Understanding Aircraft Stall Speed
A stall occurs when the angle of attack of an airfoil exceeds the critical angle of attack, resulting in a sudden decrease in lift. The speed at which this happens is known as the stall speed. However, the stall speed published in an aircraft manual is not a fixed number; it varies significantly based on several flight conditions, most notably the weight of the aircraft and the bank angle during a turn.
How Weight Affects Stall Speed
The relationship between weight and stall speed is governed by the lift equation. As an aircraft's weight decreases, the amount of lift required to maintain level flight also decreases. Consequently, a lower angle of attack is needed at any given speed, meaning the aircraft can fly slower before reaching the critical angle of attack. This calculator uses the standard aviation formula where the new stall speed is the old stall speed multiplied by the square root of the ratio of the new weight to the old weight.
The Impact of Bank Angle and Load Factor
When an aircraft enters a constant-altitude turn, the "apparent weight" or load factor (G) increases. To maintain altitude while banking, the wings must produce enough vertical lift to counteract gravity while also providing the horizontal component of lift for the turn. This increases the total load on the wings. For example, in a 60-degree level bank, the load factor is 2.0 Gs. The stall speed increases in proportion to the square root of the load factor, meaning a sharp turn can dangerously elevate your stall speed.
How to Use This Calculator
- Enter Published Stall Speed: Input the stall speed (Vs or Vs0) from your aircraft's POH based on maximum gross weight.
- Input Current Weight: Enter the current estimated weight of the aircraft (fuel, passengers, cargo).
- Set Bank Angle: If you are calculating for a turn, enter the intended bank angle in degrees.
- Review Results: The calculator will immediately update the predicted stall speed based on these variables.
Frequently Asked Questions
Why does stall speed increase in a turn? Because the wings must support both the weight of the airplane and the centrifugal force of the turn, requiring more lift and thus a higher airspeed to stay below the critical angle of attack.
Is the result in Knots or MPH? The result will be in the same units you use for the initial input. If you input knots, the result is in knots.