BASIC STALLING

LESSON OBJECTIVES:

  1. Understand the concept of stalls and the factors that affect it

  2. Enter and recover from a power-on/power-off stall

  3. Recognize the symptoms of an incipient stall

  4. Common mistakes in the manoeuvres

-Stall Definition and Stall Speed-

Lift is created when the air that flows over the upper surface of an airfoil is accelerated producing a low-pressure area that attracts the air of the high-pressure area in the bottom of the airfoil. As pilots, we can control two factors to alter lift production: airspeed and angle of attack. When airspeed or AoA increases, lift increases and vice-versa.
 

Unfortunately, both methods have a limit. Not only the aircraft has a speed limit called Vne (Never Exceed Speed) that if exceeded could cause structural damage. But the wings also have a limit on the maximum angle of attack that can be achieved to produce lift

Airfoil AoA effect.png

- When we increase the AoA we artificially increase the distance that the air flowing over the upper surface has to follow and shorten the bottom surface, maximising the pressure difference and thus the lift. 

- At low AoA, the airflow moves without any difficulty over the top of the airflow. In this state, the airflow is called laminar airflow. As the AoA increases, it becomes more difficult for the air to follow the curvature of the wing and it starts becoming turbulent, in this state, the airflow is called turbulent airflow.

 

- The point at which the laminar air turns into turbulence is called the transition point, and as the AoA increases, this point gets closer to the wing leading edge

- Finally, the AoA becomes so high that the air can no longer flow over the upper surface, the lift created reduces dramatically and the drag increases exponentially. This phenomenon is called aerodynamic stall and for most of the airfoils, the maximum AoA that can be achieved (critical AoA) is 16º

In summary, the higher the AoA, higher the lift, until one point called the critical AoA that when exceeded causes the stall, the critical AoA is constant, so the value of the CL (CLmax) at that AoA must be constant too:

- Remember the lift formula -> - Lift = 1/2dV² x CL x S

The values of density and wing area are constant, the only variables are the speed and the angle of attack.

LiftV² x CL

In S&L flight (Lift = Weight -> 1G), if the lift required is constant, then we can assume that there is a specific speed at which the critical AoA is reached, this speed is called the stall speed. All the factors that increase/decrease the lift required will increase/decrease respectively the Stall Speed

- Weight -> ↑ Weight -> ↑ Lift Required -> ↑ Stall Speed

- Power ->
In climbs or any high pitch attitude, the lift required is less than weight because the thrust vector inclined upwards do part of the job of the lift. ↑ Power -> ↓ Lift Required -> ↓ Stall Speed. As in descends, depending on the power used there are two types of stall, the power-on and power-off

- Flaps -> Flaps increase the value of CL for a fixed AoA and a fixed airspeed. ↑ Flaps Setting -> ↑ CLmax  -> ↓ Stall Speed

- Load Factor ->
LF is Lift/Weight in S&L equals 1 or 1G, but in a turn part of the lift is deflected to act as a centripetal force, and the lift required to maintain altitude increases. ↑ Load Factor -> ↑ Lift Required -> ↑ Stall Speed

- Turbulence ->
When flying in turbulent conditions, you may find that the wing may stall at higher speeds since upwards currents change the relative airflow and then, the AoA of the airfoil. Here is a link to a video of a crash of a Junkers 52 due to this phenomenon (https://www.youtube.com/watch?v=jGF4ovuSrK0)

---- Note -> Stall speed is variable and depends and all these factors, but the critical AoA remains inalterable, which means that an aircraft can stall at any airspeed, but always at the same AoA ----

-Power-Off Stall-

Power-off stalls simulate a stall under low-powered conditions like during a normal approach to landing.  This type of incident could be fatal at low altitudes where there is not enough space for the aircraft to recover

Due to the nature of the stall, it is essential for the pilot to properly detect the problem and recover the aircraft control efficiently which means losing the less altitude possible. This is the reason why the stall is practised and taught to students with an instructor at a safe altitude from the ground

In this section, we will learn how to perform a power-off stall:

- Enter the stall

- Detect the stall

- Recover from a stall

--- Enter the stall---

Start at a safe altitude from the ground, 1500ft AGL (Above Ground Level) and perform the clearing turns that we saw in Lesson Nº6 - Basic Turns, to confirm clear of traffic.

With the aircraft in S&L flight, pick a reference to maintain a constant heading during the manoeuvre. We will simulate a normal landing

-- Reduce power to idle to perform a power-off stall.

-- Counteract the effect of the power reduction by increasing back pressure on the controls to maintain the altitude while the airspeed reduces. When airspeed is below Vfe (Flaps Extended Speed), extend flaps to full.

-- Keep maintaining altitude until approach speed is obtained, then trim the aircraft as necessary and initiate a descent

-- At a predetermined altitude, which would be our simulated runway, start increasing back-pressure on the controls to reduce the descent rate, and then continue applying back-pressure to keep the aeroplane from descending

-- You will notice the first symptoms of an impending stall, when the nose pitches down by itself, the aircraft has reached the full stall

-- The coordination is essential because if the aircraft is uncoordinated, one of the wings is at a higher AoA and could stall before the other wing causing a spin (we will discuss this concept in future lessons)

 

--- Detect the stall---

The symptoms of an incipient stall, a phase before the full stall, are:

-- Low airspeed, but remember "the aircraft can stall at any airspeed but only at the critical AoA", but in normal flying conditions the low airspeed can be considered a symptom

-- High pitch attitude, much more pronounced in power-on stalls than in power-off stalls

-- Buffeting in the controls due to the turbulence of the wings hitting the tail (More pronounced in low-wing aircraft compared to high-wing aircraft)

--- Recover from a stall---

Being recognized the stall, the next step is to recover the control of the aircraft with a minimum altitude loss.

