DRAG BREAKDOWN AND EQUIVALENT FLAT-PLATE AREA

The parasite drag of an airplane can be estimated by estimating the drag of each component and then totaling the component drag while accounting for some interference drag. If CA. and Si are the drag coefficient and reference area, respectively, for the ith component, then the total drag will be

D = ipV2CDtSi + ipV^C^Sj + • • • + ipV2CDiS, + • • •

= pV2(CDtSi + CbjS 2 + • • • + CD, S, + • • •) (4.34)

Obviously, the drag coefficients of the components cannot be added since the reference areas are different. However, from Equation 4.34, the products CA. S, can be added. Such a product is referred to as the equivalent flat-plate area, /. One will also hear it referred to as the “parasite area” or simply, the “flat-plate area.” The connotation “flat plate” is misleading, since it is not the area of a flat plate with the same drag. Instead, it is the reference area of a fictitious shape having a Cd of 1.0, which has the same drag as the shape in question. / is therefore simply D/q. It is a convenient way of handling the •hag, since the /’s of the drag components can be added to give the total f of an airplane.

^=ад + ад+

4

or

f = 2 cDiSi

i= 1

= 2/. (4.35)

І = 1

This notation indicates that the flat-plate areas are to be summed for the ith component, from і = 1 to n where n is the total number of components.