Page 119 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 106

106 CHAPTER FOUR

Fig. 4.5. Northrop YB-49 Flying Wing.

Horizontal Stabilizer Sizing
How big should the horizontal stabilizer be? There are two ways to
increase the effectiveness of a horizontal stabilizer. One is to increase
the stabilizer’s area and the other is to increase its distance from the
wing. In fact, it is the distance to the horizontal stabilizer times the
wing area of the horizontal stabilizer that dictates stability.
Two stabilizer configurations, one with half the area but twice
The horizontal stabilizers on the
as far from the wing as the other, will have the same stabiliz-
Boeing 777 have approximately
ing effect. So the engineer must determine the lever arm,
the same area as the wings on a
which is the fuselage length, and the horizontal stabilizer area
Boeing 737.
to design a statically stable airplane.
The neutral point of an airplane is a function of the horizontal
stabilizer lever arm times the area. The greater this product, the
farther aft the neutral point. The advantage to having a neutral point
farther aft is that it gives increased limits on center of gravity location.
The increased limits give the airplane greater loading flexibility.
On the other hand, there is a negative impact of increasing either
the lever arm or the area of the horizontal stabilizer. Both translate
into higher weight, the airplane designer’s nemesis. Both also
contribute to higher drag due to skin friction. Another problem can be
that the airplane becomes too stable. In other words, the effectiveness
of the horizontal stabilizer is so great that the pilot has a difficult time