Napier’s temperamental 24-cylinder gem was one of the most innovative engines of its time.
Enigmatic, charismatic and, yes, a pain in the rear are just some of the descriptions that could be applied to the Napier Sabre, surely the most complex aircraft engine to see series production during World War II. Hallmarks of most Napier aviation engines included high speed, high specific power and high degree of complexity. These distinguishing characteristics started with Napier’s first foray into airplane engines, the immortal Lion, and continued on to the last of its aircraft piston engines, the formidable Nomad high-speed two-stroke diesel.
Napier’s chief designer, Arthur Rowledge, set the stage for the firm’s specialized offerings. The Lion, developed too late to see service during World War I, was an advanced, high-revving (for the time) engine with over-square cylinder dimensions. Amazingly, this versatile engine served Britain’s military until the 1950s, powering high-speed boats for the Royal Navy in marinized form. Shortly after designing the Lion, Rowledge jumped ship and went to Rolls-Royce, where he continued his work on aircraft engines.
The enterprising and talented Frank Halford could be loosely de – scribed as a hired gun when it came to designing airplane power plants. He designed engines for de Havilland as well as Napier. Following in the Napier tradition of complexity and high speed, Halford developed the 16-cylinder Napier Rapier and the even more complex 24-cylinder Napier Dagger, both of which were air-cooled. Although competently designed, neither engine came close to the success of the Lion or temperament of the Sabre. Even so, both power plants were sufficiently developed to make a contribution to the war effort.
The 24-cylinder, liquid-cooled, “H”-configured Sabre was Halford’s swan song for Napier. It represented the state of the art in the mid- to late 1930s, offering high specific power (horsepower developed for each cubic inch of displacement) and high revving. Among its design features were over-square cylinder dimensions and sleeve valves, which would prove to be the Sabre’s Achilles’ heel.
Sir Harry Ricardo was a great proponent of sleeve valve engines. Indeed, it was Ricardo who convinced Roy Fedden at Bristol Engines to drop poppet valve development in favor of sleeve valve technology. Although sleeve valve engines offered greater volumetric efficiency along with other advantages, these benefits came at a formidable cost: The cylinder sleeves were notoriously difficult to manufacture.
Innovation was the key word in describing the Sabre. Its basic configuration was two flat (horizontally opposed) 12-cylinder engines pancaked on top of each other. Each crankshaft drove a pinion that engaged with a reduction gear. Integral with the reduction gear was a helically cut gear that drove another gear mounted on the propeller shaft. With so much gearing, ensuring equal tooth loading on each gear was essential. This engineering challenge was overcome by incorporating a slight helix angle on the intermediary gears. End thrust, generated by the helix angle as power was transmitted, was opposed by a centrally pivoted, spring-loaded beam assisted with oil pressure. Two beams took care of the thrust load emanating from the four pinions. The beams would accommodate slight deviations due to manufacturing tolerances, thus ensuring that each gear tooth took its fair share of the torque being transmitted—a brilliant piece of design engineering.
Supercharging for all production Sabres was accomplished with a single-stage, two-speed design, albeit in various forms in different marks. Carburetion was via an updraft carburetor, or in later marks, single-point fuel injection. Accessories such as the starter, generator, oil pumps, etc. were arranged above or below the engine. All in all the result was a very compact design, with a remarkably small frontal area.
Initially rated at 2,060 hp at 3,700 rpm in Sabre I form, the engine eventually produced 3,055 hp at 3,850 rpm in the Sabre VII with the assistance of water/methanol injection. But that was not all the Sabre was capable of: The experimental Sabre E.122 was expected to produce 3,350 hp, but it never advanced past the design study phase. Ironically, Rolls-Royce produced a virtual copy of the E.122 called the Eagle 22, with increased displacement, intercooling and after cooling. Its only application was on prototypes of the Westland Wyvern.
Although the Sabre successfully passed its 100-hour type test in June 1940, serious problems surfaced as this complex piece of machinery entered mass production, nearly sinking the innovative engine before it had a chance to prove itself. Foremost among the manufacturing issues was reliably making cylinder sleeves. This was ostensibly a simple job consisting of producing a pipe with a few holes punched in it. But the thin-wall sleeve proved to be almost impossible to make round and stay round. Benefiting from its vast experience with this difficult manufacturing chore, Bristol had the inside track on how to make sleeves, but it was not an easy task for that firm either.
