Pressure Cushion, Pressure Deficit, Circulation, Vortices
The acceleration of air causes pressure zones (higher and lower pressure) to form around the aerofoil. The formation and dissipation is a continuous process. The pressure zones are created and localised with the speed of the aircraft. Pressure zones have a practically finite size due to the viscosity of the air. Take the underside of the wing as an example: Air particles bounce off the wing, passing on their impulse to neighbouring air particles until at some point the impulse transmission in the edge area of the "pressure cushion" has become negligibly small.
With regard to the nature of air pressure (momentum of the air molecules - momentum flow - pressure gradient), the areas of different air pressure can be divided into "pressure cushion" ("overpressure"), "pressure deficit" ("underpressure") and static air pressure. "Pressure cushion" and "pressure deficit" are merely auxiliary linguistic constructions to provide a striking image. In particular, it simplifies the visualisation of the pressure distribution as described in "Lift at the aerofoil".
![]() |
Picture: DLR Vortex wake |
A word about circulation and vortices: these occur in all sizes and different forms when flying. The tip vortices of an airliner, for example, are very impressive. Circulation and vortices are equalising movements of the air. Turbulent boundary layers and wave movements/vibrations can also be categorised as such. They are an effect and not a cause. They form at various points. The aim is to reduce them as much as possible by design (preventing them completely is only possible if you don't fly) - an exciting further topic.
Due to the practically finite nature of pressure ranges (flattening of impulse transmission), one might think that energy would be lost, which of course cannot be the case. A small proportion is converted into thermal energy, so energy is supplied to the air in various forms (ineffective impact). The pressure gradient within the pressure ranges is ignored under "Plausibility check pressure difference" because in particular the pressure conditions on the body surface are relevant. The boundary layer is also ignored. Both, pressure gradient and boundary layer, are not insignificant for the formation of the pressure areas. For a general understanding, however, they can be ignored for the time being.
Pictorial representations of pressure spreading, as we all know them, are only idealised. It depends on the profile, surface, angle of attack, speed ... ... For us as pilots, the ground effect can be a practical indicator of how extensive the "pressure cushion" can be in its effectiveness. It is, of course, somewhat more extensive, but beyond that it is hardly relevant for the pilot.
Impulse / Momentum Flow ⇐ | ⇒ Bernoulli and The Energy Conservation Principle