Born at Barnes, Surrey, on 23 March, 1902, Baynes was educated at Gresham's School, Norfolk, leaving school at the age of sixteen to join an aircraft company. His engineering skills were learnt at school and in industry.
After leaving school, Baynes started work in the fledgling aircraft industry with the Aircraft Engineering Company at Hendon. From there, he moved on to the famous firm of Short Brothers at Rochester, where he redesigned the Short Singapore flying boat.
In 1930, Baynes designed the Scud light sailplane, built at first by Brant Aircraft Limited at Croydon. The Scud was successful, and in 1931, Baynes went into partnership with E.D. Abbott as Abbott-Baynes Sailplanes Ltd, of Farnham, Surrey, to build Scud sailplanes, and later the Scud II (1932). In 1935, a Scud II flown by Mungo Buxton took the British Height Record for a glider to 8,750 feet (2,666 m).
Also in 1935, the Scud III was launched, which when fitted with an engine was called the Auxiliary. The latter was designed to the orders of Sir John Carden, an authority on tank design who was interested in gliding. He required a sailplane which could be launched unaided, and he suggested to Baynes a retractable engine.
Baynes later went into partnership with Carden (as Carden Baynes Aircraft Ltd of Heston Aerodrome, Middlesex) to build the Carden Baynes Auxiliary, essentially the Scud III Auxiliary. This carried a retractable 249 cc Villiers engine mounted in the top of the fuselage. The engine drove a pusher-propeller and produced 9 bhp, and the capacity of the fuel tank was enough to run the engine for thirty minutes. The 249 cc Auxiliary is believed to be the lowest powered aircraft in the history of powered flight.
Also in 1935, the famous Appleby Flea was rebuilt by its amateur designer, Stephen Villiers Appleby, incorporating design changes proposed by Baynes, who had witnessed the end of the first one in a disastrous maiden flight at Heston Aerodrome. The success of the Flea led to a best-selling book, The Flying Flea (1935), teaching readers how to build their own aircraft at home. Abbott-Baynes Sailplanes Ltd went into production of the Abbott Baynes Flea, which with its massed balanced front wing was a highly advanced small aircraft.
During the Second World War, Baynes was the aviation adviser to the Alan Muntz Company at Heston Aerodrome, specialists in weapons systems, and he organized an aircraft division of the company. In 1941, he put up a proposal for a detachable wing with a 100-foot wingspan which when attached to a tank would turn it into a glider. This concept was developed as far as the famous Baynes Bat prototype, with most of the test flights being piloted by Flight Lieutenant Robert Kronfeld.
Baynes also worked on designs for long-range bombers, and the V-22 Osprey was an American aircraft very similar to a bomber design submitted to the British government by Baynes during the Second World War.
After the war, in the 1940s and 1950s, Baynes was busy with research in the area of variable-sweep supersonic aircraft. He also designed interiors for airliners, invented the vertical lift plane and the high-speed hydrofoil.
Baynes designed the Youngman-Baynes High Lift, an experimental, flying test-bed for the system of slotted flaps invented by R.T. Youngman. It used components from the Percival Proctor, and was built by Heston Aircraft Company Ltd. The first flight was at Heston Aerodrome on 5 February 1948, carrying the military serial VT789.
A Scud II built in 1935 is still airworthy and is believed to be the oldest flying glider in the United Kingdom.