Page 186 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 173

Airplane Performance 173

the Ford Tri-Motor could carry only 11 passengers. The Boeing 747-400
of today can carry almost 50 times that number.
In this chapter you will learn how power, minimum drag, and
other factors are used to determine airplane performance. In the
sections that follow we discuss the performance of an airplane in
powered flight from takeoff to landing. But before that we will
prepare you by introducing the lift-to-drag ratio, the glide, and
indicated airspeed.

Lift-to-Drag Ratio

There are several parameters that are fundamental to understanding
performance. These parameters do not necessarily improve our under-
standing of how or why airplanes fly but are a useful aid to under-
stand airplane performance. The most important aerodynamic
parameter is the lift-to-drag ratio, often referred to as L over D
A high-performance glider has
and written L/D. Anyone interested in airplanes has likely
an L/D of 60:1; an albatross,
heard these words at one time or another. The L/D combines
20:1; a Boeing 747, 15:1, and a
lift and drag into a single number that can be thought of as the
sparrow, 4:1.
airplane’s efficiency for flight. Since lift and drag are both
forces, L/D has no dimensions, which means that it is just a number
with no units. A higher value of L/D means that the airplane is pro-
ducing lift more efficiently.
In still air, the L/D is the glide ratio, which is discussed in more
detail below. You can determine the L/D of a toy balsa-wood glider by
measuring its glide ratio, which is the ratio of the launch height to the
distance flown (see Figure 7.3). This ratio is the L/D of the glider. It is
unlikely that this value of L/D will be the maximum value, but one
reflecting how the trim is set for the glider.
There are two ways to look at L/D. If you are an engineer designing
an airplane, you have flexibility over both lift and drag. However, for
a pilot, in straight-and-level flight the lift equals the weight, so
maximum L/D simply means minimum drag. In this book we take the
perspective of the pilot and assume the lift is a constant, unless
otherwise stated. It is worth looking at L/D from the engineer’s
perspective first.