Santos-Dumont's final design was the Demoiselle monoplane (Nos. 19 to 22). This aircraft was employed as Dumont's personal transportation and he willingly let others make use of his design. The fuselage consisted of a specially reinforced bamboo boom, and the pilot sat beneath betweem the main wheels of a tricycle landing gear. The Demoiselle was controlled in flight partially by a tail unit that functioned both elevator and rudder and by wing warping (No. 20).
The high-wing Demoiselle aircraft had a wingspan of 5.10 m and an overall length of 8 m. Its weight was little more than 110 kg with Santos Dumont at the controls. The pilot was seated below the fuselage-wing junction, just behind the wheels, and commanded the tail surfaces using a steering wheel. The cables of sustentation of the wing were made of piano ropes. Initially, Santos-Dumont employed a liquid-cooled Dutheil & Chalmers engine with 20 hp. Later, the great inventor repositioned the engine to a lower location, placing it in front of the pilot. Santos-Dumont also replaced the former 20-hp engine by a 24-hp Antoniette and carried out some wing reinforcements. This version received the designation no. 20. Due to structural problems and continuing lack of power Santos-Dumont introduced additional modifications in Demoiselle’s design: 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. Thus, the no. 21 was born. The design of no. 22 was basically similar to no. 21. Santos-Dumont tested opposed-cylinder (he patented a solution for cooling this kind of engine) and cooled-water engines, with power settings ranging from 20 to 40 hp, in the two variants. An interesting feature of the water-cooled variant was the liquid-coolant pipeline which followed the wing lower side lofting to improve aerodynamics.
The Demoiselle airplane 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 used to perform flights with the airplane in Paris and some small 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 September 13, returning the following day, and another on the 17th, of 18 km in 16 min. The Demoiselle that was fitted with two-cylinder engine became rather popular. The French WWI-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 the Dumont's plane was better than any other that had been built to that date, 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 several units of Demoiselles, which was sold for 50,000 Francs.
The design of Demoiselle clearly influenced that of the Blériot XI airplane, which was used for the British Channel crossing in 1909.