Lift At The Airfoil
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Idealised, exaggerated representation of the pressure spread in normal flight |
In flight, a non-symmetrical "pressure cushion" builds up under the wing due to the angle of attack. The air particles are first caught in the front bottom airfoil area and accelerated downwards. They therefore already have a downward velocity vector as the underside of the profile is swept.
As a result, a stronger "pressure cushion" builds up at the front (also due to the overlap with the dynamic pressure). This "pressure cushion wedge" already influences the air in front of the airfoil. The "pressure cushion" decreases along the underside of the profile and "sands" behind the aerofoil with a downward component.
Due to the leading "pressure cushion", air particles are deflected upwards in front of the profile and accelerated slightly above the wingnoses due the "pressure deficit" at the top. The sloping upper wing profile (profile back) creates a free space. The air lifted in front of the wing, but above all the static pressure, fills this up. This temporary incomplete filling caused by the inertia therefore leads to the aforementioned "pressure deficit" (negative pressure) over the back of the profile. The pressure gradient is relatively large due to the space opening (also depending on the shape of the upper profile camber) so that the air particles pushing (shooting) from above have a downward component at the end of the profile (viscosity and mass inertia) also. The airfoil moves almost horizontally, forcing the air to deflect essentially vertically.
The wing profile moves almost horizontally, forcing the air to move essentially vertically. "Essentially" because the surface friction "entrains" air particles in the direction of flight, but this is negligible for general understanding. The known directions of the boundary layer vectors (laminar, turbulent) apply to real flow, e.g. in pipes/tubes, but they look different and lead to an opposit direction on a moving wing, which also shows that it is not possible to simply swap inertial systems.
What prevents the lower air cushion from equalising the pressure deficit of the upper side around the trailing edge of the wing? The sum of the kinetic energy and pressure energy of the upper wing surface (in the area of the trailing edge) is almost identical to the sum of the kinetic and pressure energy of the lower surface. This means that while there is less pressure on the upper side (lower pressure energy), the kinetic energy (kinetic energy with downward vector) is higher. The opposite is true at the bottom.
The two air masses moving downwards do not mix homogeneously at the end of the wing (the differences in kinetic and pressure energy are equalised) but in the form of small waves and vortices. This relatively small turbulence wake primarily affects the pressure equalisation of the airfoil.
What remains is a downward-directed air mass that subsequently mixes with the surrounding air in the form of upward-directed vortices. Mixed with the tip vortices of the wings, a larger wake vortex remains. This wake vortex behind heavy aircraft is well known and very dangerous for following aircrafts (even from the first wake turbulence category Light (L)). The aircraft flies on, the air mass behind it comes to rest after a while. What remains: Energy input into the air in the form of a thermal energy (kinetic energy is converted into heat).
Flow and The Windtunnel ⇐ | ⇒ Stall Without Flow