Tupolev 154M noise asesment (Анализ шумовых характеристик самолёта Ту-154М)

Tupolev 154M noise asesment (Анализ шумовых характеристик самолёта Ту-154М)


Noise Problem
of Noise




Influence on Health




Measurement and Rules

Effectiveness Forecast (NEF)

Perceived Noise Level (EPNL)




154M Description


Noise Calculation

Approach Noise Claculation


Jet Noise

Duct Linings

Duct Lining

1 The Noise Problem

long of concern to neighbors of major airports, aircraft noise first became a
major problem with the introduction of turbojet-powered commercial aircraft
(Tupolev 104, Boeing 707, Dehavilland Comet) in the late 1950s. It was
recognized at the time that the noise levels produced by turbojet powered
aircraft would be unacceptable to persons living under the take-off pattern of
major airports. Accordingly, much effort was devoted to developing jet noise
suppressors, with some modest success. Take-off noise restrictions were imposed
by some airport managements, and nearly all first-generation turbojet-powered
transports were equipped with jet noise suppressors at a significant cost in
weight, thrust, and fuel consumption.

         The introduction of the turbofan
engine, with its lower jet velocity, temporarily alleviated the jet noise
problem but increased the high-frequency turbomachinery noise, which became a
severe problem on landing approach as well as on take-off. This noise was
reduced somewhat by choosing proper rotor and stator blade numbers and spacing
and by using engines of the single-mixed-jet type.

2 Effects Of

Noise is often
defined as unwanted sound. To gain a satisfactory understanding of the effects
of noise, it would be useful to look briefly at the physical properties of

Sound is the result of
pressure changes in a medium, caused by vibration or turbulence. The amplitude
of these pressure changes is stated in terms of sound level, and the rapidity
with which these changes occur is the sound’s frequency. Sound level is
measured in decibels (dB), and sound frequency is stated in terms of cycles per
second or Hertz (Hz). Sound level in decibels is a logarithmic rather than a
linear measure of the change in pressure with respect to a reference pressure
level. A small increase in decibels can represent a large increase in sound
energy. Technically, an increase of 3 dB represents a doubling of sound energy,
and an increase of 10 dB represents a tenfold increase. The ear, however, perceives
a 10-dB increase as doubling of loudness.

important aspect is the duration of the sound, and the way it is distributed in
time. Continuous sounds have little or no variation in time, varying sounds
have differing maximum levels over a period of time, intermittent sounds are
interspersed with quiet periods, and impulsive sounds are characterized by
relatively high sound levels and very short durations.

The effects of
noise are determined mainly by the duration and level of the noise, but they are
also influenced by the frequency. Long-lasting, high-level sounds are the most
damaging to hearing and generally the most annoying. High-frequency sounds tend
to be more hazardous to hearing and more annoying than low-frequency sounds.
The way sounds are distributed in time is also important, in that intermittent
sounds appear to be somewhat less damaging to hearing than continuous sounds
because of the ear’s ability to regenerate during the intervening quiet
periods. However, intermittent and impulsive sounds tend to be more annoying
because of their unpredictability.

Noise has a
significant impact on the quality of life, and in that sense, it is a health
problem. The definition of health includes total physical and mental
well-being, as well as the absence of disease. Noise is recognized as a major
threat to human well-being.

effects of noise are seldom catastrophic, and are often only transitory, but
adverse effects can be cumulative with prolonged or repeated exposure. Although
it often causes discomfort and sometimes pain, noise does not cause ears to
bleed and noise-induced hearing loss usually takes years to develop.
Noise-induced hearing loss can indeed impair the quality of life, through a
reduction in the ability to hear important sounds and to communicate with
family and friends. Some of the other effects of noise, such as sleep
disruption, the masking of speech and television, and the inability to enjoy
one’s property or leisure time also impair the quality of life. In addition,
noise can interfere with the teaching and learning process, disrupt the
performance of certain tasks, and increase the incidence of antisocial
behavior. There is also some evidence that it can adversely affect general
health and well-being in the same manner as chronic stress.

