Author Archives: William Pearce

Farman 18T engine

Farman 18T 18-Cylinder Aircraft Engine

By William Pearce

The rules of the Schneider Trophy Contest stated that any country that won the contest three consecutive times would retain permanent possession of the trophy. By 1930, Britain had two consecutive victories and were favored to win the next contest scheduled for September 1931. Frenchman Jacques P. Schneider had started the contest, and France won the first competition held in 1913. The possibility of losing the contest forever spurred France to action, and the STIAé (service technique et industriel de l’aéronautique, or the Technical and Industrial Service of Aeronautics) ordered at least five aircraft types and three different engines for the 1931 contest. One of the engines ordered was the Farman 18T.

Farman 18T engine

The Farman 18T was specifically designed for installation in the Bernard flying boat. The unusual 18-cylinder engine had no other known applications.

Avions Farman (Farman) was founded in 1908 by brothers Richard, Henri, and Maurice. In October 1917, the company moved to produce engines built under license to support the war effort. The first of these engines was built in mid-1918, and production stopped after World War I. In 1922, Farman started to design their own line of engines under the direction of Charles-Raymond Waseige.

The Farman 18T was designed by Waseige and had an unusual layout. The water-cooled engine had three cylinder banks, each with six cylinders. The left and right cylinder banks were horizontally opposed, with a 180-degree flat angle across the engine’s top side. The lower cylinder extended below the crankcase and was perpendicular to the other cylinder banks. This configuration gave the 18-cylinder engine a T shape.

The engine used a two-piece cast aluminum crankcase that was split vertically. Steel cylinder liners were installed in the cast aluminum, monobloc cylinder banks that were bolted to the crankcase. The four valves of each cylinder were actuated via pairs of rockers by a single overhead camshaft. Each camshaft was driven by a vertical shaft at the rear of the engine.

The 18T used aluminum pistons and had a compression ratio of 6.0 to 1, although some sources say 8.5 to 1. The connecting rods consisted of a master rod for the lower cylinder bank and two articulated rods for the left and right cylinder banks. Each cylinder had two spark plugs, one installed in each side of the cylinder bank. The spark plugs were fired by magnetos driven from the rear of the engine. A nose case at the front of the engine contained the Farman-style bevel propeller reduction gear that turned the propeller at .384 crankshaft speed.

Farman 18T Paris Air Show 1932

The 18T (lower left) was proudly displayed as part of the Farman exhibit at the Salon de l’Aéronautique in November 1932. Note that the supercharger housing extended above the crankcase, which was otherwise the engine’s highest point.

For induction, air passed through carburetors at the rear of the engine and into a centrifugal supercharger that provided approximately 4.4 lb (.3 bar) of boost. The air/fuel mixture flowed from the supercharger into an intake manifold for each cylinder bank. The intake manifolds ran along the bottom of the cylinder bank for the left and right banks and along the right side (when viewed from the non-propeller end) of the lower cylinder bank. The exhaust ports were on the opposite side of the cylinder head from the intake.

The 18T had a 4.72 in (120 mm) bore and stroke. The engine displaced 1,491 cu in (24.4 L) and produced a maximum of 1,480 hp (1,104 kW) at 3,700 rpm. The 18T was rated at 1,200 hp (895 kW) at 3,400 rpm for continuous output. The engine was 65.98 in (1.68 m) long, 44.65 in (1.13 m) wide, 32.56 (.83 m) tall, and weighed 1,069 lb (485 kg).

Two Farman 18T engines were ordered under Contract (Marché) 289/0 (some sources state Marché 269/0) issued in 1930 and valued at 3,583,000 Ғ. The two engines were to power a flying boat built by the Société des avions Bernard (Bernard Aircraft Company). An official designation for the flying boat has not been found, and it was not among the known aircraft ordered for the 1931 Schneider Contest. There is some speculation that a lack of funds prevented the aircraft from being ordered for the 1931 race, but it would be ordered in time for the 1933 race.

Farman 18T Paris Air Show 1932 display

The display at the air show in Paris announced the 18T’s 1,200 hp (895 kW) continuous rating.

The design of the Bernard flying boat was led by Roger Robert and developed in coordination with the 18T engine. The all-metal aircraft had a low, two-step hull with sponsons protruding from the sides, just behind the cockpit. A long pylon above the cockpit extended along the aircraft’s spine, and the pylon supported the engine nacelle and wings. The engines were installed back-to-back in the middle of the nacelle. The engines’ lower cylinder banks extended into the pylon, and the left and right cylinder banks extended into the cantilever wings, which were mounted to the sides of the nacelle. Surface radiators for engine cooling covered the sides of the pylon, and extension shafts connected the propellers to the engines. The aircraft had a 36 ft 1 in (11.0 m) wingspan and was 35 ft 5 in (10.8 m) long. The engine nacelle was 17 ft 1 in (5.21 m) long. A 12.5 to 1 scale model of the flying boat was tested at the Laboratoire Aérodynamique Eiffel (Eiffel Aerodynamics Laboratory) in Auteuil (near Paris), France.

The 18T engines were bench tested in 1931, but the most power achieved was only 1,350 hp (1,007 kW). While further development was possible, at the time, the chance of France fielding a contestant in the 1931 Schneider Contest was virtually non-existent. The chances of the Bernard flying-boat being built were even worse. Although the aircraft had an estimated top speed of over 435 mph (700 km/h), and a detailed study was submitted to the Service Technique (Technical Service), the flying boat was seen as too radical and was never ordered. The limited funds were needed for the more conventional racers.

The Supermarine S.6B went on to win the 1931 Schneider Contest, giving the British permanent possession of the trophy. The 18T was marketed in 1932 and displayed at the Paris Salon de l’Aéronautique (Air Show) in November. However, there was little commercial interest in the 18T, and the project was brought to a close without the engine ever being flown; most likely, full testing was never completed.

Bernard - Farman 18T Schneider 3-view

Powered by two 18T engines, the Bernard flying boat racer had an estimated top speed of over 435 mph (700 km/h). This speed was substantially faster than the Supermarine S.6B that won the 1931 Schneider race at 340.08 mph (547.31 km/h) and went on to set an absolute speed record at 407.5 mph (655.8 km/h). However, the estimated specifications of unconventional aircraft often fall short of what is actually achieved.

Sources:
Aerosphere 1939 by Glenn D. Angle (1940)
Les Moteurs a Pistons Aeronautiques Francais Tome 1 by Alfred Bodemer and Robert Laugier (1987)
Schneider Trophy Seaplanes and Flying Boats by Ralph Pegram (2012)
Les Avions Bernard by Jean Liron (1990)
Les Avions Farman by Jean Liron (1984)

Napier-Deltic-T18-37K-Marine-Engine

Napier Deltic Opposed-Piston Diesel Engine

By William Pearce

In 1933, the British engineering firm D. Napier & Son (Napier) acquired licenses to produce the Junkers Jumo 204 and 205 aircraft engines. Napier sought to diversify and expand its aircraft engine business, and the company felt the two-stroke, opposed-piston, diesel engines would usher in an era of safe and fuel-efficient air travel. Napier made some modifications to the Jumo engines, but the internal components were mostly unchanged. The Jumo 204 was built as the Napier Culverin (E102), and the Jumo 205 was planned as the Napier Cutlass (E103). The Culverin was first run on 24 September 1934, but the engine garnered little interest and no orders. By 1936, after only seven Culverins were made and no Cutlasses, Napier halted further work on opposed-piston diesel aircraft engines. English Electric took over Napier in November 1942.

Napier-Deltic-E130-Three-cylinder-test-engine

The Napier E130 three-cylinder test engine that validated the triangular engine arrangement. Each of the engine’s crankshafts had a flywheel on the drive end (left). The six intake chamber openings are visible on the free (non-drive) end (right). Note the vertical coolant pipes on top of the engine. (Napier/NPHT/IMechE images)

In 1944, the British Admiralty desired to increase the survivability of the Motor Torpedo Boat (MTB). One of the main issues was that MTBs used gasoline engines. Gasoline liquid is highly flammable, and gasoline vapor is highly explosive. MTB safety would be improved if a switch to diesel engines could be made. Diesel fuel has non-explosive characteristics and a much higher flashpoint than gasoline. However, at the time, there were no suitable diesel engines to power MTBs.

Around 1945, Napier and other companies submitted proposals to the Admiralty for a light-weight, powerful, and compact 18-cylinder diesel engine. Napier’s new engine carried the company designation E130, and the design was influenced by their experience with the Junkers Jumo diesel engines, their work on the Culverin and Cutlass, and analyses of other Jumo six-cylinder engines captured during World War II. However, there is no mention of the Junkers Jumo 223 contributing to Napier’s engine design. In early 1946, the Admiralty selected the Napier design and issued a developmental contract that covered the construction of one single-cylinder test engine, one three-cylinder test engine, and six prototype 18-cylinder engines.

Napier-Deltic-drive-end-section

Section drawing from the drive end of a Deltic engine. The air chamber surrounds the intake end of the cylinder, and the exhaust manifolds are mounted to the outer sides of the engine. Note the rotation of the crankshafts. (Napier/NPHT/IMechE image)

Napier’s liquid-cooled, two-stroke engine used opposed-pistons, a design feature that eliminated many parts, required no cylinder head, improved thermal efficiency, and resulted in more power for a given size and weight. In an opposed-piston engine, each cylinder has two pistons that move toward each other to form a single combustion space near the center of the cylinder. Ports in the cylinder wall that are covered and uncovered by the pistons bring in air and allow exhaust gases to escape. The most unusual aspect of Napier’s design was that the engine was formed as an inverted triangle, with a crankshaft at each corner. Because of its triangular structure, the name Deltic was selected in reference to the Greek letter Delta, and the 18-cylinder engine was known as the Deltic D18 (or just 18). The triangular design resulted in a compact engine with a very rigid structure.

