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Current 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:

,

where coefficients А, В, С are different for different aircraft types and engine modes. For Tupolev-154M the coefficients А, В, С are shown in the table 6.2 in respect to Tupolev-154.

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

	Tupolev-154			Tupolev-154M
Engine mode	A	B	C	A	B	C
Maximal	145.45	-15.66	-0.81	142.53	-15.52	-0.83
Nominal	142.14	-15.56	-0.82	137.58	-14.28	-1.09
85% of nominal	140.50	-16.29	-0.76	142.84	-17.75	-0.78
Cruise	140.23	-16.35	-1.15	137.56	-16.07	-1.10
2-nd cruise	131.03	-10.38	-2.23	130.07	-11.54	-2.00
Descending	126.84	-11.86	-1.93	128.57	-14.25	-1.39
Idle	132.37	-16.36	-0.86	134.92	-17.13	-0.68

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.

Figure 6.2 Take-off 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.

Under 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

where:

m is aircraft mass;

P is thrust;

 is the angle of attack,  is the angle of engine installation;

 is climb angle which is equal to  or , 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 R_N remains constant and is equal to 119.7 m.

Figure 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,  is air density, S is wing area, C_{y max} is maximum lift coefficient determined from Fig. 6.6.

Approach speed should be 30% greater that the stall speed:

Figure 6.6 Aerodynamic characteristics of Tupolev 154M.

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

.

Corresponding drag coefficient is determined from Fig. 6.6.

Some 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  is approach path angle which is equal to 3 degrees.

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

Table 6.3 Calculation results for Tupolev 154M at approach configuration.

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 well.

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 u_c. 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 conditions.

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.

Характеристики

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