## Friday, May 18, 2012

### Thrust distribution Inclined combustion chambers & AFTERBURNING

Inclined combustion chambers
22. In the previous example (Para. 14) the flow through the combustion chamber is axial, however, if the combustion chamber is inclined towards the axis of the engine, then the axial thrust will be less than for an axial flow chamber. This thrust can be obtained by multiplying the sum of the outlet thrust by the cosine of the angle (see fig. 20-2). The cosine =  Hypotenuse /Base and for a given angle  is obtained by consulting a table of cosines. It should be emphasized that if the inlet and outlet are at different angles to the engine axis, it is necessary to multiply the inlet and outlet thrusts separately by the cosine of their respective angles.
AFTERBURNING
23. When the engine is fitted with an afterburner (Part 16), the gases passing through the exhaust system are reheated to provide additional thrust. The effect of afterburning is to increase the volume of the exhaust gases, thus producing a higher exit velocity at the propelling nozzle.+
 Fig. 20-2 A hypothetical combustion chamber showing values required for calculating thrust
24. Assuming that an afterburner jet pipe and propelling nozzle are fitted to the engine used in the previous

## Thursday, May 17, 2012

### Thrust distribution Engine

Engine
19. It will be of interest to calculate the thrust of the engine by considering the engine as a whole, as the resultant thrust should be equal to the sum of the individual gas loads previously calculated.

## Wednesday, May 16, 2012

### Thrust distribution Propelling nozzle

Propelling nozzle

17. The conditions at the inlet to the propelling nozzle are the same as the conditions at the jet pipe outlet, i.e. 16,745 lb.  Therefore, given that the propelling nozzle-- It is emphasized that these are basic calculations and such factors as the effect of air offtakes have been ignored.

18. Based on the individual calculations, the sum of the forward or positive loads is 57,836 lb. and the sum of the rearward or negative loads is 46,678 lb. Thus, the resultant (gross or total) thrust is 11,158 lb.

## Tuesday, May 15, 2012

### Thrust distribution Exhaust unit and jet pipe

Exhaust unit and jet pipe
16. The conditions at the inlet to the exhaust unit are the same as the conditions at the turbine outlet, i.e. 14,326 lb.  Therefore, given that the jet pipe--

## Monday, May 14, 2012

### Thrust distribution Turbine assembly

Turbine assembly
15. The conditions at the turbine inlet are the same as the conditions at the combustion chamber outlet, i.e. 55,417 lb.

## Sunday, May 13, 2012

### Thrust distribution Combustion chambers

Combustion chambers
14. The conditions at the combustion chamber inlet are the same as the conditions at the diffuser outlet, i.e. 21,235 lb. Therefore, given that the combustion chamber-

## Friday, May 11, 2012

### Thrust distribution CALCULATING THE THRUST OF THE ENGINE & Compressor casing

CALCULATING THE THRUST OF THE ENGINE
11. When applying the above method to calculate the individual thrust loads on the various components it is assumed that the engine is static. The effect of aircraft forward speed on the engine thrust will be dealt with in Part 21. In the following calculations 'g' is taken to be 32 for convenience. To assist in these calculations the locations concerned are illustrated by a number of small diagrams.

Compressor casing
12. To obtain the thrust on the compressor casing it is necessary to calculate the conditions at the inlet to the compressor and the conditions at the outlet from the compressor. Since the pressure and the velocity at the inlet to the compressor are zero, it is only necessary to consider the force at the outlet from the compressor. Therefore, given that the compressor-

## Thursday, May 10, 2012

### Thrust distribution METHOD OF CALCULATING THE THRUST FORCES

METHOD OF CALCULATING THE THRUST FORCES
7. The thrust forces or gas loads can be calculated for the engine, or for any flow section of the engine, provided that the areas, pressures, velocities and mass flow are known for both the inlet and outlet of the particular flow section.
8. The distribution of thrust forces shown in fig. 20- 1 can be calculated by considering each component in turn and applying some simple calculations. The thrust produced by the engine is mainly the product of the mass of air passing through the engine and the velocity increase imparted to it (i.e. Newtons Second Law of Motion), however, the pressure difference between the inlet to and the outlet from the particular flow section will have an effect on the overall thrust of the engine and must be included in the calculation.
9. To calculate the resultant thrust for a particular flow section it is necessary to calculate the total thrust at both inlet and outlet, the resultant thrust being the difference between the two values obtained.
10. Calculation of the thrust is achieved using the following formula:

Where A = Area of flow section in sq.in.

## Wednesday, May 9, 2012

### Thrust distribution DISTRIBUTION OF THE THRUST FORCES

DISTRIBUTION OF THE THRUST FORCES
2. The diagram in fig. 20-1 is of a typical single- spool axial flow turbo-jet engine and illustrates where the main forward and rearward forces act. The origin of these forces is explained by following the engine working cycle shown in Part 2.
 Fig. 20-1 Thrust distribution of a typical single-spool axial flow engine.

3. At the start of the cycle, air is induced into the engine and is compressed. The rearward accelera- tions through the compressor stages and the resultant pressure rise produces a large reactive force in a forward direction. On the next stage of its journey the air passes through the diffuser where it  exerts a small reactive force, also in a forward direction,
4. From the diffuser the air passes into the combustion chambers (Part 4) where it is heated, and in the consequent expansion and acceleration of the gas large forward forces are exerted on the chamber walls.