Design work on the Napier Deltic started under Ben Barlow, George Murray, and Ernest Chatterton, Chief Engineer of the Piston Engine Division at Napier. The project was initially overseen by Henry Nelson, with Herbert Sammons taking over in 1949. The Deltic engine formed an equilateral triangle with each of its three cylinder banks angled at 60 degrees. Cast aluminum crankcase housings were at each corner of the triangle, with the lower crankcase incorporating an oil sump and also serving as the engine’s base. Each cast aluminum cylinder bank was sandwiched between two crankcases via through bolts. The monobloc cylinder banks were identical, as were the upper two crankcases. However, various ancillary components were installed according to the casting’s position on the complete engine.

Napier-Deltic-18-Triangle-Case

The assembled cylinder banks and crankcases of an 18-cylinder Napier Deltic engine seen from the free end. Note the open space between the cylinder banks. The stadium (oval) ports are to the air chambers. The bushings visible in the upper crankcases, at the triangle’s corners, supported the shafts that drove the blower. (Napier/NPHT/IMechE image)

The forged-steel cylinder wet liners were open-ended and had a chrome-plated bore to reduce wear. Part of the bore was etched with small dimples to retain lubricating oil and reduce piston ring wear. The liner was approximately 32 in (813 mm) long and protruded some distance into the crankcases. The ends of the liner were notched to allow clearance for the swinging connecting rods. Near one end of the liner were 14 intake ports with a tangential entry to impart a swirling motion of the incoming air. The swirling air helped scavenge the cylinder through the nine exhaust ports near the other end of the liner. In each cylinder, one piston would cover and uncover the intake ports while the other piston would do the same for the exhaust ports. The exhaust ports were uncovered (opened) 34.5 degrees before the intake ports. Both sets of ports were uncovered (open) for 101.5 degrees, and the intake ports were uncovered (open) for 5.5 degrees after the exhaust ports were covered (closed). The placement of the intake and exhaust ports at opposite ends of the cylinder liner allowed for uniflow scavenging of the cylinder. The liners were shrink-fitted into the cylinder banks and secured by an annular nut on the intake side.

The two-piece pistons consisted of a cast aluminum outer body and a forged Y-alloy (nickel-aluminum alloy) inner member that held the wrist pin. The inner member was heat-shrunk to the outer piston body and secured by a large circlip. Oil flowed between the two pieces to cool the piston. Three compression rings were positioned just below the piston crown, and two oil scraper rings were located near the bottom of the piston skirt. The pistons were attached to fork-and-blade connecting rods, with the exhaust pistons mounted to the forked rods and the intake pistons mounted to the blade rods. The opposed pistons created a compression ratio of 17.5 to 1 (some sources say 15 to 1).

Napier-Deltic-assembly

Napier Deltic engine assembly, with phasing gear housings being built up in the lower right. At left is a completed phasing gear housing; note the two idler gears connecting the lower crankshaft to the central output shaft. Toward the center are Deltics in various stages of assembly. A completed engine without its blower installed is in the upper right. Note the opening in the center of the engine. (Napier/NPHT/IMechE image)

A two-piece phasing gear housing at the drive end of the engine contained the gears that connected the crankshafts to the main output shaft. The main output shaft was usually located at the center of the engine, but different phasing gear housings allowed for different output shaft locations. Each crankshaft was coupled to its drive gear via a short, flexible quill shaft. When viewed from the free (non-drive) end of the engine, the upper two crankshafts rotated clockwise and were connected to the main output shaft via one idler gear. The lower crankshaft rotated counterclockwise and was connected to the main output shaft via two idler gears. The idler gears could be repositioned to reverse the rotation of the output shaft. Each crankshaft was supported in its crankcase by seven main bearings, and each main bearing cap was secured by four studs and two transverse bolts. The crankshafts were phased so that the exhaust piston in each cylinder led the intake piston by 20 degrees. The reverse rotation of the lower crankshaft, and the crankshaft phasing was devised by Herbert Penwarden from the Admiralty Engineering Laboratory.

Via a quill shaft and bevel gears, each crankshaft also drove a camshaft for the fuel injection pumps. The camshaft was located in a housing bolted to the outer side of each cylinder bank, near its center. Each camshaft operated six fuel injection pumps, and each pump fed fuel to two injectors per cylinder. The timing of the pumps changed depending on engine RPM. The upper two crankshafts drove separate flexible drive shafts for the blower (weak supercharger). The driveshafts were positioned at the upper, inner corners of the engine triangle. They led to the opposite end of the engine and powered a single-stage, double-sided centrifugal blower. The impeller was 15.5 in (394 mm) in diameter and rotated at 5.72 times crankshaft speed, creating 7.8 psi of boost (.53 bar). The pressurized air from the blower was fed into a chamber that extended through each cylinder bank and that surrounded the intake ports in the cylinder liner. Exhaust gases were collected via a water-cooled manifold that attached to the outer side of each cylinder bank. The lower crankshaft drove a flexible drive shaft to power the engine’s two oil and two water pumps.

Napier-Deltic-T18-37K-sections-display

Basic sections of the Deltic (T18-37K) marine engine. From left to right are the blower section (turbo-blower in this case), D18-cylinder engine section, phasing gear housing, and the bi-directional gearbox. The Deltic was a powerful diesel engine for its size and weight. (Napier/NPHT/IMechE image)

When viewing the engine from the free end, the cylinder banks were designated as follows: left was Bank A; upper, horizontal was Bank B; and right was Bank C. The crankshafts were designated as follows: upper left was Crankshaft AB, upper right was Crankshaft BC, and lower was Crankshaft CA. The cylinder rows were numbered with Bank 1 at the free end, and subsequent banks were numbered consecutively with Bank 6 at the drive end. The Deltic D18’s firing order was Bank C cylinder 1 (C1), A6, B1, C5, A1, B5, C3, A5, B3, C4, A3, B4, C2, A4, B2, C6, A2, and B6.

The Napier Deltic had a 5.125 in (130 mm) bore and a 7.25 in (184 mm) stroke (x2). This gave each cylinder a displacement of 299 cu in (4.9 L), and the 18-cylinder engine displaced 5,384 cu in (88.2 L). The bare engine (without the bi-directional marine gearbox) had a maximum, 15-minute output of 2,730 hp (2,036 kW) at 2,000 rpm with a specific fuel consumption (sfc) of .380 lb/hp/hr (231 g/kW/h). The Deltic’s continuous rating was 2,035 hp (1,517 kW) at 1,700 rpm with a sfc of .364 lb/hp/hr (221 g/kW/h). With the bi-directional gearbox, the engine produced 2,500 hp (1,864 kW) at 2,000 rpm with a sfc of .415 lb/hp/hr (252 g/kW/h) and 1,875 hp (1,398 kW) at 1,700 rpm with a sfc of .395 lb/hp/hr (240 g/kW/h). The Deltic D18 was 105 in (2.67 m) long, 71.25 in (1.81 m) wide, and 80 in (2.03 m) tall. The bi-directional gearbox added another 36 in (.91 m). The engine weighed 8,860 lb (4,018 kg) without the bi-directional gearbox and 10,500 lb (4,763 kg) with it.

The single-cylinder test engine was designed from October to December 1946, with the three-cylinder engine following from January to May 1947. Testing of these engines started as soon as construction was completed. The three-cylinder engine represented just one row of a Deltic engine, but it demonstrated the validity of the components used in the triangular arrangement.

Napier-Deltic-D18-E130-Prototype

Free end of the 2,500 hp (1,864 kW) Deltic D18-1 (E130) prototype engine. Note the two intakes, one for each side of the double-sided blower. Each cylinder bank had two, large exhaust manifolds. The transverse bolts threaded into the main bearings can be seen on the side of the upper crankcase. (Napier/NPHT/IMechE image)

The first 18-cylinder Deltic Series I engine was assembled by March 1950. The engine was soon to be tested at Napier’s works in Acton, England; however, a cable broke as the engine was being mounted to the stand. It fell on the stand, damaging the engine and the test stand. Repairs were made, and engine began testing in April 1950. The 18-cylinder Deltic fired a cylinder every 20 degrees of crankshaft rotation, which resulted in smooth, nearly-constant output torque. Engine idle was around 600 rpm, and the Deltic demonstrated a gross mechanical efficiency of 85.5% at 2,000 rpm. In late 1951, two Deltics were installed in place of the three Mercedes-Benz MB 501 V-20 engines in a former German E-boat S-212 (redesignated Fast Patrol Boat P5212). By January 1952, the originally-contracted six Deltic D18 engines had been built. In 1953, an Admiralty 1,000-hr type test was completed and indicated the engine could run 2,000 hours between overhauls.

By 1954, Napier was offering a commercial version of the Deltic D18 Series I (E169). This was basically a de-rated engine. The commercial engine produced 1,900 hp (1,417 kW) at 1,500 rpm with a sfc of .363 lb/hp/hr (221 g/kW/h) and could operate for 5,000 hours between overhauls. In addition to a variety of marine applications, Deltic engines could also run power generation sets, water pumps, and be used to power traction motors for locomotives. Napier also built a nine-cylinder version with three banks of three cylinders. The Deltic 9 (E159/E165) displaced 2,692 cu in (44.1 L) and had a one-sided centrifugal blower but was otherwise of the same construction as the Deltic D18. It fired one cylinder for every 40 degrees of crankshaft rotation. Maximum output for the Deltic 9 was 1,250 hp (932 kW) at 2,000 rpm for the high-power version and 950 hp (708 kW) at 1,500 rpm for the commercial version. By late 1955, Deltic test and production engines had accumulated over 20,000 hours of operation.

