Building The Lower Surface - Thursday, September 11, 1902
The History: In Orville's personal diary he notes that the brothers finished covering the rear spar of the 1902 glider's upper surface with cloth. In the afternoon, they began construction of the glider's lower surface. Wood was spliced (pieced) together to make the long spars for the lower surface's frame. Bowed (curved) pieces were added to the ends of the frames. Now, the lower surface was ready for the ribs.
With the upper surface completed more flight tests were conducted. They raised long poles to measure the surface's angle under different wind velocities. Orville noted that these tests were "not satisfactory" and the "Measurements of angles not accurate."
These were the results.
1902 Upper Surface
Velocity Versus Angle
Orville noted: "We found that the covering of the rear side of the rear spar made the cent. [center] of pressure reverse sooner [as compared to when it was uncovered]."
Orville wrote to his sister, Katharine enclosing a voucher (a record of a business transaction or an authorization) and a check for $125. He noted Dan Tate spent the day "lathing up cracks" in their campsite building (filling up cracks with thin strips of wood).
Scientists and engineers often summarize the forces in a problem using the "center" of the force. The force is treated as if it acted through a single point. The terms center of pressure and center of gravity are still used today to explain aerodynamic forces. Pressure is a measure of force over an area (Pressure = Force/Area), while the center of gravity refers to the force created by the weight of an object.
What does Orville mean when he says "the center of pressure reversed sooner"? To understand this concept, let's examine another device which depends on aerodynamic forces - the javelin.
Modern javelins land "point-down". Earlier javelins either traveled with the point up (which was dangerous) or horizontally. When the javelin landed horizontally in competitions it was difficult to determine the exact landing point. In 1984 the International Amateur Athletic Federation (IAAF) adopted new rules to make sure future javelins were designed to land point-down, so the "touch-down" point would be known exactly.
Every javelin designed for competition has the same "center of gravity" or balance point. However, the center of pressure on a javelin remains behind the center of gravity and the direction of the center of pressure is opposite that of the gravitational force. This keeps the javelin's point slanted toward the ground.
What would happen if the center of pressure was in front of the center of gravity on a javelin? The nose of the javelin would point up.
When Orville writes about the center of pressure reversing itself, he is referring to the center of pressure changing its position (in front of or behind) in relationship to the center of gravity.