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Lee Soaring

                                Figure 30

 Model glider pilots have recently found a new way of dynamic soaring in the lee of a hill. Normally soaring is done on the windward side of a hill to take advantage of the up-draughts as the wind blows up and over the hill. The lee side of the hill is normally avoided because of down-draughts but it has been found that it is possible to maintain height and position in a circular flight path and achieve huge speeds when there is a marked shear boundary between the wind blowing over the top of the hill and a relatively still area below.

 Again the Rayleigh cycle of dynamic soaring is cited as the mechanism in use. It goes something like this: as the model flies upwind in the still air and climbs through the shear boundary, ground speed is preserved and airspeed increases. The glider turns downwind and descends through the shear boundary, ground speed is preserved and airspeed increases. The circuit is completed with a little loss of speed and the cycle is repeated.

 So what is wrong with that? Kinetic energy depends on speed relative to the ground. When moving through the shear boundary, whenever ground-speed is preserved there is no gain of  kinetic energy  and if airspeed increases then drag increases. If drag increases there must be a force to balance drag if only briefly. The wind gradient theory does not show such a force.

As explained elsewhere, potential energy is gained in a windward turn but in this case the ‘windward’ turn is made in still air so there is no gain of energy. Energy is lost in a downwind turn, in this case in a strong wind!  The wind is an obvious source of energy but where is the force that causes the acceleration? Also, when listening to the sound track of the videos of this flying, you would expect that if the Rayleigh cycle is correct then there would be audible ‘double slap’ as the glider penetrates the shear boundary twice. What you actually hear is a ‘single slap’ at the highest point of the circle.

I do not think this is either wind-gradient or windward turn dynamic soaring. I think it is more like a kind of auto-rotation, the kind of thing that keeps the rotor of an auto-gyro or the vanes of a vertical-axis windmill turning.

          Figure 31

Try this idea instead. The glider is circling with a steep angle of bank (figure 30) but with a small angle of attack and the lift and drag forces are resolved normally as in Figure 31 top.

 If the glider penetrates the shear boundary briefly at the cross-wind position (instead of into a headwind) the effect is to cause a sudden increase in angle of attack which increases lift and drag but, more importantly, tilts the lift-drag resultant forward giving the glider a kick, like a pulse of thrust,  in the direction of flight (figure 31 bottom). This is similar to the resolution of forces which enables a helicopter rotor to continue spinning in auto-rotation. The pulse of thrust, the kick, then increases the speed on every circuit, the circuit being completed in the still air below the shear boundary with a small loss of airspeed.

 It is similar to a kind of vertical axis wind turbine. Like a cup anemometer, it has a vertical axis  and three arms, but with a vertical airfoil blade at the end of each arm instead of the cup. Like the cup anemometer, this device will also rotate regardless of the wind direction. Just imagine instead of three arms, a single arm and single airfoil and there is your glider in a steep turn sipping at the wind at the upwind peak of the orbit.

Also, although these glider models are achieving very high ground-speeds, recorded by radar speed guns, it may be that they are generating and flying within, a rotating vortex in the otherwise stationary air in the lee of the hill. This would mean that the airspeed and (G loading) are not as high as the ground speed. This kind of high-speed, circling flight is not what the albatross does. However the albatross is thought to be able to soar upwind and the kick may be an explanation of how it does it.


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