Origin Rolls-Royce realised that the
Napier Lion engine used in the 1927
Supermarine S.5 Schneider Trophy winner had reached the peak of its development, and that for Britain's entrant in the next race to be competitive a new, more powerful engine design was required. The first configuration drawing of the "Racing H" engine, based on the
Buzzard, was sent to
R. J. Mitchell of
Supermarine on 3 July 1928, allowing Mitchell to start design of the new
S.6 Schneider Trophy seaplane. Shortly after this the engine's name was changed to R for "Racing". An official British Government contract to proceed with the project was not awarded until February 1929, leaving Rolls-Royce six months to develop the engine before the planned Schneider Trophy competition of that year. The wet-liner
cylinder blocks,
crankcase and
propeller reduction gear castings were produced from "
R.R 50"
aluminium alloy; which featured wet-liner cylinders and lower oil scraper rings, both originally developed for the R engine To make the R as compact as possible, several design modifications were made in comparison to the Buzzard: the propeller reduction gear housing was reshaped, and the
camshaft and
rocker covers were modified to
fair into the shape of the aircraft's nose, the air intake was positioned in the vee of the engine (which also helped to avoid the ingress of spray), and beneath the engine the
auxiliaries were raised a little to reduce the depth of the
fuselage. The engine's length was minimised by not staggering its cylinder banks fore and aft, which meant that the connecting rods from opposing cylinders had to share a short
crankshaft bearing journal known as the "
big end". This was initially achieved by fitting one connecting rod inside the other at the lower end in a blade and fork arrangement; however, after cracking of the connecting rods was found during testing in 1931, the rod design was changed to an
articulated type. Later production R engines featured
sodium-filled exhaust valve stems for improved cooling, while additional modifications included a redesigned lower crankcase casting and the introduction of an oil scraper ring below the piston
gudgeon pin; a measure that was carried over to the Merlin engine. A
balanced crankshaft was introduced in May 1931, and the
compression ratio on the "sprint" engines prepared for that year was raised from 6:1 to 7:1.
Cooling Cooling this large engine whilst minimising aerodynamic
drag posed new challenges for both the Rolls-Royce and Supermarine design teams. Traditional cooling methods using honeycomb-type
radiators were known to cause high drag in flight; consequently it was decided to use the surface skins of the S.6 wings and
floats as heat exchangers, employing a double-skinned structure through which the coolant could circulate. Engine oil was cooled in a similar manner using channels in the
fuselage and
empennage skins. The S.6 was described at the time as a "flying radiator", and it had been estimated that this coolant system dissipated the equivalent of of
heat in flight. However, even with this system in use, engine overheating was noted during the race flights, requiring the pilots to reduce the throttle setting to maintain a safe
operating temperature. A not-so-obvious cooling measure was the deliberate use of a
rich fuel mixture, which accounts for the frequent reports of black smoke seen issuing from the engine exhaust stubs. Although this robbed the engine of some power, it increased reliability and reduced the possibility of
detonation in the cylinders. By comparison the maximum boost of the earlier
Rolls-Royce Kestrel design was +6 lb (0.4 bar), this figure not being achieved until 1934. The high boost pressures initially caused the
spark plugs to fail on test, and eventually the
Lodge type X170 plug was chosen as it proved to be extremely reliable. The development of special fuel was attributed to the work of
"Rod" Banks, an engineer who specialised in fuels and engine development. After using neat
benzole for early ground test runs, a mixture of 11%
aviation petrol and 89% benzole plus 5
cubic centimetres (cc) of
tetra-ethyl lead per
Imperial gallon (4.5 L) was tried. This blend of fuel was used to win the 1929 Schneider Trophy race, and continued to be used until June 1931. It was discovered that adding 10%
methanol to this mixture resulted in a increase, with the further advantage of reduced fuel weight – particularly important for aircraft use – due to its lowered
specific gravity. For the 1931 airspeed record attempt
acetone was added to prevent intermittent misfiring; the composition of this final blend was 30% benzole, 60% methanol, and 10% acetone, plus 4.2 cc of tetra-ethyl lead per gallon. The engine was further tested and cleared for limited sprint racing at at 3,400 rpm and +21 lb (1.45 bar) of boost, but this capability was not used due to concerns with the S.6B's
airframe not being able to withstand the power, and the inability of the aircraft to lift the extra fuel required to meet the increased consumption.
Testing Ground testing The first run of engine
R1 took place at Rolls-Royce's
Derby factory on 7 April 1929 with
R7 running the next day. while considerably more trouble than expected occurred with valve springs; at one time two or three would be found broken after a 10-minute run, Ground testing of the R involved the use of three
Kestrel engines: one to simulate a headwind or
airspeed, one to provide ventilation of the test area, and another to cool the
crankcase. Superchargers could be tested on a separate rig that was driven by another Kestrel engine. Eight men were required to run a test cell, led by the "Chief Tester" who had the tasks of logging the figures and directing the other operators. One of these chief testers was Victor Halliwell who later lost his life whilst on board the water speed record contender
Miss England II. The conditions in the test cell were particularly unpleasant; deafness and
tinnitus lasting up to two days were experienced by test personnel even after plugging their ears with
cotton wool. During pre-race
scrutineering tests, metal particles were found on two of the engine's 24
spark plugs indicating a
piston failure which would require an engine re-build or replacement. The competition rules did not allow an engine change, but due to the foresight of
Ernest Hives, several Rolls-Royce engineers and mechanics that were familiar with the R had travelled down to
Southampton to witness the trials, and with their assistance one cylinder bank was removed, the damaged piston replaced and the cylinder refurbished. This work was completed overnight and allowed the team to continue in the competition. Engine starting was achieved by a combination of compressed air and a hand-turned
magneto; however, starting problems were encountered during pre-race testing at Calshot due to moisture in the air and water contamination of the fuel. A complicated test procedure was devised to ensure clean fuel for competition flights since more than 0.3% water content made it unusable. As expected, minor engine failures continued to be experienced, and to counter this engines and parts were transported at high speed between Derby and Calshot using an adapted
Rolls-Royce Phantom I motor car. Travelling mostly after dark, this vehicle became known as the
Phantom of The Night. Relationship to the Griffon and Merlin According to
Arthur Rubbra's memoirs, a de-rated version of the R engine, known by the name
Griffon at that time, was tested in 1933. This engine,
R11, but was otherwise a completely new design that first ran in the Experimental Department in November 1939. Although this single engine was never flown, the production version, the Griffon II, first flew in 1941 installed in the
Fairey Firefly. A significant difference between the R and the production Griffon was the re-location of the
camshaft and supercharger drives to the front of the engine to reduce overall length. Another length-reducing measure was the use of a single magneto (the R had two, mounted at the rear), this again was moved to the front of the engine. Further possible development work on the R engine was discussed in
The National Archives' file AVIA 13/122, which contains a proposal from the
Royal Aircraft Establishment dated October and November 1932, to test four engines to destruction. This document states that there were five engines available for test purposes, the fifth to be used for a standard
Type Test at high revolutions. Although not directly related to the
Spitfire, the
Supermarine engineers gained valuable experience of high-speed flight with the S.5 and S.6 aircraft, their next project being the
Rolls-Royce Goshawk-powered
Supermarine Type 224 prototype fighter aircraft. Technological advances used in the R engine, such as sodium-cooled valves and spark plugs able to operate under high boost pressures, were incorporated into the
Rolls-Royce Merlin design. The author Steve Holter sums up the design of the Rolls-Royce R with these words: ==Schneider Trophy use==