The Boulton & Paul P.12 Bodmin was a large experimental multi-engined aircraft, produced to test out the possibility of mounting an aircraft's engines within the fuselage.
The idea of putting an aircraft's engines in an internal engine room was originally suggested by Bristols, but their scheme was considered to be rather over-ambitious. Frank Barnwell had suggested putting two 1,500hp turbine engines in an internal fuselage engine room in the Braemar triplane, and using it to drive the propellers. The idea was to make long distance flying safer by allowing easy access to the engines while in the air. A second advantage was that each engine could be used to drive pairs of propellers, one on each side of the aircraft, so losing one engine wouldn’t cause asymmetrical thrust. Specification 1/20, for a 'Spares Carrier', was drawn up to test this idea, and Bristols were given a contract to develop the triplane Bristol Tramp.
Specification 1/20 was followed by Specification 9/20 (later modified by Specification 11/20), which called for smaller experimental aircraft, this time disguised as 'postal' aircraft. Two contracts were issued - one to Parnall for the single engined Parnall Possum and one to Boulton & Paul, for the P.12 Bodmin.
The P.12 was to carry two engines, both mounted in an engine room carried in the main fuselage. One engine drove two tractor propellers and the other powered two pusher propellers, mounted in nacelles carried between the wings.
The aircraft was a three bay biplane, with the lower wing level with the base of the fuselage. As many elements as possible were taken from the twin engined Boulton & Paul P.7 Bourges fighter bomber. The design plans showed positions for nose and dorsal guns, and the name 'Bodmin' shows that it was intended to be used as a bomber (bombers under 11,000lb in weight were to use inland towns as their names, and Boulton & Paul designs to use names starting in 'Bo').
The engine layout was very distinctive. The two engines were mounted at the top of the fuselage, in the roof of the engine room, which had a rectangular profile. All of the engine controls were placed within this engine room. The drive shafts were mounted horizontally, straight out from the sides of the aircraft, within mini-winglets that ended at the propeller nacelles. The forward engine drove the tractor propellers and the rear engine drove the pusher propeller. The propeller nacelles were braced to the base of the fuselage.
The P.12 took advantage of Boulton & Paul's work on metal aircraft, which began with the Boulton & Paul P.10. The P.12 used steel tubes that were made out of high quality steel strips, in what was known as the locked joint system. The structure was fabric covered.
The P.12 had an unusual undercarriage. The main wheels used oleo-pneumatic legs, but with spring loaded valves to let the oil in the cylinders in the legs out over certain pressure levels. When resting on the ground the cylinders were pumped full of air using a tyre pump. This raised the air pressure to the right level for takeoff. A set of smaller nose wheels were placed in front of the main wheels to prevent the aircraft from tipping over.
The P.12 had six separate fuel tanks and six radiators, each of which could be independently isolated in case of a leak. The fuel tanks and radiators were carried in the winglets between the engine and propellers.
Two prototypes of the Bodmin were built, with the first making its maiden flight early in 1924 with Frank Courtney at the controls. Because the Bodmin was officially a civil aircraft, its maiden flight gained a great deal of publicity, especially because it was such a novel design, but the first few flights went without any major problems. Inevitable the transmission caused some problems, and the engines were also prone to overheating. The one significant failure came in the undercarriage, when the starboard oleo strut collapsed while taxi-ing on 11 February 1924. This was caused by a hillock on the airfield, something that you wouldn't expect to find on an airfield, even in 1924!
The 'engine room' idea was judged to have been a failure, imposing extra weight in the drive shafts and extra gears required to get power from the engine to the propellers, as well as requiring stronger wings to compensate for the unusual location of the engines. The Bodmin could only reach 116mph at sea level and had a low service ceiling of 16,000ft.
The metal construction was considered to have been 20% lighter than an equivalent wooden structure, an improvement on the 10% estimated before construction began. However this only made up for the heavier weight of the engine room installation.
The experience gained on the P.12 did come in useful when Boulton & Paul were given a contract to produce a metal version of their P.7 Bourges, which as the Boulton & Paul P.15 Bolton was the first all-metal aircraft to be delivered to the RAF.
Engine: Two Napier Lion
Power: 450hp each
Crew: Pilot, engineer, potentially two gunners
Length: 53ft 4.5in
Empty weight: 7,920lb
Loaded weight: 11,000lb
Max speed: 116mph at sea level
Climb Rate: 8min 9sec to 6,500ft
Service ceiling: 16,000ft
Armament: Lewis guns in nose and dorsal positions.