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CHAPTER ONE
DOES THE EARTH SUPPORT THE AIRPLANE? Some people insist that since the airplane exerts a force on the earth, in straight-and-level flight, the earth is somehow holding the airplane up. This is definitely not the case. The lift on the wing has nothing to do with the presence of the surface of the earth. Examining two simple examples can show this. The first example is to consider the thrust of a propeller, which is just a rotating wing. It certainly does not develop its thrust because of the presence of the surface of the earth. Neither could the presence of the earth provide the horizontal component of lift in a steep bank. The second example to consider is the flight of the Concorde. It cruises at Mach 2 (twice the speed of sound) 55,000 ft (16,000 m) above the earth. The pressure information of the jet cannot be communicated to the earth and back faster than the speed of sound. By the time the earth knows the Concorde is there, it is long gone.
tree, the tree experiences the event, but that has nothing to do with the recoil of the gun.
The Adjustment of Lift We have said that the lift of a wing is proportional to the amount of air diverted per time times the vertical velocity of that air. And we also have stated that in a rest frame where the air is initially at rest and the wing is moving, the air is moving almost straight down after the wing passes. So what would happen if the speed of the wing were to double and the angle of attack were to remain the same? This is shown in Figure 1.8b. As you can see, the vertical velocity Vv has doubled. As we will soon see, the amount of air diverted also has doubled. As will be discussed shortly, the amount of air diverted is proportional to the speed of the airplane. Thus, in this case, both the amount of air diverted and the vertical velocity of the air have doubled with the doubling of the speed and keeping the angle of attack constant. Thus the lift of the wing has gone up by a factor of 4. In Figure 1.8c, the wing has been kept at the original speed, and the relative angle of attack has been doubled. Again, the vertical velocity