NAVWEPS 00-801-80 
AIRPLANE PERFORMANCE 
is subjected to the bending and torsion of 
the tangential impulse-reaction forces. The 
blade must wirhstand these stresses which are 
generally of a vibratory and cyclic nature 
while at high temperatures. The elevated 
temperatures at which the turbine must func- 
tion produce extreme conditions for struc- 
tural creep and fatigue considerations. Conse- 
quently, the engine speed and temperature op- 
erating limits demand very careful considera- 
tion. Excessive engine temperatures or speeds 
may produce damage which is immediately 
apparent. However, creep and fatigue damage 
is cumulative and even though damage may 
not be immediately apparent by visual inspec- 
tion, proper inspection methods (other than 
visual) must be utilized and proper records 
kept regarding the occurrence. 
Actually, the development of high tempera- 
ture alloys for turbines is a critical factor in the 
develop,mcnt of high ei%ciciicy, high output 
aircraft gas turbines. The higher the tem- 
peratute of gases entering the turbine, the 
higher can be the temperature and pressure of 
the gases at discharge from the turbine with 
greater exhaust jet velocity and thrust. 
The function of the t&pipe or exhaust no?& 
is to discharge the exhaust gases to the atmos- 
phere at the highest possible velocity to pro- 
duce the greatest momentum change and thrust. 
If a majority of the expansion occurs through 
the turbine section, there remains only to con- 
duct the exhaust gases rearward with a mini- 
mum energy loss. However, if the turbine 
operates against a noticeable back pressure, the 
nozzle must convert the remaining pressure 
energy into exhaust gas velocity. Under ideal 
conditions, the nozzle would expand the flow 
to the ambient static pressure at the exhaust 
and the area distribution in the nozzle must 
provide these conditions. When the ratio af 
exhaust gas pressure to ambient pressure is 
relatively low and incapable of producing sonic 
flow, a converging nozzle provides the expan- 
sion. The exit area must be of proper size to 
bring about proper exit conditions. If the exit 
114 
area is too large, incomplete expansion will 
take place; if the exit area is too small, an over 
expansion tendency results. The exit area can 
affect the upstream conditions and must be 
properly proportioned for overall performance. 
When the ratio of exhaust gas pressure to 
ambient pressure is greater than some critical 
due, sonic flow can exist and the nozzle will 
be choked or limited to some maximum flow. 
When supersonic exhaust gas velocities are re- 
quired to produce the necessary momentum 
change, the expansion process will require the 
convergent-divergent nozzle illustrated in fig- 
ure 2.9. With sui?icient pressure available the 
initial expansion in the converging portion is 
subsonic increasing to sonic velocity at the 
throat. Subsequent expansion in the divergent 
portion of the nozzle is supersonic and the re- 
sult is the highest exit velocity for a given 
pressure ratio and mass flow. When the pres- 
sure ratio is very high the final exit diameter 
required to expand to ambient pressure may be 
very large but is practically. limited to the 
fuselage or nacelle afterbody diameter. If the 
exhaust gases exceed sonic velocity, as is porsi- 
ble in a ramjet combustion chamber or after- 
burner section, only the divergent portion of 
the nozzle may be necessary. 
Figure 2.9 provides illustration of the func- 
tion of the various engine components and the 
changes in static pressure, temperature, and 
velocity through the engine. The conditions 
at the inlet provide the initial properties of the 
engine airflow. The compressor section fur- 
nishes the compression pressure rise with a 
certain unavoidable but undesirable increase in 
temperature. High pressure air delivered to 
combustion chamber receives heat from the 
combustion of fuel and experiences a rise in