Within the field of aviation safety and aircraft operation, the stall represents a critical aerodynamic event that commands a comprehensive understanding. This guide is designed to dissect the multifaceted factors contributing to an airplane stall, elucidate the underlying aerodynamic principles, and dispel prevalent misconceptions.
Note: For detailed information on the types of stalls and recovery procedures, please refer to the dedicated article : Stall entry and Recovery Procedures
Definition of Stall
In aviation, a stall is a condition in aerodynamics where an increase in the angle of attack leads to a reduction in the lift coefficient. This occurs when the critical angle of attack is exceeded, resulting in the airflow separating from the upper surface of the wing and a subsequent loss of lift. The critical angle is typically around 15 to 20 degrees but can vary based on the wing design.
The aerodynamics of stalling : Lift and Angle of Attack
To appreciate the intricacies of aircraft stalling, one must first understand the relationship between lift and the angle of attack. Lift is the aerodynamic force that is perpendicular to the oncoming flow of air, which allows an aircraft to ascend and remain aloft. It is primarily generated by the aircraft’s wings as they interact with the air.
The angle of attack (AOA) is defined as the angle between the chord line of the wing — an imaginary straight line from the leading to the trailing edge — and the direction of the relative airflow. The AOA is pivotal in determining the amount of lift generated by the wings. As the AOA increases, so does lift, but only to a certain extent.
At lower angles of attack, air flows smoothly over the surface of the wing and lift is increased. However, as the angle of attack continues to rise, the air can no longer flow smoothly. Once the air separates from the wing’s surface, the lift is suddenly reduced. This point of separation marks the critical angle of attack and is where the stall begins.
The critical angle of attack
The critical angle of attack is a term that signifies the precise point at which an airfoil (or wing) fails to produce the necessary lift for flight due to airflow separation. This angle is not arbitrary but is a distinct characteristic of each wing design and is usually determined during the aircraft’s certification process.
When an aircraft is in normal flight, the airflow over the wing’s surface is smooth and laminar. As the angle of attack increases, the lift generated by the wing also increases — up to this critical point. Beyond this angle, the smooth airflow is disrupted and becomes turbulent, causing the lift to decrease rapidly. This airflow separation induces a stall, regardless of the aircraft’s speed, attitude, or engine power.
It’s important to clarify that the critical angle of attack is not a fixed angle in terms of degrees, but rather a specific condition of airflow relative to the wing. Various factors such as weight, balance, airspeed, and even the shape and configuration of the wing can affect the angle at which a stall occurs. However, the critical angle of attack itself remains constant for a given wing design in a particular configuration.
Pilots must be acutely aware of the critical angle of attack because, unlike airspeed, it does not change with weight, altitude, or temperature. Aircraft are equipped with instruments that can help monitor the proximity to the critical angle, such as an angle of attack indicator, which is especially useful during maneuvers that require careful management of the AOA, such as takeoffs, landings, and steep turns.
Understanding the critical angle of attack is imperative for pilots, as exceeding it can lead to a stall at any altitude or airspeed. Proper training and experience in recognizing and respecting this aerodynamic boundary are fundamental to maintaining the safety of flight operations.
Remember : A stall is the result of exceeding the critical AOA, not of insufficient airspeed.
Factors affecting stall
The critical angle of attack
Weight and balance are pivotal elements in aircraft performance, particularly in how they influence the conditions for a stall. Central to this discussion is the center of gravity (CG), the point where the weight of the aircraft is considered to be concentrated. The position of the CG is critical because it affects the aircraft’s stability and control, especially in relation to the angle of attack and the onset of a stall.
When an aircraft is properly balanced, with the CG within the specified limits, it will have predictable stalling characteristics.
When the center of gravity shifts towards the rear (aft) of the aircraft
it affects how the plane responds to stalling. This aft CG means that the tail of the aircraft must work harder to generate lift to maintain balance. As a result, the plane can reach the critical angle of attack at a lower angle, making it susceptible to stalling sooner than if the CG were positioned more forward. Additionally, this rearward weight distribution can cause the aircraft to react more sharply to pilot control inputs and may complicate stall recovery efforts.
