Page 57 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 44

44 CHAPTER TWO

Fig. 2.17. A high-performance glider. (Photo courtesy of Motorbuch Verlag, Stuttgart,
Germany, from Cross-Country Soaring.)

operated. Because of the low density of the air, the induced power is
significant at its cruise speed.

The Physics of Efficiency

Let us now look at the physics associated with the efficiency of lift.
This will help us understand such things as why helicopters are less
efficient than fixed-wing aircraft and what affects the efficiency of
propulsion systems (discussed in Chapter 4). The first point to con-
sider is that the force producing the lift is proportional to the momen-
tum (mv, where m is the mass of the air and v is the vertical velocity)
that is transferred to the air per time. Thus, either the acceleration of
more mass or the acceleration of the air to a higher velocity will
increase the lift. The lift, of course, is the desired end product.
The induced power consumed is proportional to the amount of
2
1
kinetic energy ( ⁄2mv ) that is given to the air. For a given lift the