Stall Speed
Minimum speed for level flight, set by maximum lift coefficient.
Formula
Variables
Example
A heavier aircraft stalls at a higher speed, since more lift is needed to hold it up.
How it works
This formula gives the minimum airspeed at which a wing can keep an aircraft flying level before it stalls. It comes from the lift equation, where lift depends on air density, wing area, speed, and the lift coefficient; at stall, the wing has reached its maximum lift coefficient, so solving the lift equation for speed gives the stall speed. It shows why stall speed rises with weight and falls when the air is denser or the wing can generate more lift.
When to use it
Use this when estimating takeoff or landing speeds, comparing stall speeds at different altitudes, or analyzing how aircraft weight and configuration affect low-speed flight. It is also useful in performance and safety calculations for gliders, small aircraft, and test problems involving maximum lift.
Worked example, step by step
Given: W = 18,000 N ρ = 1.18 kg/m^3 S = 24 m^2 C_L,max = 1.6 Step 1: Substitute into the formula V_s = √(2W / (ρSC_L,max)) V_s = √(2 × 18,000 / (1.18 × 24 × 1.6)) Step 2: Calculate the denominator 1.18 × 24 × 1.6 = 45.312 Step 3: Calculate the numerator 2 × 18,000 = 36,000 Step 4: Divide and take the square root V_s = √(36,000 / 45.312) V_s = √794.4 V_s ≈ 28.2 m/s Final answer: The stall speed is about 28.2 m/s.
Common mistakes to avoid
- Using mass instead of weight without converting it to newtons
- Forgetting that stall speed changes with air density and wing configuration
- Plugging in a typical lift coefficient instead of the maximum lift coefficient
Did You Know?
A wing stalls at a critical angle, not a fixed speed — you can stall at any airspeed if you pull too hard.
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