The stall is a condition of flight in which the angle of attack of the wing exceeds its critical (stalling) angle; it is also the angle at which the lift ceases to increase. In steady, level flight, any one aerofoil will always stall at the same angle of attack, but the speed at which it stalls depends on a number of factors. When a certain angle of attack is reached, there is a breakdown—an often sudden separation of airflow resulting in a loss of lift. In the aircraft, we associate airspeed with stalling, because the manufacturer doesn’t provide an instrument for angle of attack. In level flight, the weight of the aircraft is balanced by the lift. From the lift formula, it can be seen that the lift decreases whenever any of the factors that make up the formula are decreased, and vice versa.
In level flight, if the engine is throttled back (throttle is closed), drag immediately causes a reduction in speed and, therefore, a reduction in lift. To keep the lift constant as the speed reduces, some other factor of the lift formula must be increased. For practical purposes, surface area remains constant; therefore, the only remaining factor that is variable is the lift coefficient (CL). The CL can be most readily increased by increasing the angle of attack. By doing so, the lift can be restored to its original value so that level flight is maintained. Any further reduction in speed necessitates a further increase in angle of attack. Each successive lower indicated airspeed (IAS) corresponds to a higher angle of attack. Eventually, at a certain IAS, the wing reaches its stalling angle. Beyond that point, any further increase in angle of attack in an attempt to maintain lift will result in a stall. Since there is no instrument fitted to measure the angle of attack, the only indication of the approaching stall is the airspeed indicator. The speed at which the stall occurs is known as the stalling speed.
Often aircraft are fitted with ‘washout‘. This is a gradual reduction in the angle of incidence, from the wing root, to the wing tip. This ensures that the wing root section of the wing stalls first, while the wing tip section is still ‘flying’. The pilot is still able to control the aircraft around the longitudinal axis as the ailerons are not yet affected by the stalled wing tip. The pilot can then hopefully recover the aircraft. If the wing design didn’t include washout, the whole wing would stall at the same time, and the pilot may lose control of the aircraft and be unable to roll the aircraft back to wings level.
The symptoms are:
Beyond the stall, lift is less than weight and the aircraft sinks. The centre of pressure moves rapidly rearwards, and the nose of the aircraft pitches down.
The above image depicts the airflow over the wing as well as the associate forces and their movement with an increase in angle of attack. Let’s go through it.
A: Here we have an aircraft travelling at the optimum angle of attack of 4°, cruising at 110 kt. Lift is of equal strength to weight but not in direct opposition to it, hence creating a couple moment, which pitches the aircraft down. Weight acts down towards the centre of the earth through the centre of gravity. You can see that the airflow up over the wing remains a nice laminar flow, creating little drag—represented by the drag vector at the end of the wing. This airflow creates what is known as a lift pressure envelope, as depicted above. The lift vector line simply summarises this lift envelope to act through a single point known as the centre of pressure.
B: Now we have the same aircraft slowing down to 80 kt. In order to maintain constant lift, the angle of attack is increased to 8°. The airflow travelling up over the wing has to travel further and deviate, effectively using up some of the energy, and can no longer maintain a laminar flow over the entire wing. The airflow will break away and become turbulent, thereby increasing drag. The lift envelope is modified and therefore so is the centre of pressure—it actually moves towards the leading edge. The position of the centre of gravity remains the same; the arm between the centre of pressure and centre of gravity is getting smaller, creating a smaller couple moment. Lift and weight are still of equal strength.
C: Now we have the same aircraft slowing down further to 44 kt. Again, in order to maintain constant lift, the angle of attack is increased to 15°, which is the critical angle of attack. The airflow travelling up over the wing has to deviate even further, effectively using up the energy, and can no longer maintain a laminar flow from the point of maximum camber rearwards (meaning the aircraft has stalled and the airflow is all turbulent), increasing drag even further. As a result of the airflow being unable to adhere to the surface, the lift envelope collapses, moving the centre of pressure rapidly rearwards. This creates a larger arm between the centre of pressure and centre of gravity and creates a larger couple moment. Lift has diminished and is now less than weight, resulting in loss of altitude.
It’s important to understand that, at the stall, lift reduces rapidly and drag increases rapidly. Looking at the graph below, you can see that this is in fact what happens from the critical angle onwards.
Below is another image depicting what happens to an aircraft during a stall. Lift remains constant until the aircraft stalls, but coefficient of lift is increased approaching the stall to keep lift constant.
To summarise the above image:
The single most critical aspect to recover an aircraft from a stall is to reduce the angle of attack. Let’s have a look at the coefficient of lift graph below.
We can divide the above graph into two phases of flight: normal flight and stalled flight. We know that when an aircraft stalls, it is because it has exceeded the critical angle. From the graph above, we can see that in order to recover the aircraft and get back to the normal phase of flight, we need to reduce the angle of attack. We do this by checking forward on the control stick/column. When an aircraft does stall, lift is less than weight, so height will be lost. For safety, pilots want to reduce this height loss as much as possible. To do this, the application of power is used to stop the aircraft from descending.
