Coefficient lift and Angle of Attack chart
Basic Lift Formula
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 critical 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. Flaps, leading and trailing edge, together with vortex generators will be investigated as pilots are using these devices more or less on every flight.
Stall 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.
The loss of lift and stall symptoms will be discussed as are the best rate and angle of climbs speeds typically used when the aircraft needs to clear obstacles.
We will see that stall speeds and spins are related as they can be the result of an airspeed which got too low or an angle of attack which was too high.
Lift creation
A wing creates lift based on two effects: Bernoulli's principle and Newton's third law. The curvature of the wing uses the reduction in static pressure above the wing (Bernoulli) so that the pressure below it is greater thus pushing the wing upward. When airflow passes the wing or aerofoil it is turned downward due to the angle of attack also creating an upward force (Newton).
These two effects combined create an upward force called lift and it depends on a number of factors as we shall see below.
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 CL. Where 1/2 ρ V2 is air density times airspeed resulting in dynamic energy, S is wing area and CL 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, (CL)
A given wing stalls always at the same CLmax (with a certain maximum angle of attack) for that configuration. By changing the shape of the wing by extending or retracting flaps (slats too) CLmax will have a new value at a different angle of attack. In fact, extending flaps increases CLmax 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 CL. 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, VS.
