NAVWEPS OO-SOT-RO  AIR.PLANE PERFORMANCE  excess  of the operational limits for these con-  ditions will increase the possibility of early  failure of the turbine components.  the design service life with trouble-free opera-  tion.  The following items describe  the critical  areas encountered during the operational use  of the turbojet engine:  While the turbine components are the most  critically stressed high temperature elements  they are not the only items.  The combustion  chamber components may be critical at low  altitude where high combustion chamber pres-  sures exist. Also, the airframe structure and  equipment adjacent to the engine may be sub-  ject to quite high temperatures and require  provision to prevent damage by excess  time at  high temperature.  (1) The limiting exhaust  gag tcmpcra;wcs  pro-  vide the most important restrictions to the op-  eration of the turbojet engine.  The turbine  components are subject to centrifugal loads of  rotation, impulse and reaction loads on the  blades, and various vibratory loads which may  be inherent with the design. When the turbine  components are subject to this variety of stress  in the presence  of high temperature, two types  of structural phenomena must be considered.  when a part is subject to a certain stress  at some  high temperature, weep  failure will take place  after a period of time.  Of course, an increase  in .tcmperature or stress will increase the rate  at which creep damage is accumulated and  reduce the time required to cause failure.  An-  other problem results when a part is subjected  to a repeated or cyclic stress. F&&e failure  will occur after a number of cycles of a varying  stress. An increase in temperature or magni-  tude of cyclic stress will increase the rate of  fatigue damage and reduce the number of cycles  necessary  to produce failure.  It is important  to note that both fatigue and creep damage are  cumulative.  (2) The c~mprcs~or  Jtall or surge  has the pos-  sibility of producing damaging temperatures  in the turbine and combustion chamber or un-  usual transient loads in the compressor.  While  the stall-surge phenomenon is possible with  the centrifugal compressor,  the more common  .occurrence  is with the axial flow compressor.  Figure 2.13 depicts the pressure distribution  that may exist for steady state operation of  the engine. In order to accelerate  the engine  to a greater speed, more fuel must be added to  increase  the turbine power above that required  to operate the compressor.  Suppose that the fuel flow is increased be-  yond the steady state requirement without a  change in rotative speed. The increased  com-  bustion chamber pressure due to the greater  fuel flow requires that the compressor dis-  charge pressure be higher.  For the instant  before an engine speed change occurs, an in-  crease  in compressor  discharge pressure  will be  accompanied by a decrease  in compressor  flow  velocity.  The equivalent effect is illustrated  by the flow components  onto the rotating com-  pressor blade of figure 2.13. One component  of velocity is due to rotation and this compo-  nent remains unchanged for a given rotative  velocity of the single blade. The axial flow  velocity for steady state operation combines  with rotational component to define a result-  ant velocity and direction. If the axial flow  component is reduced, the resultant velocity  and direction provide an increase in angle of  A gross overstress  or overtemperature of the  turbine section will produce damage that is  immediately apparent. However, the creep  and fatigue damage accumulated through pe-  riods of less extreme’ overstress or overtem-  perature is more subtle.  If the turbine is  sibject to repeated excessive  temperatures, the  greatly increased rate of creep and fatigue  damage wiIl produce failure early within the  anticipated service life.  Generally, the operations which produce  the highest exhaust gas temperatures are  starting, acceleration, and maximum thrust  at high altitude.  The time spent at these  temperatures must be limited arbitrarily to  prevent excessive accumulation of creep and  fatigue.  Any time spent at temperatures in  125 