maintain a nucleus of combustion in the com- 
bustion chamber. In rhe normal combustion 
process, the speed of flame propagation is quite 
low and, if the local velocities are too high at 
the forward end of the combustion chamber, 
poor combustion will result and it is likely 
rhar the flame will blow out. The secondary 
air-or cooling flow-is introduced downstream 
from the combustion nucleus to dilute the com- 
bustion products and lower the discharge gas 
temperature. 
The fuel nozzle must provide a finely 
atomized, evenly distributed spray of fuel 
through a wide range of flow rates. Very 
specialized design is necessary to provide a 
nozzle with suitable characteristics. The 
spray parrern and circulation in the combustion 
chamber must make efficient use of the fuel by 
complete combustion. The temperatures in 
the combustion nucleus can exceed 1,700” to 
1,SW’ C but the secondary air will dilute the 
gas and reduce the temperature to some value 
which can be tolerated in the turbine section. 
A pressure drop will occur through the com- 
bustion chamber to accelerate the combustion 
gas rearward. In addition, turbulence and 
fluid friction will cause a pressure drop but this 
loss must be held to the minimum incurred by 
providing complete combustion. Heat trans- 
ferred through the walls of the combustion 
chamber constitutes a loss of thermal energy 
and should be held to a minimum. Thus, the 
combustion chamber should enclose the com- 
bustion space with a minimum of surface area 
to minimize heat and friction losses. Hence, 
the “annular” typ: combustion chamber offers 
certain advantages over the multiple “can” 
type combustion chamber. 
The tur6inc section is the most critical element 
of the turbojet engine. The function of the 
turbine is to extract energy from the combus- 
tion gases and furnish power to drive the com- 
pressor and accessories. In the case of the 
turboprop engine, the turbine section must ex- 
tract a very large portion of the exhaust gas 
NAVWEPS O(L8OT-80 
AIRPLANE PERFORMANCE 
energy to drive the propeller in addition to the 
compressor and accessories. 
The combustion chamber delivers high en- 
ergy combustion gases to the turbine section at 
high pressure and tolerable temperature. The 
turbine nozzle vanes are a row of stationary 
blades immediately ahead of the rotating tur- 
bine. These blades form the nozzles which 
discharge the combustion gases as high ve- 
locity jets onto the rotating turbine. In this 
manner, the high pressure energy of the com- 
bustion gases is converted into kinetic energy 
and a pressure and temperature drop takes 
place. The function of the turbine blades 
operating in these jets is to develop a tangen- 
tial force along the turbine wheel thus extract- 
ing mechanical energy from the combustion 
gases. This is illustrated in figure 2.8. 
The form of the turbine blades may be a com- 
bination of two distinct types. The imp&c 
type turbine relies upon the nozzle vanes to 
accomplish the conversion of combustion gas 
static pressure to high velocity jets. The 
impulse turbine blades are shaped to produce 
a large deflection of the gas and develop the 
tangential force by the flow direction change. 
In such a design, negligible velocity and pres- 
sure drop occurs with the flow across the tur- 
bine rotor blades. The reaction type turbine 
differs in that large velocity and pressure 
changes occur across the turbine rotor blades. 
In the reaction turbine, rhe stationary nozzle 
vanes serve only to guide the combustion gas 
onto the turbine rotor with negligible changes 
in velocity and pressure. The reaction tur- 
bine rotor blades are shaped to provide a pres- 
sure drop and velocity increase across the 
blades and the reaction from this velocity in- 
crease provides the tangential force on the 
wheel. Generally, the turbine design is a 
form utilizing some feature of each of the two 
types. 
The turbine blade is subjected to high