Bernoulli and The Energy Conservation Principle
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Picture: Julian Moos |
The law of conservation of energy states that energy cannot be lost, but neither can it be gained from nothing. The Bernoulli formula shows that the sum of the pressure energy, potential energy and kinetic energy along the current tube is constant, i.e. no energy is added.
But without energy supply there is no flying. Where does the energy come from? From the positional energy of the aircraft, and/or hopefully usually also from the propulsion system (fuel). Example: Take-off with a full tank, landing at the take-off site with an empty tank, the aircraft now has the same positional energy again, where has the energy from the fuel gone? In the air in the form of heat, air movement (downdraft) and, a special form of air movement, in vortices.
Pulsed smoke spots in the wind tunnel show that the air particles that initially split at the leading edge at the same time do not arrive at the end at the same time. However, the simultaneous arrival of the air particles is often used to apply the "longer path" and the Bernoulli formula.
Thus, the experiment of a constricted tube, in which a lower pressure prevails in the constriction (Venturi tube), but the same energy state prevails again at the outlet, is unsuitable to explain the true cause of the lift. Bernoulli effects occur partially around the airfoil (airfoil nose) when considering the pressure equalisation movements in detail. However, the overall energy state is the same in front of and behind the flow tube.
Pressure Cushion, Pressure Deficit, Circulation, Vortices ⇐ | ⇒ Plausibility Check for Pressure Differences