Page 55 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 42

42 CHAPTER TWO

The Wright Flyer did not move In the section preceding this one we saw that the induced
to the Smithsonian until 1948. power increases as load squared. Since drag is just power
The Wright brothers were divided by speed, induced drag also increases as load squared.
snubbed when the Smithsonian Anything understood about power can be easily converted to
decided to display Samuel a similar understanding of drag by simply dividing by speed.
Langley’s Aerodrome as the “first We have said that drag is part of a pilot’s culture and
airplane capable of flight.” vocabulary. That is true. But most of the time when the term
Langley had been the secretary is used, the person really means power. Let us look at an
of the Smithsonian. example to illustrate. Take the case of a pilot flying a small
plane with retractable landing gear. If full power is applied in
straight-and-level flight, the airplane accelerates to some speed and
goes no faster. A pilot might well say that the airplane’s speed is
limited by the drag.
Let us pretend that an airplane had two meters, one that measured
total drag and another that measured the total power for flight. We
will then record both values for the airplane at its top speed. The pilot
lowers the landing gear and flaps, leaving the engine at full power.
There is now a substantial increase in the drag and power required.
This of course slows the airplane down. We would find that the
airplane slowed down to the previous total power requirement and
now the total drag is higher than before. The pilot would have to
reduce power to bring the total drag down to its previous value. Its top
speed was not determined by the total drag but by the total power
(drag times speed). So, when pilots say drag they usually mean power.
The utility of the concept of power over drag for the pilot is fairly
easy to understand. Power requirements relate simply to the demands
on the engine. Drag is a force that must be related to the airplane’s
speed in order to understand the related power requirement to
overcome it. A drag of a certain value at one speed is only half the
power drain of a drag of the same value at twice the speed. In the end,
the power available from the engine is what counts.

The Wing’s Efficiency for Lift

We have seen that the induced power requirement of a wing varies as
1/speed for a fixed load and as load squared for a fixed speed. But one