Polar Figures vs the Glide You Actually Get

Every glider comes with performance figures, and every pilot eventually discovers their wing does not achieve them. That gap is not a defect in the wing or the pilot — it is the difference between a measurement taken under ideal conditions and a flight taken in the air that actually exists.

Two ways to get a glide ratio

The glide ratio calculator accepts either.

From the polar: airspeed divided by sink rate. A wing doing 38 km/h at 1.1 m/s gives 9.6:1. At 1.2 m/s it gives 8.8:1 — a tenth of a metre per second of extra sink costs nearly a whole point of glide.

From a flight: horizontal distance over height lost. 8,000 m covered while descending 1,000 m is 8:1, and that figure already contains everything the day did to you.

The first is what the wing can do. The second is what it did.

Why they differ

• The air is not still. A glide through sinking air loses height faster than the polar predicts. Since sink is commonly found near lift, a cross-country glide is rarely through neutral air.
• Speed matters and is hard to hold. Best glide occurs at one particular airspeed. Flying faster or slower than it costs performance, and holding a precise speed in moving air is genuinely difficult.
• Turbulence costs energy. Every correction, every surge and recovery, converts height into something other than distance.
• Turns cost more than they look. Sink rate rises in a bank, so a glide with several corrections is not the same as a straight one.
• The wing is not new. Porosity, line stretch and trim drift all degrade performance, gradually enough that nobody notices.

Measure your own, and use that

The practically useful number is the one from your own logs across a season, not the one on the manufacturer's chart. Take several straight glides in reasonably neutral air, compute distance over height lost for each, and take a conservative figure from the spread rather than the best one.

That number will be lower than the polar. It is also the only one worth planning with, because it is the only one that includes your flying, your wing's current condition, and the kind of air you actually fly in.

Then apply the wind

Whatever figure you settle on, remember it is an air glide ratio and the ground sees something different. A 9.6:1 wing into a 20 km/h headwind has a ground glide of 4.55:1. Using a polar figure and ignoring wind compounds two optimistic errors in the same direction, which is the specific mistake worth not making.

The conservative habit is to plan with a measured glide, apply the headwind case, and then keep options that do not depend on the number being right. Final-glide judgement is taught and practised under supervision for good reason, and a ratio computed on the ground is a way to understand the problem rather than a way to solve it in the air.

Measuring your own glide properly

A few practical points, because a badly-measured glide is worse than the polar figure it was meant to replace.

Use straight glides in the calmest air you can find — early or late, away from active thermals. Measure over a meaningful distance rather than a few hundred metres, so that a single patch of sink does not dominate the result. Take several and look at the spread rather than the best one, because the best one is the flight where the air helped you.

Correct for wind if you can, or measure on days near still. A glide measured downwind and quoted as an air glide is the single most common way pilots end up with an optimistic figure they then trust.

Wing condition is a slow variable

Performance degrades gradually enough to be invisible. Fabric porosity increases with age and UV exposure, lines stretch unevenly — unsheathed lines particularly — and trim drifts as a result. A wing several seasons old with unchecked lines is not flying the polar it was certified with.

Line checks and porosity testing are routine service items, and a trim check after any significant number of hours is standard. If your measured glide has drifted downward over a season, that is worth investigating on the ground rather than attributing to your flying.

Why the conservative figure is the right one

There is a temptation to plan with your best measured glide, because it demonstrably happened. It happened in the most favourable air you flew in that season.

Planning is about the flight you might have, not the one you had on the best day. Taking a figure from the lower end of your spread, applying the headwind case, and keeping alternatives that do not depend on it is the arithmetic equivalent of leaving margin — and margin is the thing that makes the difference when one of the assumptions turns out to be wrong.