Number 21 was the name of the aeroplane Gustave Whitehead allegedly flew with on August 14, 1901 - two years before the Wright brothers' first flight. Gustave Albin Whitehead, born Gustav Albin Weißkopf (January 1, 1874 - October 10, 1927), was a German-American aviation pioneer. ... August 14 is the 226th day of the year in the Gregorian Calendar (227th in leap years), with 139 days remaining. ... 1901 was a common year starting on Tuesday (see link for calendar). ... First flight, December 17, 1903. ...

According to Gustave and the single eyewitness to the event, the monoplane's longest flight was 60 meters (200 feet) above ground for 2.5 kilometers (1.5 miles). However, these claims are contested. Gustave failed to keep a log book and planned for the bulk of his flights to occur at night, or "early dawn", thus limiting witnesses.

An article in the August 18, 1901 issue of The Bridgeport Herald states that the author states he witnessed a night test of the plane, at first unpiloted but loaded with sand bags, and later with Whitehead at the controls. August 18 is the 230th day of the year (231st in leap years) in the Gregorian Calendar. ... 1901 was a common year starting on Tuesday (see link for calendar). ...

Number 21 was a monoplane powered by a 20 hp engine, fueled by acetylene. It achieved takeoff speed by using the engine to power the wheels, as in an automobile; upon takeoff, power was switched to the propeller. A monoplane is an aircraft with one main set of wing surfaces, in contrast to a biplane or triplane. ... A small variety of cars, the most popular kind of automobile. ...