One curve, every answer.
Take your glider to smooth air, hold a series of constant airspeeds, and measure how fast the air carries you down at each one. Plot speed against sink and you get the speed polar — the single most useful piece of paper in soaring.
Every number your flight computer gives you — best glide speed, speed to fly, final glide altitude — is read off this curve. The instrument does nothing mystical: it stores your polar and applies geometry to it, hundreds of times per second.
Two points on the curve matter before anything else. The top of the curve is minimum sink — the speed for staying up longest, your thermalling speed. Slightly faster sits best glide — the speed that covers the most distance per meter of altitude. They are not the same speed, and confusing them costs you kilometers.
Best glide is a tangent.
Draw a straight line from the origin of the chart — zero speed, zero sink — so that it just touches the polar. The touching point is your best glide speed, and the slope of that line is your best glide ratio. For the 15-metre standard-class polar below, the tangent touches near 94 km/h at a glide ratio of about 42:1.
Here is the trick that unlocks the rest of the article: move the origin of that tangent line, and the touching point moves too. Everything MacCready discovered is contained in that one sentence.
Set your MacCready.
The blue line is the tangent drawn from a point MC metres per second above the origin. Slide MC up and watch the touching point — your speed to fly — move right. At MC 0 the tangent gives best glide; the average cross-country speed assumes your next climb is exactly MC strong.
What MacCready really means.
Paul MacCready's insight, later carried to a world championship title, was that cross-country soaring is a chain of climbs and glides — and the chain has one optimal speed. Glide too slowly and you waste time in the air between thermals. Glide too fast and you burn altitude you must pay back circling. The optimum depends on exactly one number: the strength of your next climb.
That number is your MC setting. It is not a mood dial and not a risk dial — it is a forecast. Set MC to the average climb you honestly expect in the next thermal, and the tangent construction above hands you the mathematically fastest glide speed between here and there.
The same logic runs continuously in flight: in sinking air the effective origin shifts and your computer commands more speed; in rising air it commands less. Followed smoothly, this becomes dolphin flying — climbing without ever stopping to circle.
In your cockpit.
Every LX vario system carries this construction built in. Set MC with one knob turn; the speed-to-fly vario then shows deviation from the optimum, and the audio follows it — fly by ear, eyes outside. On final glide the same tangent decides your arrival altitude: a higher MC means a faster, steeper, safer-margin glide.
Wind changes the final glide — gliding to a fixed point, a headwind shifts the optimum faster still (between thermals that drift with the airmass, it does not). How your computer even knows the wind is the next article.