Most aircraft accidents occur during the takeoff or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing do occur every now and then. In this section 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.
We discuss 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 takeoff 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.
A wing creates lift based on a number of effects: Continuity Equation, Bernoulli's principle and Newton's Third law. The curvature of the wing creates an acceleration of the air flow above the wing (Continuity) and the resulting reduction in static pressure (Bernoulli) is such that the pressure below it is greater, thus pushing the wing upward. At larger angles of attack: when airflow passes around the wing or aerofoil, it is deflected downward, thus also creating an upward force (Newton).
These effects combined create an upward force called lift and a rearward force called drag and the magnitude of both depends on a number of factors as we shall see below.
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 true airspeed resulting in dynamic energy, S is wing area and CL the coefficient lift. Change any one of these variables and the amount of lift will change too.
For example: if you were to change speed (everything else being equal), the amount of lift will change and the aircraft will change altitude. For as lift (L) opposes weight (W) and if these two forces are equal, the aircraft will remain at the same level, or altitude.
As already mentioned: air density times true airspeed results in dynamic energy, this caused by the movement of the aircraft in the surrounding air stream and it is indicated as IAS (indicated airspeed) to the pilot. See also aircraft speeds.
A given wing always stalls at the same CLmax (with a certain maximum angle of attack) for that configuration. Changing the shape of the wing by extending or retracting flaps (slats too, of course) will result in CLmax having a higher value but at a different angle of attack.
In fact, extending flaps increases CLmax but lowers the angle of attack (AOA) where the stall will occur. Extending the leading edge slats will increase both of them, AOA and CLmax.
Changing the amount of wing area changes the amount of lift too. Certain type of flaps (Fowler types, for example) extend behind the wing thus increasing the wing area (S).
During flight, the 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 alone: 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.
Definition: The angle of attack is the angle between the chord of the airfoil (determined by wing form) and the incoming relative air flow.
Now imagine that the pilot wants to reduce speed and remain at the same altitude. IAS reduces thus the AOA must increase enough so that resulting lift still equals the actual weight. This process may continue until AOA reaches its maximum angle, after which the wing will absolutely stall. And this speed is your basic level stall speed, VS, important for landings.