Napier-Deltic-C18-5-Compound-Marine-Engine

The 5,500 hp (4,101 kW) compound Deltic C18 (E185) engine was the most powerful piston engine Napier ever built. Although it is covered, the intake can be seen in the upper part of the phasing gear housing. Exhaust was routed through the three-stage turbine, which powered the eight-stage compressor inside the engine’s triangle. (Napier/NPHT/IMechE image)

In 1956, Napier built a compound diesel engine known as the Deltic C18 (E185). Serious development of the C18 occurred after the Napier Nomad II compound diesel aircraft engine was cancelled in 1955. The Deltic C18 had an eight-stage (some sources say 12-stage, which was the same number of stages as used in the Nomad II) axial compressor positioned inside the engine triangle. The compressor was driven by a three-stage turbine, which was powered by the engine’s exhaust gases. The turbine was positioned in the normal blower position on the free end of the engine. A new phasing gear housing was constructed with an opening that allowed air into the center of the engine triangle and served as the inlet for the compressor. The Deltic C18 produced 5,500 hp (4,101 kW) at 2,000 rpm. The engine was 124 in (3.15 m) long, 65 in (1.65 m) wide, and 77 in (1.96 m) tall. The C18 weighed approximately 10,700 lb (4,853 kg). The engine was tested in 1957, but only one experimental C18 was built. While undergoing power tests, a connecting rod failed at 5,600 hp (4,176 kW). The rod came through the crankcase, but the damage was never repaired due to the Navy’s increased focus on gas turbine engines.

By 1956, Napier had introduced some minor changes as the Series II Deltic engines, but one major change was the addition of a turbo-blower. These engines were known as turbo-blown, and they were designated as the Deltic T18 (E171/E239). Exhaust gases were collected and fed into an axial-flow turbine mounted behind the blower. The turbine wheel was 18.04 in (458 mm) in diameter and helped turn the blower via a geared shaft. The turbine wheel turned at .756 times the speed of the blower impeller. The blower was still driven by the upper crankshafts, but it now turned at 8.266 times crankshaft speed. The turbo-blower created 19 psi (1.31 bar) of boost. The piston was redesigned and consisted of three-pieces: a Hidural 5 (copper alloy) crown that screwed onto an aluminum skirt to form the outer body, and a Y-alloy (nickel-aluminum alloy) inner member that held the wrist pin. A third scraper ring was added to the piston skirt. The compression ratio was increased to 17.9 to 1, and the engine used one fuel injector per cylinder. The Deltic T18 had an output of 3,100 hp (2,312 kW) at 2,100 rpm and 2,400 hp (1,641 kW) at 1,800 rpm. SFC was .414 lb/hp/hr (252 g/kW/h) and .404 lb/hp/hr (246 g/kW/h) respectively. The engine was 118 in (3.00 m) long, 75 in (1.91 m) wide, and 84 in (2.13 m) tall. The T18 weighed around 13,630 lb (6,183 kg) with the bi-directional gearbox and 11,050 lb (5,012 kg) without it. The turbo-blown nine-cylinder Deltic T9 (E172/E198) produced 1,100 hp (820 kW) at 1,600 rpm.

Napier-Deltic-T18-37K-Marine-Engine

The 3,100 hp (2,312 kW) turbo-blown Deltic T18-37K (E239) engine was most widely used in Motor Torpedo Boats. Note the exhaust manifolds leading to the turbine with its large intake at the rear of the engine. The short duct connecting the blower to the upper cylinder bank is visible. (Napier/NPHT/IMechE image)

More changes were incorporated into the Series III engines, which also introduced charge-cooling with the Deltic CT18 (E263) in 1966. For the CT18, a single drive shaft passed through the center of the engine to deliver power from the phasing gear housing to the turbo-blower. The shaft turned at 5.16 times crankshaft speed, and both the blower impeller and turbine wheel were mounted to the drive shaft. The single-sided blower impeller was relocated to behind the turbine wheel. A water-filled aftercooler was mounted before each opening of the engine’s three air compartments. The aftercooler dropped the charge temperature from 259° F (126° C) to 144° F (62°C). Pistons were again redesigned, with the Hidural 5 (copper alloy) crown bolting to the aluminum skirt. For the Deltic CT18, power increased to 3,700 hp (2,759 kw) at 2,100 rpm with a sfc of .403 lb/hp/hr (245 g/kW/h) and 2,750 hp (2,051 kW) at 1,800 rpm with a sfc of .395 lb/hp/hr (240 g/kW/h). By 1968, further development had increased the output to 4,000 hp (2,983 kW) at 2,100 rpm with a sfc of .401 lb/hp/hr (244 g/kW/h) and 3,000 hp (2,237 kW) at 1,800 rpm with a sfc of .399 lb/hp/hr (243 g/kW/h). The CT18 weighed 15,382 lb (6,977 kg) with its bi-directional gearbox.

As Napier declined in the late 1960s, English Electric moved Deltic production to the newly acquired Paxman Engine Division. The General Electric Company (GEC, not related to the US company General Electric / GE) purchased English Electric in 1968. What was once Napier basically closed in 1969. In 1975, GEC reformed Paxman Engine Division as Paxman Diesels Limited. Paxman continued to support Deltic engines, developing the CT18 to 4,140 hp (3,087 kW) in 1978 and reworking the mechanically-blown Deltic 9 for production as the D9-59K (E280) in the early 1980s. The D9-59K was constructed almost entirely with non-ferrous (non-magnetic) parts for mine-sweeper duties. In 2000, MAN acquired what used to be Paxman, and Rolls-Royce was awarded a contract to support Deltic engines in 2001. The contract was carried through until 2012, but it is not clear if the contract was extended beyond that year.

Napier-Deltic-CT18-42K-Charge-Cooled-engine

A 3,700 hp (2,759 kw) charge-cooled and turbo-blown Deltic CT18-42K (E263) engine. The turbine is located between the engine and the blower. Note the large, square aftercooler in the air duct between the blower and the engine. (Napier/NPHT/IMechE image)

Deltic engines powered a number of various MTBs, including the Royal Navy’s Dark-class (18 produced). Two 3,100 hp (2,312 kW) Deltic C18 turbo-blown engines powered each Nasty-class / Tjeld-class fast patrol boat (total of 49 built), which were designed in 1959 and put in service in 1960. These boats served with the navies of Norway, the United States, Greece, Germany, and Turkey. The boats had a top speed of 52 mph (83 km/h), and some were in service until the 1990s. Deltic engines powered Ton-class minesweepers (over 100 built) as well as the pulse generators for other minesweepers. Deltics were still being installed in new military boats during the 1980s, with the 1,180 hp (880 kW) Deltic T9-powered Hunt-class minesweepers (13 built) still in service. A few commercial vessels were also powered by Deltic engines—the largest installation was four 1,850 hp (1,380 kW) engines for the 513.5-ft (156.5-m) ore carrier Bahama King in 1958.

In 1955, two 1,650 hp (1,230 kW) Deltic D18-12 (E158) engines were used in the English Electric DP1, a prototype diesel-electric locomotive. The engines powered six English Electric EE829-1A traction motors that gave the locomotive 50,000 lbf (222.4 kN) of tractive effort. The DP1 proved successful, resulting in 22 British Rail Class 55 locomotives powered by Deltic D18-25 (E169) engines being built in the early 1960s. Called Deltics, these locomotives could exceed 110 mph (177 km/h) and were in service until the early 1980s. One 1,100 hp (919 kW) Deltic T9-29 (E172) engine was used in each of the smaller British Rail Class 23 locomotives, known as Baby Deltics. The engine powered four English Electric traction motors that gave the locomotive 47,000 lbf (209.1 kN) of tractive effort. The Baby Deltics entered service in 1959, but they were not as successful as their bigger counterparts due to shorter runs and frequent stops. All Baby Deltics were withdrawn from service by 1971.

Napier-Deltic-CT18-Charge-Cooled-cutaway

Cutaway view of a Deltic CT-18 charge-cooled and turbo-blown engine. Note the shaft through the center of the engine that powered the turbo-blower from the phasing gear. (Napier/NPHT/IMechE image)

Other Deltic designs included a 735 hp (548 kW) inline six-cylinder (E164/E197) with one bank of six cylinders and a 1,420 hp (1,059 kW) 15-cylinder (E162) with three banks of five cylinders, but these engines were not built. A 24-cylinder square engine (E260) with four crankshafts and four banks of six cylinders was also designed for an output of 5,400 hp (4,027 kW). The square engine design had much more in common with the Deltic than the Jumo 223, but it was not constructed. Including the nine-cylinder version, over 600 Deltic engines were made. A number of Deltic engines survive. Some are still operational in preserved boats or locomotives, allowing the unusual roar of the triangular two-stroke Deltic to still be heard. Others engines are in various museums, and a few are privately owned.

Note: In some cases, the Napier E number is one example of the type, with additional E numbers existing for similar engines with different configurations (marine vs rail applications). Around 100 E numbers were assigned to various Deltic designs.

Napier-Deltic-T9-33-Locomotive-Rraction-Engine

A 1,250 hp (932 kW) turbo-blown nine-cylinder Deltic T9-33 (E198) under test at Napier’s factory in Acton. The engine was similar to those used in the Baby Deltic Locomotives. Note the low position of the output shaft. (Napier/NPHT/IMechE image)

Sources:
“The Napier Deltic Diesel Engine” by Ernest Chatterton, SAE Transactions Vol 64 (1956)
Opposed Piston Engines by Jean-Pierre Pirault and Martin Flint (2010)
Course Notes on the Deltic Engine Type T18-37K by D. Napier & Son Ltd. (December 1967)
“Development of the Napier Deltic Charge Cooled Engine” by R. P. Taylor and C. H. Davison, Proceedings of the Institution of Mechanical Engineers Vol 183 (1968–69)
By Precision Into Power by Alan Vessey (2007)
Napier Powered by Alan Vessey (1997)
https://www.ptfnasty.com/ptfDeltic.html
http://www.npht.org/deltic/4579702653

Fairbanks Morse Diamond stress test

Fairbanks Morse Diamond Opposed-Piston Marine Engine

By William Pearce

In the early 1930s, Fairbanks Morse & Company (FM) took an interest in two-stroke, opposed-piston, diesel engines, and they acquired a license to produce a design originally developed by the German firm Junkers. In an opposed-piston engine, each cylinder has two pistons that move toward each other to form a single combustion space near the center of the cylinder. Ports in the cylinder wall bring in air and allow exhaust gases to escape. The opposed-piston design offers some advantages over conventional engines by having fewer parts, no cylinder head, improved thermal efficiency, and more power for a given size and weight.

