Coefficient lift and Angle of Attack chart

Lift Formula and Stall Symptoms

Most aircraft accidents occur during the take-off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. On this part of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.

We take a look at the basic lift formula, aircraft stall speed and those factors which have the greatest influence on that number. This speed is important as below that speed the wing refuses to generate lift and during take-off and landing we fly close to that speed.

Lift formula

As we all (should) know, the lift formula gives us a good representation of what is going on: L = 1/2 ρ V² x S x C_L. Where 1/2 ρ V² is air density times airspeed resulting in dynamic energy, S is wing area and C_L the coefficient lift. Change any of these variables and the amount of lift will change too.

For example: if you were to change speed, the amount of lift will change and the aircraft will change altitude. For lift (L) opposes weight (W) and if these forces are equal, the aircraft remains at the same level.

Dynamic energy, (1/2 ρ V²)

Air density times airspeed is dynamic energy caused by the movement of the aircraft in the air stream and indicated as IAS (indicated airspeed). See aircraft speeds.

Coefficient lift, (C_L)

A given wing stalls always at the same C_Lmax (with a certain maximum angle of attack) for that configuration. By changing the shape of the wing by extending or retracting flaps (slats too) C_Lmax will have a new value at a different angle of attack. In fact, extending flaps increases C_Lmax but lowers the angle of attack (AOA) where the stall occurs. Extending slats will increase both.

Wing area, (S)

Changing the amount of wing area changes the amount of lift too. Certain type of flaps (fowler) extend behind the wing thus increasing the wing area (S).

During flight, wing area (S) is more or less constant for a given wing configuration so we can assume that stall speed is influenced by angle of attack (AOA) and indicated airspeed: Lift = IAS x C_L. Thus for a flight at a constant altitude there is only one IAS matched by one AOA resulting in lift equal to weight and the aircraft will not climb or descend.

Angle of Attack, (AOA)

Definition: The angle of attack is the angle between the chord of the airfoil (determined by wing form) and the incoming relative wind.

Now imagine that the pilot wants to reduce speed and remain at the same altitude. IAS reduces thus the AOA must increase so that resulting lift equals weight. This can continue until AOA reaches its maximum, after which the wing stalls. This is the basic level stall speed, V_S.

Stall symptoms

These symptoms describe the low speed non accelerated stall. Its possible to induce stall at higher speeds as in level turns and fast pitch up accelerations where some symptoms will not show at all.

Angle of Attack compared to Relative Wind

Reduced airspeed

If airspeed reduces and the pilot wants to remains at the same altitude, the aircraft will stall eventually if the airspeed keeps reducing.

Control effectiveness

As the aircraft slows down there is less airflow over the ailerons, rudder and elevator thus the amount of deflection needed is greater to control the aircraft. An indication of approaching slow speed stall.

Pre-stall buffet

When the AOA of the wing increases the separation point moves forward and the streamlined airflow wil become turbulent and separates from the wing. This turbulent wake then meets the aft fuselage and tail section of the aircraft. This will be felt by the occupants of the aircraft as a rumble or buffet. Not all aircraft have a pronounced buffet, this depends on the size and location of elevator.

Wing design is such that the wings stalls from the root toward the wingtips so that the ailerons remain effective as long as possible, for this to happen the angle of incidence of the wing is larger at the root and smaller at the wing tips. Some aircraft have a stall fence on top of the wing, others employ a discontinuous leading edge (Kodiak from Quest Aircraft) creating a vortex over the wing at high angles of attack. Both devices prevent the stall from progressing to the ailerons so that they remain effective.

Nose attitude

During a straight and level stall the nose high attitude is a good indication of an approaching stall, but remember that using flaps lowers the nose and an aircraft can be made to stall in any nose attitude. A stall occurs when angle of attack of the incoming airflow and chord line becomes too large, this has nothing to do with nose attitude and is called a high speed or accelerated stall.

Stall warning

In some aircraft a stall warning device is fitted in the form of a horn or light. This device is set to indicate a stall warning around 5 kts above actual stall speed.