NAVWEPS O&ROT-SO 
AIRPLANE PERFORMANCE 
specific fuel consumption is not adversely 
affected as long as auto-lean or manual lean 
power can be used at the cruise power setting. 
One operating characteristic of the recipro- 
cating engine is distinctly different from that 
of the turbojet. Water vapor in the air will 
cause a significant reduction in output power of 
the reciprocating engine but a negligible loss 
of thrust for the turbojet engine. This basic 
difference exists because the reciprocating 
engine operates with a fixed displacement and 
all air processed is directly associated with the 
combustion process. If water vapor enters the 
induction system of the reciprocating engine, 
the amount of air available for combustion is 
reduced and, since most carburetors do not 
distinguish water vapor from air, an enrich- 
ment of the fuel-air ratio takes place. The 
maximum power output at takeoff requires 
fuel-air ratios richer than that for maximum 
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place with subsequent loss of power. The 
turbojet operates with such great excess of air 
that the combustion process essentially is 
unaffected and the reduction of air mass flow 
is the principal consideration. As an example, 
extreme conditions which would produce high 
specific humidity may cause a 3 percent thrust 
loss for a turbojet but a 12 percent loss of BHP 
for a reciprocating engine. Proper accounting 
of the loss due to humidity is essential in the 
operation of the reciprocating engine. 
OPERATING LIMITATIONS. Recipro- 
cating engines have achieved a great degree of 
refinement and development and are one of the 
most reliable of all types of aircraft power- 
plants. However, reliable operation of the re- 
ciprocating engine is obtained only by strict 
adherence to the specific operating limitations. 
The most important operating limitations of 
the reciprocating engine are those provided to 
ensure that detonation and preignition do not 
take place. The pilot must ensure that proper 
fuel grades are used that limit MAP, BMEP, 
RPM, CAT, etc., are not exceeded. Since 
Revised January 1965 
144 
heavy detonation or preignition is common to 
the high airflow at maximum power, the most 
likely chance of detonation or preignition is at 
takeoff. In order to suppress detonation or 
allow greater power for takeoff, water injec- 
tion is often used in the reciprocating engine. 
At high power’settings, the injection of the 
water-alcohol mixture can replace the excess 
fuel required to suppress detonation, and de- 
richment provisions can reduce the fuel-air 
ratio toward the value for maximum heat re- 
lease. Thus, an increase in power will be ob- 
tained by the better fuel-air ratio. In some 
instances, a higher manifold pressure can be 1 
utilized to produce additional power. The in- 
jection fluid will require proportions of alcohol 
and water quite different from the injection 
fluid for jet engine thrust augmentation. 
Since derichment of the fuel-air ratio is de- 
sired, the anti-detonant injection (AOZ) will 
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fluid from fouling the plumbing. 
When the fuel grades are altered during oper- 
ation and the engine must be’operated on a 
next lower fuel grade, proper account must be 
made for the change in the operating limita- 
tions. This accounting must be made for the 
maximum power for takeoff and the maximum 
cruise power since both of these operating con- 
ditions are near the detonation envelope. In 
addition, when the higher grade of fuel again 
becomes available, the higher operating,limits 
cannot be used until it is sure chat no contamina- 
tion exists from the lower grade fuel remaining 
in the tanks. 
Spark plug fouling can provide certain high 
as well as low limits of operating temperatures. 
When excessively low operating temperatures