Even with Fedden leading the charge, it took an inordinate amount of time to figure out how to manufacture a thin sleeve, harden it and then maintain its roundness and concentricity. Despite Napier’s best efforts using the finest machine tools and materials, the sleeves would warp as much as .010 inch out of round—an intolerable state of affairs for a high-performance engine like the Sabre. Sleeve failures were rampant until the firm implemented corrective measures. It required the intervention of the Ministry of Aircraft Production and the use of Bristol tooling and manufacturing techniques to finally get the Sabre out of the woods.
Desperately needed tooling was shipped over from the United States, but 250 of these precision tools were lost during shipment across the Atlantic when German U-boats sunk the transports carrying them. The situation became so desperate that six Sundstrand centerless grinders needed for sleeve manufacture were redirected from their original destination, Pratt & Whitney in East Hartford, Conn., and sent instead to Liverpool.
Serious thought was given to manufacturing the Sabre in the U.S., and to that end three Sabre Is were sent to Wright Field in Dayton, Ohio, for evaluation. One look at the complex Sabre put an end to those aspirations; an unqualified “No way,” or words to that effect, described the reactions of Wright Field personnel, who were nonetheless impressed by the engine’s design and workmanship.
In retrospect, the decision not to manufacture in America was a wise one. Although it could be argued that the Sabre personified engineering excellence, wars are not necessarily won by the most elegant equipment. When the Sabre was evaluated in 1941, the 18-cylinder R-2800 radial was being spat out like popcorn by Pratt & Whitney— and soon by the likes of Ford Motor Company, Nash Kelvinator and Chevrolet. So why introduce another comparable engine that was problematic, whereas the R-2800 was bulletproof, reliable and could be overboosted to develop in excess of 2,800 hp?
Napier’s Acton, London, facility initially produced the Sabre. In order to meet the forecast demand for this critical engine, a shadow factory in Walton, near Liverpool, was built.
A number of airplanes were designed around the Sabre, but only two, both products of Sidney Camm at the Hawker Air – craft Company, saw significant production. The Hawker Typhoon followed Camm’s penchant for thick wings with substantial load-carrying capability, which paid rich dividends when it was fitted with rockets for the ground attack role. Introduced in 1942, the Typhoon was beset with significant structural issues—foremost among them the failure of the rear monocoque fuselage. Only the skill of a Hawker test pilot, who suffered a rear fuselage failure but still managed to land his crippled aircraft, allowed the problem to be diagnosed.
The Typhoon came into its own in Normandy after the D-Day landings. When the Germans attempted a breakout at the Falaise Gap, it resulted in a turkey shoot for the cannon- and rocket-armed Typhoons. But the primitive conditions at advanced airstrips on the Continent played havoc with the sensitive Sabre. Sleeve wear in particular was a constant problem. In no time Napier turned out a so-called “momentum” air filter designed to keep out most of the dust that would normally be sucked into the ram air scoop induction system.
The Tempest, essentially an improved and updated Typhoon, was introduced in January 1944. It featured thinner wings, a bubble canopy and numerous other modifications.
Several one-offs and limited production aircraft also used the Sabre. Among them was the Martin-Baker MB.3, a single-engine fighter that took the life of company cofounder Captain Valentine H. Baker when the engine failed and he crashed into a tree. The ubiquitous Fairey Battle, a miserable excuse for a military aircraft, proved its worth as a test bed for various experimental engines, including the Sabre. Blackburn Aircraft designed the Firebrand Fleet Air Arm strike plane around the Sabre, although the few produced were powered by the Bristol Centaurus.
Sabre production was quickly curtailed after the war’s end, but its high-pitched scream could be heard into the early 1950s. Alas, notwithstanding the fact that thousands of Sabres were manufactured, not one has survived in airworthy condition. That sad state of affairs may change with the completion of Kermit Weeks’ Hawker Tempest, currently undergoing restoration for display at the Fantasy of Flight museum in Polk City, Fla. Weeks has acquired two Sabre engines, and hopes to eventually return his Tempest to the air. We’ll see.
Aero engine expert Graham White is the author of R-2800: Pratt & Whitney’s Dependable Master – piece and Allied Aircraft Piston Engines of World War II, which he suggests for further reading. Also see The High Speed Internal Combustion Engine, by Sir Harry Ricardo and J.G.G. Hempson.
Originally published in the July 2010 issue of Aviation History. To subscribe, click here.
