Page 85 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 72

72 CHAPTER THREE

Dihedral angle

Fig. 3.14. The dihedral angle.

nose of the airplane swings to the left. This is called adverse yaw. This
is why the pilot must use compensating rudder to make a coordinated
turn. The reason for adverse yaw is that the wing rotating upward
experiences more drag than the wing rotating down. Whether the
rotation was caused by the ailerons or by a gust of air, the airplane
rotates because one wing has more lift than the other. We know that
lift requires work and this reflects itself in increased induced drag.
Thus the wing with the greatest lift has the greatest drag.

The Wright brothers No. 3 glider of 1902 initially had no
rudder. During longer gliding flights they practiced turns.
Much to their dismay when they banked the airplane the
result was that the airplane went into a “skid” because
the nose turned opposite to the bank.They had
discovered adverse yaw.The quick-minded brothers
deduced the need for a vertical stabilizer and rudder and
thus completed the three-axis control puzzle that made
controlled flight possible.

Dihedral adds stability because the adverse yaw results in a net
reduced angle of attack on the upper wing, and an increased angle of
attack on the lower wing. This results in an increase in lift of the lower
wing and a reduction in lift of the upper wing, as well as a restoring
yaw force. The result is a tendency to return to straight-and-level
flight. Small general-aviation aircraft and commercial transports all
have dihedral. These airplanes tend to return to level flight after gusts
or accidental control inputs. Today airplanes are so stable cross-
country trips can be quite boring.