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HERE’S A DESIGN that looks particularly unorthodox, only, guess what, it does make sense. The Blackburn B.20, a British aircraft at the beginning of World War II, was a flying boat, yet also a floatplane.
Resting on the water, the Blackburn B.20 was evidently supported by its large central float and a pair of smaller wingtip floats; the perfect definition of a floatplane.
Yet, aloft, the B.20 clearly looked like a flying boat, as characterized earlier here at SimanaitisSays in “Two Unabashedly Nautical Flying Boats.”
That is, the B.20 was neither precisely floatplane nor flying boat. It featured a unique arrangement of retracting floats, sort of a floatplane on stilts—but for very good reason.
A conventional floatplane layout has the advantage of elevating its large propellers well above the craft’s waterline. Also, the float can be designed to balance the aircraft with a large angle of attack for its wing, crucial in gaining lift for takeoff. There’s a tradeoff, though: A design’s floats and attendant struts cause high drag in flight, to the detriment of performance.
By contrast, a flying boat has an aerodynamically sleek fuselage that performs double duty in floatation. There are no drag-inducing struts, and the craft is more amenable to high performance once aloft.
However, again to provide ample clearance for large props, the flying boat must sit high in the water—thus translating into a relatively tall fuselage compared to a floatplane’s. Also, the aircraft’s fuselage design is less amenable to an optimal angle of attack for its wing. What’s optimal for takeoff carries a penalty of high drag in flight.
British aircraft engineer John Douglas Rennie led the project at Blackburn Aircraft to hybridize these two concepts of floatplane and flying boat. The fuselage design was split longitudinally, its lower portion being the primary float. Once airborne, hydraulic actuation united this float with the rest of the fuselage. Stability on the water was enhanced by smaller floats that retracted into wingtips once airborne.
The lower-fuselage float was divided into five watertight compartments, the center one serving as fuel tank as well. The upper fuselage contained what my Brit source calls a “bomb-aimer’s” location in the nose, a cabin for pilot, copilot, navigator, “wireless” operator and flight engineer. Designed for extended duty of eight-hours duration, the B.20 also had a galley and bunks located aft.
The aircraft was powered by twin Rolls-Royce Vulture engines. The Vulture had an unorthodox X-24 layout, essentially two Rolls-Royce aero V-12s aligned one atop the other in a single block sharing one crankshaft. The Vulture was chosen for its high output, 1720 hp, though teething troubles of this powerplant resulted in an eventual derating to around 1500 hp.
Only a single B.20 was built at Blackburn’s Dumbarton facility, on the north bank of the River Clyde, about 14 miles northwest of Glasgow, Scotland. The aircraft first flew on March 26, 1940. Initial test flights were successful; in particular, the retractable floats performed as designed.
However, on April 7, 1940, a test run generated severe vibration traceable to aileron flutter and the crew was forced to bail out. Three were lost, the other two picked up by HMS Transylvania, a converted merchantman. Development work ceased.
According to Wikipedia, the aircraft’s wreck still exists, remaining undisturbed and designated as a War Grave. In 1988, one of the B.20’s engines snagged a fishing boat’s net and was dragged into shallower water. The engine currently resides at the Dumfries and Galloway Aviation Museum, in Dumfries, Scotland, 60 miles southeast of Glasgow, about 340 miles northwest of London.
I learned about the Blackburn B.20 from G.R. Duval’s book British Flying-Boats and Amphibians, 1909-1952. The aircraft’s unorthodoxy made it a perfect project for Microsoft Flight Simulator 98, an earlier version of the sim. Since then, others have built virtual B.20s for subsequent versions.
Thus, through the magic of Microsoft Flight Simulator, the versatility of the B.20 can still be exhibited. ds
© Dennis Simanaitis, SimanaitisSays.com, 2017
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Here’s a link with good drawings of the steps in the fuselage, and the rudders. ( https://www.flightglobal.com/FlightPDFArchive/1938/1938%20-%200981.PDF ) For this plane, another complication, radiator shutters for the liquid cooled Junkers diesels.
One wonders how much further this 1938 non-stop flight record holder might have gone without the aerodynamic compromise, and if it had it not encountered unfavorable winds. It flew the equivalent of New York to 300 miles past the Hawaiian Islands.
According to Junkers, the engines burned a record low .341 lbs. per HP per hour, which would have been an excellent full power, much less part power, rate.