2.1  Hearing Loss

Hearing loss
is one of the most obvious and easily quantified effects of excessive exposure
to noise. Its progression, however, is insidious, in that it usually develops
slowly over a long period of time, and the impairment can reach the
handicapping stage before an individual is aware of what has happened.

exposure to noise of a certain frequency pattern can cause either temporary
hearing loss, which disappears in a few hours or days, or permanent loss. The
former is called temporary threshold shift, and the latter is known as permanent
threshold shift

Temporary threshold shift is
generally not damaging to human’s ear unless it is prolonged. People who work
in noisy environments commonly are victims of temporary threshold shift.

Figure 2.1 Temporary threshold shift
for rock band performers.

Repeated noise
over a long time leads to permanent threshold shift. This is especially true in
industrial applications where people are subjected to noises of a certain

There is
some disagreement as to the level of noise that should be allowed for an 8-hour
working day. Some researchers and health agencies insist that 85 dB(A) should
be the limit. Industrial noise level limitations are shown in the Table 2.1.

Table 2.1 Maximum Permissible Industrial Noise
Levels By OSHA

Safety and Health Act)

Sound Level, dB(A)

Maximum Duration

During Any

Working Day
















hearing loss is probably the most well-defined of the effects of noise.
Predictions of hearing loss from various levels of continuous and varying noise
have been extensively researched and are no longer controversial. Some
discussion still remains on the extent to which intermittencies ameliorate the
adverse effects on hearing and the exact nature of dose-response relationships
from impulse noise. It appears that some members of the population are somewhat
more susceptible to noise-induced hearing loss than others, and there is a
growing body of evidence that certain drugs and chemicals can enhance the
auditory hazard from noise.

Although the incidence of
noise-induced hearing loss from industrial populations is more extensively
documented, there is growing evidence of hearing loss from leisure time
activities, especially from sport shooting, but also from loud music, noisy
toys, and other manifestations of our "civilized" society. Because of
the increase in exposure to recreational noise, the hazard from these sources
needs to be more thoroughly evaluated. Finally, the recent evidence that
hearing protective devices do not perform in actual use the way laboratory
tests would imply, lends support to the need for reevaluating current methods
of assessing hearing protector attenuation.

2.2  Noise Interference

Noise can mask
important sounds and disrupt communication between individuals in a variety of
settings. This process can cause anything from a slight irritation to a serious
safety hazard involving an accident or even a fatality because of the failure
to hear the warning sounds of imminent danger. Such warning sounds can include
the approach of a rapidly moving motor vehicle, or the sound of malfunctioning
machinery. For example, Aviation Safety states that hundreds of accident
reports have many "say again" exchanges between pilots and
controllers, although neither side reports anything wrong with the radios.

Noise can disrupt face-to-face and telephone conversation, and the
enjoyment of radio and television in the home. It can also disrupt effective
communication between teachers and pupils in schools, and can cause fatigue and
vocal strain in those who need to communicate in spite of the noise.
Interference with communication has proved to be one of the most important
components of noise-related annoyance.

with speech communication and other sounds is one of the most salient
components of noise-induced annoyance. The resulting disruption can constitute
anything from an annoyance to a serious safety hazard, depending on the

Criteria for determining
acceptable background levels in rooms have also been expanded and refined, and
progress has been made on the development of effective acoustic warning

It is now dear that
hearing protection devices can interfere with the perception of speech and
warning signals, especially when the listener is hearing impaired, both talker
and listener wear the devices, and when wearers attempt to locate a signal’s

Noise can interfere with
the educational process, and the result has been dubbed "jet-pause
teaching" around some of the nation’s noisier airports, but railroad and
traffic noise can also produce scholastic decrements.

2.3  Sleep Disturbance

Noise is one
of the most common forms of sleep disturbance, and sleep disturbance is a
critical component of noise-related annoyance. A study used by EPA in preparing
the Levels Document showed that sleep interference was the most frequently
cited activity disrupted by surface vehicle noise (BBN, 1971). Aircraft none
can also cause sleep disruption, especially in recent years with the escalation
of nighttime operations by the air cargo industry. When sleep disruption
becomes chronic, its adverse effects on health and well-being are well-known.

Noise can cause the sleeper to awaken
repeatedly and to report poor sleep quality the next day, but noise can also
produce reactions of which the individual is unaware. These reactions include
changes from heavier to lighter stages of sleep, reductions in "rapid eye
movement" sleep, increases in body movements during the night, changes in
cardiovascular responses, and mood changes and performance decrements the next
day, with the possibility of more serious effects on health and well-being if
it continues over long periods.

