NAVWEPS 00-8OT-80 
AIRPLANE PERFORMANCE 
cruise power is the upper limit of power that 
can be utilized for this operation. Higher air- 
flows and higher power wirhout a change in 
fuel-air ratio will intersect the knee of the 
detonation envelope. 
The primary factor relating the efficiency of 
operation of the reciprocating engine is the 
brake specific fuel consumption, iWE%, or 
simply c. 
Brake suecific fuel consumution 
The detonation properties of a fuel are de- 
termined by the basic molecular structure of 
the fuel and the various additives. The fuel 
detonation properties are generally specified 
by the antidetonation or antiknock qualities of 
an octane rating. Since the antiknock proper- 
ties of a high quality fuel may depend on the 
mixture strength, provision must be made 
in. the rating of fuels. Thus, a fuel grade of 
IIS/ would relate a lean mixture antiknock 
rating of 115 and a rich mixture antiknock 
rating of 145. One of the most common opera- 
tional causes of detonation is fuel contamina- 
tion. An extremely small contamination of 
high octane fuel with jet fuel can cause a serious 
,decrease in the antiknock rating. Also, the 
contamination of a high grade fuel with the 
next lower grade will cause a noticeable loss of 
antiknock quality. 
The fuel metering requirements for an engine 
are illustrated by the third graph of figure 2.16 
which is a plot of fuel-air ratio versus engine 
airflow. The carburetor must provide specific 
fuel-air ratios throughout the range of engine 
airflow to accommodate certain output power. 
Most modern engines equipped with auto- 
matic mixture control provide a scheduling of 
fuel-air ratio for automatic rich or automatic 
lean operation. The auto-rich scheduling usu- 
ally provides a fuel-air ratio at or near the 
maximum heat release value for the middle 
range of airflows. However, at high airflows 
a power enrichment must be provided to sup- 
press detonation. The auto-rich schedule gen- 
erally will provide an approximate fuel-air 
ratio of 0.08 which increases to 0.10 or 0.11 at 
the airflow for takeoff power. In addition, 
the low airflow and mixture dilution that oc- 
curs in the idle power range requires enrich- 
ment for satisfactory operation. 
The schedule of fuel-air ratios with an auto- 
matic lean fuel-air ratio will automatically 
provide maximum usable economy. If manual 
leaning procedures are applicable a lower fuel- 
air ratio may be necessary for maximum possi- 
ble efficiency. The maximum continuous 
I 
engine fuel flow 
= brake horsepower 
C= lbs. per hr. 
BHP 
Typical minimum values for c range from 0.4 
to 0.6 lbs. per hr. per BHP and most aircraft 
powerplaots average 0.5. The turbocompound 
engine is generally the most efficient because 
of the power recovery turbines and can ap- 
proach values of c=O.38 to 0.42. It should be 
noted that the minimum values of specific fuel 
consumption will be obtained only within the 
range of cruise power operation, 30 to 60 per- 
cent of the maximum power output. Gen- 
erally, the conditions of minimum specific fuel 
consumption are achieved with auto-lean or 
manual lean scheduling of fuel-air ratios and 
high BMEP and low RPM. The low RPM is 
the usual requirement to minimize friction 
horsepower and improve output efficiency. 
The effect of &it&c is to reduce the engine 
airflow and power output and supercharging 
is necessary to maintain high power output 
at high altitude. Since the basic engine is 
able to process air only by the basic volume 
displacement, the function of the supercharger 
is to compress the inlet air and provide a 
greater weight of air for the engine to process. 
Of course, shaft power is necessary to operate 
the engine driven supercharger and a tempera- 
ture rise occurs through the supercharger com- 
pression. The effect of various forms of super- 
charging on altitude performance is illustrated 
in figure 2.17. 
The unsupercharged-or naturally aspi-