Page 101 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 88

88 CHAPTER THREE

wing that can change its characteristics for takeoff and landing
speeds. A second way to increase the stall angle of attack, discussed
above, is to energize the boundary layer.
The most common high-lift device is the wing flap. The next most
common is to add leading-edge devices called slots and slats. In rare
instances the deflected slipstream from the propellers or jet engine is
diverted to provide additional lift at low speeds.

In a rush to be the first to fly Flaps
nonstop from New York to Paris, Wing flaps can be found on virtually every modern airplane.
Charles Lindbergh set a record
The effect of adding flaps to the trailing edge of the wing is
with a nonstop overnight flight
equivalent to increasing the camber of the wing. Some flap
from San Diego to St. Louis.
designs also increase the chord length of the wing. This
increases the area of the wing so that more air is diverted, thus
reducing the angle of attack needed for lift.
There are many types of flaps. In the 1930s and 1940s the split flap,
shown in Figure 3.26 was introduced and was one of the first types of
flap to appear in production airplanes. Splitting the last 20 percent or
so of the wing forms this type of flap. The top surface of the wing does
not move while the bottom surface lowers. The split flap is effective in
improving the lift, but it creates a great deal of form drag, as shown in
the figure. The split flap was used on the DC-3. It was also used on
WWII-era dive-bombers because it helped increase lift at low speeds
and slowed the airplane during the dive.
The simple hinged flap (Figure 3.27) is most common on smaller
aircraft. The last 20 percent or so of the inboard section of the wing is
simply hinged so that it can increase the camber. The first 20 degrees
of flap extension increase the lift without
greatly increasing the drag of the wing at low
speeds. Many airplanes extend their flaps to 10
or 20 degrees on takeoff in order to shorten the
takeoff distance. When the flaps are extended
greater than 20 degrees, the form drag
increases rapidly with little or no increase in
lift. Increasing the drag increases the descent
Fig. 3.26. Split flap. rate, which is desirable during the approach