Principles of Flight (3)

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 xx_y-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 digts 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.

BERNOULLI'S PRINCIPLE

Daniel Bernoulli, an eighteenth-century Swiss scientist, discovered that as the velocity of a fluid increases, its pressure decreases. How and why does this work, and what does it have to do with aircraft in flight?

Bernoulli's principle can be seen most easily through the use of a venturi tube (see Animation or Figure 3-2). The venturi will be discussed again in the unit on propulsion systems, since a venturi is an extremely important part of a carburetor. A venturi tube is simply a tube which is narrower in the middle than it is at the ends. When the fluid passing through the tube reaches the narrow part, it speeds up. According to Bernoulli's principle, it then should exert less pressure. Let's see how this works.

As the fluid passes over the central part of the tube, shown in Animation or Figure 3-2, more energy is used up as the molecules accelerate. This leaves less energy to exert pressure, and the pressure thus decreases. One way to describe this decrease in pressure is to call it a differential pressure. This simply means that the pressure at one point is different from the pressure at another point. For this reason, the principle is sometimes called Bernoulli's Law of Pressure Differential.

Bernoulli's principle applies to any fluid, and since air is a fluid, it applies to air. The camber of an airfoil causes an increase in the velocity of the air passing over the airfoil.

This results in a decrease in the pressure in the stream of air moving over the airfoil. This decrease in pressure on the top of the airfoil causes lift.

RELATIVE WIND

In order to discuss how an airfoil produces lift or why it stalls, there are three terms we must understand. These are relative wind, angle of incidence, and angle of attack.

There is a noticeable motion when an object moves through a fluid or as a fluid moves around an object. If a thick stick is moved through still water or the same stick is held still in a moving creek, relative motion is produced. It does not matter whether the stick or the water is moving. This relative motion has a speed and direction.

Now let's replace the water with air as our fluid and the stick with an airplane as our object. Here again, it doesn't matter whether the airplane or the air is moving, there is a relative motion called relative wind. The relative wind will be abbreviated with the initials RW (see figure 3-3). Since an airplane is a rather large object, we will use a reference line to help in explaining the effects of relative wind. This reference is the aircraft's longitudinal axis, an imaginary line running from the center of the propeller, through the aircraft to the center of the tail cone.

Note in Figure 3-4 that the relative wind can theoretically be at any angle to the longitudinal axis. However, to maintain controlled flight, the relative wind must be from a direction that will produce lift as it flows over the wing. The relative wind, therefore, is the airflow produced by the aircraft moving through the air. The relative wind is in a direction parallel with and opposite to the direction of flight.

Let's look a little closer at how relative wind affects an airplane and its wings. As shown in Figure 3-3, the chord line of the wing is not parallel to the longitudinal axis of the aircraft. The wing is attached so that there is an angle between the chord line and the longitudinal axis. (We call this difference the angle of incidence.) Since we describe relative wind (relative motion) as having velocity (speed and direction), the relative wind's direction for the wing is different from that of the fuselage. It should be easy to see that the direction of the relative wind can also be different for the other parts of the airplane.

Very briefly, angle of attack is a term used to express the relationship between an airfoil's chord and the direction of its encounter with the relative wind. This angle can be either positive, negative, or zero. When speaking of the angle of attack, we normally think of the relative wind striking the airfoil from straight ahead. In practice, however, this is true only during stabilized flight which is in a constant direction.