Aircraft Drag Breakdown (Subsonic)

There are many variations and definitions of the bookkeeping methods for com­ponents of aircraft drag; this book uses the typical U. S. practice [2]. The standard breakdown of aircraft drag is as follows (see Equation 9.1):

total aircraft drag = (drag due to skin friction + drag due to pressure difference) + drag due to lift generation + drag due to compressibility = parasite drag (CDp) + lift-dependent induced drag (Cm)

+ wave drag (Cdw)

= (minimum parasite drag[CDpmin]

+ incremental parasite drag[ACDp])

+ induced drag (Cm) + wave drag (CDw)

Therefore, the total aircraft drag coefficient is:

Cd = Copmin+ACop + Ci2/n AR + Cdw (9.2)

The advantage of keeping pure induced drag separate is obvious because it is depen­dent only on the lifting-surface aspect ratio and is easy to compute. The total aircraft drag breakdown is shown in Chart 9.1.

It is apparent that the CD varies with the CL. When the CD and the CL relation­ship is shown in graphical form, it is known as a drag polar, shown in Figure 9.2 (all components of drag in Equation 9.2 are shown in the figure). The CD versus the CL2 characteristics of Equation 9.2 are rectilinear, except at high and low CL values (see

Total Aircraft Drag Breakdown

Parasite drag Induced drag Wave drag, CDw

CDp CDi = Cp/nAR (compressibility)

(viscous-dependent – (lift-dependent but

no lift contribution) viscous-independent)

CDpmin + CDp

(minimum) (variation of CDp

(skin friction + pressure with a change)

+ nonelliptical effect)

Chart 9.1. Total aircraft drag breakdown

Figure 9.2. Aircraft drag polar

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