Radio-control 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. However, it has been found that it is possible to maintain height and position in a circular flight path and achieve huge speeds, on the lee-side of a hill 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 into the fast moving air, 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 in still air below the shear boundary with a little loss of speed and the cycle is repeated.
So what is wrong with that? In this form of dynamic soaring both airspeed and ground-speed must increase but the theory says that airspeed will increase only if actual speed (ground-speed) does not increase. So there is something wrong with the theory. Also, the wind gradient has maximum effect only if the glider penetrates the shear boundary on upwind or downwind headings. If they are penetrating the shear boundary on more cross wind headings the effect is much reduced. In order for both airspeed and ground-speed to increase there has to a force acting in the direction of flight to achieve that acceleration. Also, 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, in albatross dynamic soaring, airspeed can be maintained in a windward turn but in the case of RC dynamic soaring the ‘windward’ turn is made in still air with reducing airspeed and ground-speed and there is no gain of energy, so that the Windward turn theory does not apply either. 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 kind of 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.
This form of dynamic soaring is neither the same as what the albatrosses do and nor is it the Rayleigh cycle. 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. Such mechanisms have axles or rotor hubs to constrain the motion of the rotor and ensure a permanent high angle of attack. The glider in free-flight can only sustain a large angle of attack briefly so that in the this case the effect is transitory.
This is how it works see figure 29 which is a plan view of the glider circling to the right with a steep angle of bank. At position 1 below the shear boundary in still air, the glider has a small angle of attack and the lift and drag forces are resolved to give resultant R.
At position 2 at the cross-wind position, the glider penetrates the shear boundary briefly. The glider continues its original curved path over the ground but the wind suddenly appears from the left and causes a change in the direction of the air-velocity and a slight increase in air-speed. The difference in the direction of the ground-velocity and the air-velocity is the same as the angle of drift but in this case the drift is caused by the vector addition of wind-velocity and ground-velocity rather than the the vector addition of wind-velocity and air-velocity.
The sudden increase in angle of attack, increases lift and drag and load-factor ie G-loading but, more importantly, tilts the lift-drag resultant R forward giving the glider a kick, like a pulse of thrust T, in the direction of its ground-velocity and component C which acts as the centripetal force to keep the turn going. 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, increases the ground-speed and the airspeed further increases as soon as the glider drops below the shear boundary. Thus both ground-speed and air-speed increase on every circuit, as recorded by radar speed detectors, 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 or a cup anemometer. The wind-turbine 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, with the RC pilot maintaining the circle.
This kind of high-speed, circling flight is not what albatrosses do. The albatrosses leeward turn is flown as a low G wing-over at a relatively small angle of attack. In that case the combination of the aerodynamic forces and the angle of drift cause the ground-speed to increase throughout the leeward turn but the air-speed still reduces slightly. The amplitude of the albatross leeward turn is relatively small, only about 60 degrees before it reverses direction into another windward turn; whereas the RC glider leeward turn is even shorter, say 10 or 20 degrees with the rest of the circle its ‘windward turn’ effectively flown in still air.
So RC dynamic soaring using a stationary hill, is quite different to classic albatross dynamic soaring which involves a continuous downwind drift. However, the albatrosses are thought to be able to soar upwind and the ‘kick’ may be an explanation of how they do that, leading to a third method of dynamic soaring which is explained in the Upwind Dynamic Soaring page.