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No-allowance shooting

A short description

"No-allowance shooting is simple - give any 19 year old a machine gun, several drums of tracer ammunition and put him in the front seat of an FE 2b and he'll discover it for himself." - overheard conversation of WWI air gunner

No-allowance shooting, also known as non-deflection shooting or non-deflection sighting is a principle of air combat first discovered and used during World War One. It heavily influenced the specifications drawn up for new fighter aircraft by the Britain's Royal Air Force (RAF) in the 1920s and 30s, being largely ignored by the air-arms of other countries. At is therefore ironic that the biggest and most successful use of the principle was made by the German Luftwaffe in the form of the Scräge Musik upward firing guns used in their nightfighters against British bombers in World War Two.

An appreciation of no-allowance shooting is essential to anyone wanting to make sense of the various RAF specifications for new designs of fighter that originated between the two world wars; and yet very little written about this subject; and what there is often confuses the matter. The term "no-deflection" is sometimes used to denote shooting at an enemy aircraft from directly behind and at very close range, so close that no allowance for bullet drop or deflection need be made - that is not the sense of "no-deflection" this web-page is about.

Beginnings and principles.

No-allowance shooting was first discovered by aviators in the early stages of World War One - Its early users were the front gunners in pusher biplanes such as the British Fe2b - where the front gun is free to be pointed in any direction that the airframe will allow. The principle was then later used by pilots flying aircraft fitted with a "Foster mount" that allowed a machine gun on the top of the wing of a biplane to be swung down into the cockpit to have its ammunition drum changed - The Foster mount also allowed the machine gun to be fired at any angle from straight ahead to vertically upwards. Aircraft that used the Foster mount included the French Nieuport 17 and the British SE5. The most famous user of no-allowance shooting being the British ace Albert Ball VC who flew both the Nieuport and SE5.

To explain no-allowance shooting imagine you are flying a Nieuport fitted with a Foster mount with the gun pointed straight ahead.  Your ammunition drum is loaded with tracer to observe the flight of the bullets. You fire the gun; what do you see?  The bullets will leave the gun and at first go straight ahead,  but after a second or so they will start to curve downward. They are not immune to the force of gravity and will therefore start to fall towards the earth. You therefore see that to hit a target at some distance ahead you have to allow for this "bullet drop". Now you use the Foster mount to pull the gun back a little so it is angled a few degrees upwards; you fire again. You see the bullets climb, but again after a second or so they start to curve down to earth, but initially not as quickly as when they were fired straight ahead. The diagram below (obviously not to scale and with the curves exaggerated) shows the situation. The red line showing the flight of bullets straight ahead and the blue when the gun is angled up a few degrees.

Now you pull the gun all the way back so that it is pointing vertically upwards. You fire again; what do you see? The bullets go up, but because the aircraft is travelling forward at nearly 100mph they are left behind. To you in the aircraft the bullet seems to curve backwards. You then push the gun forward so that is angled forward a few degrees and fire again, again the bullets curve backwards as from your point of view you catch them up and leave them behind. It is important to stress that this is caused by your forward motion.  The bullets themselves never actually start to go backwards - it is your aircraft leaving them behind that gives rise to the illusion.

Now,  if bullets fired forward curve downwards because of gravity, and bullets fired upwards curve back because they are caught up with and left behind by the forward motion of the aircraft, what happens when we pull the gun to a mid-way position?  At what point will bullets stop curving downwards and start to curve backwards?  It is a matter of common-sense that there must be a point where one effect is cancelled by the other, a "sweet spot" at which, from the pilot's point of view, the bullet just appears to keep going in the direction it was fired. You've discovered no-allowance shooting!  The angle at which you do not need to aim off to allow for deflection.

The advantages of no-allowance shooting are many. If you can formate under the tail of an enemy aircraft and fly at the same speed, with no "aiming-off" you can virtually guarantee to score hits, and you can open fire at greater range.  The position under the tail of the enemy is ideal for approaching without being seen if the target does not have a ventral gun position. The method is perfect for bringing down unescorted bombers flying in formation, with each bomber "locked" into its position within the formation it can do little to avoid an attack by no-deflection sighting, particularly if the attacker can use a longer range cannon to make the attack without getting into range of the bomber's defensive armament. Against night-bombers the no-deflection method is particularly effective, approaching from below the bomber will be silhouetted against whatever light there is remaining in the sky, whereas the attacking fighter will be lost against the "gloom" of the ground. When the bomber starts to take evasive manoeuvres it can be more easily kept in view by an aircraft below and behind, whereas an attacker directly behind can easily lose view of a bomber if it drops below its nose suddenly.

