The History: For eight hours, both Wrights worked on the construction of the lower surface of the 1902 glider today, adding the ribs and the cloth cover.

Just before noon, they tested the upper surface on Big Hill again. (See September 9, 1902 and September 10, 1902).

In his personal diary Orville writes, "Found that surface tended to duck at large angle, but on increase of wind and decrease of angle of incidence center [of pressure] seemed to move forward, and the pitching ceased." [Orville has begun to understand the mechanics and aerodynamics of a wing stalling.]

Wilbur replied to Octave Chanute's letter of September 6, 1902 informing Chanute transportation from Elizabeth City to Kill Devil Hills for Chanute's multiple-wing machine would be arranged.

Wilbur discusses the tests they have conducted on the 1902 glider's upper surface. The surface glides easily on a hill sloped at 7 3/4°. Wilbur thinks once the entire machine is constructed, it will need a greater angle, "probably 2°" more to glide.

Wilbur also tells Chanute they will heed his warning to be careful when flying, "We thank you for your caution against rashness. We will be very careful as we have special reasons for not wishing to be injured at this time."

As we read about the development of powered flight through the Wrights' work, we need to remember that in many cases they were discovering basic aerodynamic principles for this first time. Through their experiments and observations they gained insight no one else possessed. They were proving old theories incorrect.

Today we know that as the angle of attack (the angle the wing's chord makes with the relative wind) increases, the lift of the wing increases. However, at a certain point the angle of attack is so steep that the air separates from the wing. The wing loses its lift and plunges down. This is what Orville has just noticed. He has recognized that the forces on the wing have changed direction. Instead of rising in the air, the upper surface of the glider plunged down when the angle of incidence (the Wrights' term for the angle of attack) became too steep. (For more information on the center of pressure and an example using a javelin see September 11, 1902.)

The Angle Of Attack Is So Steep That The Air Separates From The Wing The Wing Loses Its Lift And Plunges Down

Wilbur has told Chanute he thinks the 1902 machine will require a greater angle (another 2°) to glide. Based on what your knowledge of flight forces and how lift is generated, why do you think the larger machine will require a steeper angle to fly?