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Using law of conservation to explain theory of lift

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Aerodynamics

Can one please explain, in simple terms, how EACH of the laws of conservation (mass, energy and momentum) affect the airflow around the airfoil, thus producing lift?

Citing from FAA's manual:

"The concept of conservation of energy states energy cannot be created or destroyed and the amount of energy entering a system must also exit. A simple tube with a constricted portion near the center of its length illustrates this principle. An example is running water through a garden hose…"

What about the concept on mass and momentum?

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1 Answers



  1. Brian on Oct 16, 2013

    The mass flow is explained one sentence later in the text you quote, except they term it mass-flow. The mass flow entering the system must be the same as the mass flow that exits the system. This explains conservation of mass’ role and why particles passing over the wing move faster than those passing beneath.

    Now as these particles pass over and under the wing each particle contains some amount of momentum and some amount of energy. For simplicity let’s skip all the impact with other particles where this energy and momentum is transferred (conserved), and jump to the collision that results in lift; pressure. When a particle with a given energy and momentum collides with a point on the airframe, the particles energy and momentum is transferred to the airframe in the form of pressure. The sum of all pressures acting on the aircraft is the aerodynamic force.

    A word of caution, the above ignores some important aerodynamic concepts in an attempt to give you the simple answer you seek. Namely boundary layer, whose existence prevents the particles described above from ever directly impacting the airplane.

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