## Tuesday, May 8, 2012

### Introduction Thrust distribution

Introduction
Distribution of the thrust forces
Method of calculating thrust forces
Calculating the thrust the engine

1. Compressor casing
2. Diffuser duct
3. Combustion chambers
4. Turbine assembly
5. Exhaust unit and jet pipe
6. Propelling nozzle
7. Engine
8. Inclined combustion cham

Afterburning

INTRODUCTION
1. Although the principles of jet propulsion (see Part 1) will be familiar to the reader, the distribution of the thrust forces within the engine may appear somewhat obscure- These forces are in effect gas loads resulting from the pressure and momentum changes of the gas stream reacting on the engine structure and on the rotating components. They are in some locations forward propelling forces and in others opposing or rearward forces. The amount that the sum of the forward forces exceeds the sum of the rearward forces is normally known as the rated thrust of the engine.

## Monday, May 7, 2012

### Noise suppression CONSTRUCTION AND MATERIALS

CONSTRUCTION AND MATERIALS
17. The corrugated or lobe-type noise suppressor forms the exhaust propelling nozzle and is usually a separate assembly bolted to the jet pipe. Provision is usually made to adjust the nozzle area so that it can be accurately calibrated. Guide vanes are fitted to the lobe-type suppressor to prevent excessive losses by guiding the exhaust gas smoothly through the lobes to atmosphere. The suppressor is a fabricated welded structure and is manufactured from heat- resistant alloys.
18. Various noise absorbing lining materials are used on jet engines. They fall mainly within two categories, lightweight composite materials that are used in the lower temperature regions and fibrous- metallic materials that are used in the higher temperature regions. The noise absorbing material consists of a perforate metal or composite facing skin, supported by a honeycomb structure on a solid backing skin which is bonded to the parent metal of the duct or casing. For details of manufacture of these materials refer to Part 22.

Noise suppression CONSTRUCTION AND MATERIALS,Introduction Noise suppression, Noise suppression Engine noise, Noise suppression Methods of suppressing noise, Noise suppression Construction and materials,

## Sunday, May 6, 2012

### Noise suppression Noise absorbing materials and location METHODS OF SUPPRESSING NOISE 1

 Fig. 19-6 Noise absorbing materials and location.

13. Deep corrugations, lobes, or multi-tubes, give the largest noise reductions, but the performance penalties incurred limit the depth of the corrugations or lobes and the number of tubes. For instance, to achieve the required nozzle area, the overall diameter of the suppressor may have to be increased by so much that excessive drag and weight results. A compromise which gives a noticeable reduction in noise level with the least sacrifice of engine thrust, fuel consumption or addition of weight is therefore the designer's aim.

## Saturday, May 5, 2012

### Noise suppression METHODS OF SUPPRESSING NOISE ypes of noise suppressor

METHODS OF SUPPRESSING NOISE

 Fig. 19-5 Types of noise suppressor.
10. Noise suppression of internal sources is approached in two ways; by basic design to minimize noise originating within or propagating from the engine, and by the use of acoustically absorbent linings. Noise can be minimized by reducing airflow disruption which causes turbulence. This is achieved by using minimal rotational and airflow velocities and reducing the wake intensity by appropriate spacing between the blades and vanes. The ratio between the number of rotating blades and stationary vanes can also be advantageously employed to contain noise within the engine.

11. As previously described, the major source of noise on the pure jet engine and low by-pass engine is the exhaust jet, and this can be reduced by inducing a rapid or shorter mixing region. This reduces the low frequency noise but may increase the high frequency level. Fortunately, high frequencies are quickly absorbed in the atmosphere and some of the noise which does propagate to the listener is beyond the audible range, thus giving the perception of a quieter engine. This is achieved by increasing the contact area of the atmosphere with the exhaust gas stream by using a propelling nozzle incorporating a corrugated or lobe-type noise suppressor (fig. 19-5).

## Friday, May 4, 2012

### Noise suppression ENGINE NOISE

 Fig. 19-4 Comparative noise sources of low and high by-pass engines.

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

## Thursday, May 3, 2012

### Rolls-Royce RM60 | Rolls-Royce Conway

 Rolls-Royce Conway

Rolls-Royce RM60

Produced in response to an Admiralty contract for a coastal-craft engine with good cruising economy, the RM60, although based on aeroengine philosophy, was designed from the first as a marine gas turbine. Two RM60s went to sea in 1953 in the former steam gunboat HMS Grey Goose, the world's first warship to be powered solely by gas turbines.

## Wednesday, May 2, 2012

### Noise suppression Introduction

Noise suppression

Introduction
Engine noise
Methods of suppressing noise
Construction and materials

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.
 Fig. 19-1 Comparative noise levels of various engine types.
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.

Introduction Noise suppression, Noise suppression Engine noise, Noise suppression Methods of suppressing noise, Noise suppression Construction and materials, Comparative noise levels of various engine types,

## Tuesday, May 1, 2012

### Thrust distribution Diffuser duct

Diffuser duct
13. The conditions at the diffuser duct inlet are the same as the conditions at the compressor outlet, i.e. 19,049 lb. Therefore, given that the diffuser--

### Noise suppression ENGINE NOISE

ENGINE NOISE
 Fig. 19-2 Exhaust mixing and shock structure.
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.