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"SmokeDog" Stu Moment
Stu Moment is an air show performer who also participates in many aviation education activities for kids and adults.
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“Simple Aerodynamics"
Part 3
copyright 1984, 2004, Sublogic Corporation 

SmokeDog's Note: In the last article we discussed drag and the use of streamlining to reduce drag. We will come back to drag in future parts in order to introduce another form of drag ... drag which is caused by lift. Before we come back to drag we will discuss lift.

I wrote the following article in 1984 as a manual which came with a computer flight simulation.

Lift

Lift is created by the wing passing through air. A cross-section of the wing (airfoil) is shown in Figure 2, where some important terms are introduced.

Figure 2. Airfoil Terminology

The mean chord line is an imaginary line that extends from the leading edge to the trailing edge of the airfoil. It is further extended in Figure 2. Relative wind is the airflow caused by passing the aircraft through an airmass. Relative wind is approximately opposite to the flight path. Angle of attack is the angle between the relative wind and the mean chord line. Figure 3 illustrates these terms in level, climbing, and descending flight.

Figure 3. Angle of Attack: Shown in level, climbing, and descending flight. (Angle of attack is the same in all three examples illustrated.)

Note that angle of attack does not have the same meaning as aircraft pitch attitude. The angle of attack in Figure 3 is purposely the same in all three examples (level, climbing, and descending flight) so as to emphasize this difference. In actual flight your angle of attack will often be different during different phases of the flight.

In order to understand how lift is produced, we must explore the theories of Bernoulli and Newton.

Bernoulli addressed the conservation of energy in fluid flow. Assuming a constant density (no compression) of the fluid, energy is held constant by decreasing pressure with increasing velocity or, conversely, by increasing pressure with decreasing velocity. This incompressibility assumption holds nearly true for airflow as well, at least at the low speeds flown by light aircraft. The classic graphic illustration of this theory usually depicts a tube of varying diameter (Figure 4).

Figure 4. Bernoulli's Favorite Tube

Figure 4 shows an enclosed tube with airflow. Mass flow within the enclosed tube is the same at all points since the air can’t escape. An incompressible fluid (or low-speed airflow) must increase velocity at the narrow point to maintain mass flow. If energy is to be conserved, then an increase in velocity (kinetic energy) must be balanced by a decrease in pressure (potential energy).

Figure 5 illustrates airflow past an airfoil at a positive angle of attack. The airflow over the top of the wing has a higher velocity than the airflow under the wing and, consequently, a lower pressure. A basic rule in physics states that when an imbalance exists, a force will result tending to relieve that imbalance. In the case of our airfoil this force is directed upwards, from the higher pressure to the lower pressure. This force is known as lift.

Figure 5. Bernoulli Explanation of Lift

The Bernoulli Controversy

As late as the mid 1960’s many flight instructors were emphasizing Bernoulli’s law as the major contributor to lift theory. This concept does go a long way in explaining lift when looking just at airflow immediately adjacent to the wing. Bernoulli’s law, however, doesn’t explain the forces of airflow deflected by the wing. Indeed, most modern instructors give credit to Newton for explaining the majority of lift production.

Newton’s third law states that for every action there is an equal and opposite reaction. Figure 6 is a repeat of Figure 5 with labels changed to emphasize action-reaction theory.

Figure 6. Newton Explanation of Lift

Down wash is caused by the airfoil altering the direction of airflow downwards. This will occur as long as there is a positive angle of attack. Downwash is easy to understand no matter what shape the airfoil takes. In this age when fighter jets use thin, symmetrical airfoils, you can see why deflected air is considered to be the major contributor to lift.

Controlling Lift

As a pilot, you must learn how to control lift during takeoff, climbs, level flight, turns, descents, and landing. You can generally increase lift in two ways; increase airspeed or increase your angle of attack.

Given a constant angle of attack, an increase in airspeed increases pressure differential and downwash, and therefore increases lift. Given a constant airspeed, an increased angle of attack increases pressure differential and downwash, thereby increasing lift. As a pilot, you must manage both airspeed and angle of attack in order to gain the desired flight goals. A good example would be an airspeed transition from fast cruise flight to slower flight when entering a crowded airport traffic pattern. You reduce power and the aircraft decelerates. Since the weight of your aircraft is unchanged, you must produce constant lift during the deceleration. In order to produce constant lift, you must increase the angle of attack slowly until the aircraft is stable at its new slower speed.

The Stall

There is a limit to the angle of attack that you can use to generate lift. You can alter the relative wind airflow only so far before the wind refuses to change anymore.

Figure 7 shows an airfoil at three different angles of attack. The top illustration shows an airfoil at the same angle of attack used in the previous discussion of lift generation. The middle airfoil shows an increased angle of attack. Notice that the airflow is separating from the surface near the upper trailing edge of the wing. The bottom airfoil is at stall angle of attack. The point of airflow separation is so far forward that we don’t even see a downwash vector; Newton’s downwash is gone. Velocity in the area aft of the separation point is very low; Bernoulli’s suction is gone. And with neither law still in effect, there is no way to maintain lift.

Figure 7. Airflow Separation With Increasing Angle of Attack

Important Note!

What is a stall? A sudden loss of lift due to airflow separation from the wing. How do you stall an airfoil? Stall is a function of angle of attack. You can stall an aircraft at any airspeed and pitch attitude if you exceed the stall angle of attack.

How do you recover from a stall? Simply reduce the angle of attack.

Stall angle of attack depends on the airfoil shape, and is usually somewhere between 10 and 20 degrees. Generally speaking, thin airfoils will stall at a lower angles of attack while thick airfoils will stall at a higher angles of attack. Also, symmetrical airfoils will stall at lower angles of attack than airfoils with more bulge on the upper surface (higher camber). Figure 8 shows the lift characteristics of a typical airfoil used on training aircraft. Lift increases steadily until stall angle is reached. After this point, lift drops off suddenly.

Figure 8. Lift versus Angle of Attack

(continued next week)

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“Simple Aerodynamics"
Part 3
copyright 1984, 2004, Sublogic Corporation