Aircraft stalls are one of the most misunderstood phenomena of aerodynamics amongst student pilots. In fact, even qualified pilots often struggle to truly understand the aerodynamic stall. Read on to dispel the myths and gain a solid understanding of this important concept.
Aircraft Stalls – what are they?
Let’s start by saying what an aircraft stall is not:
- An aircraft stall is not a loss of power, quitting of the engine, or other mechanical issue relating to the ability of the aircraft to produce thrust
- An aircraft stall is not the condition in which the airplane is flying slower than the published stall speed
The first statement may come as a surprise to many non-pilots. After all, everyone is familiar with the term stall as it relates to automobiles, where it does in fact imply that the engine has stopped unintentionally. An aircraft stall is a completely different concept to an automobile stall.
The second statement may equally confuse a novice pilot. He or she is likely to equate the concept of flying too slowly with stalling, and in fact the two concepts are related. A stall can be caused by flying too slowly, but to truly understand the topic, you must think about it differently.
So, what then is a stall?
“An aircraft stall occurs when the angle of attack of the wing(s) exceeds the critical angle of attack”
Angle of Attack
The angle of attack of any airfoil is defined as the angle made between the chord line of the wing and the relative airflow over the airfoil. In a standard fixed wing aircraft, the angle of attack of the wings increases as the nose pitches upwards, and decreases as the nose pitches downwards.
The critical angle of attack is the angle of attack which, if exceeded, results in the airflow over the airflow becoming detached from the airfoil, resulting in a dramatic or total loss of generated lift.
It is worth noting that the critical angle of attack of an aircraft wing is fixed by its design – it does not change as a result of airspeed, aircraft attitude, aircraft weight, center of gravity, or any other factor. The critical angle of attack of most airplane wings is around 17 degrees.
All airplanes I’ve ever encountered have published stall speeds for the aircraft in various configurations. If an aircraft stall is determined entirely by the angle of attack of the wings, what does this have to do with airspeed?
To understand the relationship airspeed and aircraft stalls, first consider an aircraft in straight and level flight. Because the aircraft is neither climbing nor descending, two forces must balance – lift and weight. In other words, the effects of gravity trying to pull the airplane towards the earth are exactly cancelled out by the wings generating lift and attempting to pull the aircraft up and away from the earth.
What happens when this airplane slows down? All other things being equal, the wings generate less lift as airspeed decreases. In this situation, the aircraft would begin to descend, because the lift force holding the airplane up against the force of gravity is reducing, while at the same time the effects of gravity remain constant.
However, the pilot of this aircraft wishes to maintain altitude, so what does he or she do? Increase elevator back pressure to raise the nose. What is happening here? The pilot is pitching the airplane up, which has the effect of increasing the angle of attack of the wings. A wing produces more lift for the same airspeed at a higher angle of attack, thus compensating for the loss of lift caused by the reduced airspeed. Level flight is maintained.
Now imagine that the airplane continues to slow, and the pilot continues to pitch the nose up to compensate and maintain level flight. Eventually, the wing will reach the critical angle of attack, and the stall is encountered. The stall speed is the airspeed at which this occurs.
In other words, we can consider the stall speed to be the airspeed at or below which, altitude can no longer be maintained. This is because the maximum amount of lift the wing can generate at the stall speed is not sufficient to overcome gravity. Increasing the angle of attack will take the wing beyond the critical angle of attack, which further (dramatically) reduce lift.
Got it? I hope so. If not, do not read on. Go back to the beginning and start again until you do.
What happens during the stall?
As previously stated, the airflow separates from the wing, causing a dramatic reduction in lift. In a stable, well designed aircraft, this results in the nose dropping suddenly as the lift falls away (sometimes described as the break).
Think about what happens to the angle of attack during the break as the nose drops. The angle of attack of the wing reduces. This is sometimes enough to get the airplane out of the stall condition, or at least reduce its severity, without further control inputs from the pilot.
In well designed trainer aircraft being operated within the acceptable weight and balance envelope, the aircraft stall will be gradual, rather than a sudden break, because the wings are carefully designed to stall from the fuselage first, out towards the wingtips. Think of these wings as having a designed in ‘twist’, so the angle of attack is highest at the root and gradually becoming lower towards the wingtip. This has two beneficial effects. The first is that the wing stalls gradually, allowing the pilot time to notice that a stall is developing, and to react appropriately before a full stall is encountered. The second effect is that, during a partial stall, the ailerons remain (somewhat) effective, because being at the wingtip means that they will be the last surfaces to stall as the full stall develops.
The aim of stall recovery is quite straightforward: maintain coordinated flight, reduce angle of attack, increase airspeed, and recover to straight and level flight at a safe altitude.
