Page 211 - Understanding Flight

CH07_Anderson 7/25/01 9:01 AM Page 198

198 CHAPTER SEVEN

distance. When the airplane approaches its touchdown, it has a certain
2
amount of kinetic energy (1/2 mv ). When it comes to a stop, it will
have zero kinetic energy. So the landing distance will be proportional
to the touchdown velocity squared. Once again, landing performance
benefits from a low stall speed.
Typically, the landing distance of an airplane is shorter than the
takeoff distance. This is because the airplane can decelerate with its
brakes faster than it can accelerate with its engines. As for most cars,
one can stop in a shorter distance than it takes to accelerate to the
same speed. There have been many stories of pilots who have landed
in a short field but have been unable to take off.
Once on the ground the airplane’s minimum stopping distance will
depend primarily on its ability to brake. The braking power is
proportional to the weight supported by the wheels. On a hard dry
surface, the decelerating force from the brakes can be as high as 80
percent of the weight on the wheels. Of course, this value is greatly
reduced for a slippery surface. So, for a maximum performance stop
the lift must be removed from the wings as quickly as possible
During its return to landing, the
to put the weight on the wheels. Thus, as soon as the airplane
Space Shuttle, which has no
touches down, the flaps are raised. Modern jets also employ
power during its descent, is a
spoilers on the top of the wings, which remove part of the lift.
glider with a glide ratio of 4:1.
One key factor in determining the minimum stopping
distance is the ability of the brakes to absorb energy. The brakes
of a 500,000-lb (227,000-kg) airplane landing at 170 mi/h
1
(270 km/h) must dissipate 30,000 hp (22 million watts) for 2 minute
or so! The heat from the brakes can be so great that the wheels will
literally melt. This energy that must be dissipated makes the use of
thrust reversal important to reduce the demands on the brakes.
Commercial airplane tires are filled with pure nitrogen to remove
oxygen that can contribute to a fire, in case of maximum braking.
Also, the heat will cause the tires to expand and potentially blow up.
Some airplanes use special bolts that will separate under high
temperatures so that the tire pressure is reduced before they can
explode. So normally commercial airplanes do not use maximum
braking. But even after normal landings airplanes are required to wait
for a certain length of time to let the brakes cool. Flights cannot