Ellehammer is most renown for his achievements in the field of aviation, although as a Danish inventor, he is credited with no less than 400 registered patents, the most noteworthy pertaining to motors and pumps. By 1904, his invention of a small fuel engine for a motorcycle resulted in sales of over 1000 cycles to the public as well as the military and the royal Danish family. Many of Ellehammer's engine-related inventions are still in use today including the starter valve, accelerator pump, fuel pump, hydraulic clutch and fuel-injected carbureter engine.

His keenest interests though, were to award him a place in aviation history. Although Ellehammer had heard of the Wright Brothers aviation successes by 1904, the technical data remained unpublished. This news prompted Ellehammer's initial experiments that began with the building of scale models based on his observations of kite-flying and birdwatching. In his aeroplane design, Ellehammer constructed the first air-cooled radial engine in the world. His first unmanned flight took place on January 14, 1906, followed by the first manned flight in an engine-driven plane later that year on September 12th. In 1910-11, he conducted experiments with concentric propeller blades, resulting in the construction of a full-size helicopter which was able to successfully fly with a pilot on board September 28, 1912.