On the previous web pages we learned about pressure drag and skin friction. You should recall that parasite drag is the sum of these two types of drag.
Parasite drag can be calculated using the equation given below.
Parasite Drag Equation
The equation for Parasite Drag is:
Dp = CDp x S x ½ r
Dp = Parasite Drag
An important stage in designing a new aircraft is determining the coefficient of parasite drag. Traditionally a model of the aircraft is placed in a wind tunnel. The model is adjusted so that the wings do not produce any lift, therefore, there will be no induced drag. The total parasite drag (Dp) is then measured at a variety of speeds (the wind tunnel setup must be at the correct Reynolds number.) The data is used to calculate the coefficient of parasite drag (CDp)
CDp = 2 Dp / S r V2 (this is just the Dp equation above rearranged.)
Wind tunnel experiments reveal that CDp is a constant at speeds below the critical Mach number (the speed at which some of the airflow becomes supersonic.) Therefore, we will treat the coefficient of parasite drag as a constant. (The supersonic flow situation will be dealt with in a separate chapter.)
Plot of Parasite Drag vs. Velocity
Comparison of Drag Characteristics for Different Aircraft
The best method of comparing the relative efficiency of two aircraft is by comparing the CDp for those aircraft. Since CDp is independent of the size of the aircraft it tells us how aerodynamically "clean" the design is.
Equivalent Flat Plate Area Comparison
Although the CDp is the best value for comparing the drag efficiency of one airplane to another the term Equivalent Flat Plate area (f.) is useful for comparing the absolute parasite drag of two aircraft. Equivalent flat plate area is defined as:
f = CDp x S
Note: Despite the name, f does not tell us how many square feet of
plywood would produce the same amount of drag.