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Standing on a ridge, wind hitting one side of the ridge creates lift, like seagulls zooming along cliffs at the ocean.
Behind the ridge is the "backside", where the air is still (no lift). There's a separation layer between the air zooming over the ridge and the dead air behind the ridge. Usually, if you fly back behind the ridge you crash or have to land out because there's no lift.
We found by accident if you zoom down through the separation layer behind the hill with enough speed to get back around to the frontside, that you GAIN energy. We maintain these circuits and gain energy with each lap.
It has to do with airspeed jumps over the wings. If you're going 100mph downwind in 50mph winds, airspeed over the wings is 50mph. Once you cross the boundary layer into the still air behind the ridge, you're going 100mph and the airspeed over the wings is 100mph (50mph jump in airspeed over the wings=energy gained) crank it back around up through the separation layer INTO the 50mph headwind, and the airspeed jumps to 150mph, another jump in airspeed over the wings. The planes cross the layer 2 times every lap gaining energy every time the plane crosses the layer.
If you take the round circuits, and break the circles up into "S's", you have what albatross do to cross vast ocean expanses without flapping their wings. They fly along weather fronts, crossing the boundary layers, zooming in and out, gaining energy and then "coasting" with the energy gained.
Better descriptions and info can be had at RCspeeds.com
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