Napier Nomad

Napier & Son

Industry	Aerospace, Engine manufacturing
Founded	1808
Founder	Robert Napier
Headquarters	United Kingdom
Products	Aircraft engines (including Napier Nomad)

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The Napier Nomad represents a British diesel aircraft engine developed by Napier & Son in 1949. This innovative design integrated a piston engine with a turbine component to harness exhaust energy, thereby enhancing fuel economy. Two variants underwent testing: the intricate Nomad I, which featured two propellers each powered by separate mechanical stages, and the Nomad II, which employed the turbo-compound approach to link both sections to a single propeller shaft. Notably, the Nomad II achieved the lowest recorded specific fuel consumption figures of its era. Despite these advancements, the project was discontinued in 1955 after £5.1 million had been invested, as attention shifted predominantly toward turboprop technology.

Design and development

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On display at the Steven F. Udvar-Hazy Center

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Napier Nomad I

In 1945, the Air Ministry issued a request for proposals targeting a new 6,000 hp (4.5 MW) class engine prioritising high fuel efficiency. While Curtiss-Wright was developing a comparable powerplant based on the turbo-compound engine concept, Sir Harry Ricardo, a prominent British engine designer, proposed that superior efficiency could be achieved by substituting the Curtiss petrol engine with a diesel two-stroke configuration.

Prior to World War II, Napier had acquired a licence for the Junkers Jumo 204 diesel engine to manufacture it domestically as the Napier Culverin. However, wartime priorities shifted focus to the Napier Sabre, halting Culverin development. Responding to the Air Ministry’s 1945 specifications, Napier revisited this earlier work by merging two enlarged Culverin units into an H-block layout akin to the Sabre, resulting in an imposing 75-litre powerplant. Given limited market demand for such a large engine, Napier reverted to a horizontally opposed twelve-cylinder configuration resembling the Sabre’s flat engine design, culminating in the creation of the Nomad.

The design aimed to deliver a civilian propulsion system with substantially improved fuel economy relative to emerging jet engines. Thermal efficiency is expressed as <math>1 - T_e/T_p</math>, where T_e denotes exhaust temperature in kelvins and T_p represents peak combustion temperature. Jet engines operate with relatively low combustion temperatures—around 1,000 K—compared to approximately 5,000 K typical of reciprocating engines, resulting in inherently lower thermal efficiency. The Nomad sought to replace jet engines’ low-temperature combustion chambers with efficient diesel combustion systems. However, effectively integrating diesel output back into the turbine cycle proved highly challenging. The Nomad’s maximum feasible power output was approximately 4,000 hp (3.0 MW), and its weight exceeded that of an equivalent pure jet engine. By this stage, civilian jets such as the Boeing 707 were approaching operational readiness, and consequently, no aircraft manufacturers seriously pursued adoption of the Nomad design.

Nomad I

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Diagram illustrating the Napier Nomad I and II engines.

The original Nomad design, designated E.125 or Nomad 1, was an exceptionally intricate engine, essentially combining two engines into one unit. It featured a turbo-supercharged two-stroke diesel engine, somewhat reminiscent of half of a Napier Sabre's H-24 configuration. Positioned beneath this was the rotating assembly of a turboprop engine derived from the Naiad model, which powered the forward propeller in a contra-rotating propeller set. To enhance boost pressure, the crankshaft operated a centrifugal supercharger that also facilitated scavenging necessary for starting the engine from a standstill. During takeoff, extra fuel was injected into the rear turbine section to provide increased power, which was then discontinued once cruising speed was reached.

Testing of the Nomad’s compressor and turbine components took place in 1948, with the complete engine running for the first time in October 1949. The prototype was installed in the nose of an Avro Lincoln heavy bomber to conduct flight trials; it first flew in 1950 and was showcased at the Farnborough Air Display on 10 September 1951. In total, the Nomad I accumulated just over 1,000 operational hours and demonstrated some reliability challenges; however, when functioning optimally, it delivered approximately 3,000 hp (2.2 MW) along with 320 lbf (1.4 kN) of thrust. Its specific fuel consumption (sfc) was measured at 0.36 lb/(hp⋅h) (220 g/kWh).

The prototype Nomad I is preserved and exhibited at the National Museum of Flight located at East Fortune Airfield in Scotland.

Nomad II

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Front three-quarter perspective of a Nomad II

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View from above

Prior to the operational testing of the Nomad I, its successor—the Nomad II (E.145), also known as Nomad 6—had already been developed. This iteration incorporated an additional stage to the axial compressor/supercharger section, which eliminated the need for a separate centrifugal compressor and removed the intercooler. The turbine, now featuring an extra stage itself, was dedicated solely to driving the compressor and transferring any surplus power back to the main shaft through a Beier variable-ratio gear. The distinct propeller driven by the turbine was removed along with the entire "afterburner" system and its associated valve mechanisms. Consequently, this configuration functioned as a hybrid between a mechanical supercharger and a turbocharger, without requiring any bypass arrangement. This redesign resulted in a more compact and significantly less complex engine: a single unit powering one propeller. The overall weight decreased by approximately 1,000 lb (450 kg). Additionally, the cylinders’ wet liners used in the Nomad I were replaced with dry liners.

During testing of the Nomad II, Napier utilized a prototype Avro Shackleton as a flying test platform. Despite improvements, the engine remained cumbersome like its predecessor. Meanwhile, several non-operational mock-up engines were mounted on the Shackleton for various experimental purposes.

At comparable power outputs, the Nomad II achieved a specific fuel consumption of 0.327 lb/(hp⋅h) (199 g/kWh) at a cruise altitude of 25,000 ft (7,600 m).

In 1953, Napier announced an advanced version called the Nomad Nm.7 with an output rating of 3,500 shp (2.6 MW).

By 1954, enthusiasm for further development of the Nomad had diminished. Following cancellation of its sole intended application—the Avro Type 719 Shackleton IV—work on this engine ceased in April 1955 after £5.1 million had been invested. The design had also been considered for use in the Canadair Argus, a maritime patrol aircraft under development for the Royal Canadian Air Force, but that project ultimately selected the Wright R-3350, which was originally targeted by Napier’s design.

A preserved example of the Nomad II is exhibited at the Steven F. Udvar-Hazy Center in Virginia.

Applications

• Airspeed Ambassador (intended application only)
• Avro Lincoln (used solely as a test platform)
• Avro Shackleton (proposed application)
• Bristol Britannia (planned usage)
• Canadair CP-107 Argus (planned application)

References

• https://www.thegrowler.org.uk/avroshackleton/the-nomad-proposal.htm – The Nomad Proposal, researched by Chris Ashworth – cited among original sources in article history

External links

• Aircraft Engine Historical Society photo archive of Napier piston engines