engines are challenging to make practicable at typical engine operating speeds some cam engines have been tested that offer extremely compact size (approximating to a six-inch (150mm) cube) yet producing approximately forty horsepower at c 7000 rpm, useful for light aerial applications. The attraction of lightweight and mechanically simple (far fewer major moving parts, in the form of a rotor plus twelve axial vanes forming twenty-four combustion chambers) engines, even with a finite working life, have obvious application for small unmanned aircraft. (Such a design having allegedly been tested at NAVAIR PSEF in 2003.) History Macomber In 1911 the Macomber Rotary Engine Company of Los Angeles marketed one of the first axial internal-combustion engines, manufactured by the Avis Engine Company of Allston, Massachusetts. A four-stroke, air-cooled unit, it had seven cylinders and a variable compression ratio, altered by changing the wobble-plate angle and hence the length of piston stroke. It was called a "rotary engine", because the entire engine rotated apart from the end casings. Ignition was supplied by a Bosch magneto directly driven from the cam gears. The high voltage current was then taken to a fixed electrode on the front bearing case, from which the sparks would jump to the spark plugs in the cylinder heads as they passed within 1/16 inch (1.5 mm) from it. According to Macomber's literature, it was "guaranteed not to overheat". The engine was claimed to be able to run at 150 to 1,500 rpm. At the normal speed of 1,000 rpm, it reportedly developed 50 hp. It weighed and it was long by in diameter. Pioneer aviator Charles Francis Walsh flew an aircraft powered by a Macomber engine in May 1911, the "Walsh Silver Dart". Statax In 1913 Statax-Motor of Zürich, Switzerland introduced a swashplate engine design. Only a single prototype was produced, which is currently held in the Science Museum, London. In 1914 the company moved to London to become the Statax Engine Company and planned on introducing a series of rotary engines; a 3-cylinder of 10 hp, a 5-cylinder of 40 hp, a 7-cylinder of 80 hp, and a 10-cylinder of 100 hp. It appears only the 40 hp design was ever produced, which was installed in a Caudron G.II for the British 1914 Aerial Derby but was withdrawn before the flight. Hansen introduced an all-aluminum version of this design in 1922, but it is not clear if they produced