Base Drag
The design criteria for the nozzleexit area sizing is such that at LRC, the exitplane 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 nozzleexit area being larger than required.
BoatTail Drag
The longduct contour for closure (i. e., “boattail” shape) at the aft end is shallow enough to avoid separation, especially with the assistance of entrainment effects of
Spillage drag =
scale)
the exhaust plume. Hence, the boattail drag is kept low. At the idle throttle setting, considerable flow separation can occur and the magnitude of boattail 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 exhaustpipe 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 engineinstallation losses expressed as intakerecovery loss. Intake – and exhaustduct losses are approximately 1 to 3% in engine thrust at LRC (throttle – and altitudedependent). The net thrust of the turbofan, incorporating installation losses, is computed using the enginemanufacturersupplied program and data. These manufacturers work in close liaison to develop the internal contour of the nacelle and intake. External nacellecontour design and airframe integration remain the responsibility of the aircraft manufacturer.
The longduct nacelle characteristic length, Lnac, is the length measured from the intakehighlight plane to the exitarea 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 nacellelip section is included in the intakedrag estimation; hence, it is not computed separately. Similarly, the pressure effect is included in the base/boattail 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 axisymmetric).
Wrapping:
Table 9.3. Nacelle interference drag (per nacelle)

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) Boattail/base drag (throttledependent) – includes pressure effects « 10 to 12% (higher value for smaller aircraft)
(c) Excrescence (nonmanufacturing type such as coolingair 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 wingmounted 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 Vtail, they have increased interference.
By totaling all the components, the flatplate 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
+ ACFn – excres + ACFnrough) 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 trijet nacelle of trijet
In this book, nacelle geometry is simplified to the axisymmetric shape without loss of methodology.