2.4  Noise Influence on Health

Noise has been
implicated in the development or exacerbation of a variety of health problems,
ranging from hypertension to psychosis. Some of these findings are based on carefully controlled
laboratory or field research, but many others are the products of studies that
have been severely criticized by the research community. In either case,
obtaining valid data can be very difficult because of the myriad of intervening
variables that must be controlled, such as age, selection bias, preexisting health
conditions, diet, smoking habits, alcohol consumption, socioeconomic status,
exposure to other agents, and environmental and social stressors. Additional
difficulties lie in the interpretation of the findings, especially those
involving acute effects.

Loud sounds
can cause an arousal response in which a series of reactions occur in the body.
Adrenalin is released into the bloodstream; heart rate, blood pressure, and
respiration tend to increase; gastrointestinal motility is inhibited;
peripheral blood vessels constrict; and muscles tense. Even though noise may
have no relationship to danger, the body will respond automatically to noise as
a warning signal.

3 Noise

All noise
emanates from unsteadiness – time dependence in the flow. In aircraft engines
there are three main sources of unsteadiness: motion of the blading relative to
the observer, which if supersonic can give rise to propagation of a sequence of
weak shocks, leading to the “buzz saw” noise of high-bypass turbofans; motion
of one set of blades relative to another, leading to a pure-tome sound (like
that from siren) which was dominant on approach in early turbojets; and
turbulence or other fluid instabilities, which can lead to radiation of sound
either through interaction with the turbomachine blading or other surfaces or
from the fluid fluctuations themselves, as in jet noise.

3.1  Jet Noise

fluid issues as a jet into a stagnant or more slowly moving background fluid,
the shear between the moving and stationary fluids results in a fluid-mechanical
instability that causes the interface to break up into vortical structures as
indicated in Fig. 3.1. The vortices travel downstream at a velocity which is
between those of the high and low speed flows, and the characteristics of the
noise generated by the jet depend on whether this propagation velocity is
subsonic or supersonic with respect to the external flow. We consider first the
case where it is subsonic, as is certainly the case for subsonic jets.

Figure 3.1 A subsonic jet mixing with ambient air, showing the mixing layer

followed by the fully
developed jet.

the subsonic jets the turbulence in the jet can be viewed as a distribution of

3.2  Turbomachinery Noise

generates noise by producing time-dependent pressure fluctuations, which can be
thought of in first approximation as dipoles since they result from
fluctuations in force on the blades or from passage of lifting blades past the

4 Noise
Measurement and Rules

         Human response sets the
limits on aircraft engine noise. Although the logarithmic relationship
represented by the scale of decibels is a first approximation to human
perception of noise levels, it is not nearly quantitative enough for either
systems optimization or regulation. Much effort has gone into the development
of quantitative indices of noise.

4.1  Noise Effectiveness Forecast

         It is not the
noise output of an aircraft per se that raises objections from the neighborhood
of a major airport, but the total noise impact of the airport’s operations,
which depends on take-off patterns, frequencies of operation at different times
of the day, population densities, and a host of less obvious things. There have
been proposals to limit the total noise impact of airports, and in effect legal
actions have done so for the most heavily used ones.

         One widely
accepted measure of noise impact is the Noise Effectiveness Forecast (NEF),
which is arrived at as follows for any location near an airport:

1. For each event, compute
the Effective Perceived Noise Level (EPNL) by the methods of ICAO Annex 16, as
described below.

2. For events occurring
between 10 PM and 7 AM, add 10 to the EPNdB.

3. Then NEF = , where the sum is taken over all events
in a 24-hour period. A little ciphering will show that this last calculation is
equivalent to adding the products of sound intensity times time for all events,
then taking the dB equivalent of this. The subtractor 82 is arbitrary.

4.2  Effective Perceived Noise
Level (EPNL)

perceived noisiness of an aircraft flyover depends on the frequency content,
relative to the ear’s response, and on the duration. The perceived noisiness is
measured in NOYs (unit of perceived noisiness) and is plotted as a function of
sound pressure level and frequency for random noise in Fig. 4.1.

Figure 4.1 Perceived noisiness as a function of
frequency and sound pressure level

Pure tones
(frequencies with pressure levels much higher than that of the neighboring
random noise in the sound spectrum) are judged to be more annoying than an
equal sound pressure in random noise, so a “tone correction” is added to their
perceived noise level. A “duration correction” represents the idea that the
total noise impact depends on the integral of sound intensity over time for a
given event.

