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197
Fig. 18-18 Reaction control system.
Rolls-Royce Turbomeca Adour MK151
Napier Gazelle
The Gazelle turbo-shaft engine first ran in
December 1955 at 1260 shp, a figure later
increased to 1610 shp on production engines.
Gazelles were used to power Bristol
Belvedere and Westland Wessex helicopters.
Gazelle production was taken over by Rolls-
Royce in 1961.
19: Noise suppression
Contents Page
Introduction 199
Engine noise 199
Methods of suppressing noise202
Construction and materials 205
INTRODUCTION
1. Airport regulations and aircraft noise certification
requirements, all of which govern the maximum
noise level aircraft are permitted to produce, have
made jet engine noise suppression one of the most
important fields of research.
2. The unit that is commonly used to express noise
annoyance is the Effective Perceived Noise deciBel
(EPNdB). It takes into account the pitch as well as
the sound pressure (deciBel) and makes allowance
for the duration of an aircraft flyover. Fig. 19-1
compares the noise levels of various jet engine
types.
3. Airframe self-generated noise is a factor in an
aircraft’s overall noise signature, but the principal
noise source is the engine.
ENGINE NOISE
4. To understand the problem of engine noise
suppression, it is necessary to have a working
knowledge of the noise sources and their relative
importance. The significant sources originate in the
fan or compressor, the turbine and the exhaust jet or
jets. These noise sources obey different laws and
mechanisms of generation, but all increase, to a
varying degree, with greater relative airflow velocity.
Exhaust jet noise varies by a larger factor than the
compressor or turbine noise, therefore a reduction in
exhaust jet velocity has a stronger influence than an
equivalent reduction in compressor and turbine blade
speeds.
199
Noise suppression
200
Fig. 19-1 Comparative noise levels of various engine types.
Fig. 19-2 Exhaust mixing and shock structure.
5. Jet exhaust noise is caused by the violent and
hence extremely turbulent mixing of the exhaust
gases with the atmosphere and is influenced by the
shearing action caused by the relative speed
between the exhaust jet and the atmosphere. The
small eddies created near the exhaust duct cause
high frequency noise but downstream of the exhaust
jet the larger eddies create low frequency noise.
Additionally, when the exhaust jet velocity exceeds
the local speed of sound, a regular shock pattern is
formed within the exhaust jet core. This produces a
discrete (single frequency) tone and selective amplification
of the mixing noise, as shown in fig. 19-2. A
reduction in noise level occurs if the mixing rate is
accelerated or if the velocity of the exhaust jet
relative to the atmosphere is reduced. This can be
achieved by changing the pattern of the exhaust jet
as shown in fig. 19-3.
6. Compressor and turbine noise results from the
interaction of pressure fields and turbulent wakes
from rotating blades and stationary vanes, and can
be defined as two distinct types of noise; discrete
tone (single frequency) and broadband (a wide range
of frequencies). Discrete tones are produced by the
regular passage of blade wakes over the stages
downstream causing a series of tones and
harmonics from each stage. The wake intensity is
largely dependent upon the distance between the
rows of blades and vanes. If the distance is short
then there is an intense pressure field interaction
which results in a strong tone being generated. With
the high bypass engine, the low pressure
compressor (fan) blade wakes passing over
downstream vanes produce such tones, but of a
lower intensity due to lower velocities and larger
blade/vane separations. Broadband noise is
Noise suppression
201
Fig. 19-3 Change of exhaust jet pattern to reduce noise level.
produced by the reaction of each blade to the
passage of air over its surface, even with a smooth
airstream. Turbulence in the airstream passing over
the blades increases the intensity of the broadband
noise and can also induce tones.
7. With the pure jet engine the exhaust jet noise is
of such a high level that the turbine and compressor
noise is insignificant at all operating conditions,
except low landing-approach thrusts. With the bypass
principle, the exhaust jet noise drops as the
velocity of the exhaust is reduced but the low
pressure compressor and turbine noise increases
due to the greater internal power handling.
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