**O**n 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:

**D**_{p} = C_{Dp} x S x ½ r
V^{2}

D_{p} = Parasite Drag

C_{Dp}= Coefficient of Parasite Drag

S = Wing area

r = Air density

V = Velocity

## Measuring C_{Dp}

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 (D_{p})
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 (C_{Dp})

C_{Dp} = 2 D_{p} / S r
V^{2} (this is just the D_{p} equation above rearranged.)

Wind tunnel experiments reveal that C_{Dp} 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.)

## Comparison of Drag Characteristics for Different Aircraft

## C_{Dp} Comparison

The best method of comparing the relative efficiency of two aircraft is by
comparing the C_{Dp} for those aircraft. Since C_{Dp} 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 ft^{2}, 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 ft^{2} 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.