         The 24 one-third
octave bands of sound pressure level (SPL) are converted to perceived noisiness
by means of a noy table.

Figure 4.2 Perceived noise level as a
function of NOYs

Conceptually, the
calculation of EPNL involves the following steps.

the NOY level for each band and sum them by the relation


where k denotes an interval in
time, i denotes the several frequency bande, and n(k) is the NOY
level of the noisiest band. This reflects the “masking” of lesser bands by the

total PNL is then PNL(k) = 40 + 33.3 log10N(k).

a tone correction c(k) by identifying the pure tones and adding to PNL
an amount ranging from 0 to 6.6 dB, depending on the frequency of the tone and
its amplitude relative to neighboring bands.

a duration correction according to EPNL = PNLTM + D, where PNLTM is the maximum
PNL for any of the time intervals. Here


where Dt = 0.5 sec, T = 10
sec, and d is the time over which PNLT exceeds PNLTM – 10 dB. This
amounts to integrating the sound pressure level over the time during which it
exceeds its peak value minus 10 dB, then converting the result to decibels.

All turbofan-powered
transport aircraft must comply at certification with EPNL limits for measuring
points which are spoken about in the next chapter.

5 Noise

The increasing
volume of air traffic resulted in unacceptable noise exposures near major urban
airfields in the late 1960s, leading to a great public pressure for noise control.
This pressure, and advancing technology, led to ICAO Annex 16, AP-36, Joint
Aviation Regulation Part 36 (JAR-36) and Federal Aviation Rule Part 36
(FAR-36), which set maximum take-off, landing and “sideline” noise levels for
certification of new turbofan-powered aircraft. It is through the need to
satisfy this rule that the noise issue influences the design and operation of
aircraft engines. A little more general background of the noise problem may be
helpful in establishing the context of engine noise control.

The FAA issued
FAR-36 (which establishes the limits on take-off, approach, and sideline noise
for individual aircraft), followed by ICAO issuing its Annex 16 Part 2, and JAA
issuing JAR-36. These rules have since been revised several times, reflecting
both improvements in technology and continuing pressure to reduce noise. As of
this writing, the rules are enunciated as three progressive stages of noise
certification. The noise limits are stated in terms of measurements at three
measuring stations, as shown in Fig. 5.1: under the approach path 2000 m before
touchdown, under the take-off path 6500 m from the start of the take-off roll,
and at the point of maximum noise along the sides of the runway at a distance
of 450 m.


Schematic of airport runway showing approach, take-off, and

noise measurement stations.

The noise of
any given aircraft at the approach and take-off stations depends both on the
engines and on the aircraft’s performance, operational procedures, and loading,
since the power settings and the altitude of the aircraft may vary.

The sideline
station is more representative of the intrinsic take-off noise characteristics
of the engine, since the engine is at full throttle and the station is nearly
at a fixed distance from the aircraft. The actual distance depends on the
altitude the aircraft has attained when it produced maximum noise along the
designated measuring line. Since FAR-36 and international rules set by the
International Civil Aviation Organization (ICAO annex 16, Part 2) which are
generally consistent with it have been in force, airport noise has been a major
design criterion for civil aircraft.

Stricter noise
pollution standards for commercial aircraft, established by the International
Civil Aviation Organization, came into effect worldwide on 1 April. Most
industrialized countries, including all EU states, enforced the new rules and
the vast majority of airliners flying in those states already meet the more
stringent requirements. But some Eastern European countries are facing a
problem, especially Russia. Eighty percent of its civilian aircraft fall short
of the standards, meaning it will not be able to apply the new rules for
domestic flights. Even more worrisome for Moscow is the fact that Russia could
find many of its planes banned from foreign skies. Enforcement of the new rules
could force Russia to cancel 11,000 flights in 2002, representing some 12
percent of the country’s passenger traffic.

The new rules
have been applied only to subsonic transports, because no new supersonic
commercial aircraft have been developed since its promulgation.

