The momentum / momentum flow
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| Graphic: Shot put Pendelum |
Into the "general" static air pressure, a moving wing accelerates air downwards - now it becomes dynamic. This momentum / impulse is passed on to the air and within it, called impulse flow. Due to the compressibility and viscosity of the air, thereby areas of different air pressure form around the source (wing/airfoil/glider...).
A wing in motion accelerates air downwards (dynamic, momentum, impulse, momentum flow ) which leads to pressure differences between the bottom and top of the wing. The higher pressure works in the direction of the lower pressure. The pressure difference in relation to the wing surface determines the strength of the force.
This applies not only to the wing, but in principle to all bodies such as the fuselage, provided there is a certain angle of attack in relation to the flight path. Without an angle of attack there are no pressure differences, without pressure differences there is no resulting force. Newton's laws, known as axioms, formulate the relationships between inertia, impulse and reaction. Every action causes an equally large reaction. Therefore it can be said:
The reaction effect, also known as the interaction principle, described by Newton's third axiom (actio and reactio), can be used to explain dynamic lift in terms of classical mechanics.
An aircraft is set in motion (reactio) by an external force (actio). The resulting downward acceleration of air (actio) causes different pressure ranges (reactio). Air pressure is generated by the impulse of the air molecules (actio) which causes an opposing force on the wing (reactio). The difference in pressure results in a force opposing the force of gravity (lift).
So much for the causal chain. By lift, however, we don't just mean any upward force, but at least one that is strong enough to achieve an acceptable descent rate. Better still, a horizontal flight or climb (with the support of muscle power, motor, thermals). This requires conditions such as the discussed angle of attack, the necessary speed and air with its characteristics. An equilibrium must be established for each stable flight condition. We could interweave the conditions into the causal chain mentioned above or extend it, but this would not change the core statement:
The reaction principle / the interaction principle / actio-reactio is therefore the fundamental principle or the cause of dynamic lift.
The air pressure ⇐ | ⇒ Pressure cushions, pressure deficit, circulation, air vortex