Aerodynamic theory
Kutta Condition Explained: How Airflow Leaves the Trailing Edge
HENDOI · 26 October 2026
Potential flow allows many circulations around one shape. The Kutta condition chooses the one that lets the flow leave a sharp trailing edge smoothly, at finite speed.
The condition that makes lift definite
A potential-flow solution around a closed shape can carry any circulation. Each circulation is a different lift. Experiments do not behave that way: a given section at a given angle has one lift in attached flow. The extra fact that picks the circulation is the Kutta condition. At a sharp trailing edge the velocity would be infinite if the flow tried to turn the corner from the lower surface onto the upper, or the reverse. The condition says that does not happen. The rear stagnation point sits at the trailing edge, the upper and lower streams leave along the edge, and the speed there stays finite. That requirement fixes the circulation. The Kutta–Joukowski theorem, next, turns that circulation into a force.
Sharp, cusped and blunt
| Trailing edge | Statement of the condition |
|---|---|
| Finite angle, the usual airfoil | Stagnation at the trailing edge. Both surface speeds approach the same finite limit as you reach the point along either surface. In a panel code this is often “equal pressure, or equal speed, on the last upper and lower panels.” |
| Cusped, zero angle | The two surface speeds are equal and need not be zero. The edge is already aligned with itself. |
| Blunt or missing | There is no unique Kutta point. The code is applying a substitute, often equal pressure at two points you happened to call the trailing edge. Lift then depends on that substitute. |
What this means in a calculation
If the coordinate file has a gap, a crossed tail or an extra point, you have moved the place where the condition is applied. Small changes there change circulation, so they change lift and moment even when the nose is perfect. Viscosity does not throw the condition away. It thickens the edge by the displacement thickness and moves the effective leaving point slightly. A coupled viscous method is still using a Kutta-type condition on that effective body. A massively separated wake is no longer this picture: the flow is not leaving smoothly from a single sharp point, and a Kutta condition cannot invent the stalled lift.
When a polar changes because you closed the trailing edge, believe it. You changed the boundary condition that sets the lift, not a cosmetic line on the plot.
Where this sits in the series
In simple words: what this page is for
Potential flow allows many circulations around one shape. The Kutta condition chooses the one that lets the flow leave a sharp trailing edge smoothly, at finite speed.
This lesson exists to make one engineering idea clear in plain language. Kutta Condition Explained: How Airflow Leaves the Trailing Edge is written so a student, a UAV engineer or an OEM reviewer can follow the same story.
Why the topic matters
Engineers lose time when a title sounds clear but the page only sketches the idea. This article expands Kutta Condition Explained: How Airflow Leaves the Trailing Edge so you can answer three questions without jargon: what is the idea, when do you use it, and how do you check it in a HENDOI product.
Read the sections above for the core definitions. Use the steps below to move from reading into a real project trail you can share with a teammate.
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