In 1917, after initial promising tests of the ABC Wasp air-cooled radial, Bradshaw produced a design for a larger and more powerful engine, the nine-cylinder Dragonfly. The engine was simple and easy to produce, and was predicted to give 340 hp (254 kW) for a weight of 600 lb (273 kg). One distinctive feature was the use of copper-plated cooling fins, which were claimed by Bradshaw to be so effective that water would not boil on the surface of the radiators.
On the basis of the promised performance, Sir William Weir, the Director of Aeronautical Supplies, made the decision to place large orders for the Dragonfly, with 11,500 engines having been ordered from 13 suppliers by June 1918. It was planned that a large proportion of RAF aircraft would be powered by the Dragonfly in 1919. Types designed to be powered by the Dragonfly included the Sopwith Dragon (a derivative of the existing Snipe), the Nieuport Nighthawk, and the Siddeley Siskin. Of this order 1,147 engines were built but only nine or ten actually flew.
Testing showed severe problems with the much vaunted engine. It was subject to extreme overheating, the copper-plated cooling fins proving useless; showed much poorer fuel consumption than expected; and suffered severe vibration, running at the resonance frequency of the crankshaft. These problems proved unsolvable, and the Dragonfly was eventually abandoned.
1918, 320 hp (239 kW)
360 hp (268 kW), revised pistons and cylinder heads, revised oil system.
Type: 9-cylinder air-cooled radial engine
Bore: 5½ in (139.7 mm)
Stroke: 6½ in, (165.1 mm)
Displacement: 1,389.86 in³ (22.78 L)
Dry weight: 600 lb (273 kg)
Valvetrain: 3 overhead valves, 2 inlet and 1 outlet per cylinder
Fuel system: Twin carburettors
Oil system: 2 oil pumps
Cooling system: Air-cooled
Power output: 340 hp (254 kW) at 1,650 rpm
Specific power: 0.24 hp/in³ (11 kW/L)
Specific fuel consumption: 0.56 pints per bhp/hour(0.32 L/kW)
Oil consumption: 7 pints per hour (3.97 L/hr)
Power-to-weight ratio: 0.56 hp/lb (0.93 kW/kg)