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The energy factors at the upper surface of
a wing, as we have said, are velocity and pressure—higher
velocity, lower pressure. If the velocity of the relative wind is
normally very high during 
cruising
flight of an airplane, it is not necessary for its wings to have
much camber. This is one of the reasons why fighter-type military
aircraft have thin wings. At slower speeds, such as during
takeoffs or landings, the loss of induced lift because of the low
camber is compensated for by using a high angle of attack. As you
can see, this high angle of attack causes an increase in the
dynamic lift. Even so, the airplane with low-camber airfoils must
use much higher takeoff and landing speeds than the more
conventional airplane.
To further illustrate these points, note in the top portion of Figure 4-4 that we have two examples of airfoils with the same relative wind velocity and the same dynamic lift. However, by thickening and increasing the camber of the wing, wing B's total lift is increased because of the increased induced lift.
In the lower portion of Figure 4-4, you are looking at two wings which are producing the same mount of total lift even though one wing has less amber than the other. Both wings are at the same angle of attack so they have the same amount of dynamic lift for any given airspeed (velocity of the relative wind). The only way to make the thin wing produce as much lift as the thick wing is to speed it up, and this is what we attempt to show in the figure. Wing C's relative wind is ten miles per hour faster than D's relative wind, this additional speed is needed to increase both the dynamic and induced lift so that its total lift can equal that of Wing D. We want you to understand that the examples in Figure 4-4 are just that.
We have discussed the atmosphere and how
airfoils produce lift because of their movement through the
atmosphere. We also mentioned that lift is the force that
counteracts the force of 
gravity
to allow flight. At this point, you may have concluded that lift
and gravity are the only forces involved with flight. Actually
there are two others, thrust and drag, which complete the
three-dimensional forces acting upon an aircraft in flight.
Figure 4-5 shows the basic directions of all four forces when an
aircraft is in straight and level flight at a constant speed.
Now, you should be able to see that, in this situation, the four
forces are in balance. The force of total lift equals the force
of total weight, so there is no upward or downward movement. The
force of thrust equals the force of drag, so there is no increase
or decrease in the speed of the airplane. You should also be able
to see that the moment one of these forces becomes stronger or
weaker than the others, some type of reaction must take place.
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Updated: March 12, 2004