Firepower

by Donald G Smith
Armament Engineer, Grumman Aircraft Engineering Corporation

The modern warplane is virtually a "flying gun platform" with cannon, machine guns and armor.

The present conflict has placed the airplane in its rightful position as a primary striking weapon and defensive unit. Germany has forced this recognition, to the point where air forces are as important as armies and navies. Today, the army or navy operating without air coordination becomes the target of enemy aerial units. As oft repeated, the best defense against hostile aircraft is a superior defensive air force. The "superior" air force is the one having superiority in numbers, equipment, personnel and organization.

In regards to superior equipment, the urgent demand for more firepower has been most frequently brought to public attention. Firepower means as much to the tactical airplane as the size and number of guns means to a battleship; and, in combat, can be more important than speed and performance. Actually, this word "firepower" may be applied to briefly describe an airplane's tactical qualifications. However, this article accepts the popular version and deals only with the armament units.

Increased firepower is a direct outgrowth of the recent increases in speed, caused by the necessities of war — the faster the target, the more shots per second required. Downing an airplane with gunfire is not as simple as many fiction stories may lead us to believe. We must remember that the need for increased firepower is due, mainly, to the difficulty of keeping a target in the gunsight long enough to place an effective burst of gunfire.

At the outbreak of the present hostilities, US aircraft were woefully lacking in required firepower. Standard armament for fighter craft, at that time, consisted of one .50- and one .30-caliber machine gun, with about 300 rounds of ammunition each. Such planes as the Consolidated PB-2, Northrop A-17, Republic P-35, Curtiss P-36 and Boeing F4B-4, to mention a few, became obsolete overnight, as World War II ushered in the era of winged "gun platforms." Luckily we had planes, like the Bell P-39, Curtiss P-40, Grumman F4F-3 and Boeing B-17, easily adapted to the new tactical requirements and capable of forming a backbone for our new air forces. Much credit must be extended to the various aircraft manufacturers who had the foresight to realize the importance of firepower, long before the demand became apparent.

The past two years have seen the P-35, P-36, B-17 and F4F-1 develop into the Republic P-47, P-42, Boeing B-17E and Grumman F4F-4 respectively, as our air arms kept in stride with European developments. Our American industries are, once more, proving their ability to conform with, and surpass, changing ideas and uniting in the effort to meet the demands required by existing conditions. In actual firepower, the machine guns form the mainstay, around which supporting units are installed — those being bombs and cannons. Armor plate, while being more of a protective measure, affects the firepower indirectly by limiting the target vulnerable to gunfire.

The average machine gun is capable of firing from 600 to 1,200 rounds of ammunition per minute, depending upon the strength of the drive spring, caliber, ammunition, temperature, synchronizing system (if used) and the design and location of accessory equipment (feed and ejection chutes, ammunition boxes, etc). However, "rate of fire" must not be misconstrued, as machine guns are rarely fired for a full minute. Machine guns easily overheat, causing premature and automatic firing, damaged barrels and malfunctions (jams). An air-cooled gun should not be fired in bursts exceeding 25 shots when on the ground and 125 while in flight. Due to variations between guns, ammunition and mounting methods, the resultant line of gunfire is conical (Figure 1) and may be compared to a spout of water emitted from a hose under pressure. "Effective pattern" is defined as being the diameter of a circle in which 85 per cent of the shots fired will hit at a given distance (Figure 2), amounting to about a 12-inch diameter circle at 250 yards range, for a typical gun. Muzzle velocities are usually in the neighborhood of 2,500 feet per second and vary with the type of gun and ammunition. A large caliber shell may have a higher muzzle velocity than one of smaller caliber, but will slow down sooner because it offers more wind resistance. The arc of trajectory, an important factor in "mixed" installations of machine guns and cannon, will approximate two mils or a drop of about 18 inches at 250 yards.

The foregoing figures are, necessarily, approximate and will not hold true for any one machine gun, since too many variables are concerned in the calculations. It is regrettable that a more conclusive description of machine-gun and cannon firing data cannot be published, but their listing as "confidential" implies discretion.

Structurally, an airplane must be capable of absorbing the recoil and gravity loads imposed upon it by the armament. Recoil and counter-recoil design loads approximate one ton for a .30-caliber machine gun, and about two tons for the .50-caliber type. Of course, these loads are dependent upon the gun mounting and adjacent structures. Illustratively, when a rifle is held properly and fired very little recoil (kick) is apparent, as the human body forms an excellent shock absorber. However, the same rifle, fired when the rifleman has his shoulders and back against a wall, will cause considerable "kick" and an exceedingly sore shoulder. Likewise, an airplane structure must be sufficiently rigid to prevent excessive gunfire dispersion, but flexible enough to absorb recoil loads. With the advent of multiple gun installations recoil adapters are receiving wide acclaim, as they absorb a large percentage of the recoil loads.

When aircraft are equipped with bombs, the structure and fittings must be strong enough to carry not only the bombs, but many times the weight of the bombs, to sustain gravity maneuvers.

