USA(A)F GUNSIGHTS – THE N- SERIES OF REFLECTOR GUNSIGHTS
N-1 Gunsight (experimental only)
First of the N- series of reflector gunsights, the N-1 was developed by the Armament Laboratory, Wright Field at the request of the Air Corps after conclusive testing of the German ENI sight in late 1931.
Only 2 N-1 sights were manufactured and tested on an XP-6H and a P-6E airplane early 1934 and on an A-12 of the 3rd Attack Group later in the year. Unfortunately the A-12 crashed less than 2 weeks later, killing the pilot and damaging the sight.
Tests were nonetheless satisfactory even if requests were made to provide a wider aperture and change the sight design to facilitate installation in the aircraft.
The N-2 was housed in an aluminum casting, provided an optical aperture of 2 1/8 inches and employed, thanks to a front surface mirror, an L-shaped optical system (with the lamp and reticle preceding the fold). The original reticle consisted of a single vertical line crossed by short horizontal lines indicating gravity drops for ground targets at known ranges.
The N-2 was produced in the following versions:
- N-2: electrical connector for two-wire system
- N-2A: same as N-2 except single-wire grounded electrical system and split type lamp housing for greater accessibility to the lamp(a)
- N-2B: found some reference about an N2-B, but doubtful it ever existed
- N-2AN: Navy procured N-2A. Identical to N-2A except reticle and location of nameplate. The US Navy originally ordered 1800 sights but later reduced its order to 800(b). These were mostly fitted to F4F-3 Wildcats.
In Nov 1936, in what can be considered as the first operational use of reflector sights by the AAF, eight P-26B of the 17th Pursuit squadron were fitted with N-2 sights in preparation of the annual gunnery training. This was also one of the very few occurence where the reflector sight was installed in front of the wind screen on an operational aircraft (the only other example i’ve identified so far is the AT-12). Tests were positive and the N-2 was subsequently fitted to most late 30’s fighters like the P-35, P-36, P-43 but also early versions of the P-39 and P-40 fighters.
(a) Gun Sights – Installation, Inspection and Use – Type N2 and N2A Fixed Gun Sights, Technical Order 11-35-5 dated March 1, 1939
(b) Navy Bureau of Ordnance, memo to Chief of the Air Corps dated March 8, 1941
The N-3 series of gunsights was by far the most produced and definitely the most widely used AAF gunsight of WWII as it was, at one time or another, fitted to almost every AAF fighter, attack plane, medium and light bomber, but also trainer and even observation plane that needed a fixed gunsight. It was also used briefly as a turret sight at least on B-24’s and probably on Martin top turrets.
The N-3 was definitely not a great and excellent gunsight but it had the advantages of being available (and in great numbers), cheap (3 to 6 times cheaper than the Mark 8 or N-9) and of a size suitable for most aircraft cockpits.
N-3, N-3A & N-3B gunsights were basically N-2A sights with just a few minor differences (see below) and the only big improvement finally came with the N-3C (standardized late 1943) which was lighter (redesigned aluminium casing) and on which the bulb could finally be easily changed in flight in case of failure. Note that the issue of bulb failure in flight was already partially solved before the arrrival of the N-3C with the installation of a toggle swith and the new G-9 Mazda double filament bulbs on existing Gunsights. This modiication allowed the bulb to use only one filament at a time and if it burned, you just had to switch to the other one.
- N-3 (standardized early 1939): similar to N-2A except designed for 24-volt system;
- N-3A (standardized early 1941): similar to N-3 except larger opening of reticle holder to permit the use of 70 mil (later 101 mil) reticle and relocation of data plate to the side of the gunsight to avoid light reflection.
- N-3B (standardized mid 1942): N-3A with a two wire system and uses a screw-on type electrical connector instead of the push-on type.
The N-3 series also had the distinction of using a great variety of sighting heads (which are a lot of fun of collecting) as, besides standard government procured heads, they were also fitted with a lot of different heads designed and produced directly by aircraft manufacturers. It was indeed the custom in the early years to provide the manufacturers just with the sight and let them design/install their own sight mounts and sighting heads. Some early installations, with the gunsight fixed on the cockpit floor, did not even use any sighting head as the reflector glass was fixed on the back of the instrument or directly on the windshield (P40 B&C for example).
N-4 Gunsight (never produced)
The N-4 was ordered and developped as an improved version of the N-3 featuring, amongst other(a):
- a variable density polaroid sun filter;
- a set of 5 interchangeable reticles;
- an integrated bubble level (inclinometer) and
- a manual angle of attack input.
Contracts were placed with both Bell & Howell and Fairchild Aviation in March 1941, but progress was subsequently slow and apparently unsatisfactory and both contracts were finally cancelled in 1942.
No N-4 sight or even prototype(s) were ever produced.