Fairbanks Morse 38E 5.25

The Fairbanks Morse 38E5-1/4 had characteristics common to other 38-series opposed-piston engines and was a basis for the 24-cylinder Diamond engine. (Fairbanks Morse image)

FM used the information acquired from Junkers to develop its own line of opposed-piston diesel engines. One of the first opposed-piston engines produced by FM was the Model 38, which was a two-stroke vertical engine with two crankshafts linked initially by a gear train, which was soon replaced by a drive chain. In its 38A8 form, the engine had eight cylinders with an 8 in (203 mm) bore and a 10 in (254 mm) stroke (x2). The 38A8 displaced 8,042 cu in (131.8 L) and produced 1,200 hp (895 kW) at 720 rpm. In December 1934, the United States Navy ordered eight 38A8 engines—four each for the USS Plunger (SS-179) and USS Pollack (SS-180) Porpoise-class submarines. Problems with the 38A8s led to a redesign, ultimately creating the 38D8 engine.

In 1937, FM upgraded the 38D8 to produce more power. The drive chain linking the two crankshafts was replaced with a vertical shaft and bevel gears. The bore was increased by .125 in (3 mm) to 8.125 in (206 mm), and cylinders were added to create 9- and 10-cylinder engines. The new engine was designated 38D8-1/8. With the larger bore and 10 cylinders, the engine displaced 10,370 cu in (169.9 L) and produced 1,600 hp (1,193 kW) at 720 rpm. Approximately 1,650 38D8-1/8 engines were built during World War II. The engine was eventually offered with 4, 5, 6, 8, 9, 10, and 12 cylinders and with or without turbocharging. Although changes have been incorporated over the years, the FM OP 38D8-1/8 remains in production today.

In 1939, FM developed a scaled-down version of the 38D to be used as an auxiliary power unit. This engine was designated 38E5-1/4, and it had a 5.25 in (133 mm) bore and a 7.25 in (184 mm) stroke (x 2). The engine was available with three, five, or seven cylinders. The 7-cylinder 38E5-1/4 displaced 2,197 cu in (36.0 L) and produced 467 hp (348 kW) at 1,200 rpm. Around 630 38E5-1/4 engines were built during World War II.

Fairbanks Morse Diamond sectional

Sectional drawing of the Fairbanks Morse Diamond engine shows the arrangement of its four crankshafts and opposed-piston cylinders. The output shaft is drawn with a six-hole flange and is just below the center of the engine. (Fairbanks Morse image)

Based on the development of the Model 38-series, the Navy approached FM in early 1940 with a request to design and build a 3,000 hp (2,237 kW) opposed-piston engine for submarine use. With the prospect of war looming on the horizon, FM quickly went to work on the new engine design and assigned Robert Beadle as the program’s head engineer. The engine borrowed the basic cylinder design from the 38E5-1/4, but the engine was of a diamond configuration with a crankshaft at each corner. This gave the engine four banks of six opposed-piston cylinders resulting in a total of 24 cylinders.

The FM Diamond engine was of welded steel construction, with the crankcase and four cylinder banks forming a single unit. The lower and upper bank angles were 60 degrees. The left and right bank angles were 120 degrees. A cover concealed each crankshaft, and crankshaft removal allowed access to the cylinder liners. Each forged steel crankshaft was supported by seven main bearings.

The fork-and-blade connecting rods were made from steel forgings and then polished for added strength. The rods were drilled to deliver oil from the crankshaft to the wrist pin and to the underside of the piston crown for cooling. The pistons had a concave crown and formed a somewhat hemispherical combustion space when the two pistons came together. The two-piece pistons were made of cast steel with an aluminum wrist pin carrier.

The cylinder liners were made of forged steel and had a chrome-plated bore. A water jacket was pressed on each liner’s center section, where combustion occurred. Intake and exhaust ports were cast into the cylinder liners, and movement of the pistons covered and uncovered these ports. The upper and lower crankshafts were connected to the “exhaust” pistons that controlled the exhaust ports, and the left and right crankshafts operated the “intake” pistons controlling the intake ports. The crankshafts were phased so that the exhaust pistons (upper and lower crankshafts) led the intake pistons (left and right crankshafts) by about 15 degrees. This allowed for good cylinder scavenging, with the exhaust ports being uncovered (open) before the intake ports and with the intake ports remaining uncovered (open) for a short time after the exhaust ports had been covered (closed).

Fairbanks Morse Diamond stress test

The welded crankcase of the Diamond engine undergoing stress tests before final assembly. The crankshafts and pistons are installed, and the output shaft is visible just below the engine’s center. Note the mounting pads at the top of the engine for the two centrifugal blowers. The blowers fed air into the center of the engine via the two large holes. (Fairbanks Morse image)

The upper crankshaft drove two gear-driven centrifugal blowers (weak superchargers) mounted to the drive end of the engine. The blowers forced air into a central chest inside of the engine diamond. Four compartments, one for each bank, surrounded the intake end of the cylinders and supplied air from the chest. The intake ports in the cylinder liner were tangentially cast so that the incoming air initiated a swirling motion as it entered the cylinder. This swirl helped scavenge the cylinder of exhaust gases and mix the fuel once it was injected. The exhaust end of each cylinder was surrounded by an open passageway that led outside of the engine. A water-cooled exhaust manifold made of welded steel was attached to the side of the engine and collected the exhaust gases.

Each of the left and right crankshafts drove an upper and lower camshaft. The camshafts actuated individual fuel injector pumps for the single fuel injector in each cylinder. The fuel injector was located in the center of the cylinder liner. Fuel was injected into the cylinder at approximately 3,000 psi (207 bar). All of the crankshafts were geared to a single output power shaft, located 13.75 in (349 mm) below the engine’s absolute center. The left, right, and lower crankshafts were each connected to the output shaft via one idler gear. The upper crankshaft was geared to the output shaft through three idler gears. The gears used herringbone teeth. Pressurized air fed through internal piping was used to start the engine.

In designing the engine, FM engineers spent over 6,000 man-hours on torsional vibration calculations alone. The FM Diamond engine was completed in 1942. It had a 5.25 in (133 mm) bore and a 7.25 in (184 mm) stroke (x 2). The engine’s total displacement was 7,533 cu in (123.4 L). The engine was 120 in (3.05 m) tall and 72 in (1.83 m) wide when bare, or 141.5 in (8.73 m) tall and 79.25 in (24.16 m) wide when mounted to its steel stand. Its length was approximately 90 in (27.43 m).

During testing, the Diamond engine produced 3,000 hp (2,237 kW) at 1,500 rpm with 6.88 psi (.47 bar) of scavenging pressure. At this power, the specific fuel consumption was .420 lb/hp/hr (255 g/kW/h). However, the engine experienced constant issues with excessive wear and carbon build-up in the intake and exhaust ports. The program was cancelled at the end of World War II. At the time of cancellation, the experimental Diamond engine had accumulated 2,032 hours of test running.

Fairbanks Morse Diamond test stand

The engine undergoing bench tests. Note the two centrifugal blowers providing air for scavenging and combustion. (Fairbanks Morse image)

Sources:
“Development of Diamond Opposed-Piston Diesel Engine” by R. H. Beadle (discussion of “The Napier Deltic Diesel Engine”) SAE Transactions Vol 64 (1956)
Opposed Piston Engines by Jean-Pierre Pirault and Martin Flint (2010)
Diesels for the First Stealth Weapon: Submarine Power 1902–1945 by Lyle Cummins (2007)
Submarine Main Propulsion Diesels: NavPers 16161 (June 1946)
http://www.dieselduck.info/machine/01%20prime%20movers/fairbanks_morse/fairbanks_morse.htm

Napier Nomad II rear

Napier Nomad Compound Aircraft Engine

By William Pearce

D. Napier & Son (Napier) was a British engineering firm that designed and manufactured aircraft engines since World War I. In 1931, Napier began experimental design work on a sleeve-valve, 24-cylinder, diesel (compression ignition) engine. Designated E101, the engine had a 5.0 in (127 mm) bore, a 4.75 in (121 mm) stroke, and a displacement of 2,238 cu in (36.7 L). While a two-cylinder test engine was built, and possibly a full bank of six cylinders, it is not clear if a complete H-24 E101 was constructed. However, the E101 served as the foundation for the E107, which was converted to spark ignition and became the first of the Sabre engine line. In 1933, Napier acquired licenses to produce the Junkers Jumo 204 and 205 aircraft engines as the Culverin (E102) and Cutlass (E103). Although not commercially successful, the experience with the Junkers engines provided Napier with detailed knowledge of two-stroke, high-powered diesel engines.

Napier Nomad I front

The Napier Nomad I was perhaps the most complex aircraft engine ever built. Of the contra-rotating propellers, the front set was driven by the turbine, and the rear set was driven by the 12-cyinder diesel engine. (Napier/NPHT/IMechE image)

In late 1944, the British Ministry of Aircraft Production (later, Ministry of Supply, MoS) issued a specification for an economical 6,000 hp (4,474 kW) aircraft engine to be used in large, long-range aircraft. Harry Ricardo, a prominent engine designer and researcher, suggested that combining a two-stroke diesel with a gas turbine would be the best way to create a powerful, compact, and economical aircraft engine.

Napier took Ricardo’s suggestion and combined it with their diesel engine experience. For the 6,000 hp (4,474 kW) engine, Napier proposed the E124: an H-24 diesel with a displacement of approximately 4,575 cu in (75 L) that incorporated an axial flow recovery turbine. Both of the upper and lower cylinder banks formed an included angle of 150 degrees, while the left and right banks formed an angle of 30 degrees. This spacing was done to accommodate exhaust manifolds in the 30-degree left and right Vees. Single- and twin-cylinder tests had begun, as well as tests on the axial-flow compressor, but Napier felt that such an engine would have a very limited market. The project was halted in 1946.

