Every flying boat is a compromise between two incompatible shapes. To get off the water you want a deep hull, to keep the propellers clear of spray, and a wing sitting at a high angle so it will unstick. To fly efficiently you want neither of those things.

Blackburn’s answer, in 1940, was an aircraft that changed shape. The entire bottom half of the hull dropped away on hydraulic jacks for take-off and landing, then folded back up flush for cruise. They called it the nutcracker.

It flew for twelve days.

How the nutcracker worked

The lower hull was not a fairing. It was a sealed pontoon divided into five watertight compartments, and it carried all of the aircraft’s fuel. Four hydraulic jacks lowered it, while hinged triangular frames ahead of each jack pushed it forward as it went down, so that dropping the hull also set the correct nose-up attitude for the water.

The wingtip floats did the same trick in miniature, retracting outward until they formed the wingtips themselves.

None of this was easy in 1936. British Lockheed built the hydraulics, running at around 2,500 psi at a time when 1,400 psi was considered normal practice, with a gravity system to drop the hull if the pressure failed. The engineer who wrote it up after the war was frank about how it had looked at the start.

There was an unexpected bonus. With the hull extended, the crew could stand on the pontoon, which made mooring and boarding far easier than on a conventional boat.

Twelve days

V8914 first flew from the Clyde at the end of March 1940 in the hands of Blackburn’s chief test pilot, Flt Lt Henry Bailey. On 7 April, on a high-speed run over the Sound of Bute with five men aboard, severe vibration set in. Slowing down did not help. Bailey ordered everyone out.

Ivan Waller, a Rolls-Royce flight engineer in the co-pilot’s seat, had his parachute deploy early and snag on the wireless mast, leaving him trapped half out of the hatch until he tore himself free. Fred Weeks got out of the other roof hatch and hit the rudder on the way past, an injury he carried for the rest of his life. Both were picked up from the water about a quarter of an hour later.

Bailey stayed at the controls until it was too late and jumped from too low. He drowned. The two Blackburn riggers aboard, Duncan Robertson and Samuel McMillan, were never found. Crew of a nearby ship reported seeing a large flat object fall out of the cloud — almost certainly the aileron, which had fluttered and broken away.

The idea outlived the aircraft, briefly

The Air Ministry concluded that the crash had nothing to do with the retracting hull, and that the principle was proven. Blackburn were asked for a bigger version, the B-40, to replace the Sunderland; two were ordered in September 1941 and cancelled four months later, the gain in range judged too small. A fighter floatplane derivative, the B-44, got as far as a mock-up before it too was dropped.

By then the whole question was becoming academic. Long-range land-based aircraft were making the big flying boat obsolete, and no amount of clever hydraulics was going to change that. The engineer who described the system in 1946 said as much.

He was right. Nothing else has ever used it.

The existence of the B-20 stayed classified until June 1945. The wreck lies off Bute and is a designated war grave. In 1998 a trawler snagged it and Royal Navy divers recovered one of its Vulture engines, which is now at the Dumfries and Galloway Aviation Museum — effectively the only piece of the aircraft anyone can go and look at.

Because so little survives, there are almost no photographs of it. This is the best account of the design:

And a look at the B-20 alongside the B-40 and B-44 that were meant to follow it:

Sources: H. P. Henry, "A Teaser in Hydraulics", Flight, 18 April 1946; Old Machine Press; Dinger’s Aviation Pages (citing Derek N. James, Aircraft Illustrated, February 1974); Helensburgh Heritage.