Stall Speed

Stall speed is the minimum airspeed at which a wing can generate enough lift to sustain level flight. Below this speed, the wing exceeds its critical angle of attack and lift collapses.

How It Works#

Every wing produces lift by deflecting air downward. The angle between the wing's chord line and the oncoming airflow is called the angle of attack (AoA). As AoA increases, lift increases — up to a point.

That point is the critical angle of attack. Beyond it, airflow over the upper wing surface separates and becomes turbulent. Lift drops sharply. This is a stall.

Stall speed is not a fixed number. It changes with several factors. Four key variables are:

• Weight: A heavier aircraft needs more lift. More lift requires more speed.
• Load factor: Pulling Gs in a turn multiplies effective weight. A 60-degree bank doubles the load factor, raising stall speed by about 41%.
• Flap setting: Flaps extend the wing's camber and increase lift at lower speeds. Full flaps lower stall speed.
• Altitude: Stall speed in indicated airspeed (IAS) stays roughly constant with altitude. In true airspeed (TAS), it rises — the air is thinner, so the aircraft moves faster through it to generate the same dynamic pressure.

The stall speed published in a Pilot's Operating Handbook (POH) is typically measured in a specific configuration. $V_S$ refers to stall speed in a clean configuration. $V_{S0}$ refers to stall speed in the landing configuration, with flaps and gear down.

Example in Aviation#

A Cessna 172 has a published $V_{S0}$ of approximately 40 knots indicated airspeed (KIAS) with full flaps and gear down at maximum gross weight. A student pilot on final approach slows to 55 KIAS, maintaining a safe margin above stall.

Now imagine that same pilot enters a steep 60-degree banked turn on final. The load factor doubles. Stall speed jumps from 40 knots to roughly 57 knots. The aircraft can stall at a speed that felt perfectly safe just moments before. This scenario is a leading cause of fatal accidents in the traffic pattern.

Why It Matters#

Understanding stall speed keeps pilots from entering unintentional stalls at low altitude, where there is no room to recover. Knowing that stall speed increases with load factor explains why steep turns and abrupt pull-ups are dangerous close to the ground.

Student pilots memorize $V_S$ and $V_{S0}$ for their aircraft. Experienced pilots internalize how weight, bank angle, and configuration shift those numbers in real time. That awareness is a core safety skill.

Key Takeaways#

• Stall speed is the minimum airspeed for sustained lift in level, unaccelerated flight.
• A wing stalls when it exceeds its critical angle of attack, not simply when it slows down.
• Higher weight and higher load factors increase stall speed.
• Flaps lower stall speed; steep banks raise it.
• $V_{S0}$ (landing config) and $V_S$ (clean config) are the two published reference speeds.