When the center of gravity shifts towards the front of the aircraft
it naturally tends to tip the nose downward. This forward positioning of the CG makes the plane less prone to stalling since it reduces the tendency to pitch up, which is a common precursor to a stall. However, should a stall occur, this same forward CG can make it more challenging to bring the nose down and recover because the aircraft is already predisposed to a nose-down attitude.
Maintaining the correct weight distribution is not only about avoiding stalls; it’s also about ensuring that the aircraft can be controlled effectively throughout its entire operating envelope. Overloading the aircraft, improperly distributing cargo, or failing to account for fuel consumption can shift the CG outside of its safe range, with serious implications for both stalling and general maneuverability.
Pilots must be diligent in calculating the weight and balance before every flight to ensure the CG remains within safe limits. This includes accounting for changes in fuel weight, cargo, and passenger distribution. Manufacturers provide specific weight and balance data for their aircraft, which includes the range of acceptable CG positions and the proper procedures for calculating and adjusting the CG.
Atmospheric Conditions: Turbulence, Heavy Rain, Snow, Frost, and Ice
Atmospheric conditions play a critical role in the performance and safety of flight operations. In the context of stalls, they can significantly alter the conditions under which a stall might occur and affect the pilot’s ability to recover from one.
Turbulence can disrupt the airflow around an aircraft’s wings, which can momentarily decrease lift and increase the stall speed. The erratic air movement means that the wings may not produce consistent lift, making it crucial for pilots to maintain higher speeds and smoother control inputs during turbulent conditions.
Snow, Frost, and Ice have an immediate impact on the wing’s shape and surface condition, disrupting the smooth flow of air necessary for lift. Ice formation can alter the camber and contour of the wing, leading to a significant loss of lift and an increase in drag. Even small amounts of frost can reduce lift by up to 30%, increasing stall risk, especially during takeoff or landing phases.
Heavy Rain can affect the boundary layer of the wing, the thin layer of air directly in contact with the wing surface.
In all these conditions, the importance of proper aircraft maintenance, pre-flight inspections, and the use of de-icing equipment cannot be overstated. Pilots must be trained to recognize the onset of these conditions and react accordingly, often including adjustments to speed, altitude, and configurations to ensure continued safe operation.
Aircraft Configurations
Flaps
Aircraft configuration, particularly the position of the flaps, plays a pivotal role in stalling characteristics. Flaps are hinged surfaces on the wing trailing edges, and pilots deploy them to increase the aircraft’s lift during critical phases of flight, such as takeoff and landing. While the primary function of flaps is to enhance lift at lower speeds, their impact on stall behavior is significant and multifaceted.
When extended, flaps change the camber of the wing, increasing its curvature and surface area. This alteration allows the wing to generate more lift at lower speeds, which is advantageous during takeoff and landing. However, the increased camber also changes the airflow over the wing, affecting the critical angle of attack. With flaps extended, an aircraft can fly at a higher angle of attack without stalling compared to when the flaps are retracted. This is because the enhanced camber improves the wing’s ability to produce lift even with a more significant airflow disruption.
Turns and Load Factors
Turning flight is a critical condition under which aircraft stalling characteristics can change dramatically due to the associated load factors.
When an aircraft enters a turn, the load factor—essentially the amount of force exerted on the structure of the aircraft—increases. This is due to the centrifugal force that acts upon the aircraft as it changes direction, which effectively increases the weight that the wings must support to maintain lift.
The increased load factor requires that the wings produce more lift to sustain level flight during a turn. To achieve this additional lift, the pilot must increase the angle of attack by pulling back on the control yoke. This increase, however, brings the wing closer to its critical angle of attack and therefore closer to a stall condition, particularly in steep turns where the load factor is significantly higher.
Moreover, an increased load factor also raises the stall speed of the aircraft. In straight and level flight, an aircraft may stall at a predictable speed, but when subjected to increased load factors in a turn, the stall speed will be higher. This phenomenon means that a maneuver that is safe at a certain speed in level flight may induce a stall if performed in a turn at the same speed.
