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Britain’s Blackburn B-20 attempted to overcome seaplanes’ inherent aerodynamic deficiencies via retractable center and wingtip floats.

The Blackburn B-20 was one of World War II’s most advanced but least publicized aircraft. After the single prototype built was lost in a crash in 1940,  its existence was kept secret for the next five years. Despite its promise, the B-20’s development had been hampered by bad timing to such an extent that, even if the prototype hadn’t crashed, it’s doubtful the design would have proceeded.

To understand why the Blackburn B-20 was developed in the first place, it helps to consider the state of aviation technology in the mid-1930s. The flying boat was generally regarded as the optimum configuration for a large, long-range airplane. Big landplanes required long, paved, well-maintained runways, at that time in short supply. But flying boats could take off and land wherever there was sufficient water. While landplanes were faster because their more streamlined shapes generated less aerodynamic drag, that advantage was often canceled out by the weight of their landing gear. Since flying boats had no landing gear, they could carry more fuel and cargo than comparable landplanes, which translated to longer range and greater payload—or bombload.

But there was no getting around the fact that flying boats were less aerodynamically efficient than comparable landplanes. Their fuselages had to be shaped into hulls that might be efficient hydrodynamically, but not necessarily aerodynamically. Little could be done to eliminate that problem without compromising a flying boat’s seaworthiness.

Furthermore, flying boat engines had to be mounted high enough to keep the propellers clear of the water, to prevent blade damage from spray thrown up by the hull during takeoff and landing. As a result, the engines were usually mounted high above the wings on pylons, or the aircraft had inordinately tall fuselages combined with a high straight-wing or gull-wing configuration. That design constraint adversely affected the plane’s aerodynamic efficiency. This was the problem the B-20 was designed to alleviate.

In 1936 Blackburn designer J.D. Rennie proposed a fresh approach on a newly issued RAF specification for a medium-range maritime patrol flying boat. In effect, the aircraft was to be a large twin-engine floatplane with retractable floats. The design would incorporate retractable wingtip floats similar to those later used on the Consolidated PBY Catalina, but its most novel feature was in the fuselage. In place of the usual fixed flying boat hull, it would be supported on the water by a large central float suspended beneath the fuselage on struts, similar to the way a seaplane was supported. But Rennie’s design featured a hydraulic mechanism to retract the huge central float flush with the bottom of the fuselage while in flight. The design allowed the engines to be mounted on the wing leading edges, where they generated the minimum amount of drag. The result was a flying boat combining efficient hydrodynamic properties with low aerodynamic drag.

The B-20’s large central float was subdivided into five watertight compartments and included four integral fuel tanks, holding 976 gallons. In truth, the B-20 was not the first seaplane to have retractable floats. During World War I Oskar Ursinus had devised a single-engine, single-seat floatplane fighter in which the twin floats were retracted against the bottom of the fuselage by means of a hand crank. A single prototype, powered by a 150-hp Benz engine and constructed by Gotha in 1916, was wrecked before any conclusive performance data could be obtained.

Rennie’s convertible flying boat proposal was perceived as too radical to be accepted for production. Instead the RAF received the conventional, and ultimately unsatisfying, Lerwick flying boat built by Saunders-Roe. But the Air Ministry considered Rennie’s design sufficiently promising to order two experimental prototypes from Blackburn.

The prototype was simply referred to by its design number, B-20, the 20th aircraft design produced in the current series by the Blackburn Aircraft Company. During the three years it was under construction, it acquired the unofficial nickname of “Nutcracker,” no doubt in reference to the hinged float retraction mechanism.

Constructed at Blackburn’s plant in Dumbarton, Scotland, the prototype took so long to complete because at the time the plant was also committed to producing Sunderland flying boats for the RAF under license from Shorts, as well as Botha patrol bombers for RAF Coastal Command. The Munich Crisis of 1938 and the outbreak of war in 1939 increased the priority assigned to Sunderland and Botha production.

The B-20’s design had originally called for two 1,377-hp Bristol Hercules radial engines. Due to the high performance expected of the new airplane, however, the prototypes were redesigned to house more-powerful Rolls-Royce Vulture engines. The Vulture was available in limited numbers, and the fact that two of them were earmarked for the B-20 indicates that the Air Ministry took a serious interest in the project.

A 1,720-hp liquid-cooled engine with 24 cylinders arranged in an “X” configuration, the Vulture was used in only one production aircraft, the ill-fated Avro Manchester bomber. Though the complex power plant had also been slated for use in the Hawker Tornado, due to reliability problems a Napier Sabre engine was eventually substituted—in which form the fighter achieved production as the Hawker Typhoon. The pressures of wartime production meant that Rolls-Royce had little time or effort to spare on working out bugs in the Vulture. Consequently, during 1941 development of the Vulture was dropped in favor of increased production of the company’s Merlin engine.

The B-20’s wingspan with floats extended was 76 feet; retracted, it was 82 feet 2 inches. The aircraft was 69 feet 8 inches long, and weighed 35,000 pounds fully loaded. Height with the floats lowered was 25 feet 2 inches, and 11 feet 8 inches once they were raised. Maximum speed was estimated at 306 mph, with a range of 1,500 miles at 200 mph.

The B-20 was finally flown in March 1940, after demonstrations had shown that the float-retracting mechanism worked exactly as advertised. Soon afterward, however, the aircraft suffered a bird strike, necessitating repairs. A more serious problem cropped up with the aileron trim, which had to be adjusted between preliminary test flights.

On April 7, the flying boat was finally taken up for a high-speed test run, during which it reportedly reached speeds in excess of 300 mph. Unfortunately, it also developed a severe structural vibration that did not diminish when pilot Harry Bailey throttled back the engines. Unable to control the vibration, which was threatening to shake the plane to pieces, Bailey ordered the four engineer observers to bail out.

The observers all managed to get out, though one man’s parachute briefly became entangled with a radio mast. Bailey stayed with the stricken airplane until he was sure everyone was clear. By the time he jumped, however, the B-20 was so low that his parachute didn’t open fully. Bailey lost his life, as did two of the observers, who drowned before they could be picked up.

After the Germans invaded Norway in April 1940, and France and the Low Countries on May 10, the war situation deteriorated rapidly for Britain. On May 14, new Prime Minister Winston Churchill appointed William Aitken, Lord Beaverbrook, as minister of aircraft production. Beaverbrook’s first priority was to increase production of the fighters and bombers the nation desperately needed to stave off invasion. Naturally, development of experimental and nonessential aircraft such as the B-20 was curtailed.

Rennie’s design had been successfully demonstrated. Investigators found that the crash was caused by a fixable aerodynamic problem: aileron flutter. But further development had to be suspended, and work on the second B-20 prototype was canceled.

Blackburn didn’t quite give up on Rennie’s retractable float idea, proposing a related design in 1941. Designated the B-44, it was a single-engine, single-seat fighter equipped with a retractable float arrangement similar to but smaller than the B-20’s. Powered by a 2,200-hp Napier Sabre, the B-44 was intended for operations against the Japanese from rivers and harbors in Burma, negating the need to hack airfields out of the jungle. In the end, however, sufficient airfields were secured to accommodate land-based fighters, so the B-44 was never built.

The Blackburn B-20 was an interesting conceptual test-bed that might have had a profound effect on seaplane evolution if it had been completed a year earlier. As it was, the B-20 appeared at a time when the British Air Ministry was in no position to spend money developing new aircraft that were not crucial to national defense. By war’s end, there were so many paved runways around the globe that flying boat development was effectively dead in the water.


Originally published in the January 2013 issue of Aviation History. To subscribe, click here.