While the E124 was not built, it laid the foundation for a new engine capable of 3,000 hp (2,237 kW) and designed to achieve the lowest fuel consumption under any operating conditions. The new engine was the E125 Nomad I, and Napier began preliminary design work in 1945, with the MoS giving its support by 1946. In a way, the Nomad I was half of the H-24 engine with a reworked recovery turbine. The Nomad I was a liquid-cooled, horizontally-opposed, 12-cylinder, two-stroke, valveless, diesel engine that incorporated a gear-driven, two-speed supercharger and an exhaust-driven turbine that drove a compressor integral with the bottom of the engine. Alone, the compressor could not create the high-level of boost that was desired, so the supercharger was included to reach the design goal.

Napier Nomad I org exhaust rear

Rear view of the Nomad I with its original exhaust manifold illustrates the complexity of the system with its many pipes and flexible joints. The round housing for the supercharger impeller can be seen in front of the turbine. (Napier/NPHT/IMechE image)

The engine’s two-piece magnesium-zirconium alloy crankcase was split vertically and held together by 28 through bolts. A cast aluminum, six-cylinder, monobloc cylinder bank was attached to each side of the crankcase via studs. Wet cylinder liners were installed in the cylinder banks and covered with individual cylinder heads made from aluminum. A magnesium-alloy propeller gear reduction housing was secured via studs to the front of the crankcase. The housing also incorporated air intake on each of its lower sides. The intakes led to the compressor, which had an upper housing cast integral with the bottom of the crankcase, and a lower housing that was bolted on to the crankcase. Behind the compressor was a bifurcated air outlet, an oil sump, and the lower supercharger housing—all bolted to the crankcase.

Air entered the inlets on each side of the Nomad I and flowed into the 10-stage (some sources say 11-stage) axial flow compressor, which was the first stage of supercharging. The compressor had a maximum pressure ratio of 5.62 to 1. The air then exited the compressor via the bifurcated duct, which split the air along both sides of the engine and led back to the supercharger. An air to water intercooler (never installed) was positioned on both sides of the engine, between the compressor and the supercharger. After passing through the engine-driven centrifugal supercharger, the air was ducted into two passageways—one each for the left and right cylinder banks. Pressurized at 95.5 psi (6.58 bar) absolute, the air passed through a compartment in each cylinder bank that interfaced with the intake ports for each cylinder.

Air entered the loop-scavenged cylinder via a series of intake ports around the cylinder liner wall that were uncovered by the piston. The cylinder’s compression ratio was 8 to 1. As the piston moved toward the combustion chamber, fuel was injected via an injector located in the center of the cylinder head. The injected fuel was ignited by the heat of compression as the piston moved toward the cylinder head. On its power stroke, the piston uncovered exhaust ports which were situated slightly higher in the cylinder wall than the intake ports. The high level of supercharging ensured that an ample amount of air passed through the cylinder, which also helped cool the piston crown, cylinder wall, and cylinder head.

Napier Nomad I side

The Nomad I’s original (upper) and revised (lower) exhaust system and turbine can be compared in these images. In the lower image, the compressor’s intake can be seen near the front of the engine. The polished duct between the compressor and supercharger is where the intercooler would have been installed. (Napier/NPHT/IMechE images)

The exhaust gases and scavenging air flowed from the uncovered exhaust ports in the cylinder liner into manifolds positioned above and below the cylinder bank. The two exhaust manifolds for each cylinder bank merged together at the rear of the engine. Here, fuel could be injected, mixed with the surplus air, and ignited to increase the flow of exhaust gas energy to the turbine to create more engine power (for takeoff). The hot gases then flowed to a primary axial flow turbine at the extreme rear of the engine. The gases powered the primary turbine and then flowed out the exhaust nozzle at the end of the engine, generating some thrust. If more power was being harnessed by injecting fuel into the exhaust, a valve allowed the gases to flow into a secondary axial flow turbine positioned between the engine and the primary turbine. After powering the secondary turbine, the gases flowed into the primary turbine and then out the exhaust nozzle. The turbines were mounted in a tubular frame attached to the rear of the engine.

It should be noted that the description above applies to the second version of the exhaust system that was used by 1951. An earlier, original exhaust system had two manifolds above and below each cylinder bank, with each manifold collecting exhaust from three cylinders. The four manifolds from each cylinder bank joined into pairs at the rear of the engine and then merged into a single pipe. Immediately before the exhaust pipes connected to the primary (rear) turbine, an upper and a lower pipe branched off. The upper pipes of the left and right manifolds and the lower pipes of the left and right manifolds joined together at their respective spots as they fed into the secondary (front) turbine. At this point, extra fuel could be injected and ignited for additional power, as in the previous exhaust system described above. The original exhaust system incorporated around 28 flexible joints and was far more complex than the later system. Undoubtedly, issues with the original system were encountered that led to its replacement.

The exhaust turbines were mounted coaxially to the same shaft. This turbine shaft extended forward to power the compressor and led into the propeller gear reduction housing. The turbine shaft was geared to the front (outer) propeller of a contra-rotating set. The front propeller rotated counterclockwise. The rear (inner) propeller rotated clockwise and was geared to the crankshaft. There was nothing that linked the two propeller sets together, but they could not be run independently of each other. In other words, the piston engine section was needed to power the rear propeller, and the engine’s exhaust gases powered the turbine that was needed to run the front propeller. The turbine could not power itself, and the engine’s exhaust gases could not bypass the turbine.

Napier Nomad I Avro Lincoln install

The Nomad I installed in the nose of the Avro Lincoln test bed. The installation required significant modifications to the aircraft. Note the engine’s intake duct and the reversable-pitch propeller. (Napier/NPHT/IMechE image)

The Nomad I’s compressor and turbine were based on those developed for the 1,590 ehp (1,186 kW) Napier Naiad turboprop engine. The six-throw crankshaft of the Nomad I was supported between the left and right crankcase sections by seven main journals. The front of the crankshaft was geared to the propeller and a flexible shaft that extended to the rear of the engine to drive the supercharger impeller. The connecting rods were of the fork-and-blade type. The two-piece pistons had an austenitic stainless steel crown attached to a Y-alloy (aluminum alloy) body. The steel crown was used because of the high temperatures in the cylinder, and the piston was further cooled with oil flowing between the piston body and crown. The center of the crown could reach 1,300° F (700° C) when the engine was running at full power. A camshaft just below each cylinder bank drove three fuel injection dual pumps, and each pump provided the fuel to two cylinders via a single injector in each cylinder. The front of each camshaft also drove a coolant pump. A spark plug positioned just below the injector in each cylinder was used to start the engine. The spark plugs were fired by a magneto driven from the rear of the engine.

Despite its complexity, the Nomad I was designed to be operated by a single lever in the cockpit. The Napier Nomad I had a 6.0 in (152 mm) bore and a 7.375 in (187 mm) stroke. The engine displaced 2,502 cu in (41.0 L) and was rated at 3,080 ehp (2,297 kW) at 2,050 rpm, which was 3,000 shp (2,237 kW) combined with 320 lbf (1.42 kN) of thrust from the turbine. The 3,000 shp (2,237 kW) was combined from 1,450 shp (1,081 kW) from the diesel engine and 1,550 shp (1,156 kW) from the turbine, spinning at 15,600 rpm. For estimated cruising power at 30,250 ft (9,220 m), the diesel engine produced 725 shp (541 kW) at 1,650 rpm and the turbine produced 750 shp (559 kW) at 17,000 rpm, for a combined 1,475 shp (1,100 kW). The Nomad I had a specific fuel consumption (sfc) of 0.36 lb/ehp/hr (219 g/kW/h). The engine was 126.5 in (3.21 m) long, 58.25 in (1.48 m) wide, 49.25 in (1.25 m) tall, and weighed 4,200 lb (1,905 kg).

The design of the Nomad I was laid out by a team led by Ernest Chatterton, Chief Engineer of the Piston Engine Division at Napier. The compressor and turbine sections were tested in 1948. The prototype engine was completed in 1949 and first run in October. After running for a total of 860 hours on the test stand, contra-rotating propellers were installed, and the engine underwent a further 270 hours of tests. In 1950, an Avro Lincoln bomber (serial SX973) that had been loaned to Napier’s Flight Test Department at Luton, England was modified to install the Nomad I in the aircraft’s nose. This conversion entailed a fair amount of work, with everything forward of the cockpit needing to be fabricated. SX973 made its first flight with the Nomad I in 1950. While the aircraft’s four Rolls-Royce Merlin engines were retained, they could be shut down in flight and the Lincoln held aloft solely by the Nomad I. The Nomad-Lincoln made its only public appearance at the Society of British Aircraft Constructors flying display at Farnborough in September 1951. Another Nomad I engine was also on display at the show. The Nomad I accumulated 120 hours of flight time in the Lincoln.

Napier Nomad I Avro Lincoln feathered

The Napier Nomad I had enough power to keep the Avro Lincoln aloft with the four Rolls-Royce Merlin engines shut down and feathered. (Napier/NPHT/IMechE image)

After a total of approximately 1,250 hours of operation, the Nomad I program was brought to a close in September 1952. The complex engine had proven to be temperamental, although it did exhibit very good fuel economy when it was running correctly. While Nomad I engine tests were underway, an updated and simplified version of the engine had been designed and designated E145 Nomad II. The design of the Nomad II took advantage of lessons learned from the Nomad I and the latest developments of axial compressors.

The Nomad II was designed in 1951, and the program was supervised by Chatterton and A. J. Penn, Napier’s gas turbine chief engineer. Although similar in configuration and possibly sharing some components with the Napier I, the Napier II was a new design. The Napier II retained the horizontally-opposed 12-cylinder layout incorporating a turbine and compressor, but the contra-rotating propellers and mechanically-driven centrifugal supercharger were discarded. The wet cylinder liners of the Nomad I were replaced by dry liners, which were made of chromium-copper alloy with chrome-plated bores. The crankcase was again cast of magnesium-zirconium (RZ-5) alloy.

Napier Nomad I and II geartrain

A simplified comparison of the Nomad I (top) and Nomad II (bottom) power systems. Not shown on the Nomad I was the two-speed supercharger drive. Not shown on the Nomad II was the second quill shaft to the variable-speed coupling. Neither drawing shows the engines’ accessory camshafts.

