Testing the Upper Surface - Wednesday, September 10, 1902
The History: In Orville's personal diary he notes both he and Wilbur worked 5 1/2 hours on the upper surface of their gliding machine, "tacking and sewing cloth". Although they still needed to cover part of the rear spar with cloth, at 5 P.M. they took the surface out for a test.
With a wind of 6 meters/sec (about 13 miles/hour), they placed the glider's upper surface on flat sand. Additional weight (sand) was positioned on the upper wing's front edge until the surface was horizontal (straight). They weighed the surface (36 pounds), the amount of sand they used (11 pounds) and the curvature of the surface (1/24 to 1/26).
On a hill sloped at 7 3/4° (7 3/4 degrees), the upper surface soared in a wind of 7 meters/sec (15.7 miles/hour). The angle of incidence (angle of attack) of the upper surface was about 4° to 5°. The Wrights were very pleased with the tests - this surface performed much better than last year's surface, demonstrating more lift and less "drift" (a form of drag or air resistance).
A spar is one of the main lengthwise components of the wing of an airplane which carries the ribs. Below you can see the rear and front spar of one of our glider models.
The word "tacking" is a term commonly used in sewing. Generally, tacking refers to using temporary stitches. For example, you might position your cloth a certain way and tack it to hold it in place until permanent stitches can be sewn.
Cloth was used to cover the major surfaces of the glider. Fabric is still used in some aircraft and model aircraft today. Fabric was easy to mold and conform to the frame of the glider's wing creating an airfoil (wing) shape.
The angle of attack of an airfoil (like a wing) tilts it against the airflow. For example, below are pictures of two symmetric airfoils with smoke flowing over the surfaces. In this case "symmetric" means the top of the airfoil is a mirror image of the bottom of the airfoil.
The airfoil on the left is at a 0° angle of attack and the airfoil on the right is at about a 9° angle of attack. What else do you notice about these airfoils? The smoke patterns above and below the airfoil at 0° look the same. This is a sign that the lifting forces above and below the airfoil are balanced. A symmetric airfoil at 0° has no lift.
By tilting the airfoil at an angle, a different amount of lift is created above and below the wing with a net result of a lifting force raising the airfoil. The airfoil on the right at approximately 9° is generating lift. Remember our example of a tug-of-war? In this case a lifting force is "winning".