Page 234 - The Jet Engine

Performance

In the instance of an engine operating with a non- 42. Assuming an aircraft speed (V) of 375 m.p.h.
choked nozzle (Part 20), the equation becomes: and a jet velocity (vj) of 1,230 rn.p.h., the efficiency
of a turbo-jet is:
WV v ( J − V )
2 ×
WV v ( J − V ) + 1 2 W v ( J − V ) 2 375 + 375 = approx . 47 per cent
, 1
230
2 V
Simplified to :
V + v On the other hand, at an aircraft speed of 600 m.p.h.
J
the efficiency is:
41. This latter equation can also be used for the 2 × 600
choked nozzle condition by using vj to represent the = approx . 66 per cent
jet velocity when fully expanded to atmospheric 600 + , 1 230
pressure, thereby dispensing with the nozzle Propeller efficiency at these values of V is approxi-
pressure term (P-P )A. mately 82 and 55'per cent, respectively, and from
0

Fig. 21-9 Propulsive efficiencies and aircraft speed.

224