Page 126 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 113

Stability and Control 113

rather sporty. This is a particular problem in the home-built The Wright brothers did not fly
industry, which rarely publishes qualitative handling data. from October 16, 1905, to May
One handling quality is stick force. This is a measure of how 6, 1908, to protect their pending
much force is required to make a certain change in a control patent.
surface. Suppose an airplane required 40 lb of force to roll the
airplane at 1 degree per second. This would qualify as extremely
sluggish. But, if a 1-lb force on the controls corresponds to a 180-
degrees-per-second roll rate, this would be very sporty.
Another issue is control balance. Suppose you have to put 5 lb on the
control yoke for maximum roll but 30 lb for maximum pitch. This is an
unbalanced control system. Ideally, 5 lb on the control yoke should give
roughly the same changes in both the roll and the pitch axis.
Another factor is the adverse yaw. Older airplanes had significant
adverse yaw, so a pilot had to be diligent with rudder pedals. A
modern trainer hardly needs any rudder input to counter adverse yaw.
The improvement has come primarily through the use of dihedral and
larger vertical stabilizers. Older pilots consider this sloppy flying, but
the realities of a modern trainer are that rudder pedals are barely
needed.

Fly-by-Wire

Before the days of the digital computer, airplane control surfaces were
linked to the control yoke through cables, push rods, and hydraulic
lines. These were mechanical links from the pilot’s controls to the
control surface. In the case of cables and push rods, the stick force
was a matter of designed mechanical advantage that is utilizing the
basic concept of a lever. The problem with cables and push
rods is that they can be difficult to route from the yoke to the
Jimmy Doolittle, famed air racer
control surface. For example, a cable or push rod would be
with a Ph.D. from MIT, was the
undesirable down the center of the cabin. Hydraulic lines, on
first to fly coast to coast in the
the other hand, can be routed fairly easily, since they are just
United States on Sept. 4, 1922.
tubing.
With the computer in the loop there is no need for direct
mechanical connections or running hydraulic lines through the
airplane. A fly-by-wire system is a control system where control