Maintaining coordinated flight is important, otherwise a stall can quickly develop into a spin, which requires much more altitude and pilot skill to recover from. Reducing angle of attack and increasing airspeed both restore lift.
The actual technique for achieving effective aircraft stall and spin recovery is best left to a qualified flight instructor, and so I won’t go into the details here.
Factors affecting the stall speed
We have learned so far that an aircraft stall occurs when the angle of attack of the wing is at or above the critical angle of attack, so the following statement may surprise you:
“Aircraft stalls may occur at any airspeed and at any aircraft attitude”
The key to understanding this is to remind yourself that the critical angle of attack is defined in terms of the relative angle between the wing and the airflow, rather than in terms of the attitude of the airplane relative to the earth.
Here’s an example. Imagine an airplane with a critical angle of attack of 17 degrees, with a 20 degree nose-up attitude relative to the horizon. Is the aircraft stalled? The answer is, it depends! What is the relative direction of travel between the air and the airplane? We don’t know, and so it is impossible to tell if the aircraft is stalled or not.
Now imagine that same aircraft flying with its nose pinned to the horizon. The groundspeed of the aircraft is 100 knots. Is the aircraft stalled? Again, it is impossible to tell, without knowing the relative direction of travel between the air and the airplane!
The configuration of the aircraft has an effect on stall speed, often in a beneficial way. For example, extending the flaps reduces the stall speed. This is an intended effect – it allows the aircraft to generate more lift for a given airspeed (at the expense of additional drag).
There are also factors which can work against you to increase the stall speed, most notably the load factor of the wing. This can lead to what is known as an accelerated stall – a stall that occurs at a higher airspeed than the published stall speed. I won’t go into a full explanation of load factor and accelerated stalls here, but the following example is illustrative of the effect.
Imagine an aircraft in straight and level flight at a constant airspeed of 100 knots, well above the published stall speed of 47 knots. The wind is calm. In this situation, lift and weight are equal and opposite, and the angle of attack is well below the critical angle of attack.
Now imagine that the aircraft banks to the left and begins a left turn. Has anything changed to upset the balance between lift and weight? Yes! Because the aircraft is now banked to the left, the lift generated by the wings is no longer pulling the airplane directly upwards – it is being offset to the left by the bank angle. So we have the same amount of total lift, but some of it acting upwards and some of it is pulling the aircraft to the left. Therefore, the amount of lift now acting against the effects of gravity is less than it was before the aircraft banked. Without further intervention, the aircraft would descend.
However, wanting to hold altitude, the pilot applies elevator back pressure to raise the nose, increasing the angle of attack and hence total generated lift. What is the net result? The aircraft is in level flight, turning to the left, at the same airspeed as it was before, but closer to the critical angle of attack. It should be obvious by now that the stall speed is increased in a turn, because some of the total amount of lift available is being diverted away from maintaining altitude and instead being used to turn the plane. More angle of attack is needed to maintain altitude for a given airspeed. Therefore, as the airplane slows, we will reach the critical angle of attack at a higher airspeed than we would in straight and level flight.
The effect of load factor can be dramatic. For example, the load factor in a turn involving a 60 degree bank is +2. This means that the wing is acting against forces twice as high as it would normally encounter in straight and level flight. The stall speed of an aircraft is increased by a factor of the square root of the load factor. So in our example, the normal stall speed of 47 knots will be increased by the square root of +2, resulting in a stall speed of 66 knots in a turn made with 60 degrees of bank.
Another factor that can affect stall speed is the center of gravity of the aircraft. The center of gravity can move backwards and forwards relative to its default position based on the quantity and distribution of mass in the aircraft (including fuel). As the center of gravity of the aircraft moves forward, the stall speed is increased. Conversely, the stall speed decreases as the center of gravity moves towards the aft of the aircraft. The explanation of this effect is beyond the scope of this article, but is related to the changing amounts of downforce required from the tail section to balance out the changing amounts of torque (resulting from the relative positions of the center of gravity and the center of lift) trying to pitch the nose down. More downforce from the tail section essentially adds to the weight of the aircraft that the wings see, meaning a higher angle of attack is required to maintain level flight for a given airspeed.
The danger zones
There are two critical phases of flight during which the pilot must be especially mindful of the risk of aircraft stalls – the two phases during which the airplane is both in slow flight and close to the ground. Takeoffs and landings are notoriously susceptible to stall risk due to the aircraft being flown close to the stall speed. The double whammy is that if a stall does occur during these phases, there may not be sufficient altitude to recover before striking the ground, often with tragic consequences. Be sure to discuss these dangers with your flight instructor, and develop a healthy respect for proper, coordinated flying technique when low and slow. Be especially careful on that base-to-final turn, where uncoordinated flight can quickly escalate into an accelerated stall and an unrecoverable spin into terrain.