The principle of ducted fans is well understood now. They require a duct with correct tapering at each end and a low drag but powerful engine at its core. Multiple-bladed propellers, or a fan as on a modern high-bypass turbofan are needed for efficiency. Placing a Tiger Moth engine inside a fat tube doesn't cut it. An Italian government engineer, Luigi Stipa, convinced the Caproni Company to build an aircraft to test his theory that a tubular fuselage gave significant extra thrust to a conventional engine and propeller. The resulting Caproni-Stipa aircraft had a corpulent annular fuselage, which concealed a Gipsy engine and two-bladed propeller. All this achieved was high drag and low noise, although the landing speed was reduced to 68km/h. Performance was otherwise lower than a conventional airframe with the same powerplant.
Stipa claimed that the outer fuselage was profiled to gener
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