Shooting at a bomber or at a practice drogue when it is flying straight and level, is all very well. But shooting at an evading target while manoeuvring a fighter aircraft to its limits, is obviously more difficult. The bullets from Brownings of a Hurricane or Spitfire travel at about 1800 mph, and shells from a 20 mm cannon travel at about 2000 mph. They have to arrive at exactly the same time and in the same place as the target — moving across the bullets’ line of flight at an angular velocity of from 0-100 mph. The amount of aim ahead required is gauged by the fighter pilot looking through a gunsight fixed to the mean fore-and-aft flight path of his aircraft and pointing in the same direction as the eight guns. The judgement of how much lead to allow is now made electronically. In 1938 it was made by the pilot looking through a ‘ring-and-bead’ gunsight and judging his target’s speed and its ‘angle off’. By 1939, the ring-and-bead had been substituted by a reflector gunsight — the GM II. This was a great improvement as it allowed the pilot to see his target and surroundings at the same time as taking aim and made it unnecessary for him to alter the focus of his eyes to see all three once as the ‘ring-and-bead’ images were focused at infinity. Neither did he have any need to hold his head still, but could move it about a little and with both eyes open. It made aiming easier and much quicker and gave less time for an enemy fighter to ‘jump’ him from behind while he was trying to get into a firing position.
How much aim ahead is needed? Anyone who has practised skeet shooting will know that a clay pigeon travelling from its launcher at about 30 knots will require about two barrels width aim ahead when shooting at 30 degrees angle-off its flight line, and about four barrels deflection at 90 degrees. Apart from firing within range and assessing its line of flight accurately, the main secret lies in the amount of aim ahead to allow so that gun pellets and clay pigeon arrive at the same spot at the same time.
In the case of the Hurricane or Spitfire/Seafire pilot, he could only alter the direction of aim by altering the flight direction of his entire aircraft. Not only that, but he also had to position his guns-plus-fighter within range. The skeet shooter finds that this has already been done for him. The next thing for the fighter pilot to assess is his angle-off to his target, once again already done for the skeet shooter. Finally, he has to assess the target’s speed. This, once again, is almost a standard 30 knots or so for the trap shooter. The fighter pilot’s target may be doing anything from 90 knots in a turning fight to, say, 350 knots in a chase situation.
The difficulty of doing a job can be assessed by the amount of practice needed to accomplish it well. We would spend hours of flight time in the practising of range, line-of-sight and deflection estimation, using our G45 16 mm camera guns instead of live bullets, and using each other as targets.
We would then have the film developed, in ordinary black and white negative. By projecting this film on a screen, one frame at a time, the size of the image would give its range. By modelling a shadow image to that on the film, we could also assess its angle-off and line-of-flight accurately. Knowing our bullet speed and assuming a target speed, we could then calculate, in theory, whether our bullets, had we fired them, would have hit.
The angle-off of 30 degrees obviously reduces rapidly as the target continues to fly ahead, just as the clay pigeon flies ahead past the skeet shooter. The attacking fighter gradually falls into a line astern position — unless the target evades in a steep turn — and the amount of deflection that he allows consequently reduces second by second, until it is zero in the line-astern position.
The ‘amount’ can easily be converted to a number of degrees by solving a velocity triangle, ie a four degree aim ahead is required for a crossing speed of 100 mph. This was the angle which a radius of the ‘100 mph ring sight’ subtended in the GM II.
In the case of the Seafire/Spitfire — the long nose prevented the pilot from seeing his target at greater deflection angles than four degrees — even with his seat fully raised and his shoulders clear of the armour plate behind him. A shot at 30 degrees angle-off was therefore the maximum he could attempt. However, if the target had been a Swordfish doing about 80 knots, a fighter pilot could open fire at an initial 90 degree angle-off and still hit, while keeping his target in view. His range would get too close too quickly to allow more than a couple of seconds firing. Those who describe ‘beam attacks’ on World War II targets could not have been firing in range, unless a Swordfish was encountered.
The ring-and-bead in the reflector sight was reflected back into the pilot’s eyes by means of a small semi-silvered screen. It was the first ‘Head-up display’ in the cockpit of an aircraft. (The author invented the second in 1949. This was Patent Number 622987, dated 12 May 1950. It gave readings of several cockpit instruments while looking through the windscreen. It was allowed to lapse as no one at that time saw much use for it. It is now used in all pilot-operated attack systems and by speech-makers, sometimes pretending they are speaking without notes.)
Of course there was — and is — an immense difference between theory and practice. Although, in wartime, the problem of providing money for realistic training methods in a proper operational environment is not so difficult as in peacetime, there is usually insufficient time. ‘Pipe-line’ pilots therefore had only a bare chance of survival on appointment to their squadrons.
Both skeet shooting and cinegun attacks taught the principle of deflection shooting, but when cine results were assessed, they could not allow for skid errors, the effects of high ‘g’ or the variations in speed altering the flight attitude by ten degrees or more. Films were assessed by the Wren assuming a constant target speed of about 250 knots TAS in 1 ‘g’ flight without yaw. Neither was practice firing ever carried out at high altitude, where target speed (TAS) estimation was even more difficult than at low altitudes. Neither was it carried out — as on operations — at full take-off power and high engine rpm, where yaw, due to gyroscopic effects, became serious.
With such built-in errors in the training process and where habit takes over in the stress of real-life combat, it was a wonder that any hits were obtained at all. However, many of the smaller errors were catered for by the ‘spread pattern’ of gun harmonisation, quite apart from their growing number. (The Hurricane IIb had 12 Brownings.) It was not until the GM II, ‘100 mph’ reflector sight was replaced by the Gyro Gunsight Mark IID in fighter aircraft in 1944, that most of the yaw, pitch and target speed errors were automatically allowed for by the sight and the pilot could then see by just how much he had been missing with the GM II. A camera monitor, aimed through the light itself was also available, but it was very unpopular as it obstructed the forward view.
The most serious built-in error of all when assessing a weapon’s effectiveness in the hands of a gunner is the total lack of realism in peacetime training exercises. Cost effectiveness by the gunner, by the CO of the detachment, by the manufacturer and by the Defence Minister himself has always to be demonstrated, regardless of realism, to the tax payer. It rules out any chance that any £100,000 missile fired on exercises should ever miss. The WW II drogue-towing speed of 160 mph — remarked upon by Churchill in 1939 as unrealistically slow — was half the likely target speeds of the day. When, in real-life situations, the target speeds of enemy aircraft were also found to be far greater, pilots, including the author, had the greatest difficulty in positioning for the attack and I and most others usually under-deflected when we opened fire and also failed to trim out the unaccustomed yaw experienced at the higher speeds. Over-confidence of the pilot in himself and in his weapon usually led to exaggerated claims in the number of enemy he shot down. Wartime claims can be divided by between two and four in the largest air battles, although these were made honestly at the time. Likewise, claims by AA gunnery were grossly exaggerated. True peacetime gunnery and missile firing results can bear no relation whatever to their true wartime effectiveness or reliability, unless money and time spent on realistic practice is unlimited.