The Blackburn B-20
An experimental flying boat using an interesting design concept.
In 1936 the Air Ministry issued Specification R1/36 which called for a medium-sized reconnaissance flying boat to replace the biplane Saro London and Supermarine Stranraer currently in squadron service.
All flying boat designers are beset by two constraints. First, the props must be kept well clear of spray so engines are usually mounted on or even above a high-set wing and the boat has a deep hull. Secondly, when the angle of the planing hull is level during the take-off run the wing should be at the optimum incidence to produce sufficient lift for take-off at a relatively slow speed. But at cruising speeds the wing’s angle will be reduced and the hull will cause extra drag. Blackburn’s chief designer Major J D Rennie produced an idea to meet both these constraints that was so novel that he took out a patent for it. Making the planing hull retractable allowed it to be set at the best angle for take-off and at minimum drag for cruise. An extra benefit was that the upper surface of the hull provided a very convenient platform for the mooring crew.
The B-20 didn’t win the contract but a prototype was ordered. Bristol Hercules engines were Blackburn’s first choice for the B-20 but as the weight of the design grew more powerful engines were needed and 24 cylinder RR Vulture engines were fitted. Nominally of 1720 hp they were suffering from their own development problems and were de-rated to about 1500 hp. The boat had a span of 82 ft and a maximum t/o weight of 35,000 lbs. A maximum speed of 306 mph was expected and the normal range was 1500 miles. The crew of six were housed in typical flying boat accommodation, with galley, sleeping quarters, engineer’s compartment with workbench and miscellaneous marine gear.
This prototype was produced at Blackburn’s Dumbarton works and was ready for its first flight over the Firth of Clyde in March 1940. Generally its performance was satisfactory with the retractable hull behaving well and mooring operations particularly easy. There was an evident problem with aileron trim which, without trim tabs, required ground adjustment. This took several flights to resolve and on 7 April the B-20 was readied for its first high speed test.
It proved to be quite easy to reach and exceed the expected maximum speed. On the final run the remarkable speed of 345 mph was reached. Suddenly, severe vibration was felt. Reducing power had no effect and the pilot, Flt Lt Harry Bailey, gave the order to the four crew members to bale out. Fred Weeks, the Blackburn flight test engineer, escaped from the cockpit roof hatch, his leg striking the rudder as he fell. Ivan Waller, a Rolls Royce engineer, climbed out of the hatch but opened his parachute too soon. It deployed and wrapped around the radio mast.
At this point, the vibration ceased and Ivan crawled along the fuselage and untangled his chute so that he could drop free. The pilot delayed his exit to give the crew time to escape but his parachute was too late in opening to prevent his drowning. The bodies of the other two crew members were not found.
The crew of a ship reported seeing a rectangular object falling from the sky which was believed to be an aileron. The accepted view was that aileron flutter caused the severe vibration which ceased when the aileron became detached. The loss of the only prototype ended this unique project.
The RR Vulture itself had an unhappy service career. Built from two RR Peregrine engines joined at the crankcase to produce a 24 X cylinder engine of 42.5 litres it was fitted to the Hawker Tornado and the Avro Manchester 2-engined bomber. Prolonged development problems led to its cancellation and replacement by the Napier Sabre in the Typhoon and four Merlins to create the Lancaster.
Although the Air Ministry was intrigued enough to order a prototype of the B-20, Blackburn the contract was awarded to Saunders-Roe’s Lerwick which had Bristol Hercules engines and a more traditional hull shape.
In service, the Lerwick was far from successful. It was underpowered and unstable, both in the air and on the water. It had a viscious stall, persistent problems with its hydraulics and an unfortunate tendency to shed its wing floats. Just 21 were produced, coming into service in the summer of 1939. Several were lost in accidents, they were grounded twice for safety modifications and by 1942, to the relief of its crews, the Lerwick was declared obsolete.