One of the most important considerations encountered in an aircraft armament installation is the design for maintenance and accessibility. Aircraft armament must be capable of being easily checked, repaired and reloaded under adverse conditions, such as on a reserve air field or a carrier deck at night and possibly without the aid of any lights. This requires exceptionally good access and simplified design of all accessory equipment and assemblies. Access doors and panels must be easily and quickly detachable, providing a maximum of accessibility, with a minimum number of doors. It is often wise to fasten removable panels to the airplane with hinges or light chains, to prevent their being damaged or lost (on aircraft carriers, the wind may blow loose parts overboard). In addition to the loading of ammunition boxes, gun barrels must be removed after each tactical flight for inspection and replacement.

The location of ammunition boxes, in respect to the gun, can be as important as the number of guns. The recoil action of a Browning machine gun is used to cock the bolt, eject the empty cartridge case, eject the belt link and to pull the ammunition belt into the feedway. Therefore, an excessive distance between the gun and ammunition box means additional friction to be overcome by the gun feeding mechanism. Increased gravity loads, during aerobatics, may decrease the rate of fire or, when the friction and belt load becomes too great, cause complete stoppage of fire. In many instances, electric "feed boosters" are being used to help the gun pull the ammunition belt, while another trick is to have the belt ride over rollers.

Ejected belt links and cartridge cases are carried from the gun, through chutes, to a retainer box or to an outlet through which they are dropped from the airplane. Improper designing of the ejection chutes may cause the collection of links and cases, resulting in jams. When ejected links and cases are discarded, care must be exercised in choosing the point of outlet, as the airstream around the airplane is unpredictable and might lead to damaged tail surfaces, wings, enclosures, etc.

An example of ejection chute problems was presented in the Curtiss XP-6 airplane, with its .30- and .50-caliber synchronized machine guns in the fuselage. In this particular airplane, it is said, the cases from the smaller gun hit the lower wing and bounced towards the cockpit, while those from the .50-caliber gun hit the other bottom wing, but didn't bounce, tearing holes in the fabric covering. This surprising condition was quickly corrected by rerouting the chutes and placing the outlet below the lower wing.

The charging systems for aircraft machine guns are of great import and provide the means of clearing certain types of malfunctions, as well as providing a "safety" for the guns. Charging is ordinarily accomplished by the gun recoil, but must be done by manual or hydraulic forces when a jam or a "dud" shell is encountered. Mechanical charging is executed through the use of cables and pulleys, leading from the guns to charging handles in the cockpit. Since a charging operation often requires heavy pulling loads, hydraulic charging mechanisms are receiving wide acceptance. Hydraulics, however, do not seem to be the final answer in this case, as the pressure lines must be heated to prevent the oil from becoming sluggish at low temperatures.

As previously mentioned, gun barrels will overheat, even at the low temperatures encountered at high altitudes. So their proper cooling must be insured. Conversely, the bolt and breech mechanism must be heated, to prevent the lubricants from freezing. In close connection with gun heating and cooling enters the aerodynamic fairing of wing guns to avoid a decrease in wing lift. In British aircraft, when guns are completely enclosed within the wings, the "gunports" are covered with adhesive tape, thus providing maximum lift until the guns are fired.

Very few people realize that firepower and simplicity of design go hand in hand. Yet very little explanation is needed to clarify this relationship. For instance, a battery of 12 guns might provide excellent firepower for an airplane, and several of these planes might mean similar firepower for a few squadrons. But, through simplification in design for mass production, several tactical groups might possibly have been equipped with this type of firepower in the same length of time required to equip a few squadrons. The ultimate goal is to provide an entire air force with superior firepower, not just a few units of that force.

In line with simplicity of design is the use of interchangeable assemblies and standard parts. Their use requires less skilled labor, less engineering time and fewer spare parts for maintenance crews to carry in "field" operations. It is for this reason that the older types of aircraft are assigned to foreign stations, as the parts most frequently needing replacement are known and a sufficient supply is stocked. This was evident in the replacement of the P-35 by the P-36, which in turn was replaced by the P-39 and P-40 — the replaced craft being transferred to foreign service.

The advent of aircraft cannon is not new. As is commonly known, their use was tried during World War I, both as fiexible-mounted (Voisin) and stationary (SPAD). In present aerial strategy the rapid-firing cannon is receiving wide acceptance, having rates of fire comparable to machine guns. The aircraft cannon enables the airplane to cope with lightly-armored tanks and, as larger caliber aircraft guns are developed, panzer divisions will require heavier armor, consequently decreasing their speed and mobility. When used against hostile aircraft the cannon is far more destructive than the faster-firing machine gun, as its explosive shells prove effective against self-sealing fuel tanks.

Previous to the capitulation of France, French aircraft with cannon downed enemy planes by remaining beyond machine gun range; German airplanes not ordinarily being equipped with cannon at that stage of the war. The original theory adopted by the British, concerning mixed installations of cannons and machine guns, was for the pilot to use his cannon for long-range firing and his machine guns for close-in dog-fighting. However, as might be expected, the pilots invariably blast away with all guns, whenever the chance for a possible hit presents itself.

