The vertical range of aircraft artillery is much higher. In the case of a one-pounder having the same velocity the range would be over 12,000 feet; but it is a question of luck whether the aëroplane would be hit. The great difficulty is the angle of “lead” which must be given to allow for the velocity of flight.
This angle is only constant so long as the velocity of the projectile is constant, assuming (as fairly represents the conditions) the flight speed not to vary; at extreme heights the velocity of the projectile has fallen so low that a very slight error in range-finding will be fatal to accuracy.
In regard to aëroplane artillery, Mr. W. F. Reid has collected some interesting details of the guns that Krupp has devised for the purpose of hitting aëroplanes.
The 7.5 cm. gun of this firm has seats for five men and storage for sixty-two shells. It is mounted on a car which weighs 4,300 kilos., the weight of the gun alone being 1,065 kilos. Each projectile weighs 5.5 kilos. (12 lb. 2 oz.), and the horizontal range is given as 9 km. The vertical range is 6,300 metres.
A lighter gun of 6.5 cm. gauge weighs, with car, 875 kilos., the gun weighing 352 kilos. Each projectile weighs 4 kilos. (8 lb. 13 oz.), and the extreme horizontal range is stated to be 8,650 metres (9,450 yards). The height of fire obtainable is 5,700 metres (18,700 feet). The initial velocity of the projectile is 620 metres (2,034 feet) per second. A coiled spring balances the weight of the gun when pointed above the horizontal.
For naval purposes Krupp has constructed a 10.5 cm. gun weighing 3,000 kilos, with carriage. The projectile weighs 18 kilos. (40 lb.). The muzzle velocity is 2,100 feet per second, and the shells discharge a train of smoke to facilitate aiming.
Ehrhardt, in Düsseldorf, has also built a special gun for use against aërial craft. Its bore is 5 cm., and its barrel is 30 calibres long, while the length of the Krupp barrels is 35 calibres. The weight of the Ehrhardt gun alone is 400 kilos.; with car, ammunition, and five men the weight is 3,200 kilos.
With regard to the difficult subject of armouring aëroplanes, I should like again to quote from Mr. Lanchester:—
It is manifestly not possible for an aëroplane to perform all the duties required of it in connection with tactical operations at high altitude[B], and whenever it descends below 5,000 feet or thereabouts, it is liable to attack from beneath; in fact, at such moderate altitudes it must be considered as being under fire—mainly from machine-gun and rifle—the whole time it is over or within range of the enemy’s lines. Protection from the rifle bullet may be obtained in either of two ways: the most vital portions of the machine, including the motor, the pilot, and gunner, can only be effectively protected by armour-plate; the remainder of the machine, including the wing members, the tail members, and portions of the fuselage not protected by armour, also the controls, struts, and the propeller, can be so constructed as to be transparent to rifle fire—that is to say, all these parts should be so designed that bullets will pass through without doing more than local injury and without serious effect on the strength or flying power of the machine as a whole; in certain cases components will require to be duplicated in order to realise this intention. It is important to understand clearly that any intermediate course is fatal. Either the bullet must be definitely resisted and stopped, or it must be let through with the least possible resistance; it is for the designer to decide in respect of each component which policy he will adopt. The thickness of the armour required will depend very much upon the minimum altitude at which, in the presence of the enemy, it is desired to fly; also upon the particular type of rifle and ammunition brought to bear. There is a great deal of difference in penetrative power, for example, between the round-nosed and pointed bullets used in an otherwise identical cartridge.
[B] For military purposes we may take the term “high altitude” as defined by the effective vertical range of small-arm fire, in other words, as denoting an altitude of 5,000 feet or 6,000 feet or more.