Page 212 - Understanding Flight

CH07_Anderson 7/25/01 9:01 AM Page 199

Airplane Performance 199

shorten the time before takeoff without artificially cooling the brakes.
Cooling times for normal landings are on the order of 30 minutes.
Airlines must make sure to schedule airplane turn-around to
accommodate requirement.
Light aircraft do not have a problem with the amount of energy that
the brakes must dissipate because their landing speed and weight, and
thus their kinetic energy, are so low. In addition, these airplanes are
usually landed in a high-drag configuration, such as with full flaps,
which will naturally slow the airplane down without brakes. Pilots are
always happy when they reach a nice rolling speed by the first runway
turnoff without using brakes.
Like takeoffs, wind and altitude affect landings also. In fact, they
affect landing performance in exactly the same way as they do takeoffs.
Earlier we gave an example of a head wind that was 15 percent of the
takeoff speed, shortening the takeoff distance by 30 percent. A head
wind that is 15 percent of the landing speed will shorten the landing by
the same 30 percent. Likewise, the landing distance at an altitude of
6000 ft (1800 m) will be 20 percent longer than at sea level.

Wrapping It Up

Now that we have discussed performance characteristics we can look
at the performance of some different aircraft. Let us consider a small
single-engine airplane, a large transport, and a military fighter. The
Cessna 172, the Boeing 777, and the Lockheed-Martin F-22 stealth
fighter discussed in Chapter 3 will be used as examples here.
The Cessna 172 (Figure 3.10) has very low wing loading as well as
moderate L/D, power-to-weight ratio, and specific fuel consumption
(fuel consumption divided by the thrust or power produced). With the
low wing loading, the Cessna has a very short takeoff distance when
compared to the other two examples. However, its cruise speed and
range do not compare. This is in part due to the fact that the Cessna
172 carries less than 10 percent of its weight in fuel. Because of the
low wing loading, the Cessna’s turning radius is much smaller than
that of the other airplanes. However, this is not to imply that the
Cessna can outmaneuver the F-22, since the rate of turn is low.