5.1  Noise Limits

mentioned above, all turbofan-powered transport aircraft must comply at
certification with EPNL limits for the three measuring stations as shown in
Fig. 5.1. The limits depend on the gross weight of the aircraft at take-off and
number of engines, as shown in Fig. 5.2. The rule is the same for all engine
numbers on approach and on the sideline because the distance from the aircraft
to the measuring point is fixed on approach by the angle of the approach path
(normally 3 deg) and on the sideline by the distance of the measuring station
from the runway centerline.

limits imposed by ICAO Annex 16 for certification of aircraft.

On take-off, however,
aircraft with fewer engines climb out faster, so they are higher above the
measuring point. Here the “reasonable and economically practicable” principle
comes into dictate that three-engine and two-engine aircraft have lower noise
levels at the take-off noise station than four-engine aircraft.

         There is some
flexibility in the rule, in that the noise levels can be exceeded by up to 2
EPNdB at any station provided the sum of the exceedances is not over 3 ENPdB
and that the exceedances are completely offset by reductions at other measuring

6 Noise Level

6.1   Tupolev 154M Description

For most airlines in the CIS, the
Tupolev Tu-154 is nowadays the workhorse on domestic and international routes.

Figure 6.1 Tupolev 154M main look

It was produced in two main vesions:
The earlier production models have been designated Tupolev -154, Tupolev -154A,
Tupolev -154B, Tupolev -154B-1 and Tupolev -154B-2, while the later version has
been called Tupolev -154M. Overall, close to 1’000 Tupolev -154s were built up
to day, of which a large portion is still operated.

Table 6.1 Tupolev 154M main


Medium range passenger aircraft


Produced until circa 1996, in wide spread

NATO Codename


First Flight

October 3, 1968 

First Service



3 Soloviev D-30KU (104 kN each)


47.9 m


37.5 m


3’900 km

Cruising Speed

900 km/h

Payload Capacity

156-180 passengers (5450 kg)

Maximum Take-off Weight

100’000 kg

The Tu-154 was
developed to replace the turbojet powered Tupolev Tu-104, plus the Antonov — 10
and Ilyushin — 18 turboprops. Design criteria in replacing these three relatively diverse aircraft
included the ability to operate from gravel or packed earth airfields, the need
to fly at high altitudes ‘above most Soviet Union air traffic, and good field
performance. In
meeting these aims the initial Tupolev -154 design featured three Kuznetsov
(now KKBM) NK-8 turbofans, triple bogey main undercarriage units which retract
into wing pods and a rear engine T-tail configuration.

The Tupolev
-154’s first flight occurred on October 4 1968. Regular commercial service began in February 1972. Three Kuznetsov powered
variants of the Tupolev -154 were built, the initial Tupolev -154, the improved
Tupolev -154A with more powerful engines and a higher max take-off weight and
the Tupolev -154B with a further increased max take-off weight. Tupolev -154S
is a freighter version of the Tupolev -154B.

production is of the Tupolev -154M, which first flew in 1982. The major change introduced on the M
was the far more economical, quieter and reliable Solovyev (now Aviadvigatel)
turbofans. The
Tupolev — 154M2 is a proposed twin variant powered by two Perm PS90A turbofans.

6.2  Noise Calculaions

Noise level at control points is
calculated using the Noise-Power-Distance (NPD) relationship. In practice
NPD-relationship is used in the parabolic shape:



Table 6.2 Noise-Power-Distance coefficients of
similar aircraft.



Engine mode





















85% of nominal














2-nd cruise





















6.2.1  Take-off Noise Calculation

The aircraft
begins the take-off roll at point A (Fig. 6.2), lifts off at point B, and
initiates the first constant climb at point C at an angle β. The noise abatement thrust cutback
is started at point D and completed at point E where the second constant climb
is defined by the angle γ (usually expressed in
terms of the gradient in percent). The end of the noise certification take-off
flight path is represented by aircraft position F whose vertical projection on
the flight track (extended centerline of the runway) is point M. The position
of the aircraft must be recorded for the entire interval during which the
measured aircraft noise level is within 10 dB of PNLTM. Position K is the
take-off noise measuring station whose distance AK is specified as 6500 meters.

and climb path

The take-off profile is
defined by five parameters — (A) AB, the length of take-off roll; (B) β the first constant climb angle; (C) γ, the second constant climb angle;
and (D) δ, and e, the thrust
cutback angles. These five parameters are functions of the aircraft performance
and weight, and the atmospheric conditions of temperature, pressure, and wind
velocity and direction.

reference atmospheric conditions and with maximum take-off weight, the gradient
of the second constant climb angle (γ) may not be less than 4 percent. However, the actual
gradient will depend upon atmospheric conditions, assuming maximum take-off
weight and the parameters characterizing engine performance are constant (rpm,
or any other parameter used by the pilot).

