By William Pearce
On 13 February 1935, John Douglas Rennie submitted a patent application for “Improvements in and relating to Seaplanes.” Rennie was the Chief Seaplane Designer for the Blackburn Aeroplane and Motor Company, which was renamed in 1936 as Blackburn Aircraft Ltd. Rennie’s design idea was for the lower portion of the flying boat’s hull to be sealed and extend for takeoff and landing. The extendable hull would essentially act as the aircraft’s main float.
In order to provide clearance for the propellers, traditional flying boats have some combination of a parasol or strut-mounted wing positioned above the fuselage, a gull wing, and a tall hull. In addition, the hull and wing are designed for the essential task of lifting the aircraft from the water, but they are far from optimized for cruise flight. All of these compromises add significant drag to the aircraft. With Rennie’s hydraulically-operated extendable hull, the flying boat’s cross section with the hull retracted was much more like that of a conventional aircraft, and drag was significantly reduced. In addition, when the hull was extended, the aircraft assumed the ideal angle for takeoff and landing, which allowed the aircraft’s wing to have an angle of incidence optimized for cruise flight when the hull was retracted. Rennie’s patent also included retractable wingtip floats.
Rennie was granted Great Britain patent 433,925 on 22 August 1935. In 1936 the British Air Ministry issued Specification R.1/36 for a small, general purpose flying boat capable of cruising at 230 mph (370 km/h). Rennie and Blackburn responded with a twin-engine flying boat that featured a retractable hull. Blackburn’s design carried the company designation B-20. The Air Ministry ordered the Saunders-Roe A.36 Lerwick for Specification R.1/36, but they were sufficiently intrigued by the Blackburn B-20 to order a prototype, which was later assigned serial number V8914.
Technically, the B-20 was more of a floatplane with a retractable center main float than a flying boat. However, when the float was retracted, the aircraft took on the appearance and configuration of a flying boat. The B-20 had a high wing and was of all-metal, stressed-skin construction. All of the control surfaces were fabric covered. With the exception of the extending hull, the aircraft had a conventional layout. The B-20 had a standard crew of six. The fuselage housed a bombardier’s compartment in the nose. The fight deck was located well forward of the wing attachment and provided the pilot and copilot a good view. Behind and slightly below the cockpit was the flight engineer, navigator, and observer’s compartment. Under the wing was a wardroom with sleeping accommodations for two, followed by the crew’s quarters with accommodations for four, a galley, and a lavatory.
The one-piece wing had three main spars, a straight leading edge, and a tapered trailing edge. Mounted in a nicely streamlined nacelle on each wing was a Rolls-Royce Vulture II X-24 engine capable of 1,800 hp (1,342 kW) for takeoff. The engine had an international rating of 1,780 hp (1,327 kW) at 4,000 ft (1,219 m) and 1,660 hp (1,237 kW) at 13,500 ft (4,115 m). A scoop under the engine nacelle housed the engine’s coolant radiator and oil cooler. Each engine turned a three-blade, constant-speed, de Havilland propeller. Unlike the patent design, which featured wing floats that retracted into the engine nacelles, the wing floats of the B-20 retracted outward to be flush with the wing and form the wingtip.
The extendable hull had five watertight compartments. The center compartment housed four fuel tanks with a total capacity of 1,172 US gallons (976 Imp gal / 4,437 L). The hull also housed most of the mooring equipment. Four hydraulic cylinders mounted in the fuselage controlled the extension and retraction of the 48 ft 9 in (14.86 m) hull. The hydraulic cylinders extended the hull approximately 5 ft 8.25 in (1.73 m) down from the fuselage for operating on the water’s surface. Forward of each hydraulic cylinder was a hinged triangular frame mounted to one point on the fuselage and two points on the hull. As the hull extended down, it also traversed forward. This movement of the hull gave the aircraft the proper angle for landing and taking off. Entry to the fuselage was achieved with the hull extended. Hatches under the fuselage led to the bombardier’s station, the wardroom, and the galley. A ladder that hinged down from the hatch under the bombardier’s station was the main access point.
