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Elements affecting the Angle of Attack

Flight controls and trim devices

The chord line is defined as the imaginary line that connects the leading edge with the trailing edge. The angle of attack is the angle between the direction of the airflow and chord line. Deflecting a flight control surface modifies the chord line. As shown in the figure below, the angle of attack in (a) , is smaller than in (b). An increase in the angle of attack causes an increase in both lift and drag while a decrease in the angle of attack causes a decrease in both.

(a) Angle of attack prior to control deflection. (b) Angle of attack after deflection.

The Effect of High Lift Devices

The illustration on the right demonstrate the
effect of high lift devices on the angle of attack.

The Effect of Roll on the Angle of Attack

It is a widely accepted that the angle of attack increases when a wing drops while the angle of attack decreases when a wing rises, but this is rarely explained. When an airplane is banked, there is an upward airflow on the descending wing and a downward airflow on the ascending wing in addition to the forward airflow. Airfoil (a) represents the descending wing and airfoil (b) represents the ascending wing. For each of these airfoils, there are a horizontal and a vertical airflow components which are resolved into a single resultant airflow (Magenta).

Angle of attack is larger than angle of attack . It follows that by rolling an airplane, the angle of attack of the descending wing increases and the angle of attack of the ascending wing decreases. As a result both lift and drag increase on the descending wing while decrease on the ascending wing.

The Effect of Wing Tip Vortices

As a result the differential pressure below and above the wing, there is a constant tendency of air to flow from bottom to top(a). Since the airplane is constantly moving the air is forced up at the wing tips.

Considering an airfoil at each wing tip, as demonstrated in (b), the airflow (V) is deflected upwards (w) resulting in airflow (Vres), thus increasing the angle of attack. Angle of attack is greater than angle of attack . As a result, both lift and drag are increased at the wing tips section.

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Last update May 17, 2005
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