Page 48 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 35

How Airplanes Fly 35

to the angle of attack and the speed of the wing. With this knowledge
we are in a position to understand the adjustment of lift in flight.
As our first example, let us look at what happens if the load on the
wing is increased by the airplane going into a 2g turn. In such a turn
the load on the wings has doubled. If we assume that the
speed of the airplane is kept constant, the vertical velocity of During peak WWII production,
the downwash must be doubled to compensate for the Boeing built 17 B-17s a day in
increased load. Doubling the angle of attack does this. a single plant.
Now what happens when an airplane flying straight and level
doubles its speed? If the pilot were to maintain the same angle of attack,
both the amount of air and the vertical velocity of the downwash would
double. Thus the lift would go up by a factor of 4. Since the weight of the
airplane has not changed, the increased lift would cause the airplane to
increase altitude rapidly. So to maintain a constant lift, the angle of
attack must be decreased to decrease the vertical velocity. If the speed of
an airplane were increased by 10 percent, the amount of air diverted by
the scoop would increase by 10 percent. Thus the angle of attack would
have to be decreased to give a 10 percent reduction in the vertical
velocity of the downwash. The lift of the wing would remain constant.
As our last example, let us consider the case of an airplane going to
a higher altitude. The density of the air decreases and so, for the same
speed, the amount of air diverted has decreased. To maintain a
constant lift, the angle of attack is increased to compensate for this
reduction in the diverted air. If the density of the air is reduced by 10
percent the vertical velocity of the downwash is increased by the same
10 percent to compensate. This is accomplished by increasing
the angle of attack by that amount. “Penny-planes” are rubber-band-
We now understand how the airplane adjusts the lift for powered model airplanes that
varying load, speed, and altitude. The next step in can weigh no more than a
understanding the flight of an airplane is the subject of power penny.˛
and drag. We will start with a look at power.

Power

One of the most important concepts for understanding flight is that of
the power requirements. In aeronautics textbooks the discussion of