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Noise reduction technologies for

aircraft landing gear

Noise is emitted from an aircraft and its components potential benefits being identified. This is because
during a variety of flight phases: approach to landing, the TRL is still not high enough with issues related
take-off, cruise etc. Statistically, it is only second to to their mounting, weight and complexity yet to be
traffic noise in urban areas in its unsociable levels, considered to be worth the potential noise gains. More
frequency and time of occurrence, and is often at the importantly, obstacles to safety and maintenance are
top of the list in rural areas. Due to the increasing still to be fully addressed.
popularity of air travel and the rapid development of
aeronautics in recent decades, complaints of increasing However, as aero-engine noise continues to reduce
aircraft noise are being continuously received by local and with the promise of dramatic reductions in flap
authorities, especially from communities living in and slat noise resulting from novel morphing wing
the vicinity of airports. Consequently, the reduction designs, it seems inevitable that, at least for approach
of perceived aircraft noise, in particular at approach conditions, landing gear noise will become the new
and take-off, is an essential consideration for the noise floor below which it will not be possible to break
aeronautical industry. without either a step change innovation in design or
else the implementation of low noise technologies.
As a significant contributor to aircraft noise at
landing approach, landing gear noise is currently a Looking to the future, we need to progress from
major interest in study for aeronautical engineers and “Evolutionary” designs, such as those described in
aeroacoustics researchers. Numerous technologies this paper, to “Revolutionary” configurations, such
have been proposed for the abatement of landing as might be facilitated by Mid-Fuselage Nacelle
gear noise. This article presents review of such (MFN) concepts where the engines are installed
technologies for landing gear noise reduction as well above the wings. The MFN aircraft concept is a
as potential solutions, considering the Technology NASA designed advanced aircraft configuration that
Readiness Level (TRL).These noise control retains the advantages of the conventional tube-and-
technologies can be classified as either passive or wing (T+W) layout and makes use of the benefits
active control methods. of fuselage-mounted engines. The most significant
acoustic benefit related to landing gear noise reduction
The passive control methods require no additional is the reduced ground clearance requirement, which
energy input and include different types of fairings, thus provides the opportunity for a redesigned landing
an acoustic treatment of the landing gear bay, hole gear assembly for noise reduction. This revolutionary
coverings, vortex generators, trailing edge serrations change would allow the main landing gear strut to
etc. The remaining, including the deployable ramp be shortened and to integrate the gear into “Pods”
door type fairings and the air curtain, require whose interior surfaces are treated with acoustic liners.
additional energy. Furthermore, there are both open Extending further into the future and away from the
loop and closed-loop systems, subject to whether tube-and-wing design, the “Transformational” Hybrid
feedback signals are considered. Technically, all active Wing Body (HWB) concept, incorporating ambitious
control methods can be designed with a closed-loop but realizable technology implementations, such as
system, and more importantly, it is expected that the Kruger flap, is predicted to reach a cumulative
the closed-loop can contribute to an improved noise noise reduction below the Stage 4 requirement of
reduction. However, the associated complexity is a 50.9 EPNL dB for this Far Term hybrid wing body
problem that must be considered. concept. Such step change innovations in landing gear
noise reduction using air curtains or PODS will enable
Except for the hubcaps, which are the only technology significant attenuation of landing gear noise.
so far implemented to date, these technologies
remain at a low TRL and have yet to be implemented
on commercial aircraft despite the significant

36 February 2020