Page 207 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 194

194 CHAPTER SEVEN

If the bank in the example above were increased to 60 degrees, the
load factor would be increased to 2, and the induced power and induced
drag would be increased to 4 times the values in straight-and-level flight.
But the radius of turn and the time to make the 180-degree turn would
be reduced by 42 percent. Suppose we increased the bank angle even
more. Eventually there have to be some limits, since at 90 degrees the
load on the wing and the power required would become infinite.
Suppose you were flying up a canyon (considered a very bad idea)
and you wanted to make a tight turn to get out. What is required for a
high-performance turn? How does one make a turn of minimum radius?
The minimum-turn radius is limited by three characteristics of an
airplane. These are (1) the stall speed with the flaps up, (2) the
structural strength, and (3) the propulsive power that is available. Let
us look at each of these limits individually.

Stall Speed Limit
What is the stall speed limit? This can be illustrated by considering the
extreme of an airplane flying at just above the stall speed (i.e., just
below the stall angle of attack). If this airplane tried to make a turn, it
could not increase its angle of attack to accommodate the higher load
on the wing or it would stall. So this airplane would be unable to turn
and thus have an infinite turning radius. If the airplane flew a little
faster, it could make a very gentle turn, with a very large turning
radius. The very early airplanes were so underpowered that they
could not fly much faster than the stall speed, and so flew in
Engine power grew rapidly, once
this predicament. They could only make slow turns of large
flight became a reality. By 1910,
radius. The first flights of many, including the Wright broth-
the French had built an engine
ers, could only be made in a straight line.
that could deliver 177 hp
Following the logic of the previous paragraph, lets see what
(179.5 kW).
happens when the pilot is in a turn at a speed twice the
straight-and-level stall speed. From Chapter 2 we know that at
double the speed the airplane can hold four times the load before it
stalls, due to the increase of diverted air and the increase in the
vertical velocity of that air. In a bank, this means that the airplane can
make a 4g turn (a load factor of 4), which occurs in a 75.5-degree
bank. What this shows is that, for any given speed, the maximum