Page 203 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 190

190 CHAPTER SEVEN

Fig.7.10. Boeing 747. (Used with the permission of the Boeing Management Company.)

airplane adjust for this reduction in weight to minimize the fuel
consumption on a flight? For small changes in weight an airplane
adjusts its angle of attack. But for a large jet, such as the Boeing 747
shown in Figure 7.10, the most efficient way to penetrate the air at a
high speed is with the fuselage exactly aligned with the direction of
flight. So the pilot wants to maintain the most efficient angle of
attack throughout the flight.
To keep the angle of attack constant throughout the flight,
commercial jets prefer to adjust for larger weight corrections by
reducing the amount of air the wing diverts. This is accomplished
simply by climbing to a higher altitude where the air is thinner. So, as
the fuel is consumed, the airplane wants to climb to where the air is
less dense.
It is straightforward to understand the change in altitude with
change in weight. If a large jet were to become 20 percent lighter due
to the consumption of fuel, the lift must be reduced by that amount.
Since it is undesirable to change the angle of attack of the wings or the
speed, the vertical velocity of the downwash is a constant. Therefore,
the amount of air diverted must be reduced by 20 percent.
This means a 20 percent reduction in air density. So, as a large
A KLM DC-2, which was making a
jet burns fuel, it wants to fly at an ever-increasing altitude so
commercial passenger flight, came
that its weight divided by the air density remains constant.
in second in the 11,333-mile
This is called a cruise climb.
race from England to Australia in
As an example of the change in altitude required, a large jet
1934.
initially flying at 30,000 ft (9100 m) would have to fly at about