Explain (in an instructional way) how to find key airfoil data and compare diffe

Question

Explain (in an instructional way) how to find key airfoil data and compare different airfoils from different aircraft based on their characteristic airfoil values and curves. The aircraft is a Cessna 172 and the airfoil is a NACA 2412.

Food for thought for variables to discuss:

- Stall AOA and associated CLmax

- Zero-Lift AOA

- Comparison of CLmax and Stall AOA between both aircraft airfoils

- Comparison of Zero-Lift conditions between both aircraft airfoils

- Discussion of symmetric airfoil Zero-Lift conditions and resultant application preferences

Explanation / Answer

CLmax is the maximum lift coefficient. The stall "break" is usually defined as where the pitching moment curve begins to decrease, i.e. where further increasing AoA makes the nose go down. This could be a few degrees higher AoA than CLmax.

You also need to distinguish between the stall characteristics of an airfoil, and those of a complete airplane. Many planes are set up so the elevator loses authority before the main wing goes too deeply into the stall. The CG position also strongly affects the maximum AoA for which the plane can be trimmed.

Through the "linear lift" region, air flows smoothly around the wing and remains attached to both upper and lower surfaces. As AoA increases, a point will be reached where the flow on the upper surface starts to separate. This usually happens near the trailing edge (TE) and progresses forwards as AoA increases. If the airfoil has gentle stall characteristics, there will be a range of several degrees AoA where the separated area grows from the TE to most or all of the upper surface. This is the "stall region" where the flow progresses from "partial stall" to "full stall".

Some people define the AoA for CLmax as the "stall AoA" because that's where the aircraft can fly at its lowest speed. However, if the is sufficient power and controllability, some aircraft can fly well beyond this AoA into the full stall region. Sometimes this is useful, for example to slow down quickly or make a very steep descent.

Some configurations have a "secondary stall" or "deep stall". Even though the upper surface flow is fully separated, the wing continues to produce lift, and the airplane can fly stably at high AoA (usually 40 - 60 deg). Sometimes the wing flow can make the horizontal tail or elevator ineffective at these conditions, causing a "locked-in deep stall".

Angle of Attack

Now let us look in more detail at the angle of attack of the wing.

The geometric angle of attack is defined as the angle between the mean chord of the wing (a line drawn between the leading edge and the trailing edge of the wing) and the direction of the relative wind. This is what aeronautical engineers are referring to when they discuss angle of attack.

For our discussion, we are going to use the effective angle of attack.

The effective angle of attack is measured from the orientation where the wing has zero lift. The difference between the geometric angle of attack used by most people and the effective angle of attack used here should be emphasized to prevent potential confusion by the reader.

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