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Wing Design FactorsWing design is based on the anticipated use of the airplane, cost, and other factors. The main design considerations are wing planform, camber, aspect ratio, and total wing area. Camber affects the difference in the velocity of the airflow between the upper and lower surfaces of the wing. If the upper camber increases and the lower camber remains the same, the velocity differential increases. There is, of course, a limit to the amount of camber which can be used. After a certain point, air will no longer flow smoothly over the airfoil. Once this happens, the lifting capacity diminishes. The ideal camber varies with the airplane's performance specification, especially the speed range and the load-carrying requirements. Aspect ratio is the relationship between the length and width of a wing. It is one of the primary factors in determining lift/drag characteristics. At a given angle of attack, a higher aspect ratio produces less drag for the same amount of lift. Wing area is the total surface area of the wing. Most wings don't produce a great amount of lift per square foot, so wing area must be sufficient to support the weight of the airplane. For example, in a training aircraft at normal operating speed, the wings produce only about 10.5 pounds of lift for each square foot of wing area. This means a wing area of 200 square feet is required to support an airplane weight of 2100 pounds during straight-and-level flight. Once the design of the wing is determined, the wing must be mounted on the airplane. Usually it is attached to the fuselage with the cord line inclined upward at a slight angle, which is called the angle of incidence. When wing twist, or washout, is incorporated into the wing design, the wingtip has a lower angle of incidence than the wing root. Wing twist is used by airplane designers to prevent undesirable stall characteristics in some wing designs which have a tendency to stall first at the wingtips and then stall inward toward the root. This is an undesirable characteristic, since the disrupted airflow near the wingtip can reduce aileron effectiveness to such an extent that it may be impossible to control the airplane about its longitudinal axis.
Another method sometimes used to ensure positive control during the stall is installation of stall strips, which consist of two metal strips attached to the leading edge of each wing near the fuselage. These strips disrupt the airflow at high angles of attack, causing the wing area directly behind them to stall before the wingtip stall. |
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This page was last modified June 11, 2000 |
