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The energy factors at the upper surface of a wing, as we have said, are velocity and pressurehigher 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