The angle at which no-allowance shooting works depends on two factors;   the speed of the aircraft through the air and the velocity of the bullet. For an aircraft in World War One , the angle is in the order of 45 degrees.  The increased speed of aircraft in World War Two meant the angle was reduced to about 20 degrees or less.

One of the things that confuses the issue of no-allowance shooting is a perception that somehow it made projectiles (ie bullets and cannon-shells) actually travel further. This is explained by saying that the bullet itself generates lift to combat the force of gravity. You can see this explanation of lift expressed on this webpage - Click here - part of a website devoted to the books of Anthony G. Williams and Emmanuel Gustin, the foremost writers on the subject of aircraft gun armament.

Between the wars there was a series of specifications for new fighters released by the RAF that sought to maximise the potential for no-allowance shooting.   For example specification 27/24 which resulted in the twin engined Boulton Paul Bittern fighter prototype with machine guns in revolving barbettes in the nose. Then specification 29/27 gave rise to two prototypes, one from Vickers, the other from Westlands, mounting big 37 mm Coventry Ordnance Works guns (known as "COW Guns" ) fitted to fire upwards at an angle.

The Vickers Cow gun fighter: It looks very anachronistic for an aircraft that first flew in 1931. The interceptor -style specification for which it was built stressed the ability to climb to altitude very quickly, something the large wing area of its biplane layout was ideal for. In fact the Vickers design was as fast as the monoplane COW fighter produced by Westland while possessing a much superior rate of climb. 

Specification 5/33 called for a two-seater fighter with the main armament in a turret in the nose, this did not lead to any prototypes but it did push forward turret development. This led eventually to Spec 9/35 which led to the Boulton-Paul Defiant turret fighter, and its slower naval counterpart the Blackburn Roc.  There can be no doubt that no-allowance shooting was inherent in the design of the Defiant ; for example the gun turret of the Defiant could be locked into position and then fired by the Defiant pilot.   The ultimate British no-allowance shooting project was derived from specification F9/37 which led to the Gloster G39 prototype twin engined fighter,  this was designed to have cannon in the fuselage behind the pilot firing upward at a no-allowance angle.

View of the first prototype Gloster G39 F9/37 fighter showing the bulged rear fuselage to accommodate the upward firing armament of either three 20mm cannon or four .303 machine guns; this armament was never fitted . There would also have been two 20mm cannon mounted in a conventional position under the nose. The bulged fuselage suggests that it was the intention that at least some of the rear armament could be realigned to fire forward past the cockpit. The first prototype was powered by a pair of air-cooled Bristol Taurus sleeve-valve engines of 1,050 horsepower and had a remarkable performance; being as fast as a Spitfire Mk I. The second prototype was powered by two of the lower-powered liquid-cooled Rolls-Royce Peregrine engines with chin radiators. By the time the Prototypes flew the rival Bristol Beaufighter was starting into production and to proceed with the G39 would have taken resources away from other vital projects. A follow up design with two Merlin engines called "The Reaper" was also cancelled, the design staff at Gloster concentrating on what was to become the Meteor twin-jet fighter and its single-engined stable-mate the "Ace".

Amazingly, Colin Sinnott in his masterly book "The RAF and Aircraft Design 1923-1939" shows that prior to WWII the RAF was actually calling for ways for the wing-mounted armament of Hurricanes and Spitfires to be somehow elevated to fire at no-allowance angles!  Indeed this requirement was taken forward to the specification that led to the Hawker Typhoon.

When World War II started all British interest in no-allowance shooting suddenly, and seemingly inexplicably, disappeared. Perhaps in light of its impact on Spitfire, Hurricane and Typhoon performance this is just as well, but the promising Gloster G39 was shelved and the Beaufighter developed instead. It was in Germany instead that no-allowance shooting was developed to its full combat potential as a weapon of the Luftwaffe night-fighter force. Here it does not seem to have been the result of any official development, rather it was a series of one-off trials by individual pilots and armament technicians that led to its adoption.

Links to other sites...

Westland COW gun fighter on Aviastar website

Vickers COW gun fighter on Wikipaedia

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