1011.1813010020868,000701.7
Browning .50-caliber Synchronized0.5012.760023713,00077.510
Browning .50-caliber free-firing0.5012.775029613,00096.712.5
American AA, Type F1.45663785329127,00093.51.4
German Mauser MG 151/15 free-firing0.59061595051517,85014215.8
German Mauser MG 151/20 Synchronized0.7872070063029,70017511.7
British Hispano free-firing0.7872065063232,200162.510.8
German Mauser MG 151/20 free-firing0.7872080071629,70020013.3
Aircraft Armament (Figure 1) — Most of the guns installed in Allied or German fighters and bombers are shown in the table above, ranging from the 32-hp Vickers gun of 1918 up to the new 716-hp German 20-mm Mauser cannon.
Figure 2 — Offensive Armament
(Fighters arranged in ascending order of striking horsepower)
Airplane   Number
of guns
Total
Firepower
HP
Total weight
of fire
Lb/Min
Total strikes
per Sec
Punch (KE) of
biggest projectile
ft-lb
Camel (1918)2642013.42,740
SE 5A (1918)26923.715.82,740
P-36A (1939)227992.12013,000
Defiant430811073.62,300
S-00 (Zero)44802503514,700
Me-109E4491256.23514,700
Tomahawk (P-40D)653726593.613,000
Hurricane I8616220147.22,300
Spitfire I8616220147.22,300
Me-110C56726339.488.614,700
Me-109F3806231.233.329,700
Hurricane IIB12922330220.82,300
Airacobra (37mm)71,103354.595127,000
Lightning51,378408.972.632,200
Airacobra I (20mm)71,406423.5114.432,200
Hs-12951,41043068,000
Spitfire IXA61,57544595.232,200
Me-210A141,594431.270.129,700
Fw-190A361,752606.258.429,700
Warhawk (P-40F)61,776580.27513,000
Corsair61,776580.27513,000
Wildcat (F4F-5)61,776580.27513,000
Mustang IA81,930594.89513,000
Me-106G52,238631.259.929,700
Thunderbolt82,368773.610013,000
Whirlwind42,53565043.232,200
Hurricane IIC42,53565043.232,200
Spitfire IXB42,53565043.232,200
Mosquito II82,836760116.832,200
Beaufighter102,950815153.632,200
Offensive Installations (Figure 2) — Fighter armament arranged in ascending order of armament horsepower
Figure 3 Defensive Armament
(Bombers arranged in ascending order of striking horsepower)
AirplaneNumber
of guns
Total
Firepower
HP
Max
Firepower
One
Target
Max Strikes
One Target
Per Sec
Max Punch
per Biggest
Projectile
on One
Number
of Guns
One Target
DH 4 (1918)31067418.22,2402
DH 9A (1918)31067418.22,2402
Do-17P (1939)32438118.42,4501
He-111H1 (1939)32438118.42,4501
Blenheim IV430815436.82,3002
Beaufort646223154.22,3003
Wellington II646223154.22,3003
Do-17Z2648616236.82,4502
Boston III753930873.62,3004
Wellington III8616308922,3005
Stirling8616462110.42,3006
Halifax IIA8616462110.42,3006
Lancaster10770616147.22,3008
Me-110C5780572688.614,7006
Ju-88A651,16671613.329,7001
He-111H661,23491029.129,7002
Marauder51,4807845013,0004
Do-217E261,6681,23129.129,7002
Me-210A161,9821,59470.129,7004
Mitchell72,0721,1845013,0004
Liberator B-24D113,0352,07287.513,0007
Fortress B-17E133,6202,07287.513,0007
Defensive Installations (Figure 3) — Bomber armament arranged in increasing order of armament horsepower.