It's rare for civil and military versions of similar aircraft to be developed concurrently. The Avro Lancaster developed into the peacetime York; the Comet morphed into the Nimrod, the HP O/400 bomber turned into the Type W airliner, and then back into a bomber with the Handley Page Hyderabad.
So Short's decision to develop the civil Empire alongside the military Sunderland was an unusual one. The Empire grew from a specification issued by Imperial Airways for a long-haul airliner. They required a range of at least 700 miles, with a facility to tank-up to a range of 2,000 miles. Comfortable accommodation for 24 passengers was stipulated, along with additional room for freight and mail. As this would require a far longer runway than was available anywhere in the world, the only choice available to the technology of the time was for a flying boat.
Short Brothers already had an enviable track record in designing flying boats, so were an obvious choice for fulfilling the brief. Experience had shown that the engines needed to be mounted as high as possible to avoid sucking spray into the carburettors (See for example the pylon mounted engines on the Dornier Do X) Rather than adopt this configuration or mount the wing itself on a pylon (as seen on the Consolidated PBY Catalina), Shorts opted for a deep hull. This provided extra room inside the fuselage, allowed for stronger bracing for the cantilever wings, and allowed for clean, low-drag lines. The resulting design showed a pleasing, attractive aspect for the airliner and a satisfyingly bellicose appearance for the military Sunderland.
At 114ft wingspan, the Empire and Sunderland were the largest aircraft so far built in Britain. This presented many design challenges, particularly in producing adequate lift at the relatively low speeds required in take-off and landing. The design team's leader, Arthur Gouge, conceived a completely new flap system. As it extended, the Gouge flap created a slot towards the rear of the wing, allowing high pressure air under the wing to bleed through into the low pressure above, reducing turbulence and delaying the stall. Meanwhile, the backwards movement increased the effective chord of the wing, providing extra lift with minimal extra drag. This allowed the aircraft to get up onto the hull step much sooner, reducing hydraulic drag and allowing flying speed to be reached more quickly. On landing, approach speed could be reduced by almost 10mph, shortening the deceleration sime and making for safer landings.
All of the aircraft ordered by Imperial were individually christened with names begining with a "C", hence the 'C' Class in the model title. First to fly was G-ADHL "Canopus". She took to the air on 4th July 1936. Shorts went on to produce a further 41 examples. The type was resoundingly successful for Imperial Aiways.
The Empire made its first Atlantic crossing on 5th July 1937. While it could, with the aid of ferry tanks, manage the transatlantic trip, it was not a true cross-ocean spanning airliner. To make the trip would require too much trade-off of fare-paying passengers for fuel to make the route commercially viable. In order to overcome this shortcoming, an innovative, albeit bizarre, solution was tested. If a smaller aircraft were to be mounted above the wings of the Empire, the combined lift and thrust of the two machines would suffice to carry passengers and freight across the ocean. The aircraft would separate in flight and the smaller machine would race ahead with mail and cargo. Imperial Airways' technical advisor, Robert Mayo was a highly vocal supporter of the scheme and work was begun on a creation that would be called the Short Mayo Composite.
The Mayo experiment was successful, but the intervention of WW2 meant that only one composite was built. By the end of the war, aviation technology had moved on so far that land-based aircraft could easily handle the loads and runway lengths that had defeated them before hostilities. The brief reign of the flying boat was at an end.
|'C' Class Empire by numbers|
|First Flight:||3 June 1936|
|Powerplant:||4 × Bristol Pegasus radial engines of 696kW (920hp)|
|Range:||1,223km (760 mi)|
|Maximum Speed:||322km/h (200mph)|
|Length:||26.8m (88ft 0in)|
|Height:||9.7m (31ft 10in|