At present, the cannon gives the advantage of range to the pursuit and fighter type airplanes, but a flexible-mounted cannon installation, now being developed, will probably neutralize this temporary advantage.

Bombs seldom are recognized as an important part of an airplane's firepower, especially as a form of armament to be used against other aircraft. Nevertheless, a tactical strategy that might be used by pursuits against hostile bombers could consist of the dropping of small fragmentation bombs with time fuses. When attacked by pursuit craft, the most effective defense for a bombardment squadron is a close formation, permitting the gunners to provide mutual protection for other craft in his flight. Thus, we can see, the dropping of bombs would force the bombers to open formation, lessening the chances of more than one plane being downed by a single bomb, but canceling the mutual protection strategy.

The installations for bombs and bomb racks include release and arming controls, and their positive action must be insured. The release control permits a selection of which bomb it is desired to drop (all bombs may be dropped simultaneously by throwing the release handle to "salvo" position). The arming control provides a safety position for the bombs and the control handle can be positioned in one of two selections — "armed" and "safe." When armed the bomb will explode on contact or timing and, when safe, the bomb will not explode when released. The arming operation of an aerial bomb is quite similar to that of a Mills grenade, having an arming pin that remains with the bomb rack when bomb is dropped armed, and with the bomb when dropped safe.

Gun cameras, also, form a part of the gun installation, operating in conjunction with the machine guns and making a pictorial record of all gun firing, thus serving as a means of curbing habitual gunnery mistakes.

The manufacture and use of protective armor is an industry in itself, with a history dating back to ancient Greece. In aircraft, armor plating was successfully used in the all-metal Junker airplanes during World War I and, after being rather slighted for about two decades, has now become of prime importance.

The installation of protective plating in aircraft requires careful designing since such plating is heavy, must be removable and cannot be used as a part of the airplane structure. The major design consideration is for maximum protection, with a minimum amount of armor. In general, there are two types of armor plating, one consisting of two or more fused plates of different hardness; the other consisting of a single face-hardened plate. In all cases, the hardest surface is the one receiving the initial impact. The back surface, being more ductile, tends to prevent the plate from cracking.

Armor plate, when improperly installed, can be of sufficient thickness to withstand gunfire and, at the same time, be detrimental to the safety of the pilot and his plane. The extremely hard surface of protective plating is liable to shattering when subjected to the impact of gunfire, and resultant vibrations are capable of transmitting the impact to any attaching rivets, subjecting the pilot and adjacent equipment to flying rivets and steel splinters. Vibrations, caused by gunfire impact on armor plate, will probably cause serious difficulties in the near future with the introduction of pressurized cockpits, as such vibrations within an airtight enclosure will subject the pilot to very dangerous air concussions, similar to those experienced in tanks.

Throughout the comparatively short history of aerial combat, gun location has had the greatest effect on warplane designs. World War I saw the frenzied efforts of all nations develop the synchronizing system, on the theory that guns located within reach of the pilot and in front of him would improve gunnery. In the present conflict, guns have moved to the wings, where maximum firepower is not decreased by synchronizers. However, in contradiction to the belief that synchronization slows down the rate of fire, the latest Messerschmitt Me-109F is reported to have no wing guns, its armament consisting of two synchronized fuselage guns and one cannon, the latter firing through the prop hub.

In regards to two-place and multi-place aircraft, gun locations are more dependent upon the individual airplane design and its tactical purpose. For instance, when the Boulton Paul Defiant first saw action, its tactics completely surprised German pilots — namely, flying alongside the Nazi craft and blasting away with a broadside from the four machine guns mounted in the power turret. Presumably, the Nazis were not long in locating the Defiant's weak point, as rumors were soon circulating concerning British experiments with remote control machine guns, mounted beneath the Defiant's tail and covering that blind spot.

In close relationship to the gun location on pursuit craft is the gun alignment. The importance of alignment can be readily understood and, by referring to Figure 3, the advantages and disadvantages of various methods are easily recognized. The superiority of the twin-engined fighter over the single-engined type, in respect to gun alignment, may eventually lead to its preference, as the guns and cannons require little or no convergence to obtain concentrated firepower. Also, synchronizing is not required.

The introduction of power-driven turrets has greatly increased the defensive firepower of multi-place aircraft by increasing the efficiency of the gunners. They eliminate the awkwardness encountered when attempting to reverse the direction of firing, speeding up the movement and enabling the gunner to remain seated in one position, in respect to the gun and gunsight.

Power turrets may be of mechanical, electrical or hydraulic operation and, while the gunner revolves horizontally with the turret in all present designs, he may not revolve in the vertical plane with the gun and sight. Naturally, it's most advantageous when the gunner revolves in all planes, with the sight.

Thus, we see, firepower is not accomplished as easily in practice as would be expected when theoretically conjectured. In production, the motto "Get 'em Flying" is well chosen but, in actual combat, firepower is the determining factor that will help our pilots "Keep 'em Flying."

This article was originally published in the March, 1942, issue of Flying and Popular Aviation magazine, vol 30, no 3, pp 45-46, 90, 92, 94.
The original article includes 1 photo and the three figures above.
Photo is not credited.