In operational
conditions the climb is performed without the cutback stage, and the aircraft flies
over the control point at a lower altitude, which leads to higher noise levels.

Figure 6.3 Comparison between operational and certification trajectories

The climb path
for Tupolev 154M was calculated using the following equation


m is aircraft mass;

a is the angle of attack, j is the angle of engine

q is climb angle which is
equal to b or g, depending on the climb stage.

Figure 6.4 Comparison between noise levels under different flight paths

6.2.2  Approach Noise Calculation

The approaches
must be conducted with a steady glide angle of 3°±0.5° and must be continued to
a normal touchdown with no airframe configuration change. Thus the distance from the control point to the glideslope RN
remains constant and is equal to 119.7 m.

6.5 Schematic
of approach

         Taking into account that the speed remains constant and
airframe configuration is for landing, we can calculate the stall speed:


where G is airplane weight, r is air density, S
is wing area, Cy max is maximum lift coefficient determined
from Fig. 6.6.

Approach speed should be 30% greater that the stall

Figure 6.6 Aerodynamic characteristics of Tupolev

Using the approach speed, we can calculate current
lift coefficient:


Corresponding drag coefficient is determined from Fig.

corrections must be made to calculated values of drag and lift coefficients. It
is necessary to take into account the influence of the landing gear which
creates additional drag and decreases lift. The influence of flaps and slats is
little and can be neglected.

Necessary thrust
is calculated using the following formula


where  is drag and q is approach path angle
which is equal to 3 degrees.

Calculated results for five different landing weights
are shown in the table 6.3.

Calculation results for Tupolev 154M at approach configuration.

Weight, %






Weight, kg






Vapp, m/s






Thrust, kg


















∆LA, dBA

















Thrust To Weight rt.






Tupolev 154M has the same aerodynamics as Tupolev 154, thus the necessary
thrust for both of them during approach is almost the same. Tupolev 154M has
more powerful engines and it can carry more payload. Its maximum landing weight
is 2 tons greater than that one of 154. Noise parameters are different for
these aircraft (table 6.2), and the calculated noise levels slightly differ as

7 Noise Suppression

7.1  Suppression of Jet Noise

Methods for
suppressing jet noise have exploited the characteristics of the jet itself and
those of the human observer. For a given total noise power, the human impact is
less if the frequency is very high, as the ear is less sensitive at high
frequencies. A shift to high frequency can be achieved by replacing one large
nozzle with many small ones. This was one basis for the early turbojet engine
suppressors. Reduction of the jet velocity can have a powerful effect since P
is proportional to the jet velocity raised to a power varying from 8 to 3,
depending on the magnitude of uc. The multiple small nozzles
reduced the mean jet velocity somewhat by promoting entrainment of the
surrounding air into the jet. Some attempts have been made to augment this
effect by enclosing the multinozzle in a shroud, so that the ambient air is
drawn into the shroud.

Certainly the
most effective of jet noise suppressors has been the turbofan engine, which in
effect distributes the power of the exhaust jet over a larger airflow, thus
reducing the mean jet velocity.

In judging the
overall usefulness of any jet noise reduction system, several factors must be
considered in addition to the amount of noise reduction. Among these factors
are loss of thrust, addition of weight, and increased fuel consumption.

A number of
noise-suppression schemes have been studied, mainly for turbofan engines of one
sort or another. These include inverted-temperature-profile nozzles, in which a
hot outer flow surrounds a cooler core flow, and mixer-ejector nozzles. In the
first of these, the effect is to reduce the overall noise level from that which
would be generated if the hot outer jets are subsonic with respect to the outer
hot gas. This idea can be implemented either with a duct burner on a
conventional turbofan or with a nozzle that interchanges the core and duct
flows, carrying the latter to the inside and the former to the outside. In the
mixer-ejector nozzle, the idea is to reduce the mean jet velocity by ingesting
additional airflow through a combination of the ejector nozzles and the
chute-type mixer. Fairly high mass flow ratios can be attained with such
arrangements, at the expense of considerable weight.

