How to Resample Airfoil Coordinates Without Distorting the Profile

Resampling is not fairing

To resample is to compute new stations on a curve you have already accepted. To fair is to change the curve. Keep those words separate. A resample that moves the leading-edge radius or opens a hole in the trailing edge has faired, usually by accident. The goal is a point distribution a panel code or a CAD spline can use, on the geometry you had.

Where to put the new points

Curvature is highest at the nose and, if the trailing edge is sharp, the turning is concentrated there too. Uniform steps in x spend most of the points on the flat middle and leave the nose as a few long segments. The standard fix is cosine spacing. With a parameter φ running from 0 at the trailing edge or the nose, according to the convention you document,

x = (1 − cos φ) / 2

puts points densely at both ends and sparsely at mid-chord. Apply it separately to the upper and lower surfaces after you have parameterized those surfaces. One hundred to two hundred points of this kind is a usual analysis set. A manufacturing spline may want fewer, but they should still be the cosine set, not every digitizer sample.

A procedure that keeps the shape

1. Clean the source: no duplicates, no crossings, a defined trailing edge. Interpolate with the method chosen in the previous lesson.
2. Build arc length on each surface. Sample the new distribution in arc length or in the cosine x, consistently, and evaluate the interpolant there.
3. Re-normalize. Leading edge back at the origin, chord on the x-axis, trailing edge at x = 1. Resampling can nudge the geometric nose by a fraction of a panel.
4. Recompute maximum thickness, its location, maximum camber and an estimated leading-edge radius. Differences should sit inside a tolerance you set from the source uncertainty.
5. Plot the source and the resample on top of each other, and plot curvature. If the new curvature oscillates, you interpolated noise. Go back one lesson and change the fit, not the panel count.

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Where this sits in the series

In simple words: what this page is for

Resampling places a new point set on the same curve. Cosine clustering protects the nose. Uniform spacing in x does not. Always compare thickness and leading-edge radius afterward.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. How to Resample Airfoil Coordinates Without Distorting the Profile 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 Resample Airfoil Coordinates Without Distorting the Profile 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 Resample Airfoil Coordinates Without Distorting the Profile 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?
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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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• 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.