Figure 15.8 shows the positions of noise-suppression liners placed in various areas and the jet-pipe-flow mixing arrangements for noise abatement. Exhaust-noise suppression also is achieved by using a fluted duct (which increases the mixing area) at the exit plane. Many types of liners are available on the market and there is room for improvement in liner technology. Primarily, there are two types of liners: reactive and resistive. The reactive liners have different sizes of perforations to react with matched frequency range of noise and absorb. The resistive type of liner is a noise insulator in layers with screens. The most common type of acoustic liner comprises a combination of both types. It has resistive facing sheet covering a honeycomb structure between the insulator screens with cell sizes matched to the frequency range where noise attenuation is requuired. Nacelle certification is the responsibility of the aircraft manufacturer, even when it is subcontracted to a third party, because it is covered by FAR Part 25 requirements, rather than FAR Part 33, which are for the engine.

Propeller-driven aircraft must consider noise emanating (i.e., radiation and reflection) from the propellers. Here, the noise-reflection pattern depends on the direction of propeller rotation, as shown in Figure 15.9. The spread of reflected noise also depends on the propeller position relative to the wing and the fuselage.

Inside the aircraft cabin, noise comes from the ECS and must be maintained at the minimum level. These problems are addressed by specialists. Cabin-interior design considerations are addressed in Phase 2 of a project.

15.3.1 Summary

At this stage of study, design considerations for noise reduction do not substantially affect the aircraft external configuration other than using proper filleting at

Figure 15.8. Positions of noise liners and suppression/mixing arrangements

Figure 15.9. Noise considerations for propeller-driven aircraft

two-body junctions, streamlining the projected structure, minimizing gaps, and so forth. The finalized aircraft configuration – as obtained in Chapter 6 and sized in Chapter 11 – remains unaffected because the aircraft external geometry is assumed to have accounted for these considerations. The choice of materials (e.g., nacelle liners, cabin insulators, and fatigue-resistant material) can affect aircraft mass. Enginenoise abatement is generally the responsibility of engine designers.

The advancement of CFD capabilities in predicting noise has resulted in good judgments for improving design. Substantiation of the CFD results requires testing.

In the near future (i.e., gradually evolving in about two decades), remarkable improvement in noise abatement may be achieved using a multidisciplinary design approach, taking the benefits from various engineering considerations leading to a BWB shape. Cambridge University and the Massachusetts Institute of Technol- ogy have undertaken feasibility studies that show a concept configuration in Figure 15.10 for an Airbus 320 class of subsonic-jet commercial transport aircraft. The engineers predict that the aircraft will be 25 dBs quieter than current designs – so quiet as to name it “silent aircraft.” The shaping of the aircraft is not based solely on noise reduction; it is also driven by general aerodynamic considerations (e.g., drag reduction and handling qualities). Noise reduction results from the aircraft body shielding the intake noise, minimizing two-body junctions by blending the wing and the fuselage, and eliminating the empennage. Of course, reduction in the engine noise is a significant part of the exercise. However, to bring the research to a marketable product will take time, but the author believes it will come in many sizes; heavy-lift cargo aircraft are good candidates.

Figure 15.10. Concept design of a futuristic “silent aircraft”