The improved axial flow compressor had a diameter of 10.88 in (276 mm) and was hung below the engine via four flexible mounts. The compressor had 12 stages, a maximum pressure ratio of 8.25 to 1, and a maximum mass air flow of 13 lb/sec (5.9 kg/sec). Its inlet faced forward to take full advantage of ram air. The pitch of the compressor’s inlet guide vanes automatically adjusted to improve airflow at lower speeds. The first five stages of the compressor used cobalt-steel blades, and the remaining seven stages used aluminum-bronze blades.

The Nomad II’s loop-scavenged system was improved over that of the Nomad I. Air from the compressor was routed forward in a manifold mounted below each cylinder bank. The pressurized air entered the revised cylinder banks and passed through guide vanes to flow into each cylinder via eight intake ports. Two pairs of four ports were positioned in the upper sides (top side of the engine) of the cylinder wall. The specially-designed intake ports directed the flow of air toward the hemispherical combustion chamber, where it circulated back toward the piston and the uncovered exhaust ports. The six exhaust ports consisted of three large ports, each with a smaller port below (toward the piston). The exhaust ports were positioned on the bottom side of the cylinder (lower side of the engine) and closer to the combustion chamber than the intake ports.

Napier Nomad II front

The Napier Nomad II was a simpler engine and was improved in every way compared to the Nomad I. Note the single rotation propeller shaft and simplified exhaust system. The compressor can be seen under the engine. (Napier/NPHT/IMechE image)

The exhaust gases were collected in an exhaust manifold mounted below each cylinder bank. The exhaust gases flowed back to a three-stage axial flow turbine mounted at the rear of the engine. The turbine and the compressor were mounted on separate shafts that were coaxially coupled. The turbine shaft was also connected to the crankshaft via an infinitely variable-speed fluid coupling (Beier gear). At low power (under 1,500 rpm), the turbine did not create the power needed to drive the compressor. This resulted in the variable-speed coupling delivering power from the crankshaft to drive the compressor. At high power (above 1,500 rpm), the turbine created more power than what was needed to drive the compressor. The variable-speed coupling fed the extra power back to the engine’s crankshaft. The fluid coupling drive set was mounted to the upper-rear of the engine.

While the cylinders’ compression ratio was 8 to 1, air was fed into the cylinders at 89 psi (6.14 bar) absolute for takeoff, creating an effective compression ratio of 27 to 1. A set of six fuel injection pumps were located above each cylinder bank. The pumps were driven by a camshaft from the front of the engine. The fuel injector in the center of the cylinder head had six orifices: one sprayed toward the piston, and the other five were equally spaced radially around the nozzle and sprayed toward the combustion chamber walls. The fuel was injected into the cylinder at 3,675 psi (253 bar).

Napier Nomad II cutaway

The cutaway view of the Nomad II reveals that the engine was still very complex compared to a conventional piston engine. Note the gearset at the front of the engine that powered the propeller shaft, fuel injection cams (upper), and quill shafts (lower) to the variable-speed coupling. (Napier/NPHT/IMechE image)

When the engine was viewed from the rear, the propeller turned counterclockwise. In the reduction gear housing at the front of the engine, the crankshaft drove the propeller shaft via four pinions. Although the exact gear reduction used in the test engines has not been found, a variety of reduction speeds were available: .526, .555, .569, .614, or .660 times crankshaft speed. Each of the lower two pinions were mounted to separate quill shafts that extended back to the rear of the engine and drove (or were driven by) the variable-speed gearset coupled to the turbine shaft. The crankshaft was supported by eight main bearings, with two I-beam connecting rods attached to each crankpin. The connecting rods used slipper-type bearings with two fairly-light straps securing the pair to the crankshaft. Since the engine was a two stroke, there was no downward pull on the connecting rod that required a more robust cap. The small end of the connecting rod that attached to the piston had a slipper-type eccentric bearing. As the connecting rod articulated from top dead center to bottom dead center, the bearing would rock slightly on the piston, opening a small gap for lubrication. This provided the proper oil flow that otherwise would not have occurred with the unidirectional loads of the two-stroke engine.

For starting, two ignition coils and two distributors driven from the front of the engine fired a spark plug in each cylinder. However, some photos appear to show two spark plugs in each cylinder. For installation, the engine was hung by two supports above the front cylinders and two supports above the rear casing.

The Napier Nomad II had the same 6.0 in (152 mm) bore, 7.375 in (187 mm) stroke, and 2,502 cu in (41.0 L) displacement as the Nomad I. The engine initially had a takeoff rating of 3,135 ehp (2,338 kW) at 2,050 rpm, which was 3,046 shp (2,271 kW) combined with 250 lbf (1.11 kN) of thrust from the turbine. As development continued, water injection was added that increased the Nomad II’s takeoff rating to 3,570 ehp (2,662 kW) at 2,050 rpm. This power was a combination of 3,476 shp (2,592 kW) and 230 lbf (1.02 kN) of thrust. At full power, the turbine shaft turned at 18,200 rpm, 8.88 times crankshaft speed. The engine’s maximum continuous rating was 2,488 ehp (1,855 kW) at 1,900 rpm, which was 2,392 shp and 145 lbf (1,855 kW and .64 kN). The Nomad II had a sfc of 0.345 lb/ehp/hr (210 g/kW/h). The engine was 119.25 in (3.03 m) long, 56.25 in (1.43 m) wide, 40 in (1.02 m) tall, and weighed 3,580 lb (1,624 kg).

Napier Nomad II parts

Various components of the Nomad II. Clockwise from the upper left: compressor and compressor housing, parts of the turbine, the Beier variable-speed fluid coupling, two connecting rods, and a piston with its stainless steel crown. (Napier/NPHT/IMechE images)

The Nomad II was first run in December 1952 and had accumulated 350 hours by mid-1954. The engine underwent various bench tests and tests with a 13 ft (3.96 m) diameter, constant-speed, reversable-pitch propeller. It was found that running the engine on diesel, kerosene, or jet fuel (wide-cut gasoline) resulted in little difference in power. Some tests indicated that a sfc as low as 0.326 lb/ehp/hr (198 g/kW/h) could be achieved, this being realized at 22,250 ft (6,782 m) with the engine producing 2,027 ehp (1,511 kW) at 1,750 rpm. The Nomad II maintained takeoff power up to 7,750 ft (2,362 m), and a constant boost, power, and sfc could be maintained up to 25,000 ft (7,620 m). At sea level, the turbine developed 2,250 hp (1,678 kW), but 1,840 hp (1,372 kW) was used to power the compressor. The Nomad experienced a two percent drop in power for every 20° F (11° C) increase in air temperature. Since the engine only burned 70 percent of the air passing through the cylinders, the ability to inject and ignite fuel into the exhaust manifold was experimented with, resulting in 4,095 ehp (3,054 kW) for a sfc of .374 lb/hp/hr (227 g/kW/h).

For flight tests, Napier proposed installing Nomad II engines in place of the outer two Rolls-Royce Griffons on an Avro Shackleton maritime patrol aircraft. In October 1952, the MoS loaned the second prototype Shackleton (VW131) to Napier for conversion and subsequent Nomad II flight testing. The aircraft arrived at Napier’s center at Luton on 16 January 1953. Dummy engines were first installed, and vibration tests were conducted in April 1954. The Nomad II installation and cowlings were clean and refined, but flight-cleared engines were slow to arrive. Eventually, two Nomad II engines were installed and some ground runs were made, but the Nomad program was cancelled in April 1955, before the aircraft had flown. While the Nomad II had unparalleled fuel economy for the time and was simpler, lighter, smaller, and more powerful than the Nomad I, there was little demand for the engine. Napier kept all Nomad data for a time, believing that interest in the engine might be rekindled and spark further development, but that was not the case.

Napier Nomad II rear

The 12-stage turbine was mounted in a tube frame behind the engine. The housing above the turbine contained the variable-speed coupling that linked the crankshaft to the turbine shaft. Note the single spark plug (used for starting) in each cylinder. (Napier/NPHT/IMechE image)

Before the project was cancelled in 1955, the E173 Nomad III was designed as a continuation of the engine’s development. The Nomad III incorporated fuel injection into the exhaust manifold and an air-to-water aftercooler between the compressor and the cylinders. With these changes, the engine had a wet takeoff rating of 4,500 ehp (3,356 kW) at 2,050 rpm, which was 4,412 shp (3,290 kW) combined with 230 lbf (1.021 kN) of thrust from the turbine. The Nomad III weighed 3,750 lb (1,701 kg), 170 lb (77 kg) more than the Nomad II, but a complete engine was never built.

While the Nomad demonstrated excellent economy and impressive power for its weight, the engine was overshadowed by development of turboprops and turbojets. Money for development was tight, and the Nomad program had cost £5.1 million. In cases like the Avro Shackleton, it was less expensive to use Griffon engines than continue development of the Nomad. For other projects, the turboprop offered greater potential in the long run. While the Nomad engine was designed to cruise around 345 mph (556 km/h), the turbojet offered significantly higher cruise speeds compared to any other type of aircraft engine.

The exact number of Nomad I engines constructed has not been found, but it was at least two. A nicely restored Nomad I engine is preserved and on display at the National Museum of Flight at East Fortune Airfield in Scotland. The Nomad I underwent a restoration in 1999, and it was discovered that there were no propeller gears, pistons, or a crankshaft in the engine. This engine may be the Nomad I that was displayed at Farnborough in 1951. Of the six Nomad II engines built, two are preserved and on display—one at the Steven F. Udvar-Hazy Center in Chantilly, Virginia and the other at the Science Museum at Wroughton, England.

