Angle of Attack

Why does Angle of Attack matter?

• It is critical to understand Angle of attack, as it directly relates to many aspects of aircraft performance, stability, and control.
• Every aircraft has an angle of attack where maximum lift (the stall) occurs.
• The magnitude of lift created by the wing relates directly to the density of the air, the area of the wing, wing shape, airspeed, and angle of attack.
• Remember as we discussed in the four forces of flight, total lift must overcome total weight in order for an aircraft to fly.

Angle of Attack, or “AOA” is defined as:

The angle at which the relative wind meets the chord line of the wing.

The Chord line, as shown below, travels from the leading edge to the trailing edge.

Chord Line

To define the Angle of Attack, we are looking for the difference between the angle that the “relative wind” is meeting the wing at and the chord line.  The difference between these two lines gives us our Angle of Attack.

Angle of Attack

At a high angle of attack (around 16 degrees or so for most GA airplanes), the airflow begins to become turbulent over the top of the wing instead of smooth, and thus a loss of lift occurs since most of our lift comes from the low-pressure airflow over the top of the wing.

How Angle of Attack and Airspeed relate to different phases of flight:

Remember that the amount of lift the wing produces varies with angle of attack and airspeed.

• If you maintain the same angle of attack and increase airspeed (thrust), the aircraft will start to climb.
• if you maintain the same angle of attack and decrease airspeed (thrust), the aircraft will start to descend.

If you increase airspeed (thrust) and want to maintain your current altitude, the angle of attack must be decreased. This is why when we level-off for cruise flight, we:

• Decrease the pitch attitude slowly and smoothly to the horizon, allowing the airspeed to increase.
• As the aircraft is brought to level-flight, temporarily maintain climb power to allow the airspeed to build to the desired cruising speed.
• As cruising speed is reached, reduce throttle to cruise power, and trim the aircraft as necessary.

If you reduce airspeed (thrust) and want to maintain your current altitude, the angle of attack must be increased. As the aircraft slows down, it will be necessary to continually increase the amount of backpressure exerted on the controls to maintain level flight. As the angle of attack exceeds approximately 16 degrees, the airflow over the wing becomes disrupted, resulting in loss of lift and subsequent stall.

Remember:

• A large angle of attack at a low airspeed produces the same amount of lift as a lower angle of attack would at a higher airspeed.
• When the airspeed is low, the Angle of Attack must be high to balance the required amount of lift against the weight of the airplane.

Have a close look at the video in the next TOPIC to see this in action.