Nacelle Drag

The nacelle requires different treatment, with the special consideration of throttle – dependent air flowing through as well as over it, like the fuselage. This section pro­vides the definitions and other considerations needed to estimate nacelle parasite drag (see [2], [9], and [21]). The nacelle is described in Section 10.8.

The throttle-dependent variable of the internal flow passing through the tur­bofan engine affects the external flow over the nacelle. The dominant changes in the flow field due to throttle dependency are around the nacelle at the lip and aft end. When the flow field around the nacelle is known, the parasite drag estimation method for the nacelle is the same as for the other components but must also con­sider the throttle-dependent effects.

Civil aircraft nacelles are typically pod-mounted. In this book, only the long duct is considered. Military aircraft engines are generally buried in the aircraft shell (i. e., fuselage). A podded nacelle may be thought of as a wrapped-around wing in an axi-symmetric shape like that of a fuselage. The nacelle section shows aerofoil­like sections in Figure 9.5; the important sources of nacelle drag are listed here (a short duct nacelle [see Figure 10.16] is similar except for the fan exhaust coming out at high speed over the exposed outer surface of the core nozzle, for which its skin friction must be considered):

Throttle-independent drag (external surface)

• skin friction

• wrapping effects of axi-symmetric body

• excrescence effects (includes nonmanufacturing types such as cooling ducts)

Throttle-dependent drag

• inlet drag (front end of the diffuser)

• nacelle base drag (zero for an engine operating at cruise settings and higher)

• boat-tail drag (curvature of the nozzle at the aft end of the nacelle)

Definitions and typical considerations for drag estimation associated with the flow field around an isolated long-duct podded nacelle (approximated to circular

cross-section) are shown in Figure 9.5. Although there is internal flow through the nacelle, the external geometry of the nacelle may be treated as a fuselage, except that there is a lip section similar to the LE of an aerofoil. The prevailing engine – throttle setting is maintained at a rating for LRC or HSC for the mission profile. The intake drag and the base drag/boat-tail drag are explained next.