Speed to Fly: Wind and Sink Change Everything
Best glide speed is the answer to a still-air question. The moment there is wind or the air around you rises or sinks, the airspeed that carries you farthest over the ground changes — sometimes by a lot. Speed to fly is the discipline of picking the right airspeed for the air you are actually in.
The core insight fits in one sentence: your glide over the ground depends on groundspeed and total sink, not on airspeed alone. Everything below is that sentence unpacked into a picture you can read off the polar curve.
Why wind changes your best speed
Glide into a 20 km/h headwind at a best-glide airspeed of 36 km/h and only 16 km/h remain over the ground — while you sink exactly as fast as before. Your glide ratio over the ground collapses. Every second spent in that headwind costs ground you are not covering.
Push the speedbar and your sink rate increases — but your time exposed to the headwind shrinks faster than the extra sink costs you. Up to a point, flying faster into wind gets you farther, not just there sooner. Beyond that point the steepening polar takes back more than the wind argument gives.
With a tailwind the logic flips: the wind is doing free work for you, so you want to stay in it longer. Slowing down toward minimum sink stretches your time in the helping air and flattens your glide over the ground.
The shifting-origin trick
Recall that best glide in still air is the tangent line from the origin to the polar. The beautiful generalisation: with wind, slide the origin along the horizontal axis by the wind speed — into the wind for headwind, the other way for tailwind — and draw the tangent from there. The new tangent point is your speed to fly.
A headwind moves the origin toward the curve, so the tangent touches farther right: fly faster. A tailwind moves the origin away, so the tangent touches farther left: fly slower. The picture does the algebra for you, and this app draws it live as you move the wind slider.
Sinking or rising air shifts the origin vertically instead — down for lift, up for sink. Sinking air makes the tangent touch farther right, again telling you to speed up. The two shifts combine, so any wind-plus-airmass situation reduces to one tangent line.
Sink is a headwind pointing down
Flying through air that descends at 2 m/s roughly triples a paraglider's sink rate. If you dawdle through it at minimum sink speed, you maximise the time spent losing height. The correct reflex is counterintuitive at first: push the bar and get out.
Run the numbers and it stops feeling strange. Crossing a 2 km wide sink band at 30 km/h takes four minutes; at 50 km/h it takes under two and a half. The extra sink from the bar is small compared with the 2 m/s the airmass takes from you either way — shortening the exposure wins.
In lift, the same reasoning inverts: slow down, spend longer in the rising air, and if it is strong enough, stop gliding and turn in it. The boundary between 'slow down' and 'stop and climb' is exactly what the MacCready setting formalises.
Rules of thumb for the real air
Speed up into headwind and in sink; slow down in tailwind and in lift. That one line covers most decisions. Into a stiff headwind, half to full bar is usually right; downwind, trim or a touch of brake toward minimum sink.
Respect the paraglider-specific caveat: our polars steepen quickly on bar, so the last centimetres of speedbar buy little groundspeed for a lot of sink. In a light headwind, trim or quarter bar is often nearly optimal — save full bar for strong wind and strong sink.
Do not chase the theoretical optimum km/h by km/h. The polar is shallow near the optimum, so being 5 km/h off costs almost nothing, while flying the wrong side of trim in a 30 km/h headwind costs everything. Get the direction of the correction right and be roughly right about its size.