Napier Nomad II prop test

The Nomad II setup for tests with a 13 ft (3.96 m) propeller. Note that two spark plugs appear to be installed in each cylinder. Although not finalized, the top-mounting system made it fairly easy to install or remove the engine. (Napier/NPHT/IMechE image)

Sources:
“Napier Nomad Aircraft Diesel Engine” by Herbert Sammons and Ernest Chatterton, SAE Transactions Vol 63 (1955)
“Napier Nomad” by Bill Gunston, Flight (30 April 1954)
“Napier’s Nomad Engine” The Aeroplane (30 April 1954)
“Compound Diesel Engine Design Analyzed” Aviation Week (17 May 1954)
Aircraft Engines of the World 1952 by Paul H. Wilkinson (1952)
Aircraft Engines of the World 1956 by Paul H. Wilkinson (1956)
By Precision Into Power by Alan Vessey (2007)
Turbojet: History and Development 1930–1960 Volume 1 by Antony L. Kay (2007)
Men and Machines by Charles Wilson and William Reader (1958)
Napier Powered by Alan Vessey (1997)
https://www.thegrowler.org.uk/avroshackleton/the-nomad-proposal.htm
http://www.apss.org.uk/projects/completed_projects/nomad/index.htm
http://www.apss.org.uk/projects/completed_projects/nomad/detail/index.htm

Fisher P-75A top

Fisher (General Motors) P-75 Eagle Fighter

By William Pearce

Donovan (Don) Reese Berlin had worked as the Chief Engineer for the Curtiss-Wright Corporation. He had designed the company’s successful P-36 Hawk and P-40 Warhawk fighters. Berlin also designed a number of unsuccessful fighters. He left Curtiss-Wright in December 1941 in frustration because he felt the company was not sufficiently supporting his efforts to develop a new fighter. At the request of the US government, Berlin was quickly hired by General Motors (GM) in January 1942 as the Director of Aircraft Development at the Fisher Body Division (Fisher).

Fisher XP-75 43-46950

The Fisher P-75 Eagle was supposed to be quickly and inexpensively developed by utilizing many existing components. However, many resources were expended on the aircraft. The first XP-75 (43-46950) had a uniquely pointed rear canopy. It was also the only example that used a relatively unaltered Douglas A-24 empennage. Note the fixed tailwheel and the fairings that covered the machine gun barrels in the aircraft’s nose.

Fisher was already engaged by the government to build large assembles for the North American B-25 Mitchell bomber, and plans for the manufacture of other aircraft components were in the works. It made sense to have a prominent aeronautical engineer as part of Fisher’s staff. In March 1942, Fisher was tasked to build various components (engine cowlings, outer wing panels, ailerons, flaps, horizontal stabilizers, elevators, vertical stabilizers, rudders) of the Boeing B-29 Superfortress and 200 complete aircraft. A new plant in Cleveland, Ohio would be built to support this order. Beyond Fisher, a number of other GM divisions were involved in building aircraft and aircraft engines under license from other manufacturers. However, GM wanted to design and manufacture its own products to support the war effort. Berlin was a believer in applying automotive methods to produce aircraft, which was a good match for the automotive giant GM.

On 10 September 1942, GM, through Fisher, submitted a proposal to the Army Air Force (AAF) for a new interceptor fighter. The proposal was based on an AAF request from February 1942 for such an aircraft with exceptional performance. The aircraft from Fisher was designed by Berlin, powered by an Allison V-3420 24-cylinder engine, and constructed mainly of components from other aircraft. The aircraft offered impressive performance with a top speed of 440 mph (708 km/h) at 20,000 ft (6,096 m), a 5,600 fpm (28.5 m/s) initial climb rate, a service ceiling of 38,000 ft (11,582 m), and a range of 2,240 miles (3,605 km) with only internal fuel. All of this came with a promise to deliver the first aircraft within six months of the contract being issued.

Fisher XP-75 line

The top image shows at least five XP-75A aircraft under construction. The middle image, from right to left, shows the first two XP-75 aircraft (43-46950 and 43-46951) and the first two XP-75A aircraft (44-32161 and 44-32162). The second XP-75 (second from the right) has the wide H-blade propellers installed, while the other aircraft have the narrow A-blade propellers. The bottom image is a P-75A under construction. Note the V-3420 engine. (Veselenak Photograph Collection / National Museum of the US Air Force images)

Back in February 1941, the Army Air Corps (name changed to AAF in June 1941) had considered the Allison V-3420 as a possible replacement for the Wright R-3350 engine intended for the B-29. The Allison Engineering Company was a division of GM, and at the time, development of the V-3420 was focused on creating the basic engine and not much more. However, the priority of the V-3420 program was scaled-back after the Japanese attacked Pearl Harbor on 7 December 1941.

GM had been searching for an application for its Allison V-3420 engine, and the AAF had tried to entice other manufactures to incorporate the engine in a fighter design. Fisher’s fighter project offered a solution for both entities. The AAF was sufficiently impressed with Fisher’s proposal, and they approved the construction of two prototypes (serials 43-46950 and 43-46951) on 10 October 1942. The aircraft was given the designation P-75 Eagle, with the prototypes labeled XP-75. Some believe the pursuit number “75” was issued specifically at Berlin’s request, as his “Model 75” at Curtiss-Wright became the successful P-36 and led to the P-40. Although there were some reservations with the aircraft’s design, it was believed that a team working under the experienced Berlin would resolve any issues encountered along the way.

Fisher XP-75A long-range side

Aircraft 44-32162 was the fourth of the XP-75-series and the second XP-75A with additional wing fuel tanks. Note the revised canopy and tail compared to the first prototype. The aircraft has narrow A-blade propellers, and the 10-gun armament appears to be installed.

The XP-75 was of all metal construction with fabric-covered control surfaces. The cockpit was positioned near the front of the aircraft and provided the pilot with good forward and downward visibility. The pilot was protected by 177 lb (80 kg) of armor. The cockpit canopy consisted of front and side panels from a P-40. The aircraft’s empennage, with a fixed tailwheel, was from a Douglas A-24 Banshee (AAF version of the Navy SBD Dauntless). Initially, North American P-51 Mustang outer wing panels would attach to the inverted gull wing center section that was integral with the fuselage. However, the P-51 wings were soon replaced by wings from a P-40 attached to a normal center section. The main landing gear was from a Vought F4U Corsair, and it had a wide track of nearly 20 ft (6.10 m). Four .50-cal machine guns were mounted in the aircraft’s nose and supplied with 300 rpg. Each wing carried three additional .50-cal guns with 235 rpg. Under each wing, inside of the main gear, was a hardpoint for mounting up to 500 lb (227 kg) of ordinance or a 110-US gal (416-L) drop tank.

The 2,600 hp (1,939 kW) Allison V-3420-19 engine with a two-stage supercharger was positioned in the fuselage behind the pilot. Each of the engine’s four cylinder banks had an air-cooled exhaust manifold with two exhaust stacks protruding out of the fuselage. Two extension shafts passed under the cockpit and connected the engine to the remote gear reduction box for the Aeroproducts six-blade contra-rotating propeller. Two different types of propellers were used. Initially, a 13 ft (3.96 m) diameter, narrow, A-blade design was used. Many sources state that this propeller was used on the first 12 aircraft. However, some of these aircraft flew with the second design, a 12 ft 7 in (3.84 m) diameter, wide, H-blade. The gear reduction turned the propeller at .407 crankshaft speed.

Fisher XP-75A 44-32161 crash

The empennage (left) and inverted wings and fuselage (right) of XP-75A 44-32161 after its crash on 5 August 1944. An engine explosion and inflight fire led to the empennage separating from the rest of the aircraft. Russell Weeks, the pilot, was able to bail out of the stricken aircraft. (Veselenak Photograph Collection / National Museum of the US Air Force images)

A two-section scoop was located under the fuselage, just behind the wings. The left section held an oil radiator, and coolant radiators were positioned in both the left and right sections. The aircraft’s oil capacity and coolant capacity were 28.5 US gal (108 L) and 31.5 US gal (119 L) respectively. A 485-US gal (1,836-L) fuel tank was positioned in the fuselage between the cockpit and engine. The tank was made of two sections with the extension shafts passing between the sections.

An XP-75 mockup was inspected by the AAF on 8 March 1943. On 6 July, six additional prototypes (serials 44-32161 to 44-32166) were ordered with some design modifications, including changes to the cockpit canopy, the use of a 2,885 hp (2,151 kW) V-3240-23 engine, and additional fuel tanks in each wing with a capacity of 101 US gal (382 L). The extra fuel enabled the P-75 to cover the long-range escort role, something that the AAF was desperately seeking. The long-range fighter prototypes are often referred to as XP-75As, although this does not appear to be an official designation.

Fisher XP-75A assembly

This image shows either 44-32165 or 44-32166 being completed in the Cleveland plant. These two aircraft, the last of the XP-75As, had a bubble canopy, retractable tailwheel, and a new, tall rudder and vertical stabilizer. Note the P-40-style rounded wings. (Veselenak Photograph Collection / National Museum of the US Air Force image)

Since the need for interceptors had faded, many in the AAF were optimistic that the long-range P-75 would be able to escort bombers all the way into Germany and that the aircraft would be able to outperform all German fighters for the foreseeable future. The P-75 also served as insurance if the P-51 and Republic P-47 Thunderbolt could not be developed into long-range escort fighters.

On 8 July 1943, a letter of intent was issued for the purchase of 2,500 P-75A aircraft (serials 44-44549 to 44-47048), but a stipulation allowed for the cancellation of production if the aircraft failed to meet its guaranteed performance. A definitive contract for all of the XP-75 work was signed on 1 October 1943 and stipulated that the first XP-75 prototype would fly by 30 September 1943, and the first long-range XP-75A prototype would fly by December 1943. The first production aircraft was expected in May 1944, and production was forecasted to eventually hit 250 aircraft per month. The production costs for the 2,500 P-75A aircraft was estimated at $325 million.

Fisher XP-75A 44-32165 side

XP-75A 44-32165 with the new (and final) large, angular tail and horizontal stabilizer. However, the aircraft retained the rounded wings. Note the ventral strake behind the belly scoop, and the wide H-blade propellers. The same modifications were applied to 44-32166. The stenciling under the canopy says “Aeroproducts Flight Test Ship No 4.”

