Page 125 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 112

112 CHAPTER FOUR

the pencil in position. Quick reaction is the role of the computer in the
unstable airplane.
Why would the military want a statically unstable airplane? As
discussed in Chapter 3 on “Wings,” the answer is maneuverability. If
the airplane has a natural tendency to diverge from a specific
condition, such as straight-and-level flight, then it will be
To remain stable, the flight
much more responsive when the pilot wants to make a
control system on the X-29
change. Another reason for designing a statically unstable
(Figure 3.8) has to make 40
airplane is that smaller stabilizers might be used, which
corrections per second.
decreases the weight and drag of the airplane.
The ability of a computer to solve problems quickly gives it a
tremendous advantage. The Wright brothers preferred their airplanes
slightly unstable so that the pilot would have to react and pay
attention. But the design was only slightly unstable, and controllable
by use of the control surfaces. If an airplane were highly unstable, the
pilot would not be able to react fast enough to compensate. But a
computer might be able to react fast enough. Today, inserting a
computer into the control loop can augment stability. The pilot can
manage the overall flight path while the computer manages the quick
response tasks, or the computer can do both.
With the computer in the control loop an airplane can be built to be
naturally unstable. One of the first examples of an unstable design
was the X-29, shown in Figure 3.8. The computer makes fast, tiny
adjustments that allow the pilot to focus on other tasks. If the
computer were to fail, the airplane would instantly become
uncontrollable to the pilot, with fatal consequences.

Handling

As mentioned earlier, handling qualities were not quantified until after
WWII. Before that time handling qualities relied on pilot opinion.
Words like hot, fun, smooth, fast, sluggish, and sporty are still used by
pilots to qualify an airplane’s handling properties. But what is sporty to
one pilot might be sluggish to another. The more dangerous situation is
the opposite, where the high-time fighter pilot, who considers the air-
plane to be smooth, turns the airplane over to a novice, who finds it