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Section 3.1 - AIRFOILS

   An airfoil is any part of an airplane that is designed to produce lift. Those parts of the airplane specifically designed to produce lift include the wing and the tail surface. In modern aircraft, the designers usually provide an airfoil shape to even the fuselage. A fuselage may not produce much lift, and this lift may not be produced until the aircraft is flying relatively fast, but every bit of lift helps.

   Figure 3-1 shows a cross section of a wing, but it could be a tail surface or a propeller because they are all essentially the same. Locate the leading edge, the trailing edge, the chord, and the upper and lower camber on Figure 3-1.

Leading Edge:

   The leading edge of an airfoil is the portion that meets the air first. The shape of the leading edge depends upon the function of the airfoil. If the airfoil is designed to operate at high speed, its leading edge will be very sharp, as on most current fighter aircraft. If the airfoil is designed to produce a greater amount of lift at a relatively low rate of speed, as in a Cessna 150 or a Cherokee 140, the leading edge will be thick and fat. Actually, the supersonic fighter aircraft and the light propeller-driven aircraft are virtually two ends of a spectrum. Most other aircraft lie between these two.

Trailing Edge:

   The trailing edge is the back of the airfoil, the portion at which the airflow over the upper surface joins the airflow over the lower surface. The design of this portion of the airfoil is just as important as the design of the leading edge. This is because the air flowing over the upper and lower surfaces of the airfoil must be directed to meet with as little turbulence as possible, regardless of the position of the airfoil in the air.

Chord:

   The chord of an airfoil is an imaginary straight line drawn through the airfoil from its leading edge to its trailing edge. We might think of this chord line as the starting point for drawing or designing an airfoil in cross section. It is from this baseline that we determine how much upper or lower camber there is and how wide the wing is at any point along the wingspan. The chord also provides a reference for certain other measurements as we shall see.

Camber:

   The camber of an airfoil is the characteristic curve of its upper or lower surface. The camber determines the airfoil's thickness. But, more important, the camber determines the amount of lift that a wing produces as air flows around it. A high-speed, low-lift airfoil has very little camber. A low-speed, high-lift airfoil, like that on the Cessna 150, has a very pronounced camber.

   You may also encounter the terms upper camber and lower camber. Upper camber refers to the curve of the upper surface of the airfoil, while lower camber refers to the curve of the lower surface of the airfoil. In the great majority of airfoils, upper and lower cambers differ from one another.

 

NACA AIRFOIL NUMBERING SYSTEM

Many times you will see airfoils described as NACA xxxx or NACA xxxxx or NACA xxy-xxx series.  From the book Airplane Aerodynamics, by Dommasch, Sherby and Connally, Pitman Press, 1967, the following definitions are given to this nomenclature.

                   

The NACA 4-digit airfoils mean the following: The first digit expresses the camber in percent chord, the second digit gives the location of the maximum camber point in tenths of chord, and the last two digits give the thickness in percent chord. Thus 4412 has a maximum camber of 4% of chord located at 40% chord back from the leading edge and is 12% thick, while 0006 is a symmetrical section of 6% thickness.

The NACA 5 digit series airfoil means the following:  The first digit designates the approximate camber in percent chord, the second digit indicates twice the position of the maximum camber in tenths chord, the third (either 0 or 1) distinguishes the type of mean-camber line, and the last two digits give the thickness in percent chord. Thus, the 23012 airfoil has a maximum camber of about 2% of the chord located at 15% of the chord from the leading edge (3 tenths divided by 2) and is 12% thick. 

The NACA six, seven and even eight series were designed to highlight some aerodynamic characteristic.  For example, NACA 653-421 is a 6-series airfoil for which the minimum pressure's position in tenths chord is indicated by the second digit (here, at the 50% chord location), the subscript 3 means that the drag coefficient is near its minimum value over a range of lift coefficients of 0.3 above and below the design lift coefficient, the next digit indicates the lift coefficient in tenths (here, 0.4) and the last two digits give the maximum thickness in percent chord (here, 21% of chord).  The description for this example comes from Foundations of Aerodynamics, Kuethe and Schetzer, 2nd Edition, 1959, John Wiley and Sons, New York.

There are formulas that define all the stations of the airfoil section from these digits and you can probably find those in your library in any good aerodynamics book.  Also, you are referred to two other references listed below for more information on these classifications.  HOWEVER, in all cases, the last two digits of the classification gives the thickness in percent chord.

Summary of Airfoil Data, NACA Report 824, 1945, by Abbott, von Doenhoff and Stivers.  It was originally issued as ACR L5C05.

Theory of Wing Sections, Including a Summary of Airfoil Data, by Abbott and von Doenhoff, Mc-Graw Hill, New York, 1949, in which families of airfoils constructed according to a certain plan were tested and their characteristics recorded.

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Updated: December 23, 2008