 

-- As the cause of the stall is the exceedance of the critical AoA, the first step is to "break" the stall by reducing the AoA to normal values, forward-pressure must be applied in the control column to pitch down, so the wings start producing lift again and the drag caused by the turbulence is reduced

-- Hold your heading by the use of the rudder. Even if one of the wings stall before the other, do not use the ailerons to correct it, ailerons must be neutral during the stall. Remember that these control surfaces work by increasing/decreasing the AoA of the wings, if the critical AoA is exceeded or about to be exceeded, the ailerons going down on one of the wings will cause that wing to stall first causing a spin. 

-- With the aircraft in a nose-down attitude, the altitude will decrease rapidly if not corrected. Full power must be applied while back-pressure is increased to reduce the altitude loss and regain the S&L attitude. Remember to apply right rudder to counteract the left-turning tendencies produced by the power (Do not increase back-pressure abruptly or very soon in the recovery or you could end up triggering a secondary stall while recovering from the first one)

-- As the aeroplane starts accelerating retract the first notch of flaps. The lift decrease will be minimal but the drag decrease will be substantial and will increase your climb performance

-- Establish a specific airspeed for the climb (Vx or Vy). As we are simulating a power-off stall at low altitudes, it is supposed that obstacles could be something that in a real situation would be a problem, so Vx should be used

-- At a safe altitude (400 ft above the altitude at which our imaginary landing was), clean the aircraft and regain S&L

Doing this exercise, we have simulated in a safe environment, how we must handle a stall with the aircraft in landing configuration

-Power-On Stall-

Power-on stalls simulate a stall under conditions where a high quantity of thrust is being used like during a normal takeoff and departure or a go-around.  ​

Again, it is essential for the pilot to properly detect the stall and recover the control as soon as possible to prevent the aircraft from losing excessive altitude

In this section, we will learn how to perform a power-on stall:

- Enter the stall

- Detect the stall

- Recover from a stall

--- Enter the stall---

Start at a safe altitude from the ground, 1500ft AGL (Above Ground Level) and perform the clearing turns to confirm clear of traffic.

Start with the aircraft in S&L flight, pick a reference to maintain a constant heading during the manoeuvre

-- Reduce power and increase back-pressure as we learn in Lesson Nº4 - Straight and Level to reduce airspeed while maintaining altitude. When airspeed below Vfe (Flaps Extended Speed), extend flaps to the T/O setting and keep reducing until you reach the rotation speed. Remember that the objective is to simulate a stall on take-off

-- Use the throttle to obtain full power. Remember to apply right rudder to counteract the left-turning tendencies produced by the power

-- Increase back-pressure continuously on the controls to increase the AoA of the wings until the critical AoA is reached.

 

--- Detect the stall---

 

The symptoms of an incipient stall are:

-- Low airspeed, but remember "the aircraft can stall at any airspeed but only at the critical AoA"

-- High pitch attitude, much more pronounced in power-on stalls than in power-off stalls

-- Buffeting in the controls due to the turbulence of the wings hitting the tail (More pronounced in low-wing aircraft compared to high-wing aircraft)

--- Recover from a stall---

Being recognized the stall, the next step is to recover the control of the aircraft with a minimum altitude loss.

 

-- Again, as in power-off stall, the first step is to "break" the stall by reducing the AoA to normal values, forward-pressure must be applied in the control column to pitch down, so the wings start producing lift again and the drag caused by the turbulence is reduced

-- The coordination is essential because if the aircraft is uncoordinated, one of the wings could stall before another causing a spin (we will discuss this concept in future lessons). If a wing drops do not use the aileron to pick it up

-- Check that full power is being applied and increase back-pressure to reduce the altitude loss, be careful not to cause a secondary stall. Remember to apply right rudder to counteract the left-turning tendencies produced by the power.

-- As opposite to power-off stalls, do not retract the flaps until you reach a safe altitude above our imaginary runway. Remember that now the flaps setting is in T/O configuration, so creating a high lift quantity for a small drag penalty

-- At a safe altitude (400 ft above the altitude at which our imaginary landing was), clean the aircraft and regain S&L

Doing this exercise, we have simulated in a safe environment, how we must handle a stall with the aircraft in the take-off or departure phase of the flight

-Common Mistakes-

Entry in the Stall:

  • Failure to maintain the aircraft coordinated

  • Do not properly configure the aircraft for the manoeuvre

  • Abrupt control movements

  • Failure to maintain the initial heading

Stall Recovery:​

  • Use of power first instead of reducing the AoA

  • Failure to maintain the ailerons neutral

  • Excessive nose-down input that causes an excessive airspeed gain and a higher loss of altitude

  • Excessive nose-up input to recover S&L flight causing a secondary stall