Despite its cartoonish proportions and unconventional layout, the Stipa-Caproni was a clever design which provided a crucial link between propeller-driven aircraft and jet-powered aircraft. The man behind the Stipa-Caproni prototype was an Italian engineer called Luigi Stipa. In 1932, after years of developing the aircraft in his head and on paper, he went to the Italian government in an attempt to persuede them to fund the building of an experimental aircraft which would validate his ideas. At the time the facist Italian government was keen to promote the latest technological achievements of the country, and they commissioned the Caproni aircraft company to build Stipa’s prototype.
The aircraft was called the Stipa-Caproni (or Caproni Stipa), and it was one of the more unusual aircraft designs of the time. It’s most recognisable feature was the hollow fuselage which had a propeller just inside the leading edge of the tube. However this was no ordinary tube. Stipa had spent much time calculating the best shape for the tunnel and the optimum propeller position and angle. It worked like a venturi tube, compressing the air flow from the propellor and engine exhaust before it exited the tunnel at the rear. Despite what it might look like, it actually made the engine more efficient.
But it wasn’t just this feature of the aircraft that was bizarre. The Stipa-Caproni had a stubby little tail, small wings, and the pilot and his passenger sat on top of the tube in tandem cockpits. The Stipa-Caproni was primarily constructed using wood. The engine was a British-built 120 horsepower de Havilland Gipsy III.
The Stipa-Caproni first flew on October 7 1932, piloted by Caproni test pilot Domenico Antonini. Initial flights showed that the added lift provided by the airfoil shape of the tube meant the aircraft had a very low landing speed, just 42 mph. It also had a higher rate of climb than comparable aircraft. Because the rudder and elevator were mounted at the back of the tube they benefitted from the extra airflow over them, this gave the Stipa-Caproni extremely stable handling characteristics. Almost too stable, pilots reported that it could be difficult to change direction at times.
Overall, the Stipa-Caproni proved that the “intubed propeller” idea worked. However the aircraft was not noticably better than more conventional aircraft designs. Therefore, after some additional testing, it was decided that no further aircraft would be built. Athough the motorjet-powered Caproni Campini N.1 prototype of 1940 did use some of the Stipa-Caproni’s engineering solutions.
As an interesting side note, Stipa patented his design in Italy, Germany and the US in 1938. During the Second World War, when jet engines began appearing, Stipa became convinced that countries were using some of his patents without credit or payment. The German V-1 flying bomb in particular. However, considering the vicious atrocities that occured during the conflict, it was a fairly small transgression.
Aviation experts now agree than the modern turbofan engines used in some aircraft utilize some of the principles originally developed by Stipa.
Stipa died in 1992 at the age of 92, still angry that he never got the recognition – or payment – for playing a role in advancing the world from propeller-powered aircraft, into the jet age.
But thanks to an Australian company, Aerotect Queensland, the Stipa-Caproni lives on. In 1998 they built a 3/5-scale replica of the original. It was flown at low altitude, and just twice, in 2001. The pilot reported it had similar flight characteristics to the original. Since then the aircraft has been put on static display.