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Airman 1st Class
May 28, 2008
Trentham, Victoria, AU
Nice photo of the sole surviving Fairey P.24 "Monarch" engine held at the Fleet Air Arm Museum, Yeovilton, U.K.
The Fairey P.24 Series 1 engine was a geared, medium supercharged (2-speed, single-stage), liquid-cooled twin engine unit consisting of two vertically opposed banks of 12 cylinders. The motor drove two, three-bladed propellers.
Each cylinder head had one over-head camshaft operating three valves per cylinder; two inlet and one exhaust. The inlet valves were operated by T-headed tappets lying across the cylinder head with the exhaust valves directly moved by the cams. Compression ratio was 6:1.
Four exhaust ports per cylinder bank faced outboard, the two central ports being common to two cylinders, vis 2 3 and 4 5. The end ports had only one cylinder discharging into them. Induction and exhaust was on the same side of each cylinder head.
Con-rods were of the fork-and-blade type. Bore and stroke was 5.25in X 6.0 in. Capacity - 3117 cu. ins. (51 litres).


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There was the H.16 Prince which had 16 cylinders in two halves. This was rated at 1580hp and 2800rpm.

The P.24 Monarch had 24 cylinders in two halves, a two-stage four-speed supercharger and was rated initially at 2240hp at 3000rpm with over 3000hp possible.

There's a reasonable amount of information on
this thread.

There was a similar project from Rolls-Royce for a 24-cylinder Merlin with the design moving to two blocks of flat-12s. Weight and power was expected to be double that of the standard Merlin. Theres a picture of the model in the back here.
K9370. The Monarch power plant was housed in a long, bulbous cowling from which double rows of exhaust stubs protruded on each side. A large ventral radiator was installed under the wing's centre section. There was almost no ground clearance which must have made it fun when landing.

K9370's maiden flight with the Monarch was made by Christopher Staniland, Fairey's chief test pilot, in June 1939. Further flights were made by F.H.Dixon and several RAF pilots. On 12th July 1941 it arrived at Farnborough for further trials. A total 0f 87 hours were flown in the UK.

At first Fairey had seen the Monarch as a potential power plant for naval strike aircraft, the contra-rotating propellers eliminating torque and so making carrier landings easier. Possibly the barracuda and Firefly were considered but it was also put forward for other types such as the Blackburn B20 and Hawker Tornado.

Details of the Monarch reached the USA and its high power made it a potential engine for the P-47. US authorities requested an example be made available for evaluation but few examples had been assembled so it was decided to ship the engine fitted to the Battle. To save time removing and installing it into another airframe in the US, K9370 was transported with its engine to the USA. It arrived between December 1941 and January 1942, going to Wright Field. British roundels were removed and USAAF stars painted on the wings and red and white stripes on the rudder, The serial number was retained. After 253 hours of flight testing it retuned to the RAE in 1943. Because the Air Ministry believed that too much effort was being expended on too many engine designs it ended all work by Fairey's power plant department in 1943.

A couple of photographs of K9370, before and after it went to the USA.

It was originally fitted with a cap in front of the propellers, this being replaced later with a spinner.

I think there were about seventeen Battles used as test beds for various engines,


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"The P.24 Monarch had 24 cylinders in two halves, a two-stage, four-speed supercharger and was rated initially at 2240hp at 3000rpm with over 3000hp possible."

Some more fantasy from Red Admiral! It is quite incorrect to describe the P-24 as having a "two-stage, four speed supercharger". Each bank of the engine had its own compressor and this was a single impeller unit with two speeds only. The spiral inlet to each supercharger often leads people to think it was a two-stage unit.
Normal power was 2030 HP at 2600 rpm at 5000' (Low Ratio)
1780 HP at 2600 rpm at 12,000' (High Ratio)
Source: US War Dept. Mem. Report August 22, 1941.

NB. "Prince" was the name given to the 25 litre V-12 that preceeded the "Monarch"
Different data isn't necessarily wrong, especially for an engine built in very limited quantities without much information.

US Navy Figures
1941, to be manufactured by Ford
Two stage , four speed supercharging. 100 octane fuel
max.about 2,600hp at SL,
3000rpm 36,000ft, 1,950hp
3000rpm 45,000ft, 1,150hp
Bore 5.25in stroke 6in
Compression ratio 6.0 to 1
Dry weight 2,140 lbs ( lots of different figures, 1860lb dry weight in 1938)
Propellers 570lb contra rotating
Extract from a Fairey Aviation Co. patent showing a proposed H-16 two-stroke engine with two-stage supercharging.


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Drawing from a war-time patent filed by the Fairey Aviation Co. showing probably the ultimate proposed version of the P-24 "Monarch" with two-stage, two-type superchargers but no sign of an intercooler anywhere!


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