Flight Performance, Planning and Loading
Performance fundamentals
Glider performance is described by the polar curve: sink rate versus airspeed. Minimum sink gives the longest time aloft; best glide ratio gives the greatest distance. Both shift with wing loading, and a headwind or sink moves the speed to fly faster; a tailwind or lift moves it slower. Understand the effect of mass on stall speed (proportional to the square root of the load factor), on best-glide speed, and on manoeuvring limits.
Planning
A cross-country plan includes: task length, expected meteorological conditions, terrain and outlanding options, airspace, and a final glide calculation with a safety margin. Calculate the final glide from height above the finish, expected sink, wind component, and glider polar; always keep a reserve. Diversion planning matters more than optimising the last kilometre.
Loading
Mass and balance: empty mass, pilot and parachute, ballast, and baggage must stay within the centre-of-gravity limits. Too far forward means heavy stick forces and poor flare; too far aft means light forces, poor pitch stability, and a risk of spin or tail-slide. Water ballast changes both mass and CG.
Traps
- Using still-air glide ratio with a strong headwind and assuming it holds.
- Forgetting that best glide speed increases with mass while the ratio stays roughly constant.
- Confusing minimum sink speed with best glide speed.
- Ignoring CG shift when dumping ballast or carrying a heavy pilot.
Worked example
Height 3,000 ft above the finish, still air, glide ratio 35:1, distance 15 nm. Required height = 15 × 6,076 / 35 ≈ 2,604 ft. Margin ≈ 400 ft - thin. With 10 kt headwind at 60 kt, effective ratio drops roughly to 35 × (50/60) ≈ 29, needing ~3,140 ft: the glide fails.