The Rolls-Royce R is a British aero engine that was designed and built specifically for air racing purposes by Rolls-Royce Limited. Nineteen R engines were assembled in a limited production run between 1929 and 1931. Developed from the Rolls-Royce Buzzard, it was a 37-litre (2,240 cu in) capacity, supercharged V12 capable of producing just under , and weighed . Intensive factory testing revealed mechanical failures which were remedied by redesigning the components, greatly improving reliability.
The R was used with great success in the Schneider Trophy seaplane competitions held in England in 1929 and 1931. Shortly after the 1931 competition, an R engine using a special fuel blend powered the winning Supermarine S.6B aircraft to a new airspeed record of over . Continuing through the 1930s, both new and used R engines were used to achieve various land and water speed records by such racing personalities as Sir Henry Segrave, Sir Malcolm Campbell and his son Donald, the last record being set in 1939. A final R-powered water speed record attempt by Donald Campbell in 1951 was unsuccessful.
The experience gained by Rolls-Royce and Supermarine designers from the R engine was invaluable in the subsequent development of the Rolls-Royce Merlin engine and the Spitfire. A de-rated R engine, known as the Griffon, was tested in 1933, but it was not directly related to the production Rolls-Royce Griffon of 1939, of the same exact bore/stroke and resultant displacement figures as the "R" design. Three examples of the R engine are on public display in British museums as of 2014.
Design and development
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.Eves 2001, p. 174. Shortly after this the engine's name was changed to R for "Racing".Holter 2002, p. 35. 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.Lumsden 2004, p. 168.
Description The R was a physically imposing engine designed by a team led by Ernest Hives and including Cyril Lovesey, Arthur Rowledge and Henry Royce. The R shared the Buzzard's bore, stroke and capacity, and used the same 60-degree V12 layout. A new single-stage, double-sided supercharger impeller was designed along with revised cylinders and strengthened connecting rods.Eves 2001, p. 225. The wet-liner cylinder blocks, crankcase and propeller reduction gear castings were produced from "R.R 50" aluminium alloy; and because of the short life expectancy of these engines, forged aluminium was used to replace bronze and steel in many parts.[Staff author] 2 October 1931. "The Rolls-Royce Racing Engines" Flight, p. 990. www.flightglobal.com. Retrieved: 14 November 2009.
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."[Staff author] 2 October 1931. "The Rolls-Royce Racing Engines" Flight, p. 989. www.flightglobal.com. Retrieved: 14 November 2009. 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. This was initially achieved with a blade-and-fork connecting rod arrangement; however, after cracking was found during testing in 1931, the rod design was changed to an articulated type.
The introduction of articulated connecting rods was regarded as a "nuisance" by Arthur Rubbra, a Rolls-Royce engine designer, as there were inherent problems with the arrangement. The complicated geometry meant that a pair of rods had different effective lengths, giving a longer stroke on the articulated side; consequently the cylinder liners on that side had to be lengthened to prevent the oil scraper ring from running out of the cylinder skirt. Articulated rods were used in the Goshawk engine, but were not embodied in the later Rolls-Royce Merlin, for which Arthur Rowledge had designed a revised blade and fork system.Rubbra 1990, p. 60.
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.
The ignition system consisted of two rear-mounted, crankshaft-driven magnetos, each supplying one of a pair of spark plugs fitted to each cylinder. This is common practise for aero engines, as it ensures continued operation in the case of a single magneto failure, and has the advantage of more efficient combustion over a single spark plug application.Gunston 2006, p. 60.
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.[Staff author] 16 July 1942. "Voices From The Past, Recorded talks by Schneider Trophy Contest designers and the winning pilot" Flight, p. 61. www.flightglobal.com. Retrieved 21 October 2009.
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.Eves 2001, p. 208. Although this robbed the engine of some power, it increased reliability and reduced the possibility of detonation in the cylinders.
Supercharger and fuel
The keys to the R engine's high power-to-weight ratio were its supercharger design, ability to run at high revolutions due to its structural strength, and the special blends of fuel used. The double-sided supercharger impeller was a new development for Rolls-Royce: running at a ratio of almost 8:1, it could supply intake air at up to 18 pounds per square inch (psi) (1.24 bar) above atmospheric pressure, a figure known as "boost" and commonly abbreviated as "+x lb".Holter 2002, p. 175. 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.Lumsden 2003, p. 195. 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.Eves 2001, pp. 230–231.
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.Eves 2001, p. 230. 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.
On an early test run the R engine produced and was noted to idle happily at 450 revolutions per minute (rpm). With increased boost ratings and fuel developed by Banks, the R engine ultimately developed at 3,200 rpm; well over double the maximum power output of the Buzzard.Lumsden 2003, p. 198. The engine was further tested and cleared for limited sprint racing at at 3,400 rpm and +21 lb (1.45 bar) of boost,Gunston 1989, p. 139. 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.Lumsden 2003, p. 199.
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. Many mechanical failures were experienced during bench testing including burnt valves, connecting rod breakages and main bearing seizures,Holter 2002, p. 38. 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, but the continual redesigning and testing of components reduced all these problems. Unknown to Royce himself, the engineers had also fitted "Wellworthy" pistons that were better able to withstand the 13 tons "pressure" of each firing stroke.Holter 2002, p. 38.
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. Development time was short and the deafening sound of three Kestrels and an R engine running at high power for 24 hours a day took its toll on the local population. The Mayor of Derby stepped in and asked that the people endure the noise for the sake of British prestige; subsequently testing continued for seven months.
In the course of a 25-minute test an early R engine would consume 60 Imperial gallons (gal) (270 L) of pre-heated castor oil. The majority of this was spat out of the exhaust ports and smothered the test cell walls, milk being given to staff to minimise the effects of this well-known laxative. Up to 200 gal (900 L) of the special fuel blend had to be mixed for each test, 80 gal (360 L) of which were used just to warm the engine to operating temperature. The same coarse-pitch propeller used for flight trials was fitted throughout these tests.
Flight testing