ENGINE THRUST IN FLIGHT
18. From the definitions and formulae stated in para, 17; under flight conditions, the net thrust of the Effect of forward speed
19. Since reference will be made to 'ram ratio' and Mach number, these terms are defined as follows: Ram ratio is the ratio of the total air pressure at the engine compressor entry to the static air pressure at the air intake entry. Mach number is an additional means of measuring speed and is defined as the ratio of the speed of a body to the local speed of sound. Mach 1.0 therefore represents a speed equal to the local speed of sound.
Fig. 21-4 The effect of aircraft speed onthrust and fuel consumption.
20. From the thrust equation in para. 18, it is apparent that if the jet velocity remains constant, independent of aircraft speed, then as the aircraft speed increases the thrust would decrease in direct proportion. However, due to the 'ram ratio' effect from the aircraft forward speed, extra air is taken into the engine so that the mass airflow and also the jet velocity increase with aircraft speed. The effect of this tends to offset the extra intake momentum drag due to the forward speed so that the resultant loss of net thrust is partially recovered as the aircraft speed increases. A typical curve illustrating this point is shown in fig. 21-3. Obviously, the 'ram ratio' effect, or the return obtained in terms of pressure rise at entry to the compressor in exchange for the unavoidable intake drag, is of considerable importance to the turbo-jet engine, especially at high speeds. Above speeds of Mach 1.0, as a result of the formation ofshock waves at the air intake, this rate of pressure rise will rapidly decrease unless a suitably designed air intake is provided (Part 23); an efficient air intake is necessary to obtain maximum benefit from the ram ratio effect.
21. As aircraft speeds increase into the supersonic region, the ram air temperature rises rapidly consistent with the basic gas laws (Part 2). This temperature rise affects the compressor delivery air temperature proportionately and, in consequence, to maintain the required thrust, the engine must be subjected to higher turbine entry temperatures. Since the maximum permissible turbine entry temperature is determined by the temperature limitations of the turbine assembly, the choice of turbine materials and the design of blades and stators to permit cooling are very important.
22. With an increase in forward speed, the increased mass airflow due to the 'ram ratio' effect must be matched by the fuel flow (Part 10) and theresult is an increase in fuel consumption. Because the net thrust tends to decrease with forward speed the end result is an increase in specific fuel consumption (s.f.c.), as shown by the curves for a typical turbo-jet engine in fig, 21-4.
Fig. 21-5 The effect of aircraft speed ons.h.p. and fuel consumption.
24. The effect of forward speed on a typical turbo- propeller engine is shown by the trend curves in fig. 21 -5. Although net jet thrust decreases, s.h.p. increases due to the 'ram ratio1 effect of increased mass flow and matching fuel flow. Because it is standard practice to express the s.f.c. of a turbo- propeller engine relative to s.h.p., an improved s.f.c. is exhibited. However, this does not provide a true comparison with the curves shown in fig. 21-4, for a typical turbo-jet engine, as s.h.p, is absorbed by the propeller and converted into thrust and, irrespective of an increase in s.h.p., propeller efficiency and therefore net thrust deteriorates at high subsonic forward speeds. In consequence, the turbo-propeller engine s.f.c, relative to net thrust would, in general comparison with the turbo-jet engine, show an improvement at low forward speeds but a rapid dete- rioration at high speeds. Effect of afterburning on engine thrust
25. At take-off conditions, the momentum drag of the airflow through the engine is negligible, so that the gross thrust can be considered to be equal to the net thrust. If afterburning (Part 16) is selected, an increase in take-off thrust in the order of 30 per cent is possible with the pure jet engine and considerably more with the by-pass engine. This augmentation of basic thrust is of greater advantage for certain specific operating requirements.
26. Under flight conditions, however, this advantage is even greater, since the momentum drag is the same with or without afterburning and, due to the ram effect, better utilization is made of every pound of air flowing through the engine. The following example, using the static values given in Part 16, illustrates why afterburning thrust improves under flight conditions.
. 30. This larger increase in thrust is invaluable for obtaining higher speeds and higher altitude perform-ances. The total and specific fuel consumptions arehigh, but not unduly so for such an increase in performance.
31. The limit to the obtainable thrust is determined by the afterburning temperature and the remaining usable oxygen in the exhaust gas stream. Because no previous combustion heating takes place in the duct of a by-pass engine, these engines with their large residual oxygen surplus are particularly suited to afterburning and static thrust increases of up to 70 per cent are obtainable. At high forward speeds several times this amount is achieved.