Page 200 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 187

Airplane Performance 187

consumption of an airplane depends only on the power In 1911,“Cal” Rogers made the
setting and not the ground speed. If the airplane will be flying first transcontinental flight of the
into a head wind, the ground speed will be lower, the time in United States, taking 49 days to
flight will be longer, and thus more fuel must be carried. fly from New York to Los Angeles.
Now, suppose you were an engineer and were going to
design an airplane from scratch. You have a variety of engines at your
disposal. It should be intuitive that the lower the power output of the
engine, the lower the rate of fuel consumption. So you could just
choose a small engine, right? Consider a Boeing 777 and a Cessna 172.
Can those use the same engine? Clearly the engineer must consider
some other criteria for efficiency. Engineers use a parameter called the
specific fuel consumption. This is simply the rate of fuel consumption
divided by the thrust or power produced, depending on the type of
engine. The specific fuel consumption is thus a measure of the
engine’s efficiency. The lower the value, the more efficient the engine.
So, when choosing an engine, the engineer must not only consider its
weight, the takeoff distance, cruise speed, and ceiling; the specific fuel
consumption must also be factored in.

Maximum Endurance

If an engine is burning a certain amount of fuel per hour, how long can
it stay in the air? Is there a speed at which the airplane can remain in
the air longest? Some airplanes are designed for endurance. For exam-
ple, airplanes built for surveillance may want to loiter over a particular
location for a long time. There is interest in using autonomous aircraft
that will relay local communications, such as cellular phones, rather
than using expensive satellites. These aircraft are mostly concerned
about the length of time they can stay in the air. An example is
NASA’s Pathfinder airplane, shown in Figure 7.9, which was
In April 1949, the Sunkist Lady
designed to loiter at extreme altitudes and gather atmospheric
stayed aloft for 6 weeks and 1
data. The aircraft is solar-powered so that the fuel consump-
minute (1008 h, 1 min). The
tion is zero and the endurance in principle is limitless. At night
airplane was refueled by flying
it runs on batteries charged during the day. Let us see how one
low over a Jeep that passed up
would determine how to get maximum endurance out of a
fuel cans.
fuel-powered airplane.