The De Havilland DH66 Hercules was designed to meet the needs of Imperial Airways for their Cairo to Karachi route, with a requirement to have sufficient power in reserve to maintain height following any kind of engine failure, and to cope with the elevated terrain and high temperatures found on this challenging route. It replaced the de Havilland DH10 which had been designed towards the end of World War 1 as a bomber and thus featured little refinement. Conversely, the DH66 Hercules led the way for future airline travel.
It was a large, two-bay biplane with a fuselage of steel tubular construction, powered by three Bristol Jupiter VI engines. The tail surfaces included three fins and rudders, a low-set tailplane and elevators, with a smaller fixed tailplane mounted level with the top of the fins.
The prototype (G-EBMW) was flown for the first time at Stag Lane on 30th September 1926 with the air route to Karachi being inaugurated with a flight by the second aircraft (G-EBMX) which left Croydon on 27th December 1926, arriving in Karachi on 8th January 1927.
Initial testing had revealed poor lateral control and so ailerons were fitted to all four wings. Imperial Airways ordered five aircraft, the last entering service in March 1927 and the type proved immensely reliable and successful in service.
Four additional aircraft with enclosed cockpits and a tail wheel (rather than a skid) were ordered by West Australian Airways to replace their fleet of DH50s and they were completed in 1929 although their enclosed cockpit design was later retrofitted to the earlier production machines.
Passenger accommodation in these machines was provided for up to 14 passengers as well a a quantity of mail bags and they operated bewteen Perth and Adelaide.
Two additional machines, built to the same 1929 configuration, were ordered by Imperial Airways giving a total production of eleven aircraft, seven of which were UK registered with four in Australia.
Other than their commercial use on the route to India, a notable survey flight was made in December 1931 using a Hercules (G-AARY), which flew to Cape Town to investigate the extension of the Empire Air Route into South Africa. This route began regular operations in 1932.
Alan Cobham also used a DH66 Hercules (G-ABMT leased from Imperial Airways) as part of an Air Pageant Tour to South Africa in 1932 to 1933.
Three machines (G-EBMX, G-AUJQ and G-AAJH) also served with the South African Air Force (SAAF serials 260-262) the last two of these remaining in service until they were finally retired and scrapped in 1943.
|Powerplant||Three 420 hp Bristol Jupiter VI radial engines|
|Span||79 ft 6 in|
|Maximum Weight||15,660 lb|
|Capacity||2 pilots, wireless operator and 7 passengers plus 465 cu ft mail compartment and 155 cu ft baggage compartment.|
|Maximum Speed||128 mph|
|Cruising Speed||110 mph|