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.
R = V² ÷ (g · tan φ)
scirc(V, φ) = n3/2 · s(V / √n), n = 1 / cos φ
- R
- radius of the circle you are flying
- V
- true airspeed
- φ
- bank angle
- n
- load factor
- s(V)
- your straight-flight polar
- scirc
- sink rate in the banked turn
The second line is why ballast is not free. In a banked turn the wing has to carry n times your weight, so the circling polar moves out by √n in speed and up by n3/2 in sink: the same glider sinks faster in a circle than its straight polar suggests — and a heavier glider has to fly the circle faster, which widens it. The climb penalty is what the straight polar alone cannot show, and it is the reason /tools/polar deliberately will not answer “should I carry water”.
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 checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will 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.
Common questions
When is it worth carrying water ballast in a glider?
When the thermal cores are strong enough, for their width, that water improves your cross-country speed despite the climb it costs. On the polar alone water wins at every MacCready setting above zero, because ballast leaves the glide ratio untouched and shifts the whole curve to higher speed; the threshold exists only because ballast costs you climb, and climb is not on the polar. Ballast also interacts with your flight manual's limitations and with landing weight, which are placarded matters for your aircraft.
Why does water ballast reduce a glider's climb rate?
Because a ballasted glider has to circle faster, and so on a wider circle, which sits further out in a core that is strongest at the middle, where the lift is weaker. Water multiplies the all-up mass, and so the wing loading, by the same factor, and every speed rises by the square root of that factor, stall speed included: that sets the speed you must circle at and therefore the radius of the circle you fly. None of that appears on a glide polar.
Does thermal width matter when deciding to carry water ballast?
Yes: narrow cores punish ballast hard, while wide ones barely notice it. The core strength at which water starts to pay depends on how wide the thermal is; at some widths water never pays, and at some neither a dry nor a ballasted glider can climb. Wingspan matters as well, because lift is averaged across the span, which is why a big wing struggles in a small core.
Do thermal centring errors cost a ballasted glider more climb?
Yes: centring errors cost climb whether you are dry or ballasted, but they cost the ballasted glider more, because it is flying a bigger circle in the same gradient of lift. So a ballast calculation that assumes a circle perfectly centred on the core, which nobody flies, is optimistic about water, and the real core strength at which water starts to pay sits above the one it shows.
Why does a glider sink faster when circling than in straight flight?
Because in a banked turn the wing has to carry more than the glider's weight: n times it, where n, the load factor, is 1 divided by the cosine of the bank angle. That moves the circling polar out by the square root of n in speed and up by n to the power 3/2 in sink, so the same glider sinks faster in a circle than its straight polar suggests. A heavier glider also has to fly the circle faster, which widens it, and that climb penalty is what the straight polar alone cannot show.