Page 107 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 94

94 CHAPTER THREE

Wrapping It Up

Before moving to the next chapter discussing stability and control, let
us look at some typical wings and identify what choices were made in
designing them. We will look at the following three airplanes: the
Cessna 172 from general aviation, the Boeing 777 from commercial
aviation, and the Lockheed-Martin/Boeing F22 stealth fighter from the
military.
The Cessna 172, shown in Figure 3.10, is a popular four-seat
airplane with a cruise speed of 120 knots. The wing is unswept but
has a small amount of taper toward the tip. The wing is mounted at
the top of the fuselage for stability and for structural reasons. The
wing has an aspect ratio of 7.5, dihedral for stability, hinged flaps for
landing, and round leading edges for a gentle stall. The low wing
loading gives it good low-speed performance, but its top speed is not
particularly noteworthy. The airplane is designed to be easy to fly and
sports the best safety record of all general-aviation aircraft.
The Boeing 777 (Figure 3.35) is designed to carry a heavy load over
long distances, its high cruise speed, about Mach 0.84, requiring a high
wing loading. The aspect ratio is similar to that of the Cessna 172, but
the wing is swept and tapered. In this case the sweep is necessary for
the high cruise speed. Taper, plus changes in wing section, result in a
roughly elliptic lift distribution at cruise. For takeoff and landing, the
airplane has double-slotted trailing-edge flaps and deployable leading-
edge slats. The wing is fairly thick (a person can stand in the wing
root) to accommodate the necessary structure and fuel. The wing also
has significant dihedral for stability.
The Lockheed-Martin/Boeing F-22 is the current generation stealth
fighter (Figure 3.36). The body and wing blend together for stealth. As
a result it shares the characteristics of a low-wing and a midwing
design. The wing is more highly swept and tapered than the Boeing
777. It is highly maneuverable and can cruise at speeds greater than
the sound speed. The F-22 employs many high-lift devices, including
diverting engine thrust down. Its wing loading is high and it has a
very small aspect ratio.