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

calculations, and the new conditions at the propelling nozzle are as follows- Therefore, compared with the previous calculation in para. 17, it will be seen that the negative thrust is reduced from -5,587 lb. to -2,676 lb.; the overall positive thrust is thus increased by 2,911 lb; which is equivalent to a thrust increase of more than 25 per cent.

25. To arrive at the total thrust of the engine with afterburning the calculations in para. 20 should use the above figures.