Aviation Calcs

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MacCready Speed-to-Fly Calculator

In a glider, the fastest way around a course is not always the flattest glide. Given a sink polar and an expected climb rate in the next thermal — the MacCready setting — there is a speed to fly that minimizes the time spent both gliding and climbing. This page fits a quadratic polar from two or three points (or from coefficients you type) and returns that speed, the still-air best-glide speed, and, when the polar supports it, minimum-sink speed.

Sink and the MacCready climb are positive numbers in the same unit (knots of vertical speed, or ft/min). Two polar points fit w = a + c V². A third point adds a linear term. Airspeeds are true, in still air.

Result

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Speed to fly is √((a + Mc) / c)

A quadratic sink polar w(V) = a + b V + c V² (sink positive) and a next-thermal climb Mc satisfy the classic MacCready condition w + Mc = V w′. The linear term cancels, and the speed to fly is Vstf = √((a + Mc)/c). With Mc = 0 that is also the still-air best-glide speed, √(a/c). Minimum sink, when the polar has a useful linear term, is at −b/(2c). A polar w = 1 + 0.0025 V² (knots of airspeed and sink) and a 3-knot MacCready setting therefore fly at 40 knots, with 5 knots of sink; still-air best glide is 20 knots on that same (intentionally simple) polar.

A real glider polar is measured, not invented, and speed to fly also changes in sinking or rising air between thermals and with a headwind or tailwind (a cruise-speed addition this page does not apply). Use the polar from the flight manual or a logger, not a guess. Related tools: glide distance and the drag polar / L/D max calculator.

Sources

Frequently asked questions

What MacCready setting should I use?

The expected climb rate in the next thermal, in the same units as sink. Mc = 0 is still-air best glide — useful when you only need to reach a point, not to average a high cross-country speed.

Why does the linear coefficient disappear?

The MacCready condition is w + Mc = V dw/dV. For a quadratic, that reduces to a + Mc = c V², independent of b. Minimum-sink speed still depends on b.

Does this include wind?

No. A headwind raises the speed to fly and a tailwind lowers it. This page is the still-air MacCready speed.