Airfoil Coordinate Ordering: Upper Surface, Lower Surface and Trailing-Edge Conventions

Two public conventions

Neither convention is a coordinate system. Both still require you to know whether x runs nose to tail and which side is up. A file can be in perfect Selig order and upside down, and the lift will come out negative at positive camber. Check the sign of the camber after you read the file, not only the header.

Trailing edge and winding

Close the section in one of two ways. Repeat a single trailing-edge point, sharp within the tolerance you declare. Or keep two trailing-edge points with a gap equal to the real trailing-edge thickness, and close that gap with one short face. Do not do both: a duplicated point plus a gap is how a zero-length panel or a small loop appears. Panel methods want a consistent winding so the outward normal is the outward normal. If you reverse only the lower surface during a conversion, the normals flip on that side and a source panel method will happily solve the wrong body.

Convert explicitly. Selig to Lednicer is a split at the leading-edge index and a reversal of the upper run. Lednicer to Selig is the reverse concatenation. Write the source order and the stored order in the file history. “Standard DAT” is not a specification. The coordinate-export lesson is the wider map. Ordering is the part that decides whether the next tool traces a section or a bow tie.

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Where this sits in the series

In simple words: what this page is for

Selig order walks in from the trailing edge along the upper surface and back along the lower. Lednicer lists each surface from the leading edge. A panel code needs one winding, and it needs a trailing edge that is either one point or an honest gap.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Airfoil Coordinate Ordering: Upper Surface, Lower Surface and Trailing-Edge Conventions 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 Airfoil Coordinate Ordering: Upper Surface, Lower Surface and Trailing-Edge Conventions 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 Airfoil Coordinate Ordering: Upper Surface, Lower Surface and Trailing-Edge Conventions 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
• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• How to Identify Numerical Convergence Problems in Airfoil Analysis
• How to Build an Airfoil Analysis Database with Consistent Simulation Conditions
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
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• 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.