Page 237 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 224

224 CHAPTER EIGHT

Power Required

Once the altimeter and airspeed indicator are calibrated we can use
these, along with engine-power calculations for piston-driven
airplanes or thrust for jets, to determine the power and drag curves for
straight-and-level flight. In principle one can get both curves and
determine the induced and parasitic powers and drags by measuring
the power at only two speeds.

POWER REQUIRED DATA

Though a little beyond the scope of this book, if one plots
power times speed as a function of speed to the fourth
power, the result is a straight line. This is illustrated in Figure
8.23 taken from the flight manual for a Cessna 172. Although
values for four points were used, in principle the same results
could be obtained from only two points. From this plot one
can produce a plot of power as a function of speed (the
power curve) and also a plot of drag (power/speed) as a
function of speed (the drag curve). Knowing how induced
power and drag go as speed, one is also able to separate out
the different components of power and drag. This is a lot of
information from two simple measurements.

The above measurements were made at one weight and altitude. But
we know that both weight and air density affect the power required for
straight-and-level flight. To perform the test described above for
different altitudes and different loads would be expensive and time-
consuming. Engineers have developed the equivalent weight
system to avoid these additional tests. The air density at the
The first U.S. jet fighter was the
test altitude is determined from pressure and temperature
Lockheed P80. It was conceived
measurements and related to sea-level density. Similarly, the
in 1943 to counter the German
weight of the airplane during the test is measured and related
ME-262. The P80 scored the first
to a standard weight, usually either the maximum gross weight
aerial combat victory between
or the empty weight. Known relationships already discussed
two jet fighters when one
for power requirements as a function of weight and air density
downed a MIG-15 over Korea on
allow the data to be related to any altitude and weight
November 8, 1950.
regardless of actual altitude and weight flown on the flight test.