Page 43 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 30

30 CHAPTER TWO

respect to the relative wind, which is proportional to the angle of
attack of the wing.
So what would happen if the speed of the wing were to double and
the angle of attack remained the same? This is shown in Figure 2.9b. As
you can see, the vertical velocity (Vv) has doubled. As we will soon see,
the amount of air diverted has also doubled. Since both the amount of
air diverted and the vertical velocity of the air have doubled with the
doubling of the speed, the lift of the wing has gone up by a factor of 4.
In Figure 2.9c the wing has been kept at the original speed but
the angle of attack has been doubled. Again the vertical velocity
The vertical velocity of the air is
of the air has doubled and the lift of the wing has doubled. What
proportional to both the speed
these figures show is that the vertical velocity of the air is
and the angle of attack of the
proportional to both the speed and the angle of attack of the
wing.
wing. Increase either and you increase the lift of the wing.

Angle of Attack

Now let us look in more detail at the angle of attack of the wing. In
aeronautics the geometric angle of attack is defined as the angle
between the mean chord of the wing (a line drawn between the lead-
ing edge of the wing and the trailing edge) and the direction of the rel-
ative wind. For our discussion we are going to use the effective angle
of attack. The effective angle of attack is measured from the orienta-
tion where the wing has zero lift. The difference between the geomet-
ric angle of attack used in aeronautics and the effective angle of attack
used here should be emphasized to prevent potential confusion by the
reader. Figure 2.10 shows the orientation of a cambered wing with
zero geometric angle of attack, and the same wing with a zero effective
angle of attack. A cambered wing at zero geometric angle of attack has
lift since there is a net diversion of the air down. By definition the
same wing at zero effective angle of attack has no lift and there is no
net diversion of the air. In the case of a symmetric wing, the geomet-
ric and effective angles of attack are of course the same.
For any wing, from that of a Boeing 777 to a wing in inverted flight,
an orientation into the relative wind can be found where there is zero
lift. As the wing is rotated from this position, the change in angle is the