Lift (force)
Lift consists of the sum of all the aerodynamic forces normal to the direction of the external airflow.Lift is created by forcing air downward. The pushing (accelerating) of the air downward creates an equal and opposing force upward on wing (see Newton's third law.) The displacement of air downward during the creation of lift is known as downwash. The diversion of the airflow downwards can be seen to create a higher pressure below the wing and a lower one above it. An aerofoil is so shaped to accomplish this as efficiently as possible. One puzzle is why the airflow "sticks" to the wing as it changes direction - this is known as the Coanda Effect, but the reason for it is not fully understood.
Any shape will produce lift if tilted to the air flow direction (inclined) or cambered (curved). However, most shapes will be very inefficient and waste a great deal of energy in the creation of drag. Lift itself creates drag - this is called lift-induced drag, or just induced drag.
The pressure difference is not created by differences in air speed above and below the wing, or because the air has 'further' to travel over the upper surface of a cambered wing. Classical theory of fluid dynamics does not predict lift (the correct equations (Navier Stokes Equations) are complex and until the computer were of small practical use). This explanation for lift is often cited but is erroneous - the "pathway length" theory accounts for perhaps 2% of the lift force produced. Another erroneous explanation is that of a Venturi, where an aerofoil is visualised as "half a venturi". There is no evidence that aerofoils behave in this way.
An alternative explanation of lift depends on the viscosity of air causing asymmetric flow. The effect is apparent only in a very thin layer of air adjacent to the surface of the wing at most a few centimetres thick, the boundary layer.
There are two types of flow in the boundary layer. Around the leading edge the air flows smoothly and behaves like a stack of sheets (laminae) sliding over each other - laminar flow. Further along the wing there is a transition to a turbulent flow. The laminar layer produces less drag, but the turbulent layer is less likely to move away from the surface. As flow speed increases the boundary layer starts to separate at the trailing edge of the wing and a vortex begins to form, moves back and then leaves the surface. This is the starting vortex which disrupts the symmetry of the air flow, causing differences in flow pressure and speed between the upper and lower surfaces of the wing - Lift. The vortex extends in a closed circuit of two real vortices trailing from near the wing tips (wing-bound vortex) and the starting vortex, forming a horseshoe shape and sometimes called the horseshoe vortex system.
Lift force is calculated using the Lift Equation:
where:
- is the coefficient of lift, a dimensionless number.
- is the density of air (1.225 kg/m3 at sea level)
- V is the velocity of the air over the lifting surface
- A is the surface area of the lifting surface
- L is the lift force produced.
