Base Drag

The design criteria for the nozzle-exit area sizing is such that at LRC, the exit-plane static pressure Pe equals the ambient pressure PTO (a perfectly expanded nozzle, Pe, = PTO) to eliminate any base drag. At higher throttle settings, when Pe > PTO, there still is no base drag. At lower settings – for example, idle rating – there is some base drag as a result of the nozzle-exit area being larger than required.

Boat-Tail Drag

The long-duct contour for closure (i. e., “boat-tail” shape) at the aft end is shallow enough to avoid separation, especially with the assistance of entrainment effects of

Spillage drag =


the exhaust plume. Hence, the boat-tail drag is kept low. At the idle throttle setting, considerable flow separation can occur and the magnitude of boat-tail drag would be higher, but it is still small compared to the nacelle drag.

For bookkeeping purposes and to avoid conflict with aircraft manufactur­ers, engine manufacturers generally include internal drag (e. g., ram, diffuser, and exhaust-pipe drag) in computing the net thrust of an engine. Therefore, this book only needs to estimate the parasite drag (i. e., external drag) of the nacelle. Intake – duct loss is considered engine-installation losses expressed as intake-recovery loss. Intake – and exhaust-duct losses are approximately 1 to 3% in engine thrust at LRC (throttle – and altitude-dependent). The net thrust of the turbofan, incorporating installation losses, is computed using the engine-manufacturer-supplied program and data. These manufacturers work in close liaison to develop the internal contour of the nacelle and intake. External nacelle-contour design and airframe integration remain the responsibility of the aircraft manufacturer.

The long-duct nacelle characteristic length, Lnac, is the length measured from the intake-highlight plane to the exit-area plane. The wetted area AWn, Ren, and basic CFn are estimated as for other components. The incremental parasite drag formulae for the nacelle are provided herein. The supervelocity effect around the nacelle-lip section is included in the intake-drag estimation; hence, it is not com­puted separately. Similarly, the pressure effect is included in the base/boat-tail drag estimation. These two items are addressed this way because of the special consideration of throttle dependency. Following are the relationships used to com­pute the nacelle drag coefficient ACon-

1. ACDn effects (same as the fuselage being axi-symmetric).


Table 9.3. Nacelle interference drag (per nacelle)

Wing-mounted (Figure 9.7)

Interference drag

Fuselage-mounted (Figure 9.7)

Interference drag

High (long) overhang



5% of CFn

Medium overhang

4% of CFn


5% of CFn

Low (short) overhang

7% of CFn


5% of CFn


6.5% of CFn

Straight duct (center)

5.8% of CFn

2. Other incremental effects. Drag contributions made by the following effects are given in percentage of CFn. These are typical of the generic nacelle design:

(a) Intake drag at LRC – includes supervelocity effects « 40 to 60%

(higher BPR with higher percentage)

(b) Boat-tail/base drag (throttle-dependent) – includes pressure effects « 10 to 12% (higher value for smaller aircraft)

(c) Excrescence (nonmanufacturing type such as cooling-air intakes) « 20 to 25% (higher value for smaller aircraft)

3. Interference drag. A podded nacelle near the wing or body would have interfer­ence drag as follows (per nacelle). For a wing-mounted nacelle, the higher the overhang forward of the wing, the less would be the interference drag. Typical values of the interference drag by each pylon interacting with the wing or the body) are listed in Table 9.3.

4. Surface roughness (add later, « 3%.) A long overhang in front of the wing keeps the nacelle free from any interference effects. A short overhang has the highest interference. However, there is little variation of interference drag of a nacelle mounted on a different position at the aft fuselage. Much depends on the proximity of other bodies, such as the wing and empennage. If the nacelle is within one diameter, then interference drag may be increased by another

0. 5%. The center engine is close to the fuselage and with the V-tail, they have increased interference.

By totaling all the components, the flat-plate equivalent of the nacelle drag contribution is given by the following equation (omit the term ACF„_rough in Equa­tion 9.25 if it is accounted for at the end, as shown in Equation 9.27):

fn = (CFn + ACFn_wmp + ACFnJntake + ACfn_boattail

+ A-CFn – excres + ACFn-rough) X Awn (9.25)

Converting the nacelle contribution to CDpmin in terms of the aircraft wing area, it becomes:

[CDpmin]n = fn/SW (9.26)

In the last three decades, the nacelle drag has been reduced by approximately twice as much as what has been achieved in other aircraft components. This demon­strates the complexity of and unknowns associated with the flow field around nacelles. CFD is important in nacelle design and its integration with aircraft.

Center S – duct Center strai ght duct

nacelle of tri-jet nacelle of tri-jet

In this book, nacelle geometry is simplified to the axi-symmetric shape without loss of methodology.