The Air Pressure
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Magdeburg hemisphere Picture: Ulrich Arendt |
Air is a mixture of gases and static pressure (air pressure) is generated by the air column. Air has an internal (kinetic) energy because air molecules (mass) are in motion. Gas particles in motion collide with each other, but can also act on body surfaces, such as a hollow sphere or a wing.
1 m³ of air near the ground has a weight of approx. 1.3 kg. Even more impressive is the pressure generated by the column of air above us at 1000 hPa: ~10 tons/m² weigh on the earth's surface and every m² of our body. At this high pressure, which we do not perceive (because it also acts against us from the inside), even the slightest fluctuations in air pressure can cause enormous forces.
Unimaginable forces are released in the weather, even the smallest differences in pressure can be indicated by instruments, small suction cups hold amazing loads, enormous weights float on air cushions, etc. The high energy potential of air pressure demystifies the phenomenon of flight because even the smallest differences in pressure can cause high forces on corresponding surfaces.
When you say "something is being sucked in", you mean that the vacuum (suction) is pulling. However, air pressure can only push - not pull. The higher pressure on the opposite side works against the lower pressure, the resultant determines the direction and force. Many illustrations that show the suction above the wing profile with force arrows starting from the back of the profile are therefore not physically correct. There is also air pressure in a low-pressure environment, just less. Negative pressure is by no means a vacuum. So the word vacuum has no place in our subject area, even if it is used colloquially all too often.
This is well known in pneumatics and also in carburetor technology, which is why turbos or compressors are used to help achieve high piston filling in 4-stroke engines - "suction" alone, i.e. the low-inertia ambient air pressure does not achieve 100% piston filling. In this sense, the dirt is not drawn into the hoover, but the hoover creates a negative pressure and the incoming ambient air " pushes" the dirt along with it.
The speed when pressure differences equalize (pressure equalization = flow) must not be confused with the speed of sound (wave propagation). While the speed of sound measures the spread of sound waves, the speed of pressure equalization is incomparably lower. Pressure equalization is mass displacement of air (kinematic movement).
The speed of mass displacement (pressure equalize) depends on the energy applied to the air, which is slowed down by inertia, viscosity and resistance. While the speed of sound is approx. 343 m/s, a pressure equalization can be a few m/s to a maximum of a few 10 m/s (depending on the triggering and ambient conditions). This is relevant when it comes to pressure equalization caused by the action of a moving wing.
Flying With Wings ⇐ | ⇒ Impulse / Momentum Flow