Allowing 200 pounds for the propulsion system, they estimated that the aircraft with pilot would weigh 625 pounds. Based on this estimate, they calculated power, thrust, and speed requirements and concluded they needed an 8-horsepower engine generating 90 pounds of thrust to achieve a minimum airspeed of 23 miles per hour.

A reproduction of the Wright brothers’ preliminary sketch of the 1903 Wright Flyer, drawn in pencil on brown wrapping paper. The notations are in Wilbur’s handwriting. The original is at the Franklin Institute in Philadelphia.