Subsonic Aerodynamics | The Stall | PPL(A) Principles of Flight

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What is a Stall?

A stall occurs when the wing exceeds its critical angle of attack, causing the boundary layer to separate from the upper surface. The airflow can no longer follow the wing’s curvature — it detaches, forming vortices behind the wing.

The aerodynamic consequences are immediate:

• Lift drops sharply (cL decreases)
• Drag rises sharply (CD increases)
• Smooth airflow over the wing is lost
• On a rectangular wing, the centre of pressure moves aft, producing a nose-down pitch moment — this is the aircraft’s built-in self-recovery tendency, pitching the nose down to reduce angle of attack
• Buffet, reduced control effectiveness, and major performance loss follow

Critical Angle of Attack — The Only Cause

A stall is always caused by exceeding the critical angle of attack — not by low airspeed alone. A stall can occur at any speed, any attitude, and any configuration if the critical angle is exceeded.

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Boundary Layer Separation and Vortex Formation

As the angle of attack increases toward the critical angle, the boundary layer becomes progressively more turbulent and struggles to remain attached to the upper surface. Once the critical angle is exceeded:

• The boundary layer detaches completely from the upper surface
• Separated airflow forms vortices behind the wing
• Vortices dissipate energy through turbulence, increasing drag significantly
• Lift decreases as the separated airflow can no longer generate aerodynamic efficiency
• The result is buffet, reduced control authority, and major performance loss

Strong separation → strong vortices → higher drag → lower lift

Laminar vs Turbulent Boundary Layers

There are two types of boundary layer, and their behaviour near stall is very different:

Type	Friction drag	Separation resistance
Laminar	Low	Poor — separates easily
Turbulent	Higher	Good — resists separation

Turbulent boundary layers resist separation better because the irregular motion mixes high-energy air from the free stream with the slower air near the surface. This mixing counters the adverse pressure gradient that causes separation. Laminar layers lack this energy exchange and separate more easily under the same conditions.

Artificially Induced Turbulence

Turbulence is sometimes deliberately induced on surfaces such as vertical fins and ailerons. By tripping the boundary layer from laminar to turbulent, separation is delayed — improving control surface effectiveness near stall.

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Warning Signs of an Impending Stall

As the wing approaches the critical angle of attack, several warning signs appear before the full stall:

• Aerodynamic buffet — turbulent separated airflow strikes the airframe and tail, causing vibration. This is the earliest and most reliable physical warning
• Reduced control surface effectiveness — control inputs fail to produce the expected response; the aircraft becomes sluggish
• Inability to maintain the flight path — the aircraft deviates from the intended trajectory despite control inputs
• Stall warning system activation — aural and/or visual alert in the cockpit

Stall Warning Systems

Stall warning systems on piston aircraft respond to changes in angle of attack and leading-edge pressure. As the angle of attack approaches the critical value, the stagnation point migrates downward along the leading edge. This movement triggers a flapper switch or suction sensor, closing a circuit and activating cockpit alerts.

• Stagnation point moves downward with increasing angle of attack
• Flapper or suction sensor closes an electrical circuit
• Aural and/or visual alert activates in the cockpit
• System must be tested on the ground before flight

Icing and Stall Warning

Ice can freeze stall warning flapper switches, eliminating the cockpit warning entirely. Ice also increases weight and distorts the wing profile, raising stall speed — so the aircraft may stall with no warning and at a higher speed than expected.

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Stall Recovery

The only way to recover from a stall is to reduce the angle of attack below the critical value, restoring attached airflow and lift.