The aerodynamic load on an aircraft’s wings increases proportionally to the square of the load factor, a dimensionless coefficient representing the ratio of the lift to the weight of the aircraft. In a level turn, the load factor is greater than one, which increases the stalling speed.
For instance :
- Stall speed (Vs) : 54 knots (in straight and level flight)
- Degree of bank : 60º
- Load factor : 2
Sqare of foot of 2 is 1.41
54 x 1.41 = 76.14
In a 60º banked of turn the stalling speed would be 76.14 knots.
Stall warning systems and indicators
In modern aviation, stall warning systems are essential safety features that alert pilots of an impending stall. These systems are designed to provide early warnings, allowing for corrective action before a stall occurs. The two primary types of stall warnings in aircraft are auditory and tactile systems.
Auditory Stall Warnings: These are the most common and are typically heard as a loud, distinctive horn or a synthetic voice alerting the pilot of a potential stall. The system is activated when the aircraft approaches the critical angle of attack, well before the actual stall.
Tactile Stall Warnings: Some aircraft are equipped with a stick shaker system, which physically vibrates the pilot’s control yoke (or stick) as an unmistakable tactile cue that the aircraft is nearing a stall.
Flashcards
What is an airplane stall?
An airplane stall is a reduction in lift that occurs when the wing exceeds its critical angle of attack, disrupting airflow and decreasing lift.
What is the critical angle of attack?
The critical angle of attack is the angle at which the wing can no longer produce enough lift to support the airplane, leading to a stall.
What are some factors that affect stall?
Weight, center of gravity, atmospheric conditions (turbulence, weather), aircraft configurations (flaps, gear), and turns.
How does weight affect stall speed?
Increased weight raises stall speed as more lift is needed to support the aircraft.
How does angle of attack affect lift?
As the angle of attack increases, lift initially increases up to a point. Beyond this point, if the angle continues to increase, lift diminishes, leading to a stall.
How does turning affect stall speed?
In a turn, the stall speed increases due to the increased load factor. The steeper the turn, the higher the stall speed.
What is the effect of the center of gravity on stall behavior?
- A forward CG increases stall speed but typically results in a more gradual stall.
- A rearward CG decreases stall speed but can lead to a more abrupt stall.
How do flaps affect stalls?
Extending flaps usually lowers the stall speed because they increase the wing’s lift coefficient. However, deploying flaps changes the wing’s camber and can affect the stall pattern.
How do environmental conditions affect stall risk?
Conditions like turbulence, heavy rain, and winds can affect airflow over the wings and may either mask the onset of a stall or exacerbate its severity.
What's the difference between power-on and power-off stalls?
Power-on stalls occur with engine power applied, typically during takeoff or climb. Power-off stalls happen with reduced or idle engine power, usually during approach or landing.
What is a cross-control stall, and when is it likely to occur?
A cross-control stall occurs when a pilot applies opposite aileron and rudder during a turn, disrupting the balance of lift across the wings. It’s most likely to occur during a poorly executed turn onto final approach, where the pilot is trying to align with the runway.
Why are cross-control stalls especially dangerous?
Cross-control stalls can lead to a spin if not corrected promptly, especially at low altitudes where there is insufficient height to recover.
What is a secondary stall?
A secondary stall is one that occurs during the recovery phase from an initial stall. It happens if the pilot attempts to raise the nose too quickly or aggressively without sufficient airspeed, causing a second stall.
What is aerodynamic buffeting?
Buffeting is a warning sign of an impending stall, characterized by vibrations felt from turbulent airflow as the aircraft approaches the critical angle of attack.
Under VFR, how can pilots minimize the risk of stalls?
By maintaining proper airspeed, avoiding abrupt maneuvers, and keeping a vigilant watch on the angle of attack and airspeed indicators.
How often should stall training be conducted?
Stall training should be a regular part of a pilot’s recurrent training program to ensure proficiency in recognizing and recovering from stalls under various conditions.
