The polar, and what MacCready does to it
Speed to fly is one tangent line drawn on one curve. Almost nobody sees it drawn — it arrives as a number from a flight computer, or as a rule of thumb about pushing forward in sink. Drag the handle below and watch the line move.
Speed polar
Drag the dark handle — it is the point the tangent is drawn from, at (headwind, minus the sum of your MacCready setting and the airmass sink). That is the whole construction.
Speed to fly
Sink at that speed
Glide at that speed
Achieved XC speed
Best glide
Minimum sink
Density ratio σ
Beyond the published data for this polar. The curve was fitted over a narrower speed range, so this answer is an extrapolation — and a quadratic under-predicts sink at high speed, which means the number is optimistic. Treat it as indicative and add margin.
Conditions
MacCready is the average climb you expect in the next thermal, as your averager will read it. Not the netto reading, not what the last one gave you.
Polar is at . Ballast never changes best glide — only the speed you achieve it at.
A published polar is a sea-level curve. Both axes scale by 1/√σ with altitude — work out yours.
Your glider's polar
We do not ship a database of gliders, and the reason is on the page below. Put your own aircraft's published figures in — it takes about thirty seconds with the flight manual open.
A standard-class glass ship at 33 kg/m², near enough an LS 4 — a teaching curve, not a claim about any particular aircraft. Best glide 39.8 at 96 km/h, minimum sink 0.60 m/s at 75 km/h. Use it to see how the construction behaves, then put your own numbers in.
Every flight manual prints these four, and any three of them fix the curve completely. The fourth is then a check rather than an input — so pick which one you would rather have worked out for you.
Those three speeds imply a best glide of , against the :1 you entered — .
A gap this size means the published numbers are not mutually consistent with a quadratic polar. That is common, and it is usually the headline glide ratio that is the optimistic one. The curve here is built from the two speeds and the sink rate, because those are measurements; the glide ratio is a conclusion.
Read three points off the polar graph in your flight manual. Make one of them fast — that is what fixes the high-speed end, which is the end speed-to-fly actually lives at.
Sink positive: 1.05 means descending at 1.05. The fitted range becomes your slowest to your fastest point, and the curve is dashed outside it.
The wing loading the published polar was measured at — printed next to the graph in most manuals. Your own all-up wing loading goes in Conditions above.
The numbers behind the curve
| Glide ratio | ||
|---|---|---|
Greyed rows are outside the fitted range.
What this assumes, and what it can't know
This is ground school, not a flight computer. It exists to make the geometry visible — where the tangent touches, what moves when the wind changes, why ballast does not improve your glide ratio. Fly the aircraft and the instruments in front of you.
We ship no glider database, deliberately, and here is the arithmetic behind that. Given minimum sink and best-glide speed, the best glide ratio is determined — it is not a free number. Take the Standard Discus's own published figures, 0.59 m/s at 78 km/h and best glide at 100 km/h: they imply 41.9, against a published 45. A 7% gap, with no way to know which of the three numbers is wrong. Shipping that as "the Discus polar" would mean publishing a curve that contradicts its own source, so we ask you for your manual's numbers instead — the same reason the weight and balance calculator asks you to read your own weighing report.
A quadratic is a fit, not the physics. Outside the range it was fitted over it fails in one direction: it under-predicts sink at high speed, because real drag rise outruns a parabola, and below minimum-sink speed it knows nothing about the stall. Both errors flatter the glider, which is the dangerous direction, so extrapolated segments are dashed and extrapolated answers are flagged.
The bugs slider is crude and optimistic. One multiplier on the whole curve leaves best-glide speed unchanged, whereas real contamination raises profile drag and would lower it. So the modelled speed to fly is faster than reality and any height requirement derived from it is too low. It also ignores that rain raises your stall speed. Add margin.
Glide ratio here is through the air. Over the ground it is better with a tailwind and worse with a headwind, which is exactly what the tangent construction is showing you when you move the wind.
Not yet checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.
What the polar will not tell you
Ballast leaves your glide ratio untouched and shifts the whole curve to higher speed, so on this page water always looks free. It is not: it costs you climb, and the climb penalty is the entire reason there is a decision to make. That trade-off is not on this chart.
Read: what a polar actually tells you →Common questions
How do you work out speed to fly from a glider polar?
Draw one straight line from the point at (headwind, minus the sum of your MacCready setting and the airmass sink) so that it just touches the glider's speed polar: where it touches is your speed to fly, and that is the whole construction. The MacCready setting is the average climb you expect in the next thermal, as your averager will read it, not the netto reading and not what the last thermal gave you. Airmass sink counts as negative in rising air. Change the wind, the setting or the sink and the point moves, and the tangent with it.
How many points do you need to fit a glider polar?
Three, read off the polar graph in your flight manual, and one of them should be fast, because the fast point fixes the high-speed end, which is where speed to fly lives. A curve fitted only to slow points, such as the published minimum sink and best glide, can only reach it by extrapolation. The fitted range runs from your slowest point to your fastest, and beyond it a quadratic under-predicts sink at high speed, which flatters the glider.
How can you check whether a glider's published polar figures agree?
Work out one of the four figures a flight manual prints from the other three and compare it with the printed value: minimum sink, its speed, best-glide speed and best glide ratio are not independent, because any three fix the curve completely. A big gap means they are not mutually consistent with a quadratic polar. That is common, and it is usually the headline glide ratio that is the optimistic one: the two speeds and the sink rate are measurements, the glide ratio is a conclusion.
Do bugs and rain on the wing change speed to fly?
Yes: real contamination raises profile drag, which would lower your best-glide speed. A single bugs multiplier applied to the whole polar misses that, because it leaves best-glide speed unchanged, so the speed to fly it gives is faster than reality and any height requirement derived from it is too low. Such a multiplier also ignores that rain raises your stall speed, so add margin.