Although the prototype was unarmed, the B-20’s planned armament consisted of two .303 machine guns in the nose, a dorsal turret with two .303 machine guns, and a tail turret with four .303 machine guns. In each wing, two compartments between the engine nacelle and the fuselage could each house a 500 lb (227 kg) bomb or two 250 lb (113 kg) bombs.
The Blackburn B-20 had a wingspan of 82 ft 2 in (25.04 m) with the floats retracted and 76 ft (23.16 m) with the floats extended. The aircraft was 69 ft 7.5 in (21.22 m) long, and was 25 ft 2 in (7.67 m) tall on its beaching gear with the hull extended. Without the turrets, the B-20 had a top speed of 322 mph (518 km/h) at 15,000 ft (4,572 m), 302 mph (486 km/h) at 5,750 ft (1,753 m), and 280 mph (451 km/h) at sea level. With the proposed turrets, the aircraft’s performance fell to a maximum speed of 306 mph (492 km/h) at 15,000 ft (4,572 m), 288 mph (464 km/h) at 5,750 ft (1,753 m), and 268 mph (431 km/h) at sea level. Cruising speed was 200 mph (322 km/h), and the B-20’s range was 1,500 miles (2,414 km). The aircraft had a normal weight of 35,000 lb (15,876 kg).
The B-20 was completed at Blackburn’s factory in Dumbarton, Scotland, near the River Clyde. The aircraft made its first flight on 27 March 1940, piloted by Blackburn’s test pilot Harry Bailey. Another four or five flights were made with some aileron trouble, but otherwise there were no issues. The extending hull worked well, although its extension and retraction in flight were not entirely smooth. Once extended, the hull offered an open platform from which to conduct mooring operations, and the aircraft was well-behaved on the water.
On 7 April 1940, Bailey was joined by Blackburn test engineer Fred Weeks, Blackburn aircraft riggers Sam McMillan and Duncan Roberts, and Rolls Royce flight engineer Ivan Waller. The task of the day was to complete high-speed tests in the B-20. During the fight, the aircraft reached an unofficial speed of 345 mph (555 km/h). On the next run, an aileron experienced flutter and failed, sending the B-20 out of control. The aircraft crashed in the Firth of Clyde off Garroch Head. Weeks and Waller were able to successful bail out and were picked up by the HMS Transylvania, a merchant ship converted to an auxiliary cruiser. Bailey also bailed out but was too low for his parachute to fully open. His body was recovered from the sea. However, the bodies of McMillan and Roberts were never found.
Even though its flight career was very short, the B-20 had given every indication that its hull design significantly improved performance. Based on the B-20 design, the Blackburn B-40 and B-44 were proposed. The B-40 was in response to Specification R.13/40. The aircraft was a twin-engine flying boat transport powered by two Bristol Centaurus radial engines and intended as a possible replacement for the Short Sunderland. The B-40 was larger and heavier than the B-20 and had twice the range. Two B-40 prototypes were ordered on 9 September 1941, but the aircraft’s poor single engine performance and other priorities led to its cancellation on 6 January 1942. The B-44 was a single-engine floatplane fighter designed to Specification N.2/43. The aircraft was armed with four 20 mm cannons and powered by a Napier Sabre H-24 engine turning contra-rotating propellers. Two B-44 prototypes were ordered in October 1942, but the project was cancelled shortly after a mockup was built. An analysis of the design indicated that the B-44 would be difficult to handle on the water.
In August 1998, one of the B-20’s Vulture engines was recovered after becoming tangled in the nets of a trawler. The B-20’s crash site was subsequently classified as a war grave. What remains of the Vulture engine is now on display at the Dumfries and Galloway Aviation Museum in Scotland.
– Aircraft of the Fighting Powers Volume 6 by Owen Theyford (1945/1980)
– Blackburn Aircraft since 1909 by A. J. Jackson (1989)
– British Experimental Combat Aircraft of World War II by Tony Buttler (2012)
– British Prototype Aircraft by Ray Sturtivant (1990)
– Jane’s All the World’s Aircraft 1945/46 By Leonard Bridgman (1946)
– “Improvements in and relating to Seaplanes” GB patent 433,925 by John Douglas Rennie (applied 13 February 1935).