Airfoil Design





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Published on Jun 26, 2016

When looking at a typical airfoil, such as a wing, from the side, several design characteristics become obvious. You can see that there is a difference in the curvatures (or camber) of the upper and lower surfaces of the wing. The camber of the upper surface is more pronounced than lower surface, which is usually somewhat flat.

The chord line is a reference line drawn from the center of the leading edge straight through the wing to the trailing edge. The distance from this chord line to the upper and lower surfaces of the wing shows the amount of upper and lower camber at any point. Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line. This mean line is equidistant at all points from the upper and lower surfaces.

Different airfoils have different flight characteristics. The weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for producing the greatest lift is one that has a concave, or “scooped out” lower surface

On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground.

If the wing profile were in the shape of a teardrop, the speed and the pressure changes of the air passing over the top and bottom would be the same on both sides. But if the teardrop shaped wing were cut in half lengthwise, a form resembling the familiar wing section would result

The pressure difference between the upper and lower surface of a wing alone, does not account for the total lift force produced.

The downward and backward flow of air from the top surface of a wing creates a downwash. This downwash meets the flow from the bottom of the wing at the trailing edge. Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the wing.

Lift is also generated by pressure conditions underneath the airfoil. Because of the manner in which air flows underneath the airfoil, a positive or high pressure results.

The angle of attack of a wing is the angle between the chord line and the flow of air against the leading edge of the wing

The average of the pressure variation for any given angle of attack is referred to as the center of pressure, noted as CP. All aerodynamic force acts through the Center of Pressure. At high angles of attack, the Center of Pressure moves forward, while at low angles of attack the Center of Pressure moves aft.

This Center of Pressure travel is very important, since an airplane’s aerodynamic balance and controllability are governed by changes in the Center of Pressure.

The production of lift is much more complex than a simple differential pressure between upper and lower wing surfaces. In fact, many airfoils do not have an upper surface longer than the bottom. These are called symmetrical airfoils. Symmetrical airfoils are seen in high-speed aircraft having symmetrical wings, or on symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical. With symmetrical airfoils, the relationship of the airfoil with the oncoming air stream, or angle of attack, is all that is different.

As a wing moves through air, it is inclined upward against the airflow, producing a different flow caused by the wing’s relationship to the oncoming air. Think of a hand being placed outside the car window at a high speed. If the hand is inclined in one direction or another, the hand will move upward or downward. This is caused by deflection, which in turn causes the air to turn about the object within the air stream. As a result of this change, the velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction.

So far we have only focused on the air flow across the upper and lower surfaces of an airfoil. While most of the lift is produced by these two dimensions, a third dimension, the tip of the wing also has an aerodynamic effect. The high-pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top. This action creates a rotating flow called a wing tip vortex. The vortex flows behind the airfoil creating a downwash that extends back to the trailing edge of the airfoil. This downwash results in an overall reduction in lift for the affected portion of the wing.

Airplane manufacturers have developed different methods to counteract this action. Winglets can be added to the tip of an airfoil to reduce this flow. The winglets act as a dam preventing the vortex from forming. Winglets can be on the top or bottom of the airfoil. Another method of countering the flow is to taper the airfoil tip, reducing the pressure differential and smoothing the airflow around the tip.

Adapted from the Pilot's Handbook of Aeronautical Knowledge.

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