Prerequisite Knowledge:
In the previous articles, we discussed how aerofoils produce lift, and one of the factors that affects how much lift an aerofoil produces is its angle of attack. We will examine the effects of angle of attack in more detail here.
Types of Fluid Flow
When a fluid (like air) is moving, its motion can fall under two different categories:
Angle of Attack (AoA)
To understand how the aerodynamics of an aerofoil change at high angles of attack, let's look at some wind tunnel test photos. These screenshots were taken from the following video, between 3:23 and 3:49:
In the previous articles, we discussed how aerofoils produce lift, and one of the factors that affects how much lift an aerofoil produces is its angle of attack. We will examine the effects of angle of attack in more detail here.
Types of Fluid Flow
When a fluid (like air) is moving, its motion can fall under two different categories:
- Laminar flow is where the fluid is moving along a uniform and smooth point. There is no turbulence in the laminar fluid flow. Think of water flowing slowly through a river, with the water being calm and relatively undisturbed.
- Turbulent flow is the opposite, where the motion of the fluid is chaotic and choppy. This type of fluid flow is more like the water you see trailing behind a boat as it sails through water, it's extremely turbulent and choppy.
Angle of Attack (AoA)
To understand how the aerodynamics of an aerofoil change at high angles of attack, let's look at some wind tunnel test photos. These screenshots were taken from the following video, between 3:23 and 3:49:
To begin, lets look at an aerofoil at a low angle of attack:
The distinct white lines indicate areas of smooth, laminar flow. The blurred areas indicate turbulent flow. At low angles of attack, the airflow is mostly laminar, with slight turbulent flow towards the trailing edge of the aerofoil. The flow along the upper surface of the aerofoil is attached, which means the airflow follows the contour of the aerofoil's surface. The separation point (the point where the airflow becomes detached/separated and no longer follows the contour) is located aftward along the aerofoil, towards the trailing edge.
As the angle of attack, the separation point moves forward along the aerofoil (closer to the leading edge), and the airflow becomes more and more turbulent.
At extremely high angles of attack, the separation point moves all the way up towards the leading edge of the aerofoil, and most of the airflow is detached. The airflow over the upper surface is almost completely turbulent, which would create a noticeable buffet that could be felt inside the aircraft.
At this point, the angle of attack is so great that the airflow is completely separated from the aerofoil, and the airflow over the upper surface of the aerofoil is entirely turbulent. At this point, the aerofoil is said to be "stalled" (further discussion below). Increasing the angle of attack beyond this point will not result in an increase in lift, but rather a decrease in lift.
As the angle of attack is increased, so does the drag created by the aerofoil. This increases steadily as the angle of attack.
Coefficient of Lift
In the lift generation article, the Bernoulli equation was briefly mentioned. The equation is as follows:
As the angle of attack is increased, so does the drag created by the aerofoil. This increases steadily as the angle of attack.
Coefficient of Lift
In the lift generation article, the Bernoulli equation was briefly mentioned. The equation is as follows:
The coefficient of lift (Cl) is a unitless value that varies from aerofoil to aerofoil, but it also varies with the angle of attack of the aerofoil.
This is the Cl graph for a NACA 2412 aerofoil (the one used in the Cessna 172). For the most part, the value of Cl is proprtional to the angle of attack; whenever the angle of attack increases, so does the Cl value. However, there are two turning points in the graph. The first peak, at the maximum Cl value, represents the positive critical angle of attack (for this aerofoil, it's about 17 degrees). At or above the critical angle of attack, the flow over the aerofoil becomes completely detached, and the aerofoil is considered stalled. It is important to note that the aerofoil still produces lift when its angle of attack is beyond the critical angle of attack, but as the angle of attack increases, the amount of lift produced decreases.
There is also a negative critical angle of attack, which is the same, except it marks where the aerofoil will stall when flying at negative angles of attack. For this aerofoil, it's about -16 degrees. However, you don't have to worry about the negative critical angle of attack as it is nearly impossible to stall most planes by exceeding that value (pretty much all stalls are caused by exceeding the positive critical angle of attack). |
Actual Aircraft Stalls
It is a common misconception that stalls only happen when the aircraft's airspeed gets to low, but that's no entirely true. Stalls can happen at at any airspeed, as long as the critical angle of attack is exceeded. For example, if you are in level flight flying well above the stall speed, but you yank the control column backwards, the aircraft will stall. Yanking the control column backwards will cause the aircraft's nose to rise sharply, however the aircraft's inertia will carry it in a straight line. This means the angle of attack will increase, most likely exceeding the critical angle of attack.
