Airfoil Analysis with XFOIL: Setting Reynolds Number, Mach Number and Transition

The three settings that are the case

The XFOIL method lesson described the coupled panel and boundary-layer solver. This lesson is the case you must type before a polar means anything. XFOIL, and any code in that family, will happily produce a curve from defaults. The defaults are a wind-tunnel habit, not your airplane.

• Reynolds number. Chord Reynolds number ρ V c / μ at the condition you care about. One polar cannot stand for takeoff and cruise if those Reynolds numbers differ by a factor of two. Set it explicitly. Do not leave a million in the box because the last user analysed a different wing.
• Mach number. Freestream Mach. Zero means an incompressible solution. A small UAV at Mach 0.08 can live with a small compressibility correction. A section at Mach 0.6 cannot pretend the correction is optional, and neither a correction nor XFOIL’s treatment is a shock-capturing solution past the critical Mach number.
• Transition. Ncrit is the amplification factor where the eⁿ method declares transition. The usual default is 9, a ordinary wind-tunnel disturbance level. Values near 12 to 14 are a clean, quiet assumption. Values near 4 to 6 are a noisy or dirty one. A trip location x/c on the upper and lower surfaces forces transition there if the code has not already transitioned sooner. Ncrit is not a knob for matching a drag you wish you had.

A minimum run card

1. Load a normalized geometry and repanel with clustering at the nose and the trailing edge. A bad point set will dominate the settings below.
2. Turn the viscous solution on. An inviscid XFOIL run has no business in a drag table.
3. Set Re, Mach and Ncrit, and trips if you are imitating bugs or a strip.
4. Sweep angle finely enough to see the linear range and the first break. Save the polar with those three settings in the name.
5. Repeat at a second Ncrit or with a trip before you rank the section.

HENFOIL asks for the same three ideas as polar fields: Reynolds number, Mach number, and a transition environment or an explicit trip. A VIC polar with those fields filled is the product’s version of this run card. Comparing it with an XFOIL polar is fair only when Re, Mach and the transition assumption match. The lesson on why tools differ is what to do with the gap that remains.

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Where this sits in the series

In simple words: what this page is for

An XFOIL polar is not defined until Reynolds number, Mach number and the transition setting are chosen. Those three inputs move the drag as much as a modest shape change.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Airfoil Analysis with XFOIL: Setting Reynolds Number, Mach Number and Transition 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 Analysis with XFOIL: Setting Reynolds Number, Mach Number and Transition 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 Analysis with XFOIL: Setting Reynolds Number, Mach Number and Transition 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

• HENFOIL vs XFOIL: when the browser workspace is the point
• XFOIL Airfoil Analysis: How It Works, Its Limitations and How to Interpret Results
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
• Login · Register now

• Previous: Viscous corrections to potential flow
• Next: Diagnose failed airfoil polars
• 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.