The most
promising solution, however, is some form of “variable cycle” engine that
operates with a higher bypass ratio on take-off and in subsonic flight than at
the supersonic cruise condition. This can be achieved to some degree with
multi-spool engines by varying the speed of some of the spools to change their
mass flow, and at the same time manipulating throttle areas. Another approach
is to use a tandem-parallel compressor arrangement, where two compressors
operate in parallel at take-off and subsonically, and in series at a supersonic

7.1.1  Duct Linings

It is self evident that the most
desirable way to reduce engine noise would be to eliminate noise generation by
changing the engine design. The current state of the art, however, will not
provide levels low enough to satisfy expected requirements; thus, it is
necessary to attenuate the noise that is generated.

Fan noise
radiated from the engine inlet and fan discharge (Fig. 7.1) of current fan jet
airplanes during landing makes the largest contribution to perceived noise.

illustration of noise sources from turbofan engines

Figure 7.2.
shows a typical farfield SPL noise spectrum generated by a turbofan engine at a
landing-approach power setting. Below 800 Hz, the spectrum is controlled by
noise from the primary jet exhaust. The spectrum between 800 and 10000 Hz
contains several discrete frequency components in particular that need to be
attenuated by the linings in the inlet and the fan duct before they are
radiated to the farfield.


Engine-noise spectrum

The objective
in applying acoustic treatment is to reduce the SPL at the characteristic
discrete frequencies associated with the fan blade passage frequency and its
associated harmonics. Noise reductions at these frequencies would alleviate the
undesirable fan whine and would reduce the perceived noise levels.

A promising
approach to the problem has been the development of a tuned-absorber
noise-suppression system that can be incorporated into the inlet and exhaust
ducts of turbofan engines. An acoustical system of this type requires that the internal aerodynamic
surfaces of the ducts be replaced by sheets of porous materials, which are
backed by acoustical cavities. Simply, these systems function as a series of
dead-end labyrinths, which are designed to trap sound waves of a specific
wavelength. The frequencies for which these absorbers are tuned is a function
of the porosity of flow resistance of the porous facing sheets and of the depth
or volume of the acoustical cavities. The cavity is divided into compartments by means of an
open cellular structure, such as honeycomb cells, to provide an essentially
locally reacting impedance (Fig. 7.3). This is done to provide an acoustic impedance
almost independent of the angle of incidence of the sound waves impinging on
the lining.

The perforated-plate-and-honeycomb
combination is similar to an array of Helmholtz resonators; the pressure in the
cavity acts as a spring upon which the flow through the orifice oscillates in
response to pressure fluctuations outside the orifice.

Figure 7.2 Schematic of acoustic damping cavities in an angine duct. The size of
the resonators

is exaggerated relative to
the duct diameter.

The attenuation spectrum
of this lining is that of a sharply tuned resonator effective over a narrow
frequency range when used in an environment with low airflow velocity or low
SPL. This concept, however, can also provide a broader bandwidth of attenuation
in a very high noise-level environment where the particle velocity through the
perforations is high, or by the addition of a fine wire screen that provides
the acoustic resistance needed to dissipate acoustic energy in low
particle-velocity or sound-pressure environments. The addition of the wire
screen does, however, complicate manufacture and adds weight to such an extent
that other concepts are usually more attractive.

Figure 7.3 Acoustical lining

Although the resistive-resonator lining is a
frequency-tuned device absorbing sound in a selected frequency range, a
suitable combination of material characteristics and lining geometry will yield
substantial attenuation over a frequency range wide enough to encompass the
discrete components and the major harmonics of most fan noise.

7.1.2 Duct Lining Calculation

First we have to determine
the blade passage frequency:


where z is number
of blades, n is RPM.

Blade passage frequencies
for different engine modes are given in table 7.1

Next we determine the
second fan blade passage harmonic frequency, which is two times greater than
the first one: .

Table 7.1 Fan blade passage
frequencies for different engine modes.
















harmonic freq., Hz















experimental data, we determine lining and cell geometry:

For the first harmonic,
parameters will be:

between linings 28.5 cm;

length 45 cm;

depth 2.5 cm;

length 2 cm..

For the second harmonic,
parameters will be the following:

between linings 4.5 cm;

length 5 cm;

depth 2.5 cm;

length 0.4 cm.

Figure 7.4 shows the
placement of the lining in engine nacelle.

Figure 7.4 Lining placement in the

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