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Physical Description of Lift

A jet engine and a propeller produce thrust by blowing air back. A

helicopter’s rotor produces lift by blowing air down, as can be seen in

Figure 1.1, where the downwash of a helicopter hovering over the

water is clearly visible. In the same way, a wing produces lift by diverting

air down. A jet engine, a propeller, a helicopter’s rotor, and a wing

all work by the same physics: Air is accelerated in the direction opposite

the desired force.

This chapter introduces a physical description of lift. It is based

primarily on Newton’s three laws. This description is useful for understanding

intuitively many phenomena associated with flight that one

is not able to understand with other descriptions. This approach

allows one to understand in a very clear way how lift changes with

such variables as speed, density, load, angle of attack, and wing area.

It is valid in low-speed flight as well as supersonic flight. This physical

description of lift is also of great use to the

pilot who desires an intuitive understanding of

the behavior and limitations of his or her airplane.

With the knowledge provided in this

book, it will be easy to understand why the

angle of attack must increase with decreasing


The formative period of aviation and airports:


The birth of civil aviation: 1903–1913

December 17, 1903, the day Orville and Wilbur Wright succeeded in achieving

flight with a fixed-wing, heavier-than-air vehicle at Kitty Hawk, North

Carolina, has gone down in history as being the “birth of aviation.” Their first

airplane flight occurred on a large field, with sufficient room for the aircraft

to take off and land. There were no paved runways, gates, fuel facilities,

lights, or air traffic control. There was no terminal building and there was no

automobile-parking garage. There were no rules and regulations governing

the flight. That field in Kitty Hawk was, however, the first airport.

In the 10 years following the Wright brothers’ first flight, the aviation world

evolved in a very slow and hesitant manner, with most of the advances

focusing on improving aircraft technology, and much of the efforts trying to

promote the technology. Little, if any, consideration was focused on creating

facilities for aircraft to take off and land.

As a result, by 1912, there were only 20 recognized landing facilities in the country,

all of which were privately owned and operated. The earliest operational

airfields date as far back as 1909, although they were generally indistinguishable

from, and often also functioned as, local athletic fields, parks, and golf courses.

Construction and maintenance of early airfields were, in general, considered local

responsibility, and with limited municipal funds, and the very low level of

aviation activity, priorities to build “airports” were understandably low.

World War I: 1914–1918

The outbreak of World War I in 1914 opened up initial opportunities for fixedwing

aircraft to serve in a military capacity. The effort to use aviation as a military

force in World War I resulted in the production of thousands of aircraft

(most of which were produced and served in France, Germany, and England),

and hundreds of military pilots, to first fly reconnaissance and later fighting

missions. As a result, the U.S. military built 67 airports for the war effort. These

predominantly grass fields provided facilities to base, fuel, and maintain aircraft,

as well as provide sufficient room for takeoff and landing—but required

little other infrastructure. After the war, 25 of these military airfields remained

operational, and the rest were decommissioned.

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Flight is a relatively simple and widely studied phenomenon. As
surprising as it may sound, though, it is more often than not
misunderstood. For example, most descriptions of the physics of lift
fixate on the shape of the wing (i.e., airfoil) as the key factor in
understanding lift. The wings in these descriptions have a bulge on the
top so that the air must travel farther over the top than under the wing.
Yet we all know that wings fly quite well upside down, where the
shape of the wing is inverted. This is demonstrated by the
Thunderbirds in Figure I.1, with wings of almost no thickness at all.
To cover for this paradox, we sometimes see a description for inverted
flight that is different than for normal flight. In reality, the shape of the
wing has little to do with how lift is generated, and any description
that relies on the shape of the wing is misleading at best. This assertion
will be discussed in detail in Chapter 1. It should be noted that the
shape of the wing does has everything to do with the efficiency of the
wing at cruise speeds and with stall characteristics.
Let us look at three examples of successful
wings that clearly violate the descriptions that
rely on the shape of the wing as the basis of lift.
The first example is a very old design. Figure
I.2 shows a photograph of a Curtis 1911 Model
D type IV pusher. Clearly, the air travels the
same distance over the top and under the bottom
of the wing. Yet this airplane flew and was
the second airplane purchased by the U.S.
Army in 1911.