How to Understand and Control the Angle of Attack of an Aircraft
The angle of attack (AOA) is one of the most important concepts in aerodynamics. It is the angle between a reference line on a wing or airfoil and the direction of the relative motion between the wing and the air. The AOA determines how much lift and drag are generated by the wing, and how close the wing is to stalling.
In this article, we will explain what the AOA is, how it is measured, and how it affects the performance and safety of an aircraft. We will also give some tips on how to control the AOA and avoid stalls.
What is the Angle of Attack?
The angle of attack is not the same as the pitch angle of the aircraft. The pitch angle is the angle between the longitudinal axis of the aircraft and the horizontal plane. The AOA is the angle between the chord line of the wing and the relative wind. The chord line is a straight line drawn from the leading edge to the trailing edge of the wing. The relative wind is the vector representing the relative motion between the wing and the air.
The AOA can be simply described as the difference between where a wing is pointing and where it is going. For example, if a wing is pointing up but moving down, it has a positive AOA. If a wing is pointing down but moving up, it has a negative AOA. If a wing is pointing and moving in the same direction, it has a zero AOA.
The AOA can be changed by changing either the pitch angle or the flight path angle of the aircraft. The flight path angle is the angle between the horizontal plane and the direction of motion of the aircraft. For example, if an aircraft pitches up without changing its speed or altitude, it increases its AOA. If an aircraft descends without changing its pitch angle or speed, it also increases its AOA.
How is the Angle of Attack Measured?
The AOA can be measured by using various instruments or devices on an aircraft. Some common methods are:
- Angle of attack indicator: This is a device that displays the AOA on a gauge or screen in the cockpit. It usually uses a sensor or probe mounted on the nose or wing of the aircraft that measures the pressure difference between the upper and lower surfaces of the wing.
- Stall warning system: This is a system that warns the pilot when the AOA approaches or exceeds a critical value that causes a stall. It usually uses an audible or visual signal, such as a horn or a light. Some stall warning systems also use a stick shaker or pusher that vibrates or moves the control column to alert the pilot.
- Vane: This is a simple device that consists of a small metal or plastic plate that rotates freely around an axis perpendicular to the airflow. The vane aligns itself with the relative wind and indicates its direction with respect to the chord line of the wing.
How does the Angle of Attack Affect Lift and Drag?
The AOA affects both lift and drag forces on a wing. Lift is the force that acts perpendicular to
the relative wind and supports
the weight of
the aircraft. Drag
the force that acts parallel to
the relative wind and opposes
the forward motion
Lift and drag depend on several factors, such as air density, airspeed, wing area, wing shape, and AOA. Among these factors, AOA has
the most significant influence on lift and drag.
the AOA increases,
the lift coefficient also increases,
The lift coefficient
a dimensionless number that relates
the lift force to
the air density,
the wing area.
A higher lift coefficient means more lift for
the same conditions.
The graph below shows
the relationship between
the AOA and
the lift coefficient for
a typical airfoil.
The graph shows that as
the AOA increases from