Reading the Polar Curve

Every paraglider has a performance fingerprint: for each airspeed you can fly, the wing comes down at one specific sink rate. Plot those pairs and you get the polar curve. It is the single most useful picture of what your wing can and cannot do.

You do not need mathematics to use it. Once you can find three points on the curve — minimum sink, best glide and trim — you can answer most everyday questions: how slowly can I circle, how far can I reach, and what does pushing the speedbar really cost me?

The axes and their conventions

The horizontal axis shows airspeed, usually in km/h — your speed through the air, not over the ground. The vertical axis shows vertical speed in m/s, and because a glider without an engine is always descending through the airmass, the curve lives below zero. By convention sink is drawn downward, so lower on the chart means coming down faster.

Each point on the curve is one steady flight state: pick an airspeed, and the polar tells you the sink rate that comes with it. The curve only covers speeds you can actually fly — from deep brakes near minimum speed on the left, through hands-up trim, out to full speedbar on the right.

Notice the shape: it is not a straight line but a bowed arc. Fly very slowly and sink increases because the wing is close to the stall; fly very fast and sink increases sharply because drag grows with the square of speed. The useful speeds sit in the shallow bowl between those extremes.

Three speeds worth memorising

Minimum sink is the highest point of the curve — the airspeed at which you lose the least height per second, typically with a touch of brake, a few km/h above stall. This is your speed for staying up: circling in weak thermals or scratching in ridge lift. It buys time aloft, not distance.

Best glide is where a straight line from the chart's origin just touches the curve. At that tangent point the ratio of forward speed to sink — your glide ratio — is at its maximum, so in still air this airspeed carries you the farthest per metre of height. On most paragliders it sits at or just below trim speed.

Trim speed is simply where the wing flies with brakes and speedbar released. It is your reference point: everything to the left of it needs brake, everything to the right needs bar. Knowing where trim sits relative to best glide tells you whether hands-up is already close to your optimal still-air glide — on many wings, it is.

What wing loading does to the curve

Fly the same wing heavier — top of the weight range, or with ballast — and the whole polar shifts to the right and down. Every characteristic speed increases: stall, minimum sink, best glide and trim all move to higher airspeeds, and the sink rates grow with them.

Here is the elegant part: the best glide ratio barely changes. The tangent line from the origin touches the new curve farther out, but at nearly the same angle. Loaded heavy, you glide just as far — you simply do it faster, and pay with a higher sink rate in weak lift.

That is why cross-country pilots load their wings up on strong days: more speed for the same glide, plus a more solid, collapse-resistant wing. On weak days the trade reverses — a lighter loading sinks less in gentle thermals, and staying up beats going fast.

What the polar cannot tell you on its own

The polar describes your motion through the airmass, and only that. The moment the airmass itself moves — wind pushing you back, sink pulling you down — the speed that is best over the ground is no longer the still-air best glide speed. That correction is the subject of speed to fly.

Also keep some healthy scepticism about the numbers. Published polars are measured in calm morning air with a test pilot; your wing, your harness drag and your flying weight all shift the curve. Treat the polar in this app as an archetype for learning the shapes and trade-offs, not as a certificate of performance.