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Lee-effect:

If you hold a board vertically and rotate it on your stretched arm in a circle, then a resistance (back pressure) in front of the board is clearly noticeable, simultaneously an area of low pressure (vacuum) is generated behind the board because the surrounding air can not compensate the displaced air particles quickly enough. This area behind the board is named "Lee", therefore "Lee-effect".

This effect will remain as long as the board is turned into a completely horizontal position.

If you (still turning around) turn the front edge of the board in the direction of turning only by a few degrees upwards, the effect starts again and you can feel that the board produces a clearly noticeable force upwards. First because displaced air particles from the downside of the board and thus an overpressure, respectively by "mass multiplied by acceleration", a force upwards arises. At the same time the "Lee-effect", i.e. a "vacuum" starts on the upper surface of the board and thus an additional force acting upwards is generated.

We are basically simulating not more than a blade of a propeller or a helicopter, and if we could extend our arm more and more, a wing of an airplane.

This "Lee-effect” acts in addition to the already described acceleration of air particles at the leading edge of a wing.

 

 

SR