1. Reduce angle of attack — push the control column/stick forward to unstall the wings. This is always the first and non-negotiable action — nothing else works until the wing is flying again

2. Simultaneously add power and level the wings — apply full power to arrest the descent and accelerate recovery; roll wings level to eliminate the load factor increase caused by bank, which would otherwise keep stall speed elevated

3. Allow airspeed to build — with the nose lowered, wings level, and power applied, the aircraft accelerates back through stall speed

4. Recover from the dive — once flying speed is regained, ease back smoothly to return to the desired flight path, avoiding excessive g

High Bank Angle Recovery

In a high bank angle stall, the bank must be removed simultaneously with reducing AoA — not after. The high bank increases load factor, which raises stall speed. Pulling back while still banked deepens the stall and risks entering a spin.

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Where Does the Stall Start? — Wing Planform

A stall occurs when the local lift coefficient (cL_local) reaches cLmax at any section of the wing. Where this happens first depends on the wing’s planform:

Rectangular Wing

On an untwisted rectangular wing, the stall begins near the wing root. This is because:

• The wing-fuselage junction disturbs the airflow, making the root section more prone to separation
• Root stall is the desirable design outcome — the ailerons (located near the tips) remain in attached airflow and continue to function, giving the pilot roll control throughout the stall

Tapered and Swept Wings

On tapered or swept-back wings, the stall tends to begin near the wingtips because:

• Tapered wing tips have a smaller chord — the shorter chord means the boundary layer is thinner and airflow separates sooner
• On swept wings, spanwise flow migrates toward the tips, raising the local angle of attack at the tips
• The thicker boundary layer that accumulates at the tips on swept wings further encourages early separation

Tip stall is aerodynamically undesirable because the ailerons lose effectiveness at the point of stall — the pilot loses roll control precisely when it is most needed.

Washout (Wing Twist)

To counteract the tip-stall tendency of tapered and swept wings, designers use washout — a gradual reduction in angle of attack from root to tip (negative wing twist). Washout ensures the root section reaches the critical angle before the tip, preserving aileron effectiveness.

Aileron Input Near Stall

Near the stall, sudden aileron deflection can stall one wing before the other. The down-going aileron increases the local angle of attack on that wing section, potentially pushing it past the critical angle while the other wing is still flying. This asymmetric stall can initiate a spin.

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Factors Affecting Stall Speed

The stall speed formula shows exactly which variables determine when a stall occurs:

VS = √[ 2Wn ÷ (ρ × S × cLmax) ]

Symbol	Meaning
VS	Stall speed (m/s)
W	Aircraft weight (N)
n	Load factor (1 in level flight; increases in turns and manoeuvres)
ρ	Air density (kg/m³)
S	Wing area (m²)
cLmax	Maximum lift coefficient

Weight

Stall speed increases with weight. Stall occurs when lift equals weight — a heavier aircraft requires a higher airspeed to generate sufficient lift at the critical angle of attack.

• Vstall ∝ √W — stall speed is proportional to the square root of weight
• Reducing weight (burning fuel, offloading) lowers stall speed
• Full fuel tanks increase weight → higher stall speed → at a normal climb speed, the aircraft may be closer to stall than expected

Altitude

As altitude increases, air density (ρ) decreases. To generate the same lift, the aircraft must fly faster in True Airspeed (TAS). However:

• Indicated stall speed remains constant — the ASI measures dynamic pressure, which accounts for density. The indicated reading at stall is the same regardless of altitude (assuming no compressibility effects)
• TAS at stall increases with altitude — the aircraft is physically moving faster through the thinner air

Centre of Gravity (CG)

A forward CG shifts weight toward the nose, causing a nose-down pitch tendency. To maintain level flight, a higher angle of attack is required — bringing the wing closer to the critical angle. This raises stall speed slightly.

Power Setting

Power affects stall speed through propwash:

• Power on — propwash increases airflow over the wing, allowing the wing to generate sufficient lift at a lower airspeed → lower stall speed
• Power off (idle) — no propwash benefit; more airspeed is needed to generate lift → higher stall speed

A climb-power stall occurs with high power and a steep pitch attitude. The indicated airspeed is lower than in a level-flight stall, but the pitch attitude is higher — the aircraft can stall in a steep climb at a speed that would normally be safe in level flight.

