The Blackburn B-20 was an inventive design for a flying boat where the planing bottom was carried below the main hull on the water and pulled up into it when in the air, giving it good performance on the water and in the air.
Two of the main problems faced by designers of flying boats were how to keep the propellers clear of the water and how to get a high angle of wing incidence at take off without producing a nose-down attitude in flight. The first problem was normally solved either by giving the aircraft a very tall fuselage or by mounting the wings above the fuselage
Blackburn’s solution was to split the fuselage of the B-20 in two. The planing bottom, the boat shaped part of the hull that gives some of the lift, was split from the main part of the fuselage, and attached to it by hydraulic jacks, forming a pontoon that the boat floated on in the water. Wing tip floats were installed, which folded up and out to form the wingtips when the aircraft was in flight. The required angle of wing incidence was created by the design of the pontoon, allowing the wings to be installed at the best angle for normal flight. In effect the B-20 was a massive floatplane with a retractable central float.
The advantage of the Blackburn system can be seen when the B-20 is compared to the Saunders Roe Lerwick. The Lerwick’s fuselage was 20ft tall, meaning that the propellers were five feet closer to the water than on the B-20, which was 25ft 2in tall when the bottom float was lowered, but only 11ft 8in tall when the float was retracted.
The B-20 was an all-metal high-wing cantilever monoplane. It was powered by two Rolls Royce Vulture X engines mounted in nacelles. It used stressed skin construction, with Alclad cladding over the framework. The pontoon was built more solidly than the main fuselage, and used countersunk rivets to keep it streamlined. The pontoon was divided into five watertight sections, with the central section carrying 976 gallons of fuel. The wings were of all-metal construction, with three main spars.
Normal access to the aircraft was through a folding ladder that ran from the top of the float to a hatch in the nose of the main aircraft. It had space for a bomb aimer, two pilots, navigator, observer and engineer. It also had a wardroom with sleeping space for two officers, bunks for four more crew, a galley and toilet, all required for the very long duration of maritime patrol missions. The prototype was unarmed but production aircraft would have carried two .303in machine guns in the nose, four in a tail turret and two in a powered turret amidships. Four bomb cells were built into the inner part of the wings, two on each sides (between the fuselage and engine nacelle), each capable of carrying a 500lb bomb.
The prototype took three years to build, and didn’t make its maiden flight until March or April of 1940. In early tests the planing bottom and the retracting system worked well, but the aileron control was poor. On 7 April the prototype crashed into the sea during high speed trials over the Clyde. Three of the crew escaped but two were killed including Blackburns’ chief test pilot Flt L. H Bailey.
Although the basic concept appeared to be sound, by 1940 Blackburn were busy building the Short Sunderland, which may have been less radical than the B-20 but was a perfectly good flying boat. The B-20 would probably have needed quite a bit of work to turn it into a superior combat aircraft, not least to allow it to carry heavier bombs (the Sunderland could carry 1,000lb mines). Further work on the B-20 was delayed and then cancelled. Blackburn did do some work on the B-44 retractable float fighter, using the same basic concept but this never reached the prototype stage.
Engine: Two Rolls Royce Vulture X liquid cooled engines
Power: 1,720hp each
Crew: 5 (when crashed).
Span: 84ft 2in (floats retracted), 76ft 0in (floats down)
Length: 69ft 7.5in
Height: 25ft 2in (bottom lowered), 11ft 8in (bottom retracted)
All up weight: 35,000lb
Maximum take-off weight:
Max speed (estimated): 306mph at 15,000ft
Endurance: 8 hours
Range: 1,500 miles