Fig. 16. Plan View of Plate with Long Edge to Wind. Fig. 17. Plate with Narrow Edge to Wind, Showing Loss in Lift. 17a Shows Effect of Raked Tips.
Fig. 16 shows the top view or plan of a lifting surface, with the direction of the wind stream indicated by the arrows w-w-w = w. The longer side or "span" is indicated by S, while the width or chord is C. Main lifting surfaces, or wings, have the long side at right angles to the wind as shown. When in this position, the surface is said to be in "Pterygoid Aspect," and when the narrow edge is presented to the wind, the wing is in "Apteroid Aspect." The word "Pterygoid" means "Bird like," and was chosen for the condition in Fig. 16, as this is the method in which a bird's wing meets the air. Contrary to the case with true curved aeroplane wings, flat planes usually give better lift in apteroid than in pterygoid aspect at high angles. The aspect ratio will be the span (S) divided by the chord (C), or Aspect ratio = S/C.
It will be seen from the above that the lift coefficient Ky increases with the aspect ratio, and that it generally declines after an angle of 30 degrees. The center of pressure moves steadily back with an increase in angle.
Example for Lifts. A certain flat plane has an area of 200 square feet, and moves at 50 miles per hour. The angle of incidence is 10 degrees, and the aspect ratio is 6. Find the total lift and the drag in pounds. Also the location of the center of pressure in regard to the leading edge, if the chord is 5.8 feet.
Solution. Under the table headed, "Aspect Ratio = 6" we find that Ky at 10° = 0.00173, and that the lift drag ratio is 5.2. The center of pressure is 0.333 of the chord from the front edge. The total lift then becomes: L = KyAV² = 0.00173 x 200 x (50 x 50) = 865 pounds. Since the lift drag ratio is 5.2, the drag = D = 865/5.2 = 166.3 pounds. The center of pressure will be located 5.8 x 0.333 = 1.4 feet from the leading edge.
Under the same conditions, but with an aspect ratio of 3, the lift will become: L = KyAV² = 0.0014 x 200 x(50 x 50) = 700 pounds. In this case the lift drag ratio is 5.1, so that the drag will be 137.8 pounds. Even with the same area, the aspect ratio makes a difference of 865–700 = 165 pounds. If we were compelled to carry the original 865 pounds with aspect 3 wing, we would also be compelled to increase the area, angle, or speed. If the speed were to be kept constant, we would be limited to a change in area or angle. In the latter case it would be preferable to increase the area, since a sufficient increase in the angle would greatly increase the drag. It will be noted that the lift-drag ratio decreases rapidly with an increase in the angle.
