4.3 Stall Physics, Critical Angle of Attack & High-Lift Devices

4.3 Stall Physics, Critical Angle of Attack & High-Lift Devices

An aerodynamic stall is one of the most critical phenomena in fluid dynamics and flight safety. EASA Part-66 Module 08 requires aircraft maintenance personnel to possess a thorough understanding of stall physics, boundary layer separation, variables altering stall speed, and the mechanical and aerodynamic principles of high-lift systems.

---

1. Mechanism of an Aerodynamic Stall

Lift is generated by producing a lower static pressure on the upper surface of the airfoil relative to the lower surface. As angle of attack ($\alpha$) increases, airflow over the upper surface accelerates, increasing suction and raising the lift coefficient ($C_L$).

  Low Angle of Attack (Laminar Airflow)    High Angle of Attack (Flow Separation / Stall)
       ==================>                      =========--~~~~~ (Turbulent Wake)
          /-----------\                            /-----------\  |
      --->(  Airfoil   )                  ----->  (  Airfoil   ) v Separation Point
          \-----------/                            \-----------/
       ==================>                      =================>

Adverse Pressure Gradient & Flow Separation

Beyond a certain angle of attack, air flowing along the upper surface must overcome an increasingly strong adverse pressure gradient (pressure rises from the point of maximum thickness toward the trailing edge).

1. At low angles of attack, boundary layer kinetic energy is sufficient to overcome this adverse gradient, keeping flow attached.
2. At high angles of attack, boundary layer friction depletes kinetic energy. The slow-moving boundary layer reverses direction, causing airflow separation from the upper surface.
3. As $\alpha$ reaches the Critical Angle of Attack ($\alpha_{\text{crit}}$), the separation point shifts rapidly forward toward the leading edge. The organized low-pressure vortex structure collapses into a turbulent wake, resulting in a sudden drop in lift coefficient ($C_L$) and a massive increase in pressure drag ($C_D$).

$\text{Stall Condition:} \quad \alpha > \alpha_{\text{crit}}$

Fundamental Rule for EASA Exams: An aircraft can stall at ANY airspeed, ANY pitch attitude, and ANY altitude if $\alpha_{\text{crit}}$ is exceeded.

In-Flight Warning Indicators

• Buffeting: High-energy turbulent vortices shedding from the stalled main wing strike the horizontal tailplane, causing mechanical vibrations felt in the airframe and control column.
• Sluggish Control Response: Depleted dynamic pressure over ailerons reduces roll effectiveness.
• Synthetic Warnings: Stick shakers, angle-of-attack vanes, and artificial audio horns trigger before reaching $\alpha_{\text{crit}}$.

---

2. Factors Influencing Stall Speed ($V_s$)

While $\alpha_{\text{crit}}$ is an intrinsic geometric property of a specific airfoil profile, the indicated stall speed ($V_s$) varies based on several operational factors governed by the stall speed equation:

$V_s = \sqrt{\frac{2 W}{\rho S C_{L\max}}}$

---

3. High-Lift Devices: Trailing Edge Flaps

Trailing edge flaps modify wing section geometry to increase maximum lift coefficient ($C_{L\max}$), allowing lower approach and landing speeds.

 Plain Flap               Split Flap             Slotted Flap            Fowler Flap
  \-----\                   \-----\                \-----\                 \-----\   =\-----\
       \                     \                      \---\                  \---  \       \
        v Deflection          v Plate                    v Slot & Gap           v Extended Gap

Comparison of Trailing Edge Flap Types

1. Plain Flap: Simple hinged rear wing section. Increases camber, moderate $C_{L\max}$ gain, increases drag substantially.
2. Split Flap: Plate deflected from lower surface only. Creates high profile drag with moderate $C_{L\max}$ gain; produces large pitch changes.
3. Slotted Flap: Features a high-pressure gap between main wing and flap. High-pressure air from beneath passes through the slot, re-energizing the upper boundary layer and delaying local separation.
4. Fowler Flap: Moves rearward along track guides before curving downward. Increases both wing surface area ($S$) and camber. Delivers the highest increase in $C_{L\max}$ with minimal initial drag penalty at partial deflections.

---

4. High-Lift Devices: Leading Edge Systems

Leading edge high-lift systems maintain attached airflow at extreme angles of attack.

 Leading-Edge Slot / Slat               Krueger Flap
    /---\    /------------\               /---\------------\
   ( Slat)  (  Main Wing   )             (  /  Main Wing   )
    \---/    \------------/               \/---------------/
        ^ Converging Slot Gap             ^ Deflected Lower Plate

Slats and Fixed Slots

• Slot Principle: A narrow gap near the leading edge permits high-pressure air from underneath the wing to flow to the upper surface. This re-energizes the boundary layer, delaying separation.
• Automatic Slats: Movable aerofoil segments positioned at the leading edge. At low angles of attack, aerodynamic pressure holds them flush. At high $\alpha$, low pressure at the leading edge pulls the slats forward automatically on tracks, opening a slot and increasing $\alpha_{\text{crit}}$ from $15^\circ$ up to $25^\circ$ or more.
• Krueger Flaps: Hinged structures deflected forward and downward from the lower leading edge, increasing leading-edge radius and effective camber on swept-wing transport aircraft.