Page 17 - The Jet Engine
P. 17
Basic mechanics
and turbine blades, and the design of the exhaust teristics have led to some departure from the use of
system through which the gases are ejected to form pure turbo-jet propulsion where aircraft operate at
the propulsive jet. medium speeds by the introduction of a combination
of propeller and gas turbine engine.
16. At aircraft speeds below approximately 450
miles per hour, the pure jet engine is less efficient 17. The advantages of the propeller/turbine
than a propeller-type engine, since its propulsive combination have to some extent been offset by the
efficiency depends largely on its forward speed; the introduction of the by-pass, ducted fan and propfan
pure turbo-jet engine is, therefore, most suitable for engines. These engines deal with larger comparative
high forward speeds. The propeller efficiency does, airflows and lower jet velocities than the pure jet
however, decrease rapidly above 350 miles per hour engine, thus giving a propulsive efficiency (Part 21)
due to the disturbance of the airflow caused by the which is comparable to that of the turbo-prop and
high blade-tip speeds of the propeller. These charac- exceeds that of the pure jet engine (fig. 1-10).
Fig. 1-10 Comparative propulsive efficiencies.
7
