Should you be carrying water?
On the polar alone this question has no answer. Ballast leaves your glide ratio completely untouched and shifts the whole curve to higher speed, so water wins at every MacCready setting above zero — the polar library demonstrates exactly that on real published curves. The crossover exists only because ballast costs you climb, and climb is not on the polar. That is what this page models.
In a core
Water pays above
Climb, dry
Climb, ballasted
Cross-country
The answer is a curve, not a number
How strong a core has to be before water pays, plotted against how wide it is. Narrow cores punish ballast hard; wide ones barely notice it.
The thing the polar cannot show you
Ballast raises your wing loading by — a factor of — and every speed rises by the square root of that ratio, about . That includes your stall speed, and therefore the speed you must circle at, and therefore the radius of the circle you fly. A wider circle sits further out in a core that is strongest at the middle. That is the entire mechanism, and none of it appears on a glide polar.
Climb achieved, by speed and bank
In a core of radius.
No optimum is marked, deliberately. The speed and bank that maximise climb are a recommended airspeed and bank angle, and this site does not publish those. The height of the curve is a performance figure and is published; where your own peak sits is yours to read.
- At ° you would have to fly to keep × the stall speed at that bank, which is above the ceiling this model explores.
Lower the stall margin below and they come back — which is the honest demonstration that the margin is not a detail.
The thermal
Where the lift falls to zero. Strength is taken as parabolic from the middle out.
The strength right in the middle, not what your averager reads.
The aircraft
All-up mass ×, so every speed rises about .
From your flight manual, at the dry mass above. Never looked up for you.
Lift is averaged across the span, which is why a big wing struggles in a small core.
How you fly it
Against the stall speed at the bank angle flown, not the wings-level figure — those differ by 19% at 45°, and the difference is the margin. This input moves the answer more than any other.
The polar
A standard-class curve at 33 kg/m², dry. A worked example so the page works before your manual is open — not a claim about any particular type.
Read three points off the polar graph in your flight manual, at the dry mass you entered above. The ballasted curve is derived from it, never entered separately — a hand-entered wet polar that disagreed with the dry one would silently break the comparison this page exists to make.
What this assumes, and what it can't know
This is ground school, not a flight computer. It exists to show why the ballast decision is a decision at all, and where the trade-off actually sits. It does not know what today's thermals are doing, and neither do you until you are in one.
The circle is assumed perfectly centred on the core, which nobody flies. Real centring errors cost climb in both configurations, and they cost the ballasted one more, because it is flying a bigger circle in the same gradient. So the model is optimistic about ballast, and the real crossover sits above the one shown.
The thermal model is one parabola. Strength falls as the square of the distance from the middle and is clamped to zero past the stated radius — no shear, no drift with height, no ragged edges, no gusts. Real thermals have all of those, and lift is averaged across your span rather than sampled at the wingtip, which is a further simplification.
The stall margin is doing more work than any other input. It is pinned to the stall speed at the bank angle flown, which is the conservative reading, and at the default of 1.30 a ballasted standard-class ship has to circle fast enough that steep bank leaves the modelled speed range entirely. Real pilots thermal closer than that. Move the slider and watch the answer move with it — that sensitivity is a finding, not a defect.
Cross-country speed here is the closed-loop MacCready result: the climb each configuration achieves is used as its own MacCready setting, and the cruise is flown accordingly. That is the only self-consistent way to rank two aircraft that climb differently, but it assumes every thermal is the one described above and that you take all of them.
Nothing here is about the decision to dump. Ballast interacts with your flight manual's limitations, with wave and rotor, with a wet field, and with landing weight. Those are placarded matters for your aircraft and this page carries none of them.
Not yet reviewed by anyone but us. If you instruct, or one of these assumptions is wrong, we would genuinely rather hear it than not — tell us and we'll credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.
Where these numbers come from
Why ballast leaves your glide ratio untouched, what the tangent construction is actually showing, and how two published polars can differ by less than the gap between a factory figure and a measured one.