Factors Affecting Stall Speed

Stall Speed Fundamentals

Stall speed is the minimum speed at which an aircraft can maintain level flight before the wing reaches its critical angle of attack and airflow separates. This speed is influenced by several key parameters, each of which can raise or lower the stall threshold.

Aircraft Weight and Wing Loading

Stall speed increases with aircraft mass. Heavier aircraft require more lift to sustain flight, which means the wing must generate more lift at a given angle of attack, resulting in a higher stall speed. The relationship is proportional to the square root of the weight—if weight doubles, stall speed increases by about 41%.

Load Factor (n)

Load factor is the ratio of lift to weight and increases during manoeuvres such as turns or pull-outs from dives. As load factor rises, so does the required lift, and thus stall speed increases by the square root of the load factor. For example, at 2g, stall speed increases by about 41% compared to straight-and-level flight.

Flap Setting

Extending flaps increases the wing's maximum lift coefficient (CLmax), allowing the aircraft to fly slower before stalling. Flaps lower the stall speed and reduce the angle of attack at which the stall occurs. Different flap settings and aircraft designs affect the degree of stall speed reduction.

Centre of Gravity (CG)

A forward CG position requires a higher angle of attack to maintain level flight, increasing stall speed. An aft CG allows for a lower angle of attack and a reduced stall speed. Stall speeds published in manuals are typically based on the most forward CG for safety.

Wing Contamination

Ice, frost, or other contamination on the wing surface reduces lift and increases stall speed. Even small amounts of contamination can significantly degrade performance and alter stall characteristics, sometimes asymmetrically.

Other Influences

• Thrust and Slipstream: Increased thrust or slipstream (from propeller wash) can improve airflow over the wing, reducing stall speed.
• Wing Sweep: Swept wings generally stall at higher speeds than straight wings.
• Altitude: At typical operating altitudes, indicated stall speed remains nearly constant, but at high altitudes, compressibility effects can increase the true stall speed.

Stall Speed Calculation

The stall speed (VS) can be derived from the lift equation:

VS = √[ (2 × Weight) / (Air density × Wing area × CLmax) ]

This formula shows how weight, air density, wing area, and maximum lift coefficient all play roles in determining stall speed.