10.9 Exhaust Nozzle and Thrust Reverser

At the throat, if the Mach number is high (e.g., reaches the local sonic speed), there is more loss and a longer diffuser is needed to decrease air velocity to around Mach 0.5 at the fan face. It is best to keep the average Mach number at the throat just below 1.0. Care must be taken that at yaw and/or high angles of attack, the fan-face flow distortion is minimized.

Finally, to make a proper divergent part of the subsonic intake acting as the diffuser, the internal contour shows an inflection point (at around 0.5 to 0.75 Ldiff). At that point, to avoid separation, the maximum wall angle θ should not exceed 8 or 9 deg.

Typically, the nacelle length, LN, is 1.5 to 1.8 times the bare engine length, LE. The maximum diameter is positioned around 0.25 to 0.40 of the nacelle length (LFB) from the front end. The nacelle external cross-section is not purely circular but rather has a “pregnant-belly” shape at the keel cut to house engine accessories. Use the maximum radius at the crown cut as 1.1 to 1.4 times the engine fan-face radius and at the keel cut as 1.2 to 1.6 times the engine fan-face radius. The side cuts are faired between the two.

For the worked-out Bizjet example, use the following values (see Section 10.10.3):

Given fan-face diameter, Dfan = 0.716 m (2.35 ft)

R1 = 0.9 × Rfan

At maximum cruise, MFR = 1 (A∞ = AH)

At cruise, MFR = 0.7 (A∞ < AH)

LCR = 1.12

DMAX = 1.5 × 0.716 = 1.074 m (3.52 ft)

Lower-lip fineness ratio (a/b) = 3

Upper-lip fineness ratio (c/d) = 5

Ldiff = 0.65 Dfan

(b + d)/Ldiff = 0.18 LFB = 1.4 times Dfan

Engine manufacturers supply the data for bare engines. A bare engine may come with an exhaust duct as a nozzle that fits within the nacelle exhaust.

10.8.2 Military Aircraft Intake Design

This extended section of the book can be found on the Web site www.cambridge

.org/Kundu and discusses the important consideration for typical military aircraft intake design involving supersonic intakes. The associated figure is Figure 10.25.

Figure 10.25. Types of ideal supersonic intake demand conditions [21]

10.9 Exhaust Nozzle and Thrust Reverser

The thrust reverser (TR) is part of an exhaust nozzle and both are addressed in this section; an empirical sizing method for a nozzle is discussed but not the size and

Figure 10.26. TR efflux pattern

design of the TR, which is a separate technology. Before explaining exhaust nozzles, it is helpful to understand TRs.

The role of a TR is to retard aircraft speed by applying thrust in the forward direction (i.e., in a reversed application). The rapid retardation by the TR application reduces the landing-field length. In a civil aircraft application, the TR is applied only on the ground. Because of its severity, certification rules require to either design for deployment in flight (e.g., Concorde) or prevent in-flight deployment. However, the latter is the more common approach. A TR reduces the wheel-brake load so there is less wear and fewer heat hazards. A TR is effective on slippery runways (e.g., ice and water) when braking is less effective. A typical benefit of having sufficient stopping distance at landing on an icy runway with TR application is that it reduces the field length by less than half. A midsized jet-transport aircraft stops at about 4,000 ft with a TR or at about 12,000 ft without it. Without a TR, the energy that was depleted to stop the aircraft is absorbed by the wheel brake and aerodynamic drag. Application of the TR also provides additional intake momentum drag (at full throttle), contributing to energy depletion. A TR is useful for an aircraft to go in reverse (e.g., a C17) on the ground for parking, alignments, and so forth – most aircraft with a TR do not use reverse but rather a specialized vehicle that pushes it.

A TR is integrated on the nacelle and it is the responsibility of an aircraft manufacturer to design it or it may be subcontracted to specialist organizations devoted to TR design. The next section introduces the TR in detail so that coursework can proceed on the nacelle without undertaking the detailed design.

10.9.1 Civil Aircraft Thrust Reverser Application

TRs are not required by the regulatory authorities (i.e., FAA and CAA). The components are expensive, heavy, and only applied on the ground, yet their impact on an aircraft’s operation is significant due to additional safety through better control and reduced time for stopping, especially during aborted takeoffs and other related emergencies. Most airlines want their aircraft to have TRs even with the increased DOC.

Aircraft designers must ensure that TR efflux is well controlled – there should be no adverse impingement of on aircraft surface or reingestion in the engine. Figure 10.26 shows a typical satisfactory TR efflux pattern.

In general, there are two types of TRs: (1) operating on both the fan and core flow, and (2) operating on the fan flow only. The choice depends on the BPR, nacelle location, and customer specifications.

The first type, which operates on the total flow (i.e., both fan and core), is shown at the top of Figure 10.27. There are two types: (1) the sliding-port aft-door type, in

Figure 10.27. Types of thrust reversers

which the doors slide to the aft end as they open up to deflect the exhaust flow; and (2) the fixed-pivot type, in which the doors rotate to a position that deflects the exhaust flow.

The second type of TR operates on the fan flow only. There are two types: (1) the petal-cowl type, shown in the middle of Figure 10.27; and (2) the cascade-cowl type, shown at the bottom of Figure 10.27. There are two cascade types: the conventional type and the natural-blockage type. The Bombardier CRJ700/900 aircraft uses a petal cowl TR of the natural-blockage type. The external cowls translate back, blocking the fan flow when it escapes through the fixed cascades that reverse the flow. This design is attractive with a low parts count, scalability, easier maintenance, and a relatively higher retarding energy. The petal-cowl type operating on the fan flow is suitable for short-duct nacelles, as shown in the figure. The petal doors open on a hinge to block the secondary flow of the fan when it deflects to develop reverse thrust.

TRs are applied below 150 kts and are retracted at around 50 kts (to avoid reingestion of engine efflux), when the wheel brakes become effective. The choice of the TR type depends on a designer’s compromise with the available technology.

10.9.2 Civil Aircraft Exhaust Nozzles

Civil aircraft nozzles are conical in shape, on which the TR is integrated. Small turbofan aircraft may not need a TR but regional jet (RJ) aircraft and larger use a TR. Inclusion of a TR may slightly elongate the nozzle, but this is not discussed in this book.

In general, the nozzle exit area is sized as a perfectly expanded nozzle (pe = p∞) at LRC condition; at higher engine ratings, it is pe > p∞. The exit nozzle of a longduct turbofan does not run choked at cruise ratings. At takeoff ratings, the back pressure is high at a lower altitude; therefore, a long-duct turbofan does not need to run choked (i.e., the low-pressure secondary flow mixes in the exhaust duct). An exhaust nozzle runs in a favorable pressure gradient; therefore, its shaping results