Ronald Ward Harker was born at Tynemouth, where his father was chief medical officer for the Tyne ports. He was educated at Shrewsbury School and joined Rolls-Royce in 1925 as an engineering apprentice. In 1927 a visit to the Hendon Air Display gave him the impulse to fly. He joined Newcastle flying club in 1927. He finished his apprenticeship in 1929, but by that time the Depression was biting and there was no job for him at Rolls-Royce. With parental support he kept up his flying at the Lympne flying club in Kent, and in 1934 was invited back to Rolls-Royce on the aero-engine side. When Rolls-Royce formed its first test flight he became its first test pilot and was soon evaluating various types of RAF aircraft. He also joined No 504 City of Nottingham Squadron, Royal Auxiliary Air Force, based at Hucknall, where Rolls-Royce was soon to move its test flying. When war came in 1939 he joined his squadron on a full-time basis, but in the spring of 1940, with the squadron ordered to France, he was ordered back to Rolls-Royce to resume test flying at Hucknall, liaising closely with the RAF. Harker was at Hucknall when, in April 1942, he received a telephone call from the CO of the RAF's Air Fighting Development Unit at Duxford, to tell him that they had acquired an Allison-engined Mustang and would he like to evaluate it? Having spent 30 minutes throwing the aircraft round the sky he reported that it closely resembled the Messerschmitt 109F, but with a Merlin 61 engine ought to prove much faster than that aircraft and the Spitfire V. The Air Ministry, however, wanted to put all the available Merlin 61s into the new Spitfire IX to combat the threat of the latest German fighter, the Focke Wulf 190, which was proving vastly superior to the Spitfire V in combat. There was therefore a good deal of concerted scepticism about Harker's observations. But he persisted and the first Merlin-engined Mustang flew in October 1942, giving the radical improvements in performance that he had predicted. News of the Merlin Mustang's performance spread like wildfire and was greeted as manna from heaven in Washington. Indeed, the Americans were the chief beneficiaries of Harker's initiative, since the new escort fighter enabled the USAAF to resume daylight bombing raids which had been discontinued, since the "invulnerable" B17 Flying Fortress had proved incapable of defending itself against the Luftwaffe's fighters.
Throughout the war Harker was involved in a variety of other projects for improving the performance of RAF aircraft. Improvements in superchargers increased the speed of the Spitfire; Merlins were put into the Whitley bomber; and - the greatest bomber success of all - the disastrous Vulture-engined Avro Manchester became the superlative Merlin-engined Lancaster. But the Mustang remains his supreme achievement. By the end of the war 15,582 of the aircraft had been built. Harker was appointed OBE and given the Air Efficiency Award (AE) for his wartime work.After the war Harker continued his liaison work with the RAF, testing new types. In 1974 he moved to London as Rolls-Royce's aero-export manager and from 1957 as the company's military adviser. He retired from the firm in 1971 when it went bankrupt over the financial problems caused by the escalating cost of the RB211 engine for the Lockheed TriStar. Over the years he had spent an increasing amount of time in New Zealand, pursuing his passion for fishing - and flying - and he finally settled there with his second wife in 1993. He had his last flight in a Mustang in New Zealand in 1997 at the age of 88.
Ronnie Harker has his place in the history of aviation for the role he played in the evolution of the Mustang fighter into one of the great warplanes of history. The North American P-51 Mustang was originally built for the British Purchasing Commission and the early aircraft had many pleasing qualities but its Allison V-1710 engine gave it poor performance at higher altitudes and its range was somewhat short.he Mustang entered service with the RAF in 1941 but, because its performance did not challenge that of the latest marks of Spitfires, it was relegated to Army co-operation and recon work where it performed admirably. However, Ronnie Harker, RollsRoyce's senior test liaison pilot, was offered the opportunity to test the Mustang by the RAF. He liked the aircraft's handling qualities but not its engine, which did not have the performance to exploit the fighter's advanced aerodynamics. He was convinced the airplane would be another animal entirely if fitted with the Rolls-Royce Merlin.
Harker pressed strongly for the American engine to be replaced by the Merlin and, after a good deal of official reluctance, largely from the Air Ministry, he got his way. The result was a transformation. The Mustang's top speed went from 390 mph to 440 mph and the range from 450 miles to as much as 2000 with various configurations of drop tanks. A great escort fighter had been born and ever afterwards Harker was known as "the man who put the Merlin in the Mustang."