The polar you bought is not the polar you fly.
A factory polar comes from a flight-test campaign flown with sealed gaps, taped joints, a waxed wing and a test pilot holding speed to half a knot. It is an honest measurement — of the best day of that airframe's life. A club glider with worn gelcoat, unsealed control gaps and a canopy that whistles starts a few percent below the book before the first insect of the morning.
That would hardly matter if the polar were only a chart on the wall. It is not. It is the reference curve inside your flight computer, and every number the instrument produces — speed to fly, netto, arrival altitude — is geometry applied to it. If the stored curve is optimistic, every one of those numbers inherits the optimism.
Bugs are a leading-edge problem.
Modern glider airfoils earn their performance by keeping the boundary layer laminar over a large fraction of the wing. Laminar flow is fragile: a squashed insect a millimetre tall, sitting exactly on the leading edge where the flow is still deciding what to be, trips it into turbulence early. Each bug drags a turbulent wedge behind it, and a summer evening at a lowland site can paste a full row of them along the span.
Measurements on contaminated laminar wings put the damage at anywhere from a few percent to 10–25% of glide ratio for heavy contamination. The insidious part is the trend: the degradation accumulates one invisible increment at a time, over hours, while your reference for "normal" drifts along with it.
Rain is worse.
A water film does everything a row of bugs does, everywhere at once — and then it attacks the other end of the polar too. Rain does not just add sink: on some airfoils it degrades maximum lift, which raises the stall speed and softens stall behaviour warnings. A few well-known wing sections are notoriously rain-sensitive; all of them are worse wet than dry.
The operational answer in rain is blunt: fly faster, add margin to every approach, and stop trusting the low-speed end of the polar entirely. The computed best-glide speed of the dry curve means little on a wing that is carrying its own weather.
The same glider, dirtier.
Watch what the slider does not move: the red dot stays at the same airspeed. Uniform degradation leaves the best-glide speed where it was — the speed barely changes; the distance is what dies. From 2000 m, twenty percent of bugs is fourteen kilometres of reach.
Tell the computer the truth.
This is exactly what the bug setting on your vario system is for. It scales the stored polar's sink by the percentage you set, and every downstream number — speed to fly, netto, required altitude — updates to match the wing you actually have. A final glide computed clean and flown dirty arrives low by precisely the "kilometres lost" figure in the tool above.
The discipline that works is to raise the setting through the day, as the leading edge collects its evidence — not after the first scare on a marginal glide. If the vario shows persistent sink the airmass cannot explain, or the achieved glide keeps falling short of the computed one, the instrument is telling you the polar it was given is no longer the wing it is flying on.
In your cockpit.
Set the bug factor with the same seriousness as MacCready — it is the other half of the arrival equation. A few percent after a clean morning, ten or fifteen by a buggy afternoon, more in rain, and back to zero only after the wing has actually been cleaned. The numbers the system gives you are only ever as honest as the polar you let it use.
Where those numbers finally get spent is the glide home — and that arithmetic, margins included, is the final glide article.
One bug setting corrects speed-to-fly, netto and arrival altitude across every display on the bus.
The airspeed and vario data that show a wing underperforming its stored polar in flight.
Polar and degradation editable per flight, not per firmware — tell the computer about today.