Calculating and Plotting Parasite Drag
Text Contents ] Chapter Contents ] Overview ] Pressure Drag ] Skin friction ] [ Calculating and Plotting Parasite Drag ] Reducing Parasite Drag ] Induced Drag ] Reducing Induced Drag ] Calculating and Plotting Induced Drag ] Total Drag ]

 

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 V2

 Dp = Parasite Drag
CDp= Coefficient of Parasite Drag
S = Wing area
r
= Air density
V = Velocity

Measuring CDp

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

The graph to the right shows a plot of Parasite Drag vs. Velocity.

This graph is plotted based on the assumption that  S and r are held constant.

Parasite Drag vs. Velocity

 

Comparison of Drag Characteristics for Different Aircraft

CDp Comparison

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.

The advantage of using f is that it allows comparison of the total drag from one airplane to another. For example a C-172 has a CDp of about 0.035 (poor) and S=174 ft2, which gives an equivalent flat plate area of 6 square feet. But a B-747 has a CDp of about 0.022 (much better)  but the equivalent flat plate area is 121 ft2 due to the large wing area (5500 square feet.) The comparison between 6 and 121 is useful for visualizing the total drag, but the comparison between 0.035 and 0.022 is more useful for comparing how streamlined each airplane is.

 

Text Contents ] Chapter Contents ] Overview ] Pressure Drag ] Skin friction ] [ Calculating and Plotting Parasite Drag ] Reducing Parasite Drag ] Induced Drag ] Reducing Induced Drag ] Calculating and Plotting Induced Drag ] Total Drag ]