Specifications for No. 20
Length
8.0 m
Wingspan
5.10 m
Powerplant
1 × Darracq, 26 kW (35 hp)

The Santos-Dumont Demoiselle ("Damselfly") was an airplane built in France by Brazilian aviation pioneer Alberto Santos-Dumont in the early XX century. It was a light-weight monoplane with a wire-braced wing mounted atop an open-framework fuselage built around a reinforced bamboo truss. Pilot's seating place was located below the wing, and between the main wheels of the undercarriage. At the rear end of the bamboo truss a tailwheel and a conventional empennage found place. As originally designed (No. 19) Santos-Dumont used a liquid-cooled Dutheil & Chalmers flat-twin engine rated at 15 kW (20 hp) mounted on the leading edge of the wing. Later, the aviation pioneer repositioned the engine to a lower location, placing it in front of the pilot. The Demoiselle was controlled in flight by a tail unit, pivoting on a form of universal joint that functioned both as elevator and rudder, which the pilot operated with a steering wheel.

The No. 19 was damaged beyond repair when crashed by Hélène Dutrieu in 1908. Santos-Dumont's next Demoiselle, the No. 20, featured an 18-kW (24-hp) Antoinette engine, later replaced with a Darracq-built liquid-cooled opposed twin engine of approximately 3.2 litre displacement and similar power output, and some additional wing reinforcements. Both the Antoinette and Darracq-powered versions had a pair of lightweight thin-tube radiators mounted under the wing roots and just below the lower layer of fabric wing covering, running the entire 1.70 meter chord of the wing root panels. It also utilised wing warping for lateral control, with control cabling that only pulled down alternately on the outer section of the rear wing spar with no "upwards" warp capability. Due to structural problems and continuing lack of power Santos-Dumont introduced additional modifications in the No. 21: a triangular and shortened fuselage made of bamboo; the engine was moved back to its original position, in front of the wing; and increased wingspan. The design of No. 22 was similar to No. 21. Santos-Dumont tested opposed-cylinder (he patented a solution for cooling this kind of engine) and water-cooled engines, with power settings ranging from 15–30 kW (20–40 hp) in the two variants. A feature of the water-cooled variant was the liquid-coolant pipeline which followed the wing lower side lofting to improve aerodynamics.

The Demoiselle could be constructed in only fifteen days. Possessing outstanding performance, easily covering 200 m of ground during the initial flights and flying at speeds of more than 100 km/h, the Demoiselle was the last aircraft built by Santos-Dumont. He performed flights with it in Paris, and made trips to nearby places. Flights were continued at various times through 1909, including the first cross-country flight with steps of about 8 km, from St. Cyr to Buc on 13 September 1909, returning the following day, and another on 17 September 1909 of 18 km in 16 min. The Demoiselle, fitted with a two-cylinder engine, became rather popular. The French World War I ace Roland Garros flew it at the Belmont Park, New York, in 1910. The June 1910 edition of the Popular Mechanics magazine published drawings of the Demoiselle and affirmed that "This machine is better than any other which has ever been built, for those who wish to reach results with the least possible expense and with a minimum of experimenting." American companies sold drawings and parts of Demoiselle for several years thereafter. Santos-Dumont was so enthusiastic about aviation that he released the drawings of Demoiselle for free, thinking that aviation would be the mainstream of a new prosperous era for mankind. Clément-Bayard, an automotive maker, constructed Demoiselles, they planned a production run of 100 units, built 50 and sold only 15 for 7,500 francs for each airframe. (or 50,000 francs) It was the world's first series production aircraft. By 1909 it was offered with a choice of 3 engines, Clement 20 hp; Wright 4-cyl 30 hp (Clement-Bayard had the license to manufacture Wright engines); and Clement-Bayard 40 hp designed by Pierre Clerget. It achieved 120 km/h.

An examplear of a No. 21 with a Darracq engine is preserved in the Musée de l'Air et de l'Espace. Several flyable replicas were built by Personal Plane Services Ltd for the 1965 film 'Those Magnificent Men in Their Flying Machines' and others have been built since then.