Glossary term
Stall Speed
Aircraft speed at which the required lift reaches the available maximum lift for a specified weight, configuration, load factor and flight condition.
Definition
quantityStall speed is the aircraft speed at which the lift required for the current condition reaches the available maximum lift.
Stall speed is not a single universal speed. It depends on weight, reference area, maximum lift coefficient, density, load factor, configuration, contamination, thrust effects, Mach number, Reynolds number, center of gravity, control authority, air-data calibration and the airspeed reference being used. It is often estimated from the lift equation, but engineering use requires a stated condition and validation evidence.
Stall speed is the speed at which the lift required for the current aircraft condition reaches the available maximum lift. For a simple steady condition where lift must balance load factor times weight:
with:
the stall speed estimate is:
For wings-level unaccelerated flight, n=1:
This equation is a screening model. It is useful because it shows the controlling dependencies, but it does not replace approved flight data, stall testing, calibrated airspeed schedules or aircraft-specific limitations.
Engineering Role
Stall speed connects aerodynamic lift capability to operating margin. It is used in takeoff and landing analysis, approach-speed selection, maneuver margin, envelope protection, handling-quality review, icing assessment, flight-test planning and structural load interpretation.
A higher weight, higher load factor or lower available maximum lift coefficient increases stall speed. Lower density increases true stall speed for the same lift requirement, while equivalent airspeed can be a better way to compare dynamic-pressure exposure. Configuration matters strongly: flaps, slats, gear, surface contamination, ice, stores, damage, thrust effects and control-law mode can all change C_{L,max} and stall behaviour.
Stall speed is related to angle of attack, but it is not the same quantity. Stall occurs when the wing or aircraft reaches a maximum usable lift condition, usually associated with high angle of attack and flow separation. The speed at which that happens depends on weight, density, load factor and configuration.
Worked Example: Clean and Contaminated Stall Margin
An aircraft approach review uses:
| Parameter | Value |
|---|---|
| Aircraft weight, W | 68{,}000\ \text{N} |
| Wing reference area, S | 30.2\ \text{m}^2 |
| Local density, \rho | 1.02\ \text{kg/m}^3 |
| Clean maximum lift coefficient, C_{L,max,clean} | 2.10 |
| Contaminated maximum lift coefficient, C_{L,max,cont} | 1.55 |
Clean wings-level stall speed is:
With contamination reducing maximum lift coefficient:
The stall-speed ratio is:
The estimated stall speed has increased by about 16\%. If the aircraft banks to 30^\circ during the approach, the load factor is approximately:
The contaminated turning stall speed becomes:
Engineering comment: a clean target speed that looked comfortable before contamination may no longer provide the intended margin after reduced C_{L,max} and bank angle are included. The calculation does not authorize an operating speed; it shows why the condition must be checked against approved data, handling evidence, air-data validity and release criteria.
Distinction from Related Quantities
Stall speed is not angle of attack. Angle of attack is a geometric aerodynamic angle. Stall speed is the speed at which the aircraft reaches the maximum usable lift condition for a stated weight, density, load factor and configuration.
Stall speed is not lift coefficient. Lift coefficient describes normalized lift at a flight condition; C_{L,max} is an input to a stall-speed estimate. A lower C_{L,max} from ice, roughness, flap malfunction, damage or Reynolds-number effects raises stall speed.
Stall speed is not equivalent airspeed, calibrated airspeed or indicated airspeed by itself. A report must state whether the speed is TAS, EAS, CAS, IAS or another reference. Performance, cockpit procedures and structural-envelope reviews may use different speed references.
Stall speed is also not a guarantee of controllability. The aircraft may encounter buffet, roll-off, tailplane stall, actuator limits, sensor disagreement, control-law limiting or inadequate climb margin before or near the nominal stall-speed estimate.
What Changes Stall Speed
Important dependencies include:
- aircraft weight and load factor;
- wing reference area and chosen reference geometry;
- C_{L,max} for the actual configuration;
- flap, slat, gear, spoiler, store and ice state;
- density, Mach number, Reynolds number and dynamic pressure;
- center of gravity, trim condition and tail download;
- thrust setting, propeller slipstream or jet effects;
- maneuver rate, gusts, turbulence and unsteady aerodynamics;
- air-data calibration and the chosen speed reference;
- stall-warning, buffet, protection-law and certification definitions.
Because these dependencies are strong, a single stall-speed number should never be reused outside its stated condition.
Validation and Common Mistakes
Stall-speed evidence can come from approved flight manuals, flight-test expansion, calibrated air-data reconstruction, wind-tunnel or CFD support, high-lift-system testing, icing evidence, handling-quality assessment and uncertainty analysis. A defensible value states weight, configuration, density or speed reference, load factor, C_{L,max} basis, air-data source, units, margins and limitations.
Common mistakes include:
- using a clean-wing stall speed after ice, roughness, damage or flap malfunction;
- forgetting the \sqrt{n} increase in stall speed during turns or pull-up maneuvers;
- mixing TAS, EAS, CAS and IAS without stating the reference;
- treating C_{L,max} as constant across Reynolds number, Mach number and configuration;
- using stall speed as if it proved climb, controllability or structural margin;
- applying a flight-manual speed outside its certified weight, configuration or operating condition;
- ignoring sensor calibration, pitot-static faults, filter lag or warning-threshold uncertainty.