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

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.

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.

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--

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.

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-

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-

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.

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.

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.

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--