Increased weight causes a higher stall speed, since for any angle of attack—including the stalling angle of attack—a higher airspeed is necessary to provide the extra lift to balance the increased weight of the aircraft. In effect, the stall is reached sooner.
By increasing the camber of the aerofoil, flaps provide a higher CLMAX, and the result is a lower stall speed. In effect, the stall is delayed.
The use of power enables a lower stall speed, because the slipstream from the propeller passing over part of the wing tends to eliminate the onset of turbulent airflow on that section of the wing. Clearly, this isn’t a consideration for aircraft with aft-mounted or jet engines. Also, if the nose is inclined upwards, then a portion of the thrust will be contributing to the lift, again delaying the stall.
The effect of loading (apparent weight), as in a turn, will have the same effect as increased weight, so once again, the stall speed will be higher.
Icing or Damage
Since an accumulation of ice or damage will normally upset the smooth airflow over the wing, the stall speed will be higher. With the air breaking away sooner, this encourages an earlier stall. An accumulation of clear ice will have the effect of increased weight.
If all the factors remain constant, the aircraft stalls at the same indicated airspeed at all altitudes.
A wing drop stall is the situation where one wing stalls before the other does, and it drops due to the lift imbalance.
Let’s have a look at some of the reasons why a wing drop stall occurs.
Weight Imbalance: This can occur when we have an uneven fuel load from the left to right wing tank.
Ice and/or Damage: Unequal ice build-up and/or damage can change the shape of the aerofoil, which can lead to one wing stalling before the other does.
Turbulence: The changes in vertical velocities of the air in turbulence can cause one wing to stall prior to the other. Also, the use of aileron near the stall to counter a wing drop can result in the wing with the down-going aileron having an increased angle of attack.
Rigging: Rigger’s angle of incidence*. If one wing has been rigged at a larger angle of attack than the other has, then this wing will tend to stall first.
*The angle of incidence is the angle formed between the chord line and the longitudinal axis (fore–aft datum line) of the aircraft. It is also known as the rigger’s angle of incidence—literally, the angle at which the wings are attached to the aircraft’s fuselage.
Power/Slipstream: An increase in power approaching a stall causes the slipstream to strike down on the starboard wing and upwards on the port wing. This in effect increases the angle of attack on the port wing while decreasing the angle of attack on the starboard wing.
Pilot Induced: Flying the aircraft out of balance means that one wing will travel faster than the other will. The other wing will be lower and travelling slower with a higher effective angle of attack, causing it to stall first. The use of aileron near the stall will have a similar effect, as already discussed.
The recovery for a wing drop stall is similar to that for a normal stall but there are some important differences that you must note. The recovery procedure for a wing drop stall is as follows:
Note: Procedures 2–5 are performed simultaneously.
A recovery from the onset of a stall refers to recovering the aircraft before a full stall has taken place, meaning the stall warning or buffet. Recovering here will significantly reduce the height loss that the aircraft would lose should a full stall occur.
The fact that one wing may drop at the point of stall, either intentionally or otherwise, is the basic cause of a spin. When this occurs, the angle of attack on the down-going wing is increased, which aggravates the stalled condition of that wing. The angle of attack on the up-going wing is reduced, thereby tending to decrease its stalled condition. Combined with the above, the accentuated stalled condition of the down-going wing is accompanied by a corresponding increase in drag, which will yaw the nose towards the down-going wing. The result is a stabilised condition known as an autorotation.
Note: The longer the rudder’s arm, the more effective the rudder will be in the recovery.
Autorotation can be categorised into:
Auto roll: when the down-going wing is more deeply stalled, i.e., there is less lift than there is under the opposite wing; this is a condition where the wing keeps dropping and the aircraft continues to roll.
Auto yaw: when the down-going wing causes more drag and continues to yaw in the direction of the roll.
A spin is a stalled condition of flight, developing from autorotation, in which the aircraft follows a spiral descent path about a vertical axis. There are three main stages of a spin: incipient, fully developed, and recovery stage. Incipient stage is the beginning of a spin, i.e., autorotation. Once established in a spin, the aircraft is in the fully developed stage and the aircraft is stalled. It is rolling, yawing, and pitching rapidly and losing altitude at a low airspeed. The only way to recover the aircraft is to perform the recovery procedure. A spin can be avoided by not letting a wing drop during a stall. This isn’t to say that a wing-drop stall will lead to autorotation. But this is the danger of using aileron near, during, and in the recovery of a stall. It can induce a wing-drop stall, leading to a spin.
You should have a good working knowledge of the forces in a spin. Your instructor will explain the image below.
(Image courtesy of CAA; used with permission)
CAUTION: Gyroscopic Instruments may topple and, therefore, become unreliable.
There are two instruments that remain reliable in a spin:
1) The airspeed indicator (low and fluctuating airspeed)
2) The turn coordinator (will indicate the direction of the spin)
The best way to avoid spins is prevent them! Knowing which flight conditions can induce a spin can save you from entering one in the first place.
The following flight conditions can induce a spin:
There is a great publication released by the CAA on spinning. Click here to download it.
The standard steps for spin recovery are:
Discuss the following questions in class now.