Around the airfoil
 |
| Idealised, exaggerated representation of the pressure spread in normal flight |
As the air accelerates downwards, a non-symmetrical "pressure cushion" builds
up under the wing. The air particles are first caught in the front airfoil area
and accelerated downwards. They therefore already have a downward velocity
vector as they continue down the underside of the profile.
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, supported by the "pressure deficit", they are
accelerated slightly above the wing. 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 (incomplete) filling
(dilution of the upper air mass - negative pressure) leads to a "pressure
deficit" above 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 also
have a downward component at the end of the profile (viscosity and mass inertia).
If the airfoil, angle of attack and speed (equilibrium) are optimised, both the upper and lower air particles at the end of the airfoil have a downward vector. I.e. the upper equalising movement of the air particles (re-shooting) is still so energetic that the "pressure cushion" cannot yet fill the "pressure deficit" over the trailing edge of the airfoil.
The two downward-directed air masses do not mix homogeneously at the end of
the wing, but in the form of small waves and vortices. This relatively small
wake of turbulence primarily affects the pressure equalisation of the airfoil.
Mixed with the vortices of the wingtips, a larger wake vortex remains. This
wake vortex behind heavy aircraft is well known and very dangerous for our air
sports devices (even from the first Wake Turbulence Category Light (L)).
The lift is not only the result of the acceleration of the air mass downwards,
but also increases in effect because there is a
"pressure deficit" at the top. The force on the wing arises from the pressure
difference. Regardless of how the pressure areas expand, air forces (lift, drag)
only act directly on the physical surface of the body / wing / fuselage - see
also "Pressure cushion....".
Wind tunnel flow ⇐ | ⇒ Stall without flow