In the early 1960s the RAF issued a requirement to replace the current wooden gliders used by the Air Training Corps with a new tandem two-seater.
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The T.20 was fitted with special instrumentation and large Zap flaps, fixed at 30°, at Wombleton aerodrome where initial trials were carried out at low altitude towed behind a car.
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The glider, scientists, John Sproule and winch were loaded onto HMS Pretoria Castle at the Clyde docks and on 29 May 1945 the T.20 took off, tethered to a winch on the flight deck.
The T.21 was popularly named "The Barge", after its boat-like hull and sedate flying qualities, while the T.31 was often referred to as "The Brick", again after its flying qualities.
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This was the T.21B, which first flew in December 1947 and went into quantity production both for the RAF (named the Sedbergh TX.1 after the public school of that name) and for civilian clubs.