Prof. Frank Cotton of Sydney University, Australia, designed the world's first successful gas-operated anti-G suit. Research commenced late in 1940, and a suit was designed with rubber sacs covered externally by inextensible material. The sacs automatically inflated when G forces increased during flying. Initially tested on a centrifuge specially built at Sydney University, and then flight-tested in a Hurricane, Kittyhawks, and Spitfires, the suit provided about 2 G protection. The Canadians had earlier developed a water-filled suit, which the RAF adopted, but comparative trials in 1944 by the Royal Air Force concluded that: "There is no doubt the Cotton Suit gives the best protection." By the time the Cotton Aerodynamic Anti-G Suit was operational, Japanese attacks on Darwin had virtually ceased, and the suit was never used in combat. However, the principle of gas-inflatable bladders is still used in the modern anti-G suit.