Saturday, June 9, 2012



8. The thrust of the turbo-jet engine on the test bench differs somewhat from that during flight. Modern test facilities are available to simulate atmospheric conditions at high altitudes thus providing a means of assessing some of the performance capability of a turbo-jet engine in flight without the engine ever leaving the ground. This is important as the changes in ambient temperatur and pressure encountered at high altitudes consider-ably influence the thrust of the engine.

9. Considering the formula derived in Part 20 for engines operating under 'choked' nozzle conditions, it can be seen that the thrust can be further affected by a change in the mass flow rate of air through the engine and by a change in jet velocity. An increase in mass airflow may be obtained by using wate injection (Part 17) and increases in jet velocity by using afterburning (Part 16).
10. As previously mentioned, changes in ambien pressure and temperature considerably influence the thrust of the engine. This is because of the way they affect the air density and hence the mass of ai entering the engine for a given engine rotationa speed. To enable the performance of similar engines to be compared when operating under differen climatic conditions, or at different altitudes, correction factors must be applied to the calculations to return the observed values to those which would be found under I.S.A. conditions. For example, the thrus  correction for a turbo-jet engine is: Thrust (lb.) (corrected) =

11. The observed performance of the turbo- propeller engine is also corrected to I.S.A. conditions, but due to the rating being in s.h.p. and not in pounds of thrust the factors are different. For example, the correction for s.h.p. is: S.h.p. (corrected) = In practice there is always a certain amount of jet thrust in the total output of the turbo-propeller engine and this must be added to the s.h.p. The correction for jet thrust is the same as that in para. 10.

12. To distinguish between these two aspects of the power output, it is usual to refer to them as s.h.p. and thrust horse-power (t.h.p.). The total equivalent horse-power is denoted by t.e.h.p. (sometimes e.h.p.) and is the s.h.p. plus the s.h.p. equivalent to the net jet thrust. For estimation purposes it is taken that, under sea- level static conditions, one s.h.p. is equivalent to approximately 2.6 lb. of jet thrust. Therefore :

13. The ratio of jet thrust to shaft power is influenced by many factors. For instance, the higher the aircraft operating speed the larger may be the required proportion of total output in the form of jet thrust. Alternatively, an extra turbine stage may be required if more than a certain proportion of the total power is to be provided at the shaft. In general, turbo-propeller aircraft provide one pound of thrust for every 3.5 h,p. to 5 h.p. Comparison between thrust and horse-power

Fig. 21-2 provides a diagrammatic explanation.
14. Because the turbo-jet engine is rated in thrust and the turbo-propeller engine in s.h.p., no direct comparison between the two can be made without a power conversion factor. However, since the turbo- propeller engine receives its thrust mainly from the propeller, a comparison can be made by converting the horse-power developed by the engine to thrust or the thrust developed by the turbo-jet engine to t.h.p.; that is, by converting work to force or force to work. For this purpose, it is necessary to take into account the speed of the aircraft. Since one horse-power is equal to 550 per sec. and 550 ft. per sec. is equivalent to 375 miles per hour, it can be seen from the above formula that on lb. of thrust equals one t.h.p. at 375 m.p.h. It is also common to quote the speed in knots (nautical milesper hour); one knot is equal to 1.1515 m.p.h, or one pound of thrust is equal to one t.h.p. at 325 knots.

16. Thus if a turbo-jet engine produces 5,000 lb. of net thrust at an aircraft speed of 600 m.p.h. the t.h.p. However, if the same thrust was being produced by a turbo-propeller engine with a propeller efficiency of 55 per cent at the same flight speed of 600 m.p.h., then the t.h.p. would be  Thus at 600 m.p.h. one lb. of thrust is the equivalentof about 3 t.h.p.

Fig. 21-2 The balance of forces and expression for thrust and momentum drag.

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