|Thermodynamics and Propulsion|
If we consider our expressions for thrust and propulsive efficiency together,
we see that
Also note that for
where is the inlet area of an engine, shown in Figure 11.1.
The balance between propulsive efficiency and specific thrust ( thrust per unit mass flow) is shown in Figure 11.2.
For fighter aircraft that need high thrust/weight and fly at high speed, it is typical to employ engines with smaller inlet areas and higher thrust per unit mass flow,
The small inlet of one of the F-22 Raptor's two engines is visible just below the cockpit in Figure 11.3.
However, transport aircraft that require higher efficiency and fly at lower speeds usually employ engines with relatively larger inlet areas and lower thrust per unit mass flow,
The large inlets of a Boeing 777-200's engines are shown in Figure 11.4.
At low flight velocities, the highest propulsive efficiency is typically obtained with a propeller or an unducted fan. Figure 11.5 shows a propeller craft, and Figure 11.6 shows a sketch of a jet engine with an unducted fan. Figure 11.7 shows propulsive efficiency as a function of airspeed for different engine bypass ratios.