at very high velocity and high kinetic energy. 
A pressure rise is produced by subsequent ex- 
pansion in the diffuser manifold by converting 
the kinetic energy into static pressure energy. 
The manifold then distributes the high pres- 
sure discharge to the combustion chambers. 
A double entry impeller allows a given diam- 
eter compressor to process a greater airflow. 
The major components of the centrifugal com- 
pressor are illustrated in figure 2.7. 
The centrifugal compressor can provide a 
relatively high pressure ratio per stage but the 
provision of more than one or two stages is 
rarely feasible for aircraft turbine engines. 
The single stage centrifugal compressor is 
capable of producing pressure ratios of about 
three or four with reasonable efficiency. &es- 
sure ratios greater than four require such high 
impeller tip speed that compressor efficiency 
decreases very rapidly. Since high pressure 
ratios are necessary to achieve low fuel con- 
sumption, the centrifugal compressor finds 
greatest application to the smaller engines 
where simplicity and flexibility of operation are 
the principal requirements rather than high 
efficiency. 
The axial flow compressor consists of altet- 
nate rows of rotating and stationary airfoils. 
The major components of the axial flow com- 
pressor ate illustrated in figure 2.7. A pressure 
rise occurs through the row of rotating blades 
since the airfoils cause a decrease in velocity 
relative to the blades. Additional pressure 
rise takes place through the row of stationary 
blades since these airfoils cause a decrease in 
the absolute velocity of flow. The decrease 
I in velocity, relative or absolute, eEeLts a com- 
1 ptession of the flow and causes the increase in 
static pressure. While the pressure rise pet 
stage of the axial compressor is relatively Jo%-, 
the efficiency is very high and high pressure 
ratios can be obtained efficiently by successive 
axial stages. Of course, the eficient pressure 
rise in each stage is limited by excessive gas 
velocities. The multistage axial flow com- 
pressor is capable of providing pressure ratios 
NAWEPS 00-8OT-80 
AIRPLANE PERFORMANCE 
from five to ten (or greater) with efficiencies 
which cannot be approached with a multi- 
stage centrifugal compressor. 
The axial flow compressor can provide 
efficiently the high. pressure ratios necessary 
for low fuel consumption. Also, the axial 
compressor is capable of providing high air- 
flow with a minimum of compressor diameter. 
When compared with the centrifugal com- 
pressor, the design and construction of the 
axial compressor is relatively complex and 
costly and the high efficiency is sustained over 
a much narrower range of operating conditions. 
For these reasons, the axial compressor finds 
greatest application where rhe demands of 
efficiency and output predominate over con- 
siderations. of cost, simplicity, flexibility of 
operation, etc. Multispool compressors and 
variable statot blades serve to improve the 
operating characteristics of the axial com- 
pressor and increase the flexibility of operation. 
The combustion chamber must convert the fuel 
chemical energy into heat energy and cause a 
large increase in the total energy of the engine 
airflow. The combustion chamber will opet- 
ate with one principal limitation: the dis- 
charge from the combustion chamber must be 
at temperatures which can be tolerated by the 
turbine section. The combustion of liquid 
hydrocarbon fuels can produce gas temperatures 
which are in excess of 1,700 to 1,800° C. 
However, the maximum continuous turbine 
blade operating temperatures rarely exceed 
NO0 to J,OOO” C and considerable excess air 
must be used in the combustion chamber to 
prevent exceeding these temperature limits. 
While the combustion chamber design may 
.take various forms and configurations, the 
main features of a typical combustion chamber 
ate illustrated by figure 2.8. The combustion