Stalls can also happen at low speeds, and have been responsible for several prominent accidents (e.g. Turkish Airlines flight 1951). When the aircraft's speed drops, the amount of lift produced by the wing also decreases, as per the Bernoulli equation (lift is proportional to velocity-squared). If left uncorrected, the plane will start descending due to the lack of lift, so the pilot corrects this by gently pulling back on the control column, which increases the wing's angle of attack. This increase in angle of attack increases the Cl value, which then increases the amount of lift produced by the wing. As the plane slows down further and further, the pilot will need to pitch up and increase the angle of attack further and further to compensate, but eventually the angle of attack will reach the critical angle of attack, and the wing will stall.
The aircraft may begin to shake noticeable when getting close to a stall, due to the turbulent airflow over the wing. When the wing is stalled, the aircraft will begin descending due to the loss of lift, and this descent can often be quite rapid. A quick recovery from a stall is essential, especially when close to the ground (e.g. takeoffs or landings).
It is a common misconception that stalls only happen when the aircraft's airspeed gets to low, but that's no entirely true. Stalls can happen at at any airspeed, as long as the critical angle of attack is exceeded. For example, if you are in level flight flying well above the stall speed, but you yank the control column backwards, the aircraft will stall. Yanking the control column backwards will cause the aircraft's nose to rise sharply, however the aircraft's inertia will carry it in a straight line. This means the angle of attack will increase, most likely exceeding the critical angle of attack.
Stalls can also happen at low speeds, and have been responsible for several prominent accidents (e.g. Turkish Airlines flight 1951). When the aircraft's speed drops, the amount of lift produced by the wing also decreases, as per the Bernoulli equation (lift is proportional to velocity-squared). If left uncorrected, the plane will start descending due to the lack of lift, so the pilot corrects this by gently pulling back on the control column, which increases the wing's angle of attack. This increase in angle of attack increases the Cl value, which then increases the amount of lift produced by the wing. As the plane slows down further and further, the pilot will need to pitch up and increase the angle of attack further and further to compensate, but eventually the angle of attack will reach the critical angle of attack, and the wing will stall.
The aircraft may begin to shake noticeable when getting close to a stall, due to the turbulent airflow over the wing. When the wing is stalled, the aircraft will begin descending due to the loss of lift, and this descent can often be quite rapid. A quick recovery from a stall is essential, especially when close to the ground (e.g. takeoffs or landings).
The first step in recovering from any stall is to decrease the wing's angle of attack, in order to un-stall it. Generally, this means releasing any backpressure on the controls, or even applying a swift but smooth forward control column movement in order to reduce the angle of attack. Since most stalls are caused by low airspeed, applying full power quickly is also essential, as it will increase the aircraft's speed. The recovery is often contradictory to instinct; when the plane stalls and starts dropping, the pilot's instinct is to pull the controls back, and try to climb out of the stall. However, this will only worsen the situation, as it will cause the aircraft to pitch up farther and further exceed the critical angle of attack. The pilot must lower the nose of the aircraft in order to get out of the stall.
A secondary stall occurs when the pilot is too quick to recover. The pilot will successfully decrease the angle of attack to unstall the wing, but they will be to aggressive on the controls post recovery, causing another stall. Being swift but smooth is essential to a proper stall recovery.
Remember, forward on the controls to reduce the angle of attack, and full power to increase the airspeed. Keep all inputs smooth to avoid the risk of a secondary stall.
That's it for stalls! The next article will cover wake turbulence.
A secondary stall occurs when the pilot is too quick to recover. The pilot will successfully decrease the angle of attack to unstall the wing, but they will be to aggressive on the controls post recovery, causing another stall. Being swift but smooth is essential to a proper stall recovery.
Remember, forward on the controls to reduce the angle of attack, and full power to increase the airspeed. Keep all inputs smooth to avoid the risk of a secondary stall.
That's it for stalls! The next article will cover wake turbulence.