| Nomad | |
|---|---|
| Napier Nomad II | |
| Type | Turbo-compound aero-engine |
| National origin | United Kingdom |
| Manufacturer | Napier & Son |
| First run | October 1949 |
| Major applications | Avro Lincoln (test bed only) |
| Program cost | £5.1 million |
The Napier Nomad was a complex British compression-ignition aircraft engine designed and built by Napier & Son in 1949. Two versions were flight tested:
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In 1945 the Air Ministry asked for proposals for a new 6,000 horsepower (4,500 kW) class engine with good fuel economy. Curtiss-Wright was designing an engine of this sort of power known as the Turbo-compound engine, but Sir Harry Ricardo, one of Britain's great engine designers, suggested that the most economical combination would be a similar design using a diesel two-stroke in place of the Curtiss's petrol engine.
Before World War II Napier had licensed the Junkers Jumo 204 diesel design to set up production in the UK as the Napier Culverin, but the onset of the war made the Sabre all-important and work on the Culverin was stopped. In response to the Air Ministry's 1945 requirements Napier dusted off this work, combining two enlarged Culverins into an H-block similar to the Sabre, resulting in a massive 75-litre design. Markets for an engine of this size seemed limited, however, so instead they reverted to the original Sabre-like horizontally opposed 12 cylinder design, and the result was the Nomad.
The objective of the design was to produce a civilian power plant with far superior fuel efficiency to the emerging jet engine. Thermal efficiency is given by 1-(Tx/Tp), where Tx is the exhaust temperature (any absolute scale) and Tp is the peak combustion temperature. Jet engines always have a Tp of no more than about 1000 Kelvin, much less than the typical 5000 Kelvin of a reciprocating engine, and so jets have very poor thermal efficiency. The Nomad design focused on replacing the low temperature combustion chambers of the jet engine with highly efficient Diesel combustion chambers. In practice, it was much too difficult to couple the Diesel power output back into the turbine cycle. The maximum practical power of the Nomad was 4,000 horsepower (3,000 kW), and it was much heavier than a pure jet of the same power. By this time civilian jets such as the Boeing 707 were nearing completion, and the Nomad was never seriously considered by any aircraft manufacturer.
The initial Nomad design (E.125) or Nomad 3 was incredibly complex, essentially two engines in one. One was a turbo-supercharged diesel similar to the upper half of a Napier Sabre. Mounted below this was a complete turboprop engine, based on their Naiad design, the output of which was geared to a shaft driving the front propeller of a contra-rotating pair, as well as the axial compressor. To achieve higher boost, a centrifugal supercharger was used. This was driven by the crankshaft, which also provided the scavenging needed for starting this two-stroke engine from rest, before the turbine section was up to speed. As if that were not enough complexity, during takeoff additional fuel was injected into the rear turbine stage for more power, and turned off once the aircraft was cruising.[3]
The compressor and turbine assemblies of the Nomad were tested during 1948, and the complete unit was run in October 1949. The prototype was installed in the nose of an Avro Lincoln heavy bomber for testing: it first flew in 1950 and appeared at the Farnborough Air Display on 10 September 1951.[4] In total the Nomad I ran for just over 1,000 hours, and proved to be rather temperamental, but when running properly it could produce 3,000 horsepower (2,200 kW) and 320 lbf (1.4 kN) thrust. It had a specific fuel consumption (sfc) of 0.36 lb/(hp·h) (0.22 kg/(kW·h)).
The prototype Nomad I is on display at the National Museum of Flight at East Fortune Airfield in Scotland.
Even before the Nomad I was running, its successor, the Nomad II (E.145) Nomad 6, had already been designed. In this version an extra compressor stage was added, replacing the original supercharger. This stage was driven by an additional stage in the turbine, so the system was now more like a turbocharger and the compressed air for the Diesel was no longer "robbing" power for the turboprop. In addition the propeller shaft from the turbine was eliminated, and geared using a hydraulic clutch into the main shaft. The result was smaller and considerably simpler: a single engine driving a single propeller.[5] Overall about 1,000 lb (450 kg) was taken off the weight. The wet liners of the cylinders of the Nomad I were changed for dry liners.[6] While the Nomad II was undergoing testing, a prototype Avro Shackleton was lent to Napier as a testbed. The engine proved bulky, like the Nomad I before it, and in the meantime several dummy engines were used on the Shackleton for various tests.
A further development, the Nomad Nm.7, of 3,500 shp (2,600 kW) was announced in 1953.[7]
By 1954 interest in the Nomad was waning, and after the only other project, the Avro Type 719 Shackleton IV, based on it was cancelled, work on the engine was ended in April 1955, after an expenditure of £5.1 million.
A Nomad II is on display at the Steven F. Udvar-Hazy Center in Virginia.
Data from Flight 1954[8]
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