How to Detect Duplicate and Invalid Points in Airfoil Coordinate Files

Invalid means the polyline is not an airfoil

A panel method builds one panel between each successive pair of points. If two successive points are the same, the panel has zero length and the influence calculation divides by that length or invents a direction. If the points jump backward, the surface folds. If the upper and lower surfaces cross, the section has negative thickness and the “inside” is ambiguous. None of these are aerodynamic results. They are file defects. The cleaning lesson told you to remove them. This lesson is how to detect them on purpose.

Checks that catch the usual files

1. Non-numeric or blank tokens. A header line read as a coordinate becomes a huge jump. Reject rows that are not a pair of finite numbers.
2. Exact duplicates. Same x and y as the previous point. Delete them, except when you intentionally keep a trailing-edge pair and you have labelled it as a face rather than as a glitch.
3. Near duplicates. Distance much smaller than the local spacing, for example a point left behind by a bad spline knot. Use a tolerance tied to chord, such as a small fraction of the mean spacing, and report how many points you removed.
4. Order. After you split at the leading edge, x on the upper surface should move steadily from trailing edge to nose, or nose to trailing edge, according to the format. A reversal is a loop or a reversed stretch.
5. Closure and crossing. The first and last points should meet at a defined trailing edge. A segment of the upper surface should not intersect a segment of the lower surface.
6. Spikes. A point whose turning angle is far sharper than its neighbours is usually a digitizer hit, not a hinge, unless you know there is a hinge.

A short report, not a silent repair

HENFOIL’s geometry gate is this kind of report turned into ACCEPT, ACCEPT_WITH_WARNINGS or REJECT. A polar computed on a file that fails the duplicate and crossing checks is a polar of the defect.

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Where this sits in the series

In simple words: what this page is for

A zero-length panel, a repeated trailing edge, a crossed surface or a point that jumps backward will quietly ruin a panel solution. Detect them before the solver runs.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. How to Detect Duplicate and Invalid Points in Airfoil Coordinate Files helps you keep a clean trail from shape to polar to choice.

Why the topic matters

Engineers lose time when a title sounds clear but the page only sketches the idea. This article expands How to Detect Duplicate and Invalid Points in Airfoil Coordinate Files 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.

How to apply this in HENDOI

In practice: create a HENDOI account or log in, open HENFOIL, bring a section in (create, import DAT, or digitize), pass the geometry gate, climb the analysis ladder only as far as the decision needs, then compare and archive. Every calculated number should keep its method label.

When the section choice is ready for the aircraft, continue in HENSYNTH for mission, compliance and trades. Same account. Same plans.

What this page will not claim: a teaching polar or geometry note is not a flight-Re certificate. HENFOIL labels methods so you know what you bought. Tunnel data and professional judgement still sit outside any single software run.

Checklist before you leave this page

• Can you state the idea of How to Detect Duplicate and Invalid Points in Airfoil Coordinate Files in one sentence?
• Do you know which product owns the next action — HENFOIL or HENSYNTH?
• Will every number you keep carry a method label or a stored-run source?
• If you are new here, have you opened Register or Login?

Related reading on this site

• Active Flow Control on Airfoils: Suction, Blowing and Synthetic Jets
• How Adverse Pressure Gradients Affect Airfoil Boundary Layer Separation
• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• What Is Airfoil Aerodynamic Efficiency Across a Range of Flight Conditions?
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• HENSYNTH product page
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• Next: Airfoil coordinate interpolation
• Index: all HENFOIL airfoil lessons

These pages teach the ideas. They are not wind-tunnel data and not a certification analysis. When a number matters, compute it at a stated Reynolds number, Mach number and transition setting, and keep the method label on the result. In HENFOIL that production polar is a VIC polar — see the analysis ladder and the polar field guide.