Load Factor

In manoeuvres, the wings must support more than just the aircraft’s weight — the effective load increases. Load factor (n) is defined as:

n = Lift ÷ Weight = 1 / cos φ (in a coordinated turn, where φ = bank angle)

Stall speed rises with the square root of the load factor:

V_stall(new) = V_stall(original) × √n

Example — 60° banked turn:

• n = 1 / cos 60° = 1 / 0.5 = 2
• √2 ≈ 1.41
• If clean stall speed = 60 kt → stall speed in the turn = 60 × 1.41 ≈ 85 kt

Stall Speed in Turns

In a 60° banked turn, load factor doubles and stall speed increases by 41%. A pilot accustomed to a stall speed of 60 kt in level flight may be surprised to stall at 85 kt in a steep turn. This is a significant cause of loss-of-control accidents in the circuit.

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Stall in Specific Situations

Climbing Turn

In a climbing turn, two effects combine to increase stall risk:

• Load factor increases — the wings must support more than the aircraft’s weight, requiring a higher angle of attack
• Airspeed may be lower — the climb reduces airspeed, further reducing the margin above stall
• Asymmetric stall risk — one wing may stall before the other, potentially leading to a spin

Steep Turn

In a steep turn, the higher load factor raises stall speed significantly. If one wing stalls before the other due to uneven lift distribution, the aircraft can roll rapidly toward the stalled wing and enter a spin.

• Steep turn → higher load factor → higher stall speed
• Asymmetric lift at stall → spin risk

T-Tail Aircraft

In a deep stall on a T-tail aircraft, the turbulent separated airflow from the stalled wing rises and engulfs the horizontal stabiliser and elevator. The elevator loses effectiveness, making pitch control extremely difficult and stall recovery much harder. This is a known hazard of T-tail configurations.

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Environmental Factors

Icing

Ice contamination is one of the most dangerous stall-related hazards:

• Ice distorts the wing profile, reducing cLmax and raising stall speed
• Ice increases weight, further raising stall speed
• Ice can freeze stall warning switches, removing the cockpit warning
• The stall may occur at a significantly higher speed than placarded, with no warning

Heavy Rain

Heavy rain primarily increases drag and reduces performance:

• Raindrops disrupt the boundary layer, raising profile drag
• Can reduce visibility and affect sensors or avionics
• Does not directly cause major lift loss, jam control surfaces, or shift CG significantly

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Asymmetric Stall and the Spin

How an Asymmetric Stall Develops

An asymmetric stall occurs when one wing stalls before the other. This can be triggered by:

• Sudden aileron input near stall speed — the down-going aileron increases the local angle of attack on that wing section, stalling it while the other wing continues to fly
• An uneven upward gust — one wing experiences a greater increase in angle of attack than the other
• A banked attitude at low speed — the lower wing may be at a higher angle of attack

The wing that stalls loses lift while the other continues to generate it. This lift imbalance causes the aircraft to roll and yaw toward the stalled wing — the entry condition for a spin.

The Incipient Spin

The incipient phase is the period immediately after entering a spin, before full rotation develops. During this phase:

• One wing is stalled, the other is not
• The aircraft rotates around its vertical (normal) axis
• Altitude and airspeed are being lost
• Recovery is easiest at this stage — before the spin becomes fully developed

Spin Recovery

1. Reduce angle of attack — push the control column/stick forward to unstall both wings

2. Apply opposite rudder — full opposite rudder to stop the yaw driving the rotation

3. Neutralise rudder — once rotation stops, centralise the rudder

4. Recover from the dive — ease back on the controls to return to level flight, avoiding excessive g

Spin Awareness

Lack of spin training can lead to startle and delayed recovery. The instinctive reaction — pulling back — worsens the spin by increasing the angle of attack. The correct first action is always to push forward to reduce angle of attack, even though this feels counterintuitive close to the ground.