(a) Air Corps Spec XR-24649, Sight fixed gun Type N-4, 3 April 1940, notice N°1 20 March 1941
The N-5 was simply a standard N-3 sight fitted with a specific smaller (lower) reflector head and a new reticle designed for flexible gunnery. The N-5 was ordered specifically for use in the Martin upper turret whose smaller size didn’t allow the use of standard N-3 reflector heads or K- series sights available at that time.
Whereas the N-5 was actually produced and used operationally is unclear. I have personally never seen one and do not know of any collector who has one or even seen one. Moreover, period photos of Martin turrets do not permit to see if they are equipped with standard N-3 or the new N-5 sight.
N-6 & N-6A Gunsights
The N-6 was the first of the N- series to be specifically designed for flexible gunnery for use in turrets not equipped with compensated sights. AAF Material Command also requested(a) the N-6 to replace the old iron sights on all other gun positions in medium and heavy bombers (but most period photos clearly show it didn’t happen).
The sight and optical system, also very similar to the ones of the N-3, were more compact. Also, it was equipped with:
- a flipping green sun filter ;
- an attachable ring and bead back-up sight in case of electrical failure of the sight;
- an easily replaceable bulb;
- a single ring with central dot orange reticle.
Early N-6s were fitted with a 29.7 mil ring reticle later replaced by a more standard 70 mil ring reticle.(b)
The N-6 was most widely used in Martin and Bendix upper turrets, B-24 turrets (nose and tail), B-17 chin turrets and on the remote control sighting station of the P-61 night fighter. Adapter rings were also available for fitment of either A-1 or A-2 tilting heads for use as a fixed gunnery gunsight on attack and light bomber plane(c), but if these combinations were ever used operationally is unclear.
N-6 was also field modified to be used as a fixed gunsight on B-25 in the PTO. One of the most famous being “PAPPY’S FOLLY”, the very own B-25 of Col. Paul “Pappy” Gunn.
(a) AAF Material Command memo dated June 10, 1943.
(b) Handbook of Instructions with Parts Catalog for Type N-6 Gunsight, T.O. 11-35-12 dated March 1, 1944.
(c) The N-6 with A-1 head was even tested for potential use by the B-24 bomber for bombing through broken clouds.
N-7 Gunsight (experimental only)
The sight was designed by the L A.B. laboratory, New Jersey which, besides a few prototypes and test units, also produced a very limited production quantity of 20 units.
This new sight included new and improved features such as:
- sealed optical chamber against condensation and entrance of moisture;
- a lamp bulb that can easily be replaced in flight;
- sight head reflector glass supported at the bottom only, reducing interference to pilot visibility to a minimum (later replaced by the standard A-2 tilting head);
- a 70 mils reticle (later changed to 100 mils);
- an integral sun filter;
- a reticle mask to be used for night fighting (reducing the reticle to a single dot).
Tests conducted in November 1942 and July 1943(a), including flight tests in P-39, P-47 and B-25, were satisfactory but it was concluded that improvements the N-7 sight offered compared to the N-3A sight (especially if fitted with the new A-2 tilting head) did not warrant production(b) and the N-7 was finally placed in the obsolete classification in September 1943 as a better and more suitable sight (i.e. the N-9) could be purchased and delivered at an earlier date(c)
(a) Proof Department AAF Proving Ground Command, Eglin Field – Reports on Service Test of Type N-7 Fixed Gunsight – dated 2 December 1942 and 12 July 1943 respectively.
(b) AAF Material Command memo dated 24 July 1943.
(c) AAF Material Command memo dated 20 April 1944.
N-8 & N-8A Gunsights
Designed for use both as a fixed gunsight for fighters and flexible gunnery gunsight for hand-held guns and turrets(a), the N-8 “Retiflector “(b) gunsight design departed from other AAF sights in that it employed a reflecting instead of a refracting collimator and used a Mangin mirror instead of a conventional lens.(c) Light from the lamp, mounted in a removeable holder at the bottom of the sight, pass through a reticle fixed directly above it, then through the reflector plate to the Mangin collimating mirror at the top of the sight. After collimation, the light returns to the reflector plate from which it is reflected to the gunner’s eye.
Early N-8 were fitted with a 70 mil single ring with central dot reticle soon replaced by 100 mil three-ring orange reticle. As for the N-6, the N-8 sights were also fitted with a flipping sun filter (plain green on early ones replaced a by a Polaroid variable density later on) and an attachable ring and bead sight in case of failure of the sight.
Despite its qualities, the N-8 finally saw limited use: mostly in the waist and tail gun installations of late B-25, B-26 Marauders and B-17’s. As a fixed gunsight, it was only used (up to my knowledge) in some late B-26 Marauder trainers in the US and never on a fighter plane.
(a) AAF Proving Ground, Eglin Field, Final Report on Test of N-8 Gun Sight, 15 June 1943.
(b) As the light is folded back on itself by the mirror, this type of gunsight was given the name “retiflector”.
(c) Developed by the French company Optique et Précision de Levallois and used operationally on late 30s French fighters with the RX- series gunsights.
N-9 & N-9-1 Gunsights
- have superior optical performance (reduced aberrations and parallax);
- be as compact as possible;
- bulbs can readily be exchanged during flight;
- have at least a 101 mils reticle;
- sealed optical chambers as a protection against condensation and moisture;
- reflector glass secured at the bottom only to minimize interference with pilot visibility;
- have an inclinometer that can easily be seen by the pilot;
- improved electrical system to avoid potential compass deflection when the gunsight is turned on and
- be economical to produce (but it still cost the US taxpayer around 160$ a piece or at least 4 times the price of an N-3).
Inspection of the experimental N-9 early July 1943 showed “considerable” promise and from then on Material Command was repeatedly requested to make every effort to obtain necessary critical materials and resources to insure quick and uninterrupted production. Initial contracts for 4.900 sights were placed in October and November 1943 with Bell & Howell and Robinson Houchin Optical Co. with additional orders of 14.100 sights (including orders to a 3rd supplier Sperti) following later on.
The N-9 was built in 2 versions: the type N-9 Gun Sight (by all three manufacturers) and the type N-9-1 Gun and rocket Sight (contrary to what documentation says, it was made not only by Sperti but also at least by Robinson Houchin Optical Co.). The 2 versions were identical except for the reticle slits.(a)
Initially planned to equip all fighter and most light and medium bomber type aircrafts(b), delay in deliveries, space limitations in some of the intended aircrafts (P-38 and P-61) and the arrival of the K-14 / Mk-21 sights (US procured copies of the British gyroscopic gunsight) the N-9 was finally factory fitted to a limited number of aircrafts:
- all P-51D up to P-51D-20-NT 44-12852 and P-51D-20-NA 44-72226 (c). Contrary to what has often been written, the N-9 was never fitted to the P-51B & C as the required number of modifications to the cockpit and additional parts to be fabricated was considered ineffective. (d)
- P-63 from P-63A-10 through P-63C-5 replacing the N-3C gunsight.
- A-26 (later B-26) replacing the N-3B or N-3C gunsight.
- Prototypes of the XP-40Q and XP-75.
The N-9 could be used without a sighting head (P-63) or be fitted with either a B-1 fixed or an A-3 variable sight head. Fighter type aircrafts were initially planned to be equipped with the A-3 variable reflector head (used for low level and skip bombing and rocket firing) but after many reports were received from operational units that fighter pilots were not using the tilting feature of their N-3 gunsights and were always leaving their sights set at “zero”, decision was taken to fit the N-9 with only B-1 fixed heads on fighter planes.(e)
The N-9 also equipped a few post war aircrafts as it was fitted at least to early F-86D, most RF-84F (with a specifically designed sight head), T-28A advanced trainers and finally, in a most upgraded form and renamed the XM73, to the AH-1 Cobra attack helicopter.
(a) Handbook of instructions for GUNSIGHTS type N-9 and N-9-1, AN 11-35-18 revised 1 June 1945
(b): B-25, B-26, A-26, A-36, P-40, P-61, P-63, P-75, P-38, P-47 and P-51. Sources: USSTAF Ordnance Memorandum N° 4-25 AIRCRAFT GUNSIGHT GUIDE and AAF Material Command memo dated 22 December 1943.
(c) Source: P51sig.com
(d) AAF Material Command instructions dated 25 May 1944
(e) Headquarters AAF – Variable Reflector Heads for Fighter Airplane Gun Sights – 28 Dec 1943
N-10 Gunsight (experimental only)
Last of the N series of reflector gunsights, the N-10 was a scaled down version of the successful N-9 and was intended for use in fighter planes were the N-9 or other gunsights could not easily be fitted due to space limitations (i.e. the P-38).
Based on the proposal of Bell & Howell dated July 1943, a total of (only) 7 gunsights were ordered for flight tests and trial installation in engineering mock-ups of P-38 airplanes(a). Flight tests conducted at the AAF Proving Ground, Eglin Field in May 1944 were positive(b) even if no perfect mounting installation could be found for The P-38 cockpit. However, by the end of 1944, with production not yet launched and the P-38 planned to be equipped with the K-14 (which never happened), the N-10 was cancelled.
The N-10 was fitted with the same 101 mils reticle with identical image as the one in the N-9 sight. Service Tool & Eng. designed both a fixed (the B-2) and a variable (the A-4) reflector heads for the N-10. These were almost identical miniature version of the B-1 and A-3 heads fitted on the N-9.
Note: at least one N-10 was preserved and is currently in the collections of the AF museum.