The Fisher XP-75A had a wingspan of 49 ft 1 in (14.96 m), a length of 41 ft 4 in (12.60 m), and a height of 14 ft 11 in (4.55 m). The aircraft’s performance estimates were revised to a top speed of 434 mph (698 km/h) at 20,000 ft (6,096 m) and 389 mph (626 km/h) at sea level. Its initial rate of climb was 4,200 fpm (21.3 m/s), with 20,000 ft (6,096 m) being reached in 5.5 minutes, and a service ceiling of 39,000 ft (11,887 m). The aircraft had an empty weight of 11,441 lb (5,190 kg) and a fully loaded weight of 18,665 lb (8,466 kg). With the fuselage tank, a total of 203 US gal (768 L) of fuel in the wings, and a 110-US gal (416-L) drop tank under each wing, the XP-75A had a maximum range of 3,850 miles (6,196 km).

The AAF gave the XP-75 priority over most of Fisher’s other work, particularly that of constructing 200 B-29 bombers. Construction of the first two prototypes was started at Fisher’s plant in Detroit, Michigan. The other six XP-75 aircraft were built at the new plant in Cleveland, Ohio, which opened in 1943. Production of the aircraft would occur at the Cleveland plant.

Fisher P-75A assembly line

The production line with P-75A numbers two through four (44-44550 through 44-44553) under construction. While the aircraft have square wingtips, at least the first one still has the rounded horizontal stabilizer. Note the V-3420 engine by the first aircraft. The wing of an XP-75A is visible on the far right.

Flown by Russell Thaw, the XP-75 prototype (43-46950) made its first flight on 17 November 1943, and it was the first aircraft to fly with the V-3420 engine. Almost immediately the aircraft ran into issues: the center of gravity was off; the ailerons were heavy; the controls were sluggish; the aircraft exhibited poor spin characteristics; and the V-3420 engine was down on power and overheating. The trouble is not very surprising considering the aircraft consisted of parts from other aircraft and was powered by an experimental engine installed in an unconventional manner. The V-3420’s firing order was revised for smoother operation. Modifications to the second prototype (43-46951) included changes to the ailerons and a new rear canopy. The size of the rudder was decreased, but the surface area of the vertical stabilizer was increased by extending its leading edge. The second XP-75 prototype first flew in December 1943.

The first of the six XP-75A long-range aircraft (44-32161) flew in February 1944. The last two of these aircraft, 44-32165 and 44-32166, were finished with a bubble canopy and a new empennage. The new empennage had a retractable tailwheel and a taller vertical stabilizer and rudder. Lateral control was still an issue, and these two aircraft were later modified with larger and more angular vertical and horizonal stabilizers. These changes were also incorporated into most of the P-75A production aircraft.

Fisher P-75A 44-44549

The first production P-75A (44-44549) with its square wingtips and original rounded tail. Note the ventral strake and the fins attached to the horizontal stabilizer. It is not known when the picture was taken (possibly 22 September 1944), but the aircraft and pilot were lost on 10 October 1944.

The third long-range XP-75A aircraft (44-32163) crashed on 8 April 1944, killing the pilot, Hamilton Wagner. The crashed may have been caused by the pilot performing unauthorized aerobatics. On 7 June 1944, the AAF issued the contract for 2,500 P-75A aircraft. Official trials were conducted in June 1944 and indicated that the XP-75A aircraft was well short of its expected performance. A top speed of only 418 mph (673 km/h) was achieved at 21,600 ft (6,584 m), and initial climb rate was only 2,990 fpm (15.2 m/s). However, the engine was reportedly not producing its rated output. On 5 August 1944, XP-75A 44-32161 was lost after an inflight explosion, which separated the empennage from the rest of the aircraft. The pilot, Russell Weeks, successfully bailed out.

In addition to other changes made throughout flight testing of the prototypes, the P-75As incorporated extended square wingtips with enlarged ailerons, the controls were boosted to eliminate the heavy stick forces, and a ventral strake was added that extended between the scoop exit doors and the tailwheel. The P-75A had a wingspan of 49 ft 4 in (15.04 m), a length of 41 ft 5 in (12.62 m), and a height of 15 ft 6 in (4.72 m). The aircraft’s performance estimates were revised down, with a top speed of 404 mph (650 km/h) at 22,000 ft (6,706 m). Its initial rate of climb dropped to 3,450 fpm (17.5 m/s), and the service ceiling decreased to 36,400 ft (11,095 m). The aircraft had an empty weight of 11,255 lb (5,105 kg) and a fully loaded weight of 19,420 lb (8,809 kg).

Fisher P-75A runup

P-75A 44-44550 with the new (and final) square tail and horizontal stabilizer. Note the two-section belly scoop and the F4U main landing gear.

The first two P-75As (44-44549 and 44-44550) were not originally finished with the latest (angular) empennage. Rather, they used the tall, round version that was originally fitted to the last two XP-75A prototypes. A dorsal fillet was later added to the vertical stabilizer. The first Fisher P-75A (44-44549) took flight on 15 September 1944, with the second aircraft (44-44550) following close behind. Aircraft 44-44550 was later altered with the enlarged, square-tipped vertical and horizontal stabilizers, but it is not clear if 44-44549 was also changed. At some point (possibly late September 1944), aircraft 44-44549 had stabilizing fins added to the ends of its horizontal stabilizer. Both aircraft were sent to Eglin Field, Florida for trials. On 10 October 1944, aircraft 44-44549 was lost with its pilot, Harry Bolster. The crash was caused by the propellers becoming fouled by either a nose-gun tube failure or by part of the spinner breaking free. The damaged propellers quickly destroyed the gear reduction, and once depleted of oil, the propeller blades went into a flat pitch. Bolster attempted a forced landing but was not successful.

By the time of the last crash, the AAF had realized it would not need the P-75A aircraft. The P-51B/D and P-47D/N had proven that they were up to the task of being long-range escort fighters, and the war’s end was in sight. The P-75A was larger, heavier, slower, and sluggish compared to fighters already in service. The production contract for the 2,500 P-75As was cancelled on 6 October 1944, and further experimental work was stopped on 8 November. Five P-75A aircraft were completed, with an additional, nearly-complete airframe delivered for spare parts. Construction of approximately 20 other P-75A production aircraft had started, with some assemblies being completed.

Fisher P-75A top

A top view of 44-44550 provides a good illustration of the square wingtips and horizontal stabilizer. The wings were only slightly extended, but the area of the ailerons was increased by a good amount. The square extensions to the horizontal stabilizer increased its area significantly. Note that the machine gun armament is installed.

P-75A 44-44550 was later transferred to Moffett Field, California where it underwent tests on the contra-rotating propellers. The aircraft was scrapped after the tests. In an attempt to produce more power, a new intercooler was installed in 44-44551, and the aircraft was lent to Allison on 28 June 1945. Later, a 3,150 hp V-3420 was installed. Aircraft 44-44552 and 44-44553 were sent to Patterson Field, Ohio and stored for further V-3420 development work. None of the aircraft were extensively flown. The last completed P-75A, 44-44553, was preserved and is currently on display in the National Museum of the US Air Force in Dayton, Ohio. The aircraft went through an extensive restoration in 2008. All other P-75 aircraft were eventually scrapped.

The eight prototype aircraft had cost $9.37 million, and the manufacturing contract, including the six production aircraft, construction of the Cleveland plant, and tooling for production, had cost $40.75 million. This gave a total expenditure of $50.21 million for the 14 P-75 aircraft. In the end, the expeditious and cost-saving measure of combining existing components led to delays and additional costs beyond that of a new design. It turned out that the existing assemblies needed to be redesigned to work together, essentially making the P-75A a new aircraft with new components.

Fisher P-75A side

The pilot under 44-44550’s bubble canopy helps illustrate the aircraft’s rather large size. The P-75’s sluggish handling and lateral instability did not endear the aircraft to test pilots. Note the nearly-wide-open rear shutter of the belly scoop.

An often-cited story states that then Col. Mark E. Bradley, Chief of Aircraft Projects at Wright Field, was so dissatisfied with the XP-75 after making a test flight, that he requested North American add a large fuel tank in the fuselage of the P-51 Mustang. This act led to the ultimate demise of the XP-75 and the ultimate success of the P-51. However, that sequence of events is not entirely accurate.

Bradley initiated North American’s development of the P-51 fuselage tank in July 1943, after evaluating the XP-75’s design. Experiments with the P-51’s 85-gallon (322-L) fuselage tank were successfully conducted in August 1943. In early September 1943, kits to add the tank to existing P-51s were ordered, and about 250 kits arrived in England in November. At the same time, the fuselage tank was incorporated into the P-51 production line. These events preceded the XP-75 prototype’s first flight on 17 November 1943. Bradley’s later flight in the XP-75 solidified his view that the P-51 with the fuselage tank was the best and quickest option for a long-range escort, and that the XP-75, regardless of its progression through development, would not be superior in that role.

Fisher P-75A USAFM

Fisher P-75A 44-44553 has been preserved and is on display in the National Museum of the US Air Force. (US Air Force image)

Sources:
U.S. Experimental & Prototype Aircraft Projects Fighters 1939–1945 by Bill Norton (2008)
Vees For Victory!: The Story of the Allison V-1710 Aircraft Engine 1929-1948 by Dan Whitney (1998)
P-75 Series Airplanes Advance Descriptive Data (20 May 1944)
P-51 Mustang: Development of the Long-Range Escort Fighter by Paul A. Ludwig (2003)
Development of the Long-Range Escort Fighter by USAF Historical Division (1955)
“Le Fisher XP-75 Eagle” by Alain Pelletier, Le Fana de l’Aviation (August 1996)
“A Detroit Dream of Mass-Produced Fighter Aircraft: The XP-75 Fiasco” by I. B. Holley, Jr. Technology and Culture Vol. 28, No. 3 (July 1987)
http://usautoindustryworldwartwo.com/Fisher%20Body/fisherbodyaircraft.htm
http://www.alexstoll.com/AircraftOfTheMonth/3-00.html
https://en.wikipedia.org/wiki/List_of_accidents_and_incidents_involving_military_aircraft_(1943%E2%80%931944)