Minimum Parasite Drag

The aircraft CDpmin can now be obtained from faircmft. The minimum parasite drag of the entire aircraft is CDpmin — (1 /Sw)J2fi, where is the sum of the total fs of

the entire aircraft:

CDpmin — faircraft/Sw

9.10 ACDp Estimation

Equation 9.2 shows that ACDp is not easy to estimate. ACDp contains the lift – dependent variation of parasite drags due to a change in the pressure distribution with changes in the angle of attack. Although it is a small percentage of the total air­craft drag (it varies from 0 to 10%, depending on the aircraft Cl), it is the most diffi­cult to estimate. There is no proper method available to estimate the ACDp-versus – Cl relationship; it is design-specific and depends on wing geometry (i. e., planform, sweep, taper ratio, aspect ratio, and wing-body incidence) and aerofoil character­istics (i. e., camber and t/c). The values are obtained through wind-tunnel tests and, currently, by CFD.

During cruise, the lift coefficient varies with changes in aircraft weight and/or flight speed. The design-lift coefficient, Cld, is around the mid-cruise weight of the

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LRC. Let CLP be the lift coefficient when ACDp = 0. The wing should offer CLP at the three-fourths value of the designed CLD. This would permit an aircraft to oper­ate at HSC (at McrU; i. e., at the lower CL) with almost zero ACDp. Figure 9.8a shows a typical ACDp-versus-CL variation. This graph can be used only for coursework in Sections 9.18 and 9.19.

For any other type of aircraft, a separate graph must be generated from wind – tunnel tests and/or CFD analysis. The industry has a large databank to generate such graphs during the conceptual design phase. In general, the semi-empirical method takes a tested wing (with sufficiently close geometrical similarity) ACDp – versus-CL relationship and then corrects it for the differences in wing sweep (ф), aspect ratio (ф), t/c ratio (t), camber, and any other specific geometrical differences (Figure 9.8a).

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