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Airfoils

An Airfoil is a structure designed to create a reaction upon its surface from the air through which it moves. The wings, control surfaces and other parts of the airplanes are shaped as airfoils.

Typical Airfoil Designs. (a) subsonic airfoil (b) supersonic airfoil

The most forward point of the airfoil is defined as the leading edge and the most rearward point is defined as the trailing edge. The imaginary line that connects the leading edge with the trailing edge is called the chord line.

The Airfoil's Leading and Trailing edges, Chord line and Camber.


Angle of Attack, lift and drag

As shown in Figure below, the angle of attack is defined as the angle between the chord line and the relative wind. The angle of attack should not be confused with the angle of incidence which is the angle between the chord and the longitudinal axis of an airplane.

(a) Angle of Attack. (b) Angle of Incidence.

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The Flow of Fluids

The ideal flow of fluid about objects is shown in next figure. Although ideal flow does not exist, it is a helpful in developing an understanding of lift and drag.

(a) Ideal flow around a circular cylinder. (b) Ideal flow around an airfoil
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Lift

NASA Glenn Research Center published a Lift Theory that claims that the following theory is incorrect.
The author of this article is not a scientist and is merely republishing the lift theory as taught for decades by schools and government agencies.
Interested readers are encouraged to do their own research to form an educated opinion.
The next figure illustrates an uninterrupted airflow over an airfoil. The air above (blue) travels longer distance than the air below(red). The time it takes a particle to travel between A and B is constant, therefore the airflow above the airfoil is faster than below it. By applying the Bernoulli's principle the following is observed:

a. Higher speed - lower pressure above.
b. Lower speed - higher pressure below.

The velocity and pressure as air flows below and above an airfoil

The Glenn Research Center Lift Theory teaches that the following is the largest contributor to lift.
The air below (blue) the airfoil is deflected downward. The deflection has a horizontal and a vertical components(green). By applying Newton's Third Law of Motion we find that a force (red) that is equal in size to the vertical component of the deflection but in the opposite direction is acting on the wing. From Newton's Second Law of Motion F=ma we learn that a change of the velocity or the the vertical component generates lift. "Lift is a force generated by turning a moving fluid" - Glenns Research Center Theory.

Lifting effect of the deflection

The differential pressure combined with the reaction force of the deflection (Newton's first law of motion), generates lift. The total lift of the airfoil is noted by a single vector that is perpendicular to the airflow.

Lift Notation

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Drag

As objects move through air they excert force onto the air. From Newton's First Law of Motion we learn that the air reacts at the same force but in an opposite direction. The reaction force on objects as they move through air is defined as drag.


Drag Notation
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Resulting Lifting Force

As shown in this section, two forces result from the movement of an airfoil through air:

a. Lift
b. Drag
A graphic resolution of these two forces demonstrates a single resultant force that is a sum of the lift and drag. The resulting lifting force is perpendicular to the chord.

Lift and Drag Vector Resolution
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Pressure Distribution

The understanding of airflow about aerodynamic objects is required for analyzing their aerodynamic characteristics. Bernoulli's principle does not cover the distribution of pressure above or below an airfoil. Because an airfoil's shape is curved by design, the airflow about it is subject to Circular Motion laws. The Centripetal Force* causes variations in the pressure over an airfoil as shown in next figure.


Momentum Influence Airflow over an Airfoil

The following illustrates a typical pressure distribution over an airfoil as its Angle of Attack varies. The (-) and (+) represent the pressure and the arrows represent the resulting forces.

Pressure Distribution versus Angle of Attack
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Center of Pressure

The resultant force (green vector) of the pressures around an airfoil is shown in the next Figure. The point of the application of this force (lift) is noted as the Center of Pressure. For any Angle of Attack, the center of pressure is the point where the resultant force crosses the chord line.
It should be noted that the center of pressure is not a fixed point but is changing with the change in the Angle of Attack. An increase in angle of attack causes the center of pressure to move forward while a decrease in angle of attack moves the center of pressure backwards. The location of the center of pressure with respect to Center of Gravity is an important factor in airplane stability. The effect of the locations of the center of pressure and of the center of gravity will be discussed later.

The Center of Pressure and its Variations as Angle of Attack Changes.



*The Centripetal Force is the inward force acting of objects which move in a circular motion.

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Last update May 17, 2005
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