The Royal Aircraft Factory S.E.5 was a British biplane fighter aircraft of the First World War. Although the first examples reached the Western Front before the Sopwith Camel, and it had a much better overall performance, problems with its Hispano-Suiza engine meant that there was a chronic shortage of S.E.5s until well into 1918 and fewer squadrons were equipped with the type than with the Sopwith fighter. Together with the Camel, the S.E.5 was instrumental in regaining allied air superiority in the summer of 1917 - and maintaining this for the rest of the war - ensuring there was no repetition of "Bloody April" 1917, when losses in the Royal Flying Corps were much heavier than in the German Air Service.
The S.E.5 (Scout Experimental 5) was designed by Henry P. Folland, J. Kenworthy and Major F.W. Goodden of the Royal Aircraft Factory, in Farnborough. It was built around the new 150-hp (112 kW) Hispano-Suiza 8a V8 engine which, while it provided excellent performance, was initially under-developed and unreliable. The first of three prototypes flew on 22 November 1916. The first two prototypes were lost in crashes (the first killing one of its designers, Major F.W. Goodden, on 28 January 1917) due to a weakness in their wing design. The third prototype underwent modification before production commenced - the S.E.5 was known in service as an exceptionally strong aircraft which could be dived at very high speed, so these changes were certainly effective.
Like the other significant Royal Aircraft Factory aircraft of the war (B.E.2, F.E.2 and R.E.8) the S.E.5 was inherently stable, making it an excellent gunnery platform, but it was also quite manoeverable. It was one of the fastest aircraft of the war, at 138 mph (222 km/h) equal at least in speed to the SPAD S.XIII, and faster than any standard German type of the period. The S.E.5 was not as effective in a dog fight as the Camel, as it was less agile, but it was easier and safer to fly, particularly for novice pilots.
The S.E.5 had only one synchronised .303-in Vickers machine gun to the Camel's two. However it did have a wing-mounted Lewis gun on a Foster mounting, which enabled the pilot to fire at an enemy aircraft from below, as well as forward. This was much appreciated by the pilots of the first S.E.5 squadrons, as the new "C.C." synchronising gear for the Vickers was unreliable at first. The Vickers gun was mounted on the left side of the fuselage with the breech inside the cockpit. The cockpit was set amidships, making it difficult to see over the long front fuselage, but otherwise visibility was good. Perhaps its greatest advantage over the Camel was its superior performance at altitude - so that (unlike most Allied fighters) it was not outclassed by the Fokker D.VII when the German fighter arrived at the front.
Only 77 original S.E.5 aircraft were built before production settled on the improved S.E.5a. The S.E.5a differed from late production examples of the S.E.5 only in the type of engine installed - a geared 200 hp Hispano-Suiza 8b replacing the 150 hp model. In total 5,265 S.E.5s were built by six manufacturers: Austin Motors (1,650), Air Navigation and Engineering Company (560), Curtiss (1), Martinsyde (258), the Royal Aircraft Factory (200), Vickers (2,164) and Wolseley Motor Company (431). A few were converted as two-seat trainers and there were plans for Curtiss to build 1,000 S.E.5s in the United States but only one was completed before the end of the war. At first, airframe construction outstripped the very limited supply of French-built Hispano-Suiza engines, and squadrons earmarked to receive the new fighter had to soldier on with Airco DH 5s and Nieuport 24s until early 1918.
The introduction of the 200 hp (149 kW) Wolseley Viper, a high-compression version of the Hispano-Suiza made under licence by the Wolseley Motor Company, resolved the S.E.5a's engine problems, and was adopted as the standard powerplant.
The S.E.5 entered service with No. 56 Squadron RFC in March 1917 although the squadron did not deploy to the Western Front until the following month, among other reasons so that the very large "greenhouse" windscreens, unpopular with pilots, could be replaced with small rectangular screens of conventional design. The squadron flew its first patrol with the S.E.5 on 22 April. While pilots, some of whom were initially disappointed with the S.E.5, quickly came to appreciate its strength and fine flying qualities, it was universally held to be under-powered, and the more powerful S.E.5a began to replace the S.E.5 in June. At this time 56 squadron was still the only unit flying the new fighter; in fact it was the only operational unit to use the initial 150 hp S.E.5 - all other S.E.5 squadrons used the 200 hp S.E.5a from the outset.
In spite of the initial very slow build up of new S.E.5a squadrons, due to a chronic shortage of the type that lasted well into 1918, by the end of the war the type equipped 21 British Empire squadrons as well as two U.S. squadrons. Many of the top Allied aces flew this fighter, including Billy Bishop, Cecil Lewis, Edward Mannock and James McCudden. Legendary British ace Albert Ball was initially disparaging of the S.E.5 but in the end claimed 17 of his 44 victories flying it. McCudden wrote of the S.E.5 "It was very fine to be in a machine that was faster than the Huns, and to know that one could run away just as things got too hot."