What's stall speed?

Stall speed is the minimum steady flight speed at which an aircraft can maintain controlled flight. Below this speed, the wing cannot generate enough lift to counteract the aircraft's weight, resulting in a stall condition where the airflow separates from the wing's upper surface. This causes a sudden decrease in lift and increase in drag, potentially leading to loss of control.

How to calculate stall speed

The stall speed (Vs) can be calculated using the following formula:

Vs = √(2 × W / (ρ × S × CL_max))

Where:

• Vs = Stall speed (in knots or m/s)
• W = Aircraft weight (in pounds or kg)
• ρ (rho) = Air density (in slugs/ft³ or kg/m³)
• S = Wing area (in ft² or m²)
• CL_max = Maximum coefficient of lift

Wing loading

Wing loading is the weight of an aircraft divided by the area of its wing. It's typically expressed in pounds per square foot (lb/ft²) or kilograms per square meter (kg/m²).

Wing Loading = W / S

Higher wing loading generally results in higher stall speeds, faster cruise speeds, and reduced gust sensitivity. Lower wing loading typically provides better low-speed handling, shorter takeoff and landing distances, but potentially more susceptibility to turbulence.

Altitude

As altitude increases, air density (ρ) decreases, which affects stall speed. Since stall speed is inversely proportional to the square root of air density, an aircraft will have a higher stall speed at higher altitudes compared to sea level (assuming all other factors remain constant).

The correction factor for altitude can be approximated using the air density ratio:

Vs(altitude) = Vs(sea level) × √(ρ(sea level) / ρ(altitude))

CL_max

CL_max is the maximum coefficient of lift that an airfoil can generate before stalling. It depends on several factors:

• Airfoil design: Different airfoil shapes have different lift characteristics
• Flap configuration: Extending flaps increases CL_max
• Slat/slot configuration: Leading edge devices can increase CL_max
• Surface condition: Contamination (ice, rain) can reduce CL_max
• Reynolds number: Affects boundary layer behavior

Typical CL_max values:

• Clean wing: 1.2 - 1.6
• With flaps: 1.8 - 3.0 (depending on type and deflection)
• With full high-lift devices: up to 4.0