Cross-Country Soaring 2004
User Guide 
7.2 Ridge Soaring
As air (wind) flowing across the earth’s surface encounters a mountain (slope), the air is deflected upwards in order to pass over that mountain. Likewise, as the air passes the top of the mountain, it descends on the other side to the valley below. Thus the combination of wind and slopes results in upward and downward air movement. The soaring pilot can use the upward movement of air (i.e., slope lift) to stay aloft or even climb. While slope lift generally isn’t as strong as and doesn’t offer as high of altitude gains as thermal lift, it can be used for prolonged periods of time (as long as the wind keeps blowing at about the same strength and direction). Generally speaking, slope lift isn’t dependent on time of day or sky condition, only on wind speed and direction.
The most obvious factor that influences the strength and vertical extent of slope lift is the wind’s speed. Generally speaking, the stronger the wind (i.e., the higher the wind’s speed), the stronger the slope lift and the greater its vertical extent. The “vertical extent” is simply how high above the slope the slope lift reaches. In lighter winds, there is little momentum in the deflected air, so it is quickly “turned” downward, returning to horizontal movement (no lift). Higher winds take longer to “turn”, so the slope lift in higher winds reaches a higher altitude above the slope.
In any given wind conditions, the slope lift strength will be greatest at the lowest altitude above the slope. The lift strength will gradually diminish with altitude, until an altitude is reached at which you can maintain a constant altitude but can’t climb any higher (without a thermal, anyway). To climb as high as possible in slope lift, fly at minimum sink speed. To race in slope lift, fly as low as possible over the slope, flying as fast as you can without getting dangerously close to the ground.
Another factor that influences the strength and vertical extent of slope lift is the direction faced by the slope. Ideally, the wind will blow straight in your face, if you stand atop the slope facing directly downhill. Soaring pilots refer to this as “blowing straight in”, which gives the maximum slope lift possible for the given slope steepness and wind speed. If the wind is somewhat in your face but is also somewhat from the side, the wind is said to be “crossing”. The greater the “cross”, the weaker the slope lift. Crossing wind has a compounding effect to weaken slope lift, as the wind will try to go “around” the slope rather than over it.
Another factor is the steepness of the slope. A sheer, or vertical, cliff is the steepest slope possible (let’s not get into undercut cliffs). The obvious rule of thumb is… the steeper the slope, the stronger the slope lift and the greater its vertical extent. Use this knowledge to maximize the slope lift over a given slope by finding and using the steepest parts of the slope. The best slope lift won’t necessarily be at the highest point on a ridge, if that ridge gets shallow (less steep) at its top.
While wind can generate consistent, easy to predict slope lift, it can also produce consistent, easy to predict slope sink. Air that flows up as it passes over the upwind slope of a ridge will flow back down as it passes over the downwind slope of the ridge. After all, what goes up must come down. Avoid flying low over the downwind side of a ridge, and you’ll avoid this unwelcome “down elevator”.
Ridges often have “gaps”, or sections where the continuous shape of the ridge is interrupted. Use all the above knowledge about slope lift to most efficiently cross these gaps. Gaps are most challenging in crossing winds, where one side of the gap will produce nice lift but the other side will produce no lift – and maybe even sink. If crossing a large gap while flying in a crossing headwind, you may be forced to either wait on a thermal to lift you high enough to guarantee the glide across or just cross your fingers and go for it, hoping to find a thermal while crossing the gap.
Fore more about Cross-Country Soaring (CCS) slope lift/sink, see Creating and Using Slope Lift.
Simulation Details
CCS employs a more realistic simulation of slope lift/sink than is included with Flight Simulator or can be created using scenery (BGL) lift/sink with the same level of effort. The CCS model, however, isn’t perfect. The CCS model considers only the following factors:
· wind (speed and direction) at your current position (including altitude)
· __________ the ground directly beneath you
o your altitude above
o the steepness of
o the direction faced by any slope in
In reality, slope lift and sink are affected by more terrain than just the point directly beneath you. For example, you may find sink over flat ground just downwind of a slope, or you may find lift over flat ground just upwind of a slope. This latter case is common at many real-world cliff soaring sites. Also, in reality, the wind conditions at ground level on the slope are more important than the wind conditions at your altitude (probably hundreds of feet AGL). However, the only wind conditions CCS can measure are those at your current position. Also, in reality, solar heating/cooling can affect slope lift/sink. For example, at my home flying site, late day solar cooling causes the air to cool in a gap just downwind of the landing field. This cooled air “slides” down the gap and causes a reversal of the wind direction in the lowest hundred feet or so of air over the field. Likewise, direct solar heating on a slope can improve the lift on that slope.
The CCS model simulates the terrain as a collection of three second (latitude/longitude) square areas. Three seconds is a maximum of about 300 feet, so this approximation can be as coarse as to treat every point within each 300x300 foot area as having roughly the average slope characteristics of the whole area. Also, this approximation can cause slight (less than 150 foot) misalignments with the terrain. Doubling the precision of this approximation (using 1.5 second square areas) would quadruple the size of CCS slope data files and the amount of time required to generate slope data for an area.
Future versions of CCS may improve slope lift/sink modeling. Some improvements will simply not be practical, however, if the cost (development, disk space, processing efficiency, etc.) is simply too great for the expected benefit. This explanation is given for two reasons. First, it gives the typical user enough of an understanding to optimize slope effects using CCS and to determine if CCS is functioning properly regarding slope effects. Second, it gives interested developers some “food for thought” on how to go about improving the simulation model.