What is the foil effect? This eagle is exploiting it beautifully.
The foil’s axis of yaw is controlled by the foiler swinging their hips and footing. The foiler’s feet can be in binding or not depending if they want to ride their foil more like a snowboard or a surfboard.
One of the greatest dangers of flight is the possibility of stalling, which is the loss of aerodynamic lift, and in an airplane means crashing into the ground. But because a foiler is secured to a zip line stalling a foil will never cause the foiler to crash to earth. In fact, depending on the skill of the foiler, stalling a foil will allow the foiler to do some mind blowing, thrilling stunts for their enjoyment.
As in skiing where there is freestyle and slalom, foiling will have Freestyle and GE Foiling, with GE standing for Ground Effect. With Freestyle Foiling the foiler will be suspended high above the ground and will need a larger foil than with GE Foiling. GE Foiling the foiler will be skimming only a foot or two off the ground, and the foil will be smaller than the one used for Freestyle Foiling because of the added lift created by ground effect. The first foil runs will most likely be Freestyle Foiling runs because this type of foiling will be easier to implement, but GE foiling will best mimic skiing and snowboarding, so the long range goal is to have most foiling runs be adapted to be GE foiling runs.
Even if you are not familiar with the term ground effect, you have experienced it when coming in for a landing on a commercial jet: Your jetliner is coming down toward the landing strip, and just when you think the pain is over and you are going to land the flight path of the jet flattens out and you go on for a while before you finally touchdown, and you exhale with relief. That was ground effect in action, and in simple terms it is the area that is the distance equal to the length of the wing that the plane is off the ground that produces twice as much lift as if you were flying high in the sky.