A short description
"No-allowance shooting is simple; give any 19 year old a machine gun, some 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. It 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 night fighters 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 is 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.
Foster Mount - It allowed the machine gun to be mounted on top of the wing to fire outside the arc of the propeller, yet allowed the gun to be pulled back for the ammunition drum to be changed - But it also allowed the gun to be fired at an angle upwards and take advantage of no-deflection shooting.
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? As soon as the bullet leaves the barrel gravity will start to act on the bullet, causing it to start arcing downward. The speed of the bullet will mean that from the pilots point of view this "bullet drop" will be negligible at first, but as the bullet starts to slow because of air resistance the effect will become more pronounced. You therefore see that to hit a target at some distance ahead you have to allow for this "bullet drop" by "aiming-off" and firing at a point above your target. 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 it 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, however if you were firing at an enemy you would have to allow for this deflection and "aim-off" to achieve hits.
Now, if bullets fired forward curve downwards because of gravity, and bullets fired upwards appear to 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 even as little as 10 degrees.
One of the things that confuses the issue of no-allowance shooting is a perception that the technique owes its existence to the fact that projectiles (i.e. bullets and cannon-shells) actually travel further when fired at an oblique angle from moving aircraft. 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 web page - 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. However the resulting increase in range would be very small indeed and has no bearing on the ability of the pilot to fire on a target without having to allow for deflection.
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 ( Its slower naval counterpart the Blackburn Roc was covered by 15/37). 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.*
Specification F9/37 which led to the Gloster G39 prototype, which was designed to have 3 cannon in the fuselage behind the pilot firing upward at a no-allowance angle (12 degrees), and another two in the nose, also firing up at at the same angle.
View of the first prototype Gloster G39 F9/37 fighter showing the bulged rear fuselage to accommodate the upward firing armament of three 20mm cannon, . There would also have been two 20mm cannon mounted under the nose, but also at an angle to fire upwards. This 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 first prototype was never fitted with any armament. The second prototype was powered by two of the lower-powered liquid-cooled Rolls-Royce Peregrine engines with chin radiators and had a lower performance, but it was fitted with full armament. Photographs in Tony Buttlers book "British Experimental Combat Aircraft of World War II" show the armament fitted.. 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".
Side view of the Gloster F9/37 showing the alignment of the cannon. Two were in the nose and three amidship.
The Gloster G39 F9/37 painted as if it had gone into production.
Another expression of confidence in the no-allowance shooting philosophy was Air Ministry specification F11/37 which specified a cannon armament in a power operated turret -Of the three companies who tendered to this specification, two (Armstrong Whitworth and Bristol) had turrets that could only traverse through the forward hemisphere and hence were plainly designed with no-allowance attacks in mind.
The Armstrong Whitworth design for Air Ministry Specification F11/37 was powered by two Merlin engines in "pusher" configuration. Its armament of four 20mm cannon was accomodated in a turret which could be rotated only around the forward hemisphere. The cannons could not be depressed far enough to fire horizontally, clearly showing it was intended to be used in no-allowance attacks.
The Bristol tender to specification F11/37 came in two versions. The first with a pair of Hercules engines and a second "scaled up" version with two of the bigger Centaurus engines. Both features a turret with four 20mm cannon that could only be operated in the forward quadrant. The aircraft layout is similar to the Grumman XF5F Skyrocket although much larger.
F22/39 was an Air Ministry Specification written around the Type 414 twin-engined fighter design being developed by Vickers. This clearly has no-allowance sighting at the heart of the thinking behind the design and can be seen as the rational successor to the Vickers COW gun fighter of the previous decade. It envisioned a streamlined monoplane two-seater fighter (looking a little like a DH Mosquito but with a twin-rudder tail) mounting a large 40 mm cannon in the nose in a remotely controlled turret which could be elevated for no-allowance shooting.
The Vickers Type 414 design tendered to specification F6/39. Successor to the Vickers "COW" fighter of a decade before. Powered by two RR Griffon engines and mounting its 40mm gun in the nose. The gunner sat beside the pilot with his head in a perspex blister. The gun installation and sighting installation were tested successfully in place of the front turret of a Wellington bomber.
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.
Something that you have to keep in mind when reviewing British interest in no allowance shooting is that it was seen as a way to counter enemy bombers attacking the UK. It was never anticipated that those bombers might be escorted by nimble single-engined fighters. It was the German territorial gains of over-running France, Belgium and Holland that put Britain within range of German Bf109 escort fighters. The presence of these escorts during the Battle of Britain quickly dampened enthusiasm for no-allowance shooting for daylight fighters. However for night-fighters no-allowance shooting was still seen as desirable and this led to specification F18/40 calling for a fighter with forward-facing cannon armament with a turret with machine-guns capable of being used against night-bombers from below.
The Boulton Paul P97B was one of the tenders for F18/40. It was a big twin-boom design powered by two Napier Sabre engines. Armament was a type-A four gun turret as used on the Defiant along with two forward firing 20mm cannon. Quite similar in layout to the US Northrop P-61 Black Widow night-fighter.
During the night-blitz of the winter of 1940-41 the Defiant turret-fighter did achieve some success and many of these successes were achieved in a no-allowance shooting scenario, but the new Bristol Beaufighter with it's on-board interception radar started to achieve much greater success, and although there were plans to fit Beaufighters with turrets, and prototypes were flown, these plans were dropped and production was concentrated on the version with only forward-firing armament and over the course of that winter of 40/41 British interest in the no-allowance shooting method faded and disappeared.
It was in Germany instead that upward 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.
* I've had some people express doubts about the fact that the Defiant's guns could be fired by the pilot - Can I just point out this quote from the Defiant Pilot's Notes (AP1592B) - Section 1 Paragraph 2: "The control column is operated in the normal way and is mounted on the forward edge of the pilot's seat. The spade-grip is of standard type and incorporating a brake operating level (47) , and a gun-firing push-button (48) for use when the gun-firing control is taken over by the pilot." The numbers 47 + 48 refer to numbers on the associated diagram of the pilots cockpit. The turret had a switch to transfer control of firing the guns to the pilot. This link <click here> takes you to an on-line copy of the Defiant Pilots notes on the WWW2 aircraft .net forum (you'll have to register to view it). If you look at this RAF training film on youtube at the link here <click here> you can see the three position switch clearly marked "Pilot- Off - Gunner".
For details of the many remarkable designs covered in F9/37, F11/37, F22/39 and F18/40 can I recommend "British Secret Projects: Fighters & Bombers 1935-1950" by Tony Buttler ISBN 1 85780 179 2 It was published by Midland Counties Publishing (an imprint of Ian allan Publishing).
"The British Aircraft Specifications File" by KJ Meekcoms and EB Morgan ISBN 0 85130 220 3, published by Air Britain also covers these designs but in much less detail and without any illustrations or diagrams.
Links to other sites...
Westland COW gun fighter on Aviastar website
Vickers COW gun fighter on Wikipaedia