Page 238 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 225

Aerodynamic Testing 225

8000

7500

Power x speed 7000

6500

6000

5500

5000

90 100 110 120 130 140 150
4
Speed (x 1,000,000)
Fig.8.23. Flight data of power times speed as a function of speed to the fourth
power for a Cessna 172.

Takeoff and Landing
Flight testing of takeoff performance is one of the more extensive tests
required for commercial airplanes. Certain aspects, such as friction with
the ground, depend on runway conditions such as the presence of ice or
water. The designer made assumptions about friction, which have to be
verified in flight test. Once a few conditions are verified, usually calcu-
lations are used to fill in the rest of the operating procedures.
Takeoff techniques also have to be established. For example, at
what speed should a pilot begin rotation? At what speed should the
airplane lift off? If the speed is too low, the airplane might be in
ground effect on the backside of the power curve and unable to climb.
Two tests are particularly exciting. They are the VMU (velocity-
minimum-unstuck) test and the maximum braking test. VMU is the
minimum speed at which the airplane can leave the ground (that is
“unstuck”). This occurs at approximately the stall angle of attack. In
order to achieve this goal, part of the airplane tail may actually drag
on the ground. When aluminum hits concrete, sparks tend to fly. It