Lecture 13 - Airfoil Characteristics

Main Ideas

Why an Airfoil Has Its Shape

An airfoil is the cross-section of a wing or other lifting surface.

A thin, flat plate at a small angle of attack turns the airflow downward. The plate exerts a downward force on the air, and the equal-and-opposite reaction produces an upward force on the plate: lift. The horizontal component of the aerodynamic force is drag.

The lecture describes viscosity and skin friction as helping the flow follow the plate’s surface. A plate with sharp corners can cause the flow to separate because it must turn abruptly. Rounding the leading edge helps the airflow turn smoothly and remain attached, while a sharp trailing edge lets the flow leave in a more orderly way. If the airfoil is tilted too far, the flow may no longer follow its surface and can separate.

Symmetric and Cambered Airfoils

A symmetric airfoil has a straight centerline and matching upper and lower shapes. In the lecture’s lift-curve illustration, it produces zero lift at zero angle of attack.

A cambered airfoil has a curved centerline. Its shape turns the airflow further, so it can produce lift even at zero angle of attack. Camber can increase flow turning without requiring a larger angle of attack, but excessive camber can also cause separation.

Airfoil Geometry and Terminology

• Leading edge: The edge facing the incoming flow.
• Trailing edge: The edge where the flow leaves the airfoil.
• Chord length, (C): The straight-line distance from the leading edge to the trailing edge.
• Thickness, (T): Normally the airfoil’s maximum thickness; its location along the chord is another geometric property.
• Camber: The maximum deviation of the airfoil’s centerline (camber line) from the straight chord line. Camber and its location are commonly expressed relative to chord length.

NACA Four-Digit Example

For the example NACA 2412, the digits indicate:

• First digit (2): Maximum camber is 2% of the chord.
• Second digit (4): Maximum camber occurs at four-tenths, or 40%, of the chord.
• Last two digits (12): Maximum thickness is 12% of the chord.

Airfoil dimensions are often expressed as fractions or percentages of chord so the same profile can be scaled to different aircraft sizes. The lecture gives a typical thickness-to-chord range of about 8–12%. It also notes that the stated location of maximum thickness is approximate and may vary among airfoils.

Design Trade-Offs and Airfoil Information

A larger thickness-to-chord ratio makes an airfoil stouter. The lecture describes a trade-off: aerodynamic designers generally prefer less thickness to reduce drag, while structural designers may prefer more thickness to reduce structural weight. The final design balances these considerations.

To characterize an airfoil, the lecture says to specify its chord, thickness-to-chord ratio, whether it is symmetric or cambered, and—if cambered—the amount and location of maximum camber. Designers also need the lift-coefficient-versus-angle-of-attack curve ((C_L)–(\alpha)). The lecture contrasts the symmetric airfoil’s curve, which passes through zero lift at zero angle of attack, with the cambered airfoil’s curve, which is offset and gives lift at zero angle of attack.

Speakers and Sources

• Lecturer: Unidentified instructor presenting the lecture for IITM BS Aerospace.
• No other spoken speakers are identifiable in the subtitles.