Hill soaring uses the band of rising air that forms where wind blowing across level ground is pushed up the windward face of a slope, and a glider stays up only if its own sink rate is smaller than the air’s rise. Sink rate equals airspeed divided by glide ratio, so at 70 km/h (19.4 m/s) a glide ratio of 8 sinks 2.4 m/s and a ratio of 20 sinks 0.97 m/s. A heavy, draggy aircraft converted from a powered design may sink faster than any ridge lifts it, and the lee side of the hill is dangerous turbulent air.
Part of Aerodynamics and Flight
Staying aloft in the rising air on the windward side of a ridge, and the hazards that go with flying close to a slope.
How Ridge Lift Forms
When wind meets a slope it cannot pass through, so it flows up and over. In front of the face, air rises and the glider can use it. The band is limited: it extends upward by a height related to the hill’s height, grows with the wind’s strength and depends on the wind meeting the ridge close to square-on. On the lee side the air sinks and swirls, and behind a ridge in a strong wind it forms rotor turbulence that no light aircraft should enter. The corpus’s Wind Direction Reading, Wind Speed Estimation, Wind Patterns and Terrain Reading cover reading the wind and the ground; the strength of lift at a given hill is found by watching it, not from a number here.
The Test Is Sink Versus Lift
Sink rate = airspeed ÷ glide ratio. At 70 km/h = 19.4 m/s: with a glide ratio of 8 the sink is 19.4 ÷ 8 = 2.4 m/s; with a ratio of 20 it is 19.4 ÷ 20 = 0.97 m/s. The two ratios are illustrations; the aircraft’s own ratio is measured in still air (Glider Testing), and its minimum flying speed is above its stall speed. If the lift on your hill is smaller than the sink, you will descend whatever you do. The carrier’s engine-converted airframe, with struts, wires and a fixed gear, is likely to be draggy and heavy, and slow flight near the stall is not the way to make up the difference. Whether it can soar is an open question that testing decides.
Rules If A Slope Is Used At All
- Fly along the face, in front of it, and turn away from the slope at the end of each leg, never toward it. Turning into the hill, low, has no escape.
- Keep clear of the lee side and of the hill’s top in strong wind.
- Stay high enough to reach the landing field at the bottom, which you have walked and measured.
- Do not go in gusty or strong wind; the wind speed has to be within what the aircraft’s speed allows with margin, since a wind approaching the glider’s flying speed means it cannot make progress and can be blown backward over the hill.
- Never fly ridge lift alone; have a person watching and first-aid cover (Bleeding Control).
- Weather can change fast (Storm Safety, Weather Forecasting).
Common Mistakes
- Counting on lift that is not there.
- Soaring a slope because a book says it works.
- Flying too slowly to stay safe from a stall while turning near the slope.
- Landing in the lee.
Safety
A slope leaves no room for error
Close to terrain, a stall, a gust, a turn into the hill or a patch of sink ends in a collision. Rotor behind the ridge can throw a light aircraft out of control. Do not soar a slope in an unproven aircraft, and do not do it at all without having flown low, controlled, repeated tows and glides first (Envelope Expansion).
What’s Next
- Learn next: Envelope Expansion, how far to go and how slowly
- Combine with: Tow Launches, the alternative way up