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| An excellent view of the Blackburn B.20 flying boat highlighting the aircraft’s extended hull, retracted wingtip floats, and well-engineered cowlings for the Vulture engine. |
The Blackburn B.20 was an experimental aircraft, first flying in 1940, that attempted to drastically increase the performance of flying boat designs. Blackburn Aircraft undertook an independent design study based on a patent filed by their chief designer, John Douglas Rennie for a retractable pontoon float that formed the planing hull.
Design and Development
The B.20 was an attempt to combine the best features of both the flying boat and the floatplane. While on the water, the B.20 was essentially a floatplane, using a large float under the fuselage for buoyancy, and two smaller floats near the wingtips for stability. In flight, the main float retracted upwards towards the fuselage, fitting into a "notch" to become streamlined as a part of the fuselage. The wing floats folded outwards, somewhat like those on the American Consolidated PBY flying boat design, to become the wingtips. This configuration gave the correct wing incidence for takeoff and for flight and in the latter a much reduced drag compared to the deep hulls of flying boats.
Blackburn, along with Supermarine, Shorts and Saunders-Roe tendered designs against Air Ministry Specification R1/36. The Supermarine was chosen initially but Supermarine could not start work soon enough (due to their work on the Spitfire) and what would enter service as the Saunders Roe Lerwick became the chosen aircraft. However, the Ministry was interested enough to authorize and contract for the construction of a prototype of the B.20, serial number V8914, to test the concept.
Testing
The prototype, built at Dumbarton, flew for the first time on 26 March 1940. On 7 April, during a test run, the aircraft experienced extreme vibration due to aileron flutter and the crew bailed out. Three were lost, the other two were picked up by HMS Transylvania, a converted merchantman. Development ceased when the first prototype crashed, as Blackburn's resources were dedicated to the war effort. The Ministry felt the concept had been proven and the crash was not due to the pontoon design.
The aircraft's wreck still exists, but remains undisturbed as it is designated a War grave. In 1998, one of the engines was raised as it had been caught in a fishing boat's nets and dragged away from the wreck, into shallower water. It is currently an exhibit in the Dumfries and Galloway Aviation Museum.
B.40
The B.40 was an improved variant of the B.20 with Bristol Centaurus engines to meet a requirement for a small general purpose flying boat and specification R.13/40 was raised for it. Two prototypes were ordered in September 1941 but the situation was reconsidered in December. Its range was insufficient improvement over the Sunderland III, performance on one engine was unacceptable, and land-based patrol aircraft were capable of covering longer ranges. Further, there was little value as an experimental design because the principle had been proven in the B.20 and for an aircraft the size of the B.40 there would not be a significant improvement in drag. With no operational requirement for the B.40, it was therefore cancelled.
Type: Prototype flying boat
National origin: United Kingdom
Manufacturer: Blackburn Aircraft
Designer: John Douglas Rennie
Primary user: Royal Air Force
Number built: 1
First flight: 26 March 1940
Specifications (B-20, as designed)
Crew: 6
Length: 69 ft 8 in (21.23 m)
Wingspan: 82 ft 0 in (24.99 m) floats retracted
Height: 25 ft 2 in (7.67 m) hull extended
Wing area: 1,066 sq ft (99.0 m2)
Maximum takeoff weight: 35,000 lb (15,876 kg)
Powerplant: 2 × Rolls-Royce Vulture X-24 liquid-cooled piston engines, 1,720 hp (1,280 kW) each
Propellers: 3-bladed Rotol constant-speed feathering propellers
Maximum speed: 306 mph (492 km/h, 266 kn) at 15,000 ft (4,572 m)
Range: 1,500 mi (2,400 km, 1,300 nmi)
Armament:
Guns: provision for two turrets and other defensive positions
Bombs: bomb-cells in center section
Bibliography
"Blackburn B.20". British Aircraft Directory. 20 July 2003.
Bussy, Geoffrey (2006). "Blackburn's Amazing B.20 Flying Boat". Air Enthusiast (124). Stamford, UK: Key Publishing: 28–29.
Buttler, Tony (2004). British Secret Projects: Fighters & Bombers 1935–1950. Hinckley, UK: Midland Publishing.
Jackson, A. J. (1989). Blackburn Aircraft Since 1909. London: Putnam.
Meekcoms, K. J.; Morgan, E. B. (1994). The British Aircraft Specification File. Tonbridge, UK: Air-Britain.
Townend, David R (2010).
Clipped Wings – World War Two Edition. Markham, UK: Aerofile Publications.
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| This side view of the B.20 illustrates how the hull moved forward as it was extended. The rear member of each of the four hull mounts was a hydraulic cylinder that actuated the extension and retraction of the hull. |
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| The B.20 on the water looked a little ungainly with its hull extended. Note the access ladder between the hull and the fuselage. |
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| The B.20 providing a good view of the wing float design. Note the Short Sunderland and what appears to be a Short Empire framed nicely between the B.20’s hull and fuselage. |
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Rear view of the B.20 helps visualize the defense the four .303 machine guns in the turret would have provided. |
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| Blackburn B.20. |
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| The diagram for J.D. Rennie's patent for a retractable bottom for a flying boat (patent number 433925). Note that at this stage it was envisaged that the wing floats would retract into the rear of the engine nacelles. |
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| Major John Douglas Rennie, Chief flying boat designer for the Blackburn Aircraft company and designer of the B.20. |
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| The Blackburn B20. It is likely to be a composite image, airbrushed to show the B.20 as it would have appeared in flight. |
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| The B.20 pontoon, showing the ladder to gain entrance. There were another two hatches in the "floor" of the aircraft to allow escape onto the pontoon in an emergency. Mooring equipment (boat hooks and anchors) was stored under panels in the pontoon. The fuel tanks were accommodated in the center of the pontoon which made fuel replenishment much simpler than on a traditional flying boat. |
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| Flt Lt Henry "Harry" Bailey, A.M.I.C.E., A.F.R.Ae.S, pilot of the B.20, who lost his life in the crash. |
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| How a Hercules powered version of the Blackburn B.20 might have looked. |
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| How the B.20 might have looked in the air-sea rescue role. Its speed would have made its chance of survival in this task over the North Sea much greater than the types which actually carried it out. The pontoon float would have greatly simplified the task of picking up survivors. |
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| Blackburn B.20. |
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| Blackburn B.20. |
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| On the water, B.20 prototype V8914 showing retracting wingtip floats. Mooring could be done from the deck of the retractable pontoon. |