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Calculation of Inclined Planes

Lift and Drag Co-efficients

For the calculation of the drag, we will use a new expression—the "Lift-Drag Ratio"—or as more commonly given, "L/D." This shows the relation between the lift and drag, so that by knowing the lift and the ratio for any particular case, we can compute the drag without the necessity of going through the tedious calculation D = KxAV². The lift-drag ratio for a flat plate varies with the angle of incidence, and the aspect ratio, and hence a separate value must be used for every inclination and change in aspect. To obtain the drag, divide the lift by the lift-drag ratio. Hence if the lift is 1200 pounds, and the ratio equals 6.00, the drag will be: 1200/6=200 pounds, or in other words, the lift is 6 times the drag force. Changing the angle of incidence through angles ranging from 1 degree to 7 degrees, the lift-drag ratio of a flat plate will vary from 1.5 to 7.5. When the plane is parallel to the wind stream and gives no lift, the drag is computed from Zahm's skin friction formula.