Selection & compare fields: filters, cruise CL, weights and why-#1

What Selection is for

Selection ranks candidate airfoils under shared assumptions so the winner has a sentence you can defend. Peak CL alone is not a process. HENFOIL’s path is: geometry filters → equal-CL viscous points on the top set → score (geometry + cruise L/D + forced-transition robustness + thickness) → gate from leader evidence and rank stability.

Compare / envelopes feed the same story. This guide focuses on the fields you fill and the results you read.

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Before you open Selection

• Several airfoils already created / imported / digitized
• Geometry gate understood for each (FAIL foils should not compete)
• At least a plan for Re and cruise CL from the mission (UAV example in UI: Re ~1.5e5–3.5e5, cruise CL ≈ 0.55, t/c ≥ 12% is a typical screen — not a certified choice)

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Geometry filters

Purpose. Drop sections that cannot meet structural or packaging constraints before spending viscous solves.

Result. A filtered set. Name is never a score — geometry comes first.

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Reynolds number (selection condition)

Purpose. The Re at which equal-CL viscous points are evaluated. Why match flight/tunnel. Ranking at 3e6 when you fly at 2e5 will crown the wrong foil. Result. Viscous metrics labelled at that Re.

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Cruise CL

Purpose. Target lift coefficient for trimmed equal-CL comparison. Why. Wings fly at CL, not at a random α. Comparing at equal CL is how you compare drag fairly. How the product uses it. α bracket to hit cruise CL on top candidates, then score cruise L/D and related metrics. Result. Rankings that answer “at our cruise lift, who is quieter in drag?”

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Forced-transition / robustness inputs

Purpose. Re-score with trips or dirtier transition so a laminar-looking winner is not brittle. Why. Manufacturing bugs and LE roughness happen. Result. Robustness contribution in the mixed score — winners that survive both free and forced assumptions rise.

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Weights (compare / decision study)

When you rescore a study, weights say which metrics matter more (cruise L/D vs thickness vs moment vs robustness).

Purpose. Encode programme priorities explicitly. Why not leave defaults forever. Defaults are a starting opinion, not your OEM’s. Result. A new ranking and an updated why-#1 when weights change — the trail stays auditable.

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What results look like

Ranked list

Each candidate shows score, t/c, camber and a short why string list.

Best / why-#1

• Headline winner with metrics
• Bullet why reasons (what drove the rank)
• Tradeoffs when the product surfaces them
• Confidence language from evidence + rank stability — not a certification stamp

What you should export

Datasheet or design-review PDF, or a HENFOIL design package with coordinates, metrics, polar/validity and provenance. Selection remains engineering judgement; HENFOIL makes the early decision repeatable.

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Field checklist

1. Filter t/c to the real spar/packaging band
2. Set Re from mission chord and speed
3. Set cruise CL from weight and dynamic pