Page 204 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 191

Airplane Performance 191

35,000 ft (10,600 m) in order to correct for a 20 percent reduction in
weight. Ideally this change in altitude would be accomplished by a
slow climb over the entire flight. Of course, for safety reasons, the FAA
would not allow this. Unfortunately, for fuel consumption, jets are
only allowed to increase their altitudes in 4000-ft (1200-m) steps as
they fly.
A pilot can request an altitude change during the flight and thus
approach a cruise-climb situation with a stepped-altitude climb. Most
long flights will perform a stepped-altitude climb, and it is not
uncommon for the pilot to announce such a change in flight. The
purpose is to decrease the amount of fuel consumed.
Pilots of small airplanes fly at a constant altitude and speed. If you
are a pilot of smaller aircraft, this is the easiest condition to maintain.
As fuel is consumed and the airplane gets lighter, the angle of attack
will have to be adjusted. Unfortunately, changing the angle of attack
also changes the airplane’s location on the drag curve. The airplane is
no longer flying at maximum efficiency. The lost range from the lost
efficiency is on the order of 10 percent over that of a long cruise-climb
flight. This means that if an airplane could travel 1000 miles (1600
km) in cruise-climb, it would only go 900 miles (1440 km) at fixed
altitude and airspeed.
One might ask whether large airplanes fly at the best fuel
consumption. The answer is “almost.” Take a look at Figure 7.11,
which shows the distance traveled per amount of fuel as a
function of the airplane’s speed. Large airplanes try to fly on
As early as 1946, the U.S. Air
the high side of the maximum. They fly at 99 percent of the
Force was considering nuclear-
maximum range since the 1 percent loss in range gives them a
powered airplanes. In 1955, the
several percent increase in cruise speed. This is considered
NB-36H actually flew a nuclear
a good exchange.
reactor, although it was
You should recognize that the range a particular pilot gets
conventionally powered.
from an airplane is dependent on the pilot’s particular flying
habits and how fast he or she wants to get to the destination. Most
pilots of general-aviation airplanes fly considerably faster than the
speed for maximum range. This is because the cost of the fuel is a
smaller fraction of the total operating cost and a good part of the
expense of flying is tallied by the hour.