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Aviation Words: Angle of Attack (AOA, Alpha)

From Bits & Pieces Newsletter, March 2015 Issue

By Ian Brown, Editor – Bits and Pieces, EAA 657159

Anyone who has sailed knows that the apparent wind direction changes as the boat speeds up. If it is at 90 degrees to the boat when stationary, it will move toward the bow with increasing forward motion. So what about in the vertical plane?

There are varying descriptions of angle of attack (AOA) out there. All seem to agree that a baseline needs to be established; with simple wing shapes, it’s generally the chord of the wing from the trailing edge to some point on the leading edge. The angle, alpha, is measured between that baseline and the apparent wind direction. The faster you are moving, the smaller the angle of attack in still air.

Conversely, the slower you are moving, the greater will be the angle of attack. When angle of attack increases, the lift increases, but then so does drag. At some critical point, airflow detaches from the wing and the aircraft stalls. Measuring AOA is useful both in landing at the slowest speed and taking off at the best angle of climb.

It’s easy to think of the direction of the airflow as being parallel to the ground, but it’s not. It’s parallel to the direction of motion of the aircraft. I found a great graphic to help explain this.

Angle of attack indicators are a significant safety enhancement. Note to self: Get one.

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