A stall occurs when a wing exceeds its critical angle of attack — the angle at which airflow can no longer follow the curvature of the upper surface. The boundary layer separates, the pressure difference that generates lift collapses, and drag increases sharply. The aircraft pitches nose-down (or, if asymmetric, drops a wing and enters a spin).
Stall speed — the minimum airspeed at which level flight is possible — is determined by maximum lift coefficient and wing loading:
V_stall = √(2W / (ρ × S × C_L_max))
Stall behavior varies by airfoil type and wing planform:
- Thin airfoils (common at low Reynolds numbers in small UAVs) tend to stall abruptly — the entire upper surface separates almost simultaneously. This can cause sudden, unrecoverable loss of control.
- Thick airfoils stall gradually from the trailing edge forward, giving progressive warning and partial lift retention.
- Delta wings stall at very high angles of attack (30°+), with separated flow generating vortex lift from the leading edge — a different and more progressive stall mechanism than conventional airfoils.
- Swept wings tend to stall at the tips first, which can cause pitch-up (the tips are behind the center of gravity) — a dangerous characteristic countered by washout or wing fences.
For autonomous UAVs, stall protection is typically implemented in the flight controller as a pitch or airspeed limit rather than relying on a pilot’s physical sensation of airframe buffet.
Related terms
- Angle of Attack — the variable whose exceedance causes stall
- Wing Loading — the parameter that determines stall speed
- Boundary Layer — the thin air layer whose separation defines stall