Soaring — One afternoon, no engine, and an atmosphere that is not on your side.

You are the glider pilot. You are released at six hundred metres over a ridge at one in the afternoon. There is no engine. Everything after this is a negotiation with air you cannot see: thermals that build and die on a twenty-minute clock, a ridge that works only while the wind holds, and a polar curve that tells you exactly what every decision costs. Stay up, go somewhere, and land where you meant to.

Timescale: one afternoon, in real time. Model: Three-degree-of-freedom point mass flown off published polars, in a thermal field with a lifecycle, a sheared wind, and first-order ridge lift.

What you will learn

• How a polar curve works, and why the fastest way round a task is not to fly as fast as you can.
• Why MacCready theory says to fly faster when the lift is strong — and slower when it is not.
• What a thermal actually looks like from the inside: a core, a sink ring, and a life of about twenty minutes.
• Why cumulus marks lift but is not lift, and why the cloud you set off for is so often finished when you arrive.

Scenarios

• Sandbox — Any glider, any day, nothing scored.
• First Thermal — Six hundred metres, one ridge, and thirty minutes to survive.
• Ridge Run — A strong wind across the ridge. Fifty kilometres without a single circle, if you are good.
• 100 km Triangle — A proper task on a proper day. Round the three turnpoints and home.
• Final Glide — Half past five, fifteen hundred metres, and forty kilometres to go.

What you control

• Type — Each has a published polar, and the whole flight follows from it.
• Country — Decides where the lift comes from: ridges make their own in a wind, flat country needs the sun.
• Thermal strength (m/s) — Peak core climb at the best part of the day. Two is a weak day; four is a very good one.
• Afternoon temperature (°C) — With the dewpoint, this sets cloudbase: about 125 m of height for every degree between them.
• Dewpoint (°C) — How dry the air is. A low dewpoint means a high cloudbase and a long working day.
• Wind at 1 000 m (m/s) — Ridge lift needs it. Thermals drift with it, which is why circling in a strong wind takes you somewhere.
• Wind direction (°) — The direction the wind is blowing towards, in degrees clockwise from east on the map.
• Blue day — No cumulus. The thermals are still there; nothing marks them, and you have to read the ground instead.
• Release time (h) — The convection peaks about an hour after noon and is finished by the evening.
• Length of the soaring day (h) — How long the convection lasts either side of its peak.

Questions

Why do I keep sinking even when I fly through a cloud?

Because a cumulus marks a thermal that reached cloudbase some minutes ago, and a thermal lives fifteen or twenty minutes. By the time it is a well-formed cloud it is often past its best. The wisps forming on the upwind edge are worth more than the solid cloud beside them.

What is the total-energy variometer doing?

It measures the rate of change of your total energy — height plus kinetic energy — rather than height alone. Pull up and a plain variometer shouts about lift that is only your own speed being converted; a total-energy vario stays quiet, and tells you about the air instead.

Why does circling more steeply cost so much?

Bank angle sets the load factor, and the polar scales as V → V·√n and sink → sink·n^1.5. At 45° that is about 40 % more sink; at 60° it is nearly triple. The trade is against turn radius: a wider circle in a narrow core misses the best of it.

What is the MacCready setting for?

It is your estimate of the climb rate in the next thermal. The theory then gives the cruise speed that maximises average cross-country speed, and the height you need for a final glide. Set it to what you are actually climbing at, not to what you would like to be.

What this model cannot do

• A point mass has no attitude and no stall. It flies at whatever speed the pilot asks for, down to a stall speed that is a hard floor rather than a departure.
• The thermal field is a sum of independent columns. Real convection organises itself into streets, convergence lines and shear-tilted cores, and knowing that is most of what a good pilot knows.
• Ridge lift is a local slope calculation with no flow model behind it. Real ridge lift depends on the whole upwind profile, the stability, and where the wave is.
• There is no wave, no rotor turbulence, no sea-breeze front and no overdevelopment. Each of them is the whole story on the days when it happens.
• The air is smooth. Real thermals are rough, and a great deal of real centring is done by feel through the seat rather than by watching an instrument.

Sources

• Reichmann, H. (1978) Cross-Country Soaring — The standard text. Speed-to-fly, thermal centring, final glides.
• MacCready, P. B. (1958) Optimum airspeed selector — Soaring, 22, 10–11. Two pages that reorganised the sport.
• Lenschow, D. H. & Stephens, P. L. (1980) The role of thermals in the convective boundary layer — Boundary-Layer Meteorology 19, 509–532. Where the thermal profile and lifecycle numbers come from.
• Wallington, C. E. (1977) Meteorology for Glider Pilots — Cloudbase, convective depth, and why a blue day is different.
• Welch, A. & Irving, F. (1977) New Soaring Pilot — Ridge and wave lift, and the practical arithmetic of both.