Angle of Attack
The Aerodynamics of AOA
As an airfoil cuts through the relative wind, an aerodynamic force is produced. This force can be broken down into two components, lift and drag. The lift produced by an airfoil is the net force produced perpendicular to the relative wind. The drag incurred by an airfoil is the net force produced parallel to the relative wind.
The angle of attack is the angle between the chord line and the relative wind.
The angle of attack is the angle between the chord line and the relative wind.
AOA Impact on Lift
The coefficient of lift is a measure of how much lift the wing can produce and can only be changed by changing the shape of the wing or the angle of attack at which it cuts through the relative wind. We can change the shape of the wing using flaps, slats, or other similar leading and trailing edge devices. We change the angle of attack using our flight controls and, in some cases, power settings.
For any given wing, an increase in angle of attack leads to an increase in the coefficient of lift up until the point it doesn't. In the graph shown, we see that a typical cambered wing produces a higher coefficient of lift but when it gets to its critical angle of attack, the lift drops off quickly. A symmetrical wing, on the other hand, produces less total lift but when it drops off, it drops off slowly.
[Dole, pg. 36] The importance of AOA in determining aircraft performance cannot be overemphasized. We have discussed stall AOA, but these facts are of equal importance: an aircraft has its maximum climb angle at a certain AOA, will achieve maximum rate of climb at another AOA, and will get maximum range at still another AOA.
For any given wing, an increase in angle of attack leads to an increase in the coefficient of lift up until the point it doesn't. In the graph shown, we see that a typical cambered wing produces a higher coefficient of lift but when it gets to its critical angle of attack, the lift drops off quickly. A symmetrical wing, on the other hand, produces less total lift but when it drops off, it drops off slowly.
[Dole, pg. 36] The importance of AOA in determining aircraft performance cannot be overemphasized. We have discussed stall AOA, but these facts are of equal importance: an aircraft has its maximum climb angle at a certain AOA, will achieve maximum rate of climb at another AOA, and will get maximum range at still another AOA.
Boundary Layer
To understand airfoil performance at high angles of attack, one must first consider the airflow at just about any angle of attack. When an airfoil passes through an airstream, the particles of air right next to the skin of the airfoil are pulled along at the same speed of the airfoil. As you get further away from the skin of the airfoil, the particles are less apt to "grab on" to the wing and at a certain distance they do not "grab on" at all. This layer of air, the particles that grab on to the wing completely to those that don't, is known as the boundary layer.
The behavior of the boundary layer determines, in great measure, the maximum lift coefficient and the stalling characteristics of the airfoil.
The beginning of airflow at the leading edge of a smooth airfoil surface produces a very thin layer of smooth airflow. This type of airflow is called laminar flow and is characterized by smooth regular streamlines. Fluid particles in this region to not intermingle. As the airflow moves back from the leading edge, the boundary layer thickens and becomes unstable. Small pressure disturbance cause the unstable airflow to tumble, and intermixing of the air particles takes places. This type of airflow is called turbulent flow.
The behavior of the boundary layer determines, in great measure, the maximum lift coefficient and the stalling characteristics of the airfoil.
The beginning of airflow at the leading edge of a smooth airfoil surface produces a very thin layer of smooth airflow. This type of airflow is called laminar flow and is characterized by smooth regular streamlines. Fluid particles in this region to not intermingle. As the airflow moves back from the leading edge, the boundary layer thickens and becomes unstable. Small pressure disturbance cause the unstable airflow to tumble, and intermixing of the air particles takes places. This type of airflow is called turbulent flow.
Development of a Boundary Layer on a Smooth Flat Plate
Adverse Pressure Gradient
As we saw in our discussion of lift, the pressure over and under the wing decreases as the velocity of the air increases. The pressure differential over the top and under the bottom of the wing generate the aerodynamic force that becomes lift and drag.
The point of minimum pressure divides the airfoil into two. Forward of that point the pressure gradient is helping to produce lift and, to a lesser degree, a pulling force forward. Aft of that point we have what is called the adverse pressure gradient. The pressure here contributes to drag and a separation of the air flowing over the wing.
The point of minimum pressure divides the airfoil into two. Forward of that point the pressure gradient is helping to produce lift and, to a lesser degree, a pulling force forward. Aft of that point we have what is called the adverse pressure gradient. The pressure here contributes to drag and a separation of the air flowing over the wing.
As the angle of attack is increased, the point of minimum pressure moves forward and the size of the adverse pressure gradient increases. Three things happen as a result:
- The lift component of aerodynamic force increases, up to a point.
- The drag component of aerodynamic force increases.
- The turbulent flow area increases, encouraging separation of the boundary layer.
Airflow Separation
The friction along the airfoil tends to reduce the velocity of the air particles right next to the surface to zero, forming a thin boundary layer of air. The adverse pressure gradient tends to expand the boundary layer to the point the air particles separate and are neither flowing with the relative wind or sticking to the airfoil.
The result of the air slowing creates a stagnated region close to the surface of the object. Airflow from outside the boundary layer will overrun the point of stagnation and cause the the boundary layer to separate from the surface. A flow reversal results, and the airflow moves forward, lift is destroyed, and drag becomes excessively high.
The result of the air slowing creates a stagnated region close to the surface of the object. Airflow from outside the boundary layer will overrun the point of stagnation and cause the the boundary layer to separate from the surface. A flow reversal results, and the airflow moves forward, lift is destroyed, and drag becomes excessively high.
Stall
The airflow separation can happen under conditions of slow speed flight, high-G maneuvering, or high speed shock waves.
While we are on the subject of AOA, however, it may be helpful to look at two aircraft systems for detecting and displaying AOA to the pilot.
While we are on the subject of AOA, however, it may be helpful to look at two aircraft systems for detecting and displaying AOA to the pilot.
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