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. 
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. 
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 
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. 
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. 
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