NAVWEPS OO-ROT-RO  AWIANE  PERFORMANCE  higher altitude or a lower engine speed would  produce less supercharging and a given MAP  would require a greater throttle opening.  Generally, the most important critical alti-  tudes will be specified for maximum, rated,  and maximum cruise power conditions.  manifold pressure any greater than the induc-  tion system inlet pressure. As altitude is  increased with full throttle and a governed  RPM, the airflow through the engine is  reduced and BHP decreases.  The first forms of  supercharging were of relatively low pressure  ratio and the added airflow and power could  be handled at full throttle within detonation  limits.  Such a “ground  boosted” engine  would achieve higher output power at all  altitudes but an increase in altitude would  produce a decrease  in manifold pressure, air-  flow, and power output.  A change of the blower to a high speed will  provide greater supercharging but will require  more shaft power and incur a greater tempera-  ture rise. Thus, the high blower speed can  produce an increase in altitude performance  within the detonation limitations.  The vari-  ation of BHP with altitude for the blower at  high speed shows an increase in critical alti-  tude and greater BHP than is obtainable in low  blower. Operation below the high blower  critical altitude requires some limiting mani-  fold pressure to remain within  detonation  limits.  It is apparent that the shift to high  blower is not required just past low blower  critical altitude but at the point where the  transition from low blower, full throttle to  high blower, limit hiAP will produce greater  BHP.  Of course, if the blower speed is  increased without  reducing  the  throttle  opening, an  “overboost” can occur.  More advanced forms of supercharging with  higher pressure ratios can produce very large  engine airflow.  In fact, the typical case of  altitude supercharging will produce such high  airflow at low altitude operation that full  throttle operation cannot be utilized within  detonation limits.  Figure 2.17 illustrates this  case for a typical two-speed engine driven  altitude supercharging installation.  At sea  level, the limiting manifold pressure  produces  a certain amount of BHP. Full throttle oper-  ation could produce a higher MAP and BHP  if detonation were not the problem. In this  case full  throttle  operation is unavailable  because of detonation limits.  As altitude is  increased with the supercharger  or “blower”  at low speed, the constant MAP is maintained  by opening the throttle and the BHP increases  above the sea level value because of the re-  duced exhaust back pressure. Opening the  throttle allows the supercharger inlet to re-  ceive the same inlet pressure and produce the  same MAP.  Finally, the increase of altitude  will require full throttle to produce the con-  stant MAP with low blower and this point is  termed the “critical altitude” or “full throttle  height.”  If altitude is increased beyond the  critical altitude, the engine MAP, airflow, and  BHP decrease.  Since the exhaust gases have considerable  energy, exhaust turbines provide a source of  supercharger power. The turbosupercharger  (TB.S) allows control of the supercharger  speed and output to very high altitudes with  a variable discharge exhaust turbine (PDT).  The turbosupercharger  is capable of providing  the engine airflow with increasing altitude by  increasing turbine and supercharger speed.  Critical altitude for the turbosupercharger is  usually defined by the altitude which produces  the limiting exhaust turbine speed.  The minimum specific fuel consumption of  the supercharged  engine is not greatly affected  by altitudes less than the critical altitude.  At  the maximum cruise power condition, specific  fuel consumption will decrease  slightly with  an increase in altitude up to the critical  altitude.  Above critical altitude, maximum  ,cruise power cannot be maintained but the  The critical altitude with a particular super-  charger installation is specific to a given com-  bination of MAP and RPM.  Obviously, a  lower MAP could be maintained to some  143 