QUALITATIVE COMPARISON OF THE PERFORMANCE OF TURBOJET, TURBOFAN, AND TURBOPROP ENGINES

Figure 6.30a, 6.30b, and 6.30c presents a qualitative comparison of the turbojet, turbofan, and turboprop engines, each having the same core engine.

The specific fuel consumption for a turbojet or turbofan engine is expressed as a thrust specific fuel consumption (TSFC). In the English system of units, one states TSFC as pounds of fuel per hour per pound of thrust, so that TSFC actually has the dimensions of 1/time. Thus its numerical value is the same in the SI system as in the English system. In the SI system, TSFC is given as N/hr/N. The characteristics of the three engines are seen to be quite different with the turbofan, not surprisingly, lying between the turboprop and turbojet. The relative differences in these curves are explained mainly by the momentum and energy considerations undertaken previously for the pro­peller. “Disc loadings” for turbojet engines are of the order of 81,400 Pa (1700psf), while turbofans operate at approximately half of this loading and propellers at only approximately 4% of the disc loading for a turbojet. If we assume that the core engine is delivering the same power to each engine configuration then, from Equation 6.17, for static thrust one obtains,

Note that thrust for a turbojet engine is denoted by F instead of Г, since T is understood to refer to temperature when working with a gas turbine. Thus, with its appreciably lower disc loading, one would expect the static thrust of a turboprop to be significantly higher than the corresponding turbojet, possibly even more so than that shown in Figure 6.29a (taken from Ref. 6.6).

The rapid decrease in thrust with airspeed for the turboprop and the more gradual changes for the turbofan and turbojet engines are also explained in part by the relative disc loadings. Combining Equations 6.13, 6.14, and 6.15 gives

(6.77)

If the power to produce the thrust is assumed to be constant, then Equation

Comparative net thrust at sea level.

Comparative thrust specific fuel consumption.

Relative maximum continuous thrust comparison during climb.

6.77 can be written

(6.78)

where Fa is the static thrust and w0 is the static-induced velocity given by Equation 6.16. This implicit relationship between F/F0 and VI w0 can be easily solved iteratively using a programmable calculator. The solution is presented graphically in Figure 6.31. Thus, it is not V per se that determines the ratio of F to F0 but, instead, the ratio of V to w0. For a high disc loading with a concomitant w0, a given V will have a lesser elfect on F than for the case of a low disc loading.

Disc loading is not the total explanation for the relative dilferences in T as a function of V shown in Figure 6,29a. Consider a typical turbojet with a static disc loading of around 81,400 Pa (1700 psf). For this engine at sea level, w0 will equal approximately 180 m/s (600 fps). An airspeed of 400 kt in this case gives and

F

= 0.64

F0

However, Figure 6.30a shows only а 20% decrease in the thrust. This is because the gas generator power is not-constant but also increases with V

because of the increased mass flow and ram pressure. If we tacitly assume the power proportional to the product of F and TSFC then, from Figure 6.30b, one would predict the core engine power to have increased by about 10%. This results in a decreased value of VI w0 of 1.08, giving a new T/T0 of 0.66. However, T0 corresponds to the core engine power at 400 kt. Based on the original F0 corresponding to the core engine power at V = 0, F/F0 becomes 0.73. Figure 6.30a, 6.30b, and 6.30c is, of course, not too accurate and is really intended only to show relative differences. You may wish to apply Figure 6.31 to the performance curves of the JT9D-3 turbofan engine that follow. In this case, the predicted variation of F and V will be found to match closely the results given in the installation handbook.