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Suppose you have two wings with different shapes that you would like to
compare. You want to see which one makes more lift, or which one makes less
drag at the same lift value. ( To learn more about drag, look at the discussion
section on "Drag" in the main document)
You need to find a fair basis for comparison, even if the wings are different size, or are tested at different speeds. The way to do this is to adjust the lift and drag measurements for the specific area and dynamic pressure. For example, we know that the lift varies in proportion to the wing area. So, if we divide the lift and drag by the wing area, then we have a value of lift-per-unit-area and drag-per-unit-area, which now provides a basis of comparision for different wings. In the same way, we know that the lift is proportional to the dynamic pressure, q = 1/2 r V2, so again, we can divide the lift and drag by the dynamic pressure to get the lift-per-unit-dynamic pressure. Whats left is a "non-dimensional" lift coefficient and drag coefficient that provide the fair comparison we want. The lift coefficient is defined as CL = L/qS, whereL is the lift, q is the dynamic pressure and S is the wing area. In the same way, the drag coefficient is CD = D/qS. Here are some plots showing typical characteristics of a wing, with lift coefficient and drag coefficient varying as the angle of attack is changed. Excercise: Two wings with different airfoil shapes have been tested at the same angle of attack. The first wing produced 10 lbs of lift, 0.75 lb of drag. It has a wing area of 5 square feet, and was tested at a velocity of 50 ft/s with an air density of 0.002377 slugs/ft3. The second wing produced 12 lbs of lift, 1 lb of drag, and has a wing area of 7.5 square feet. What is the dynamic pressure? Determine which wing has a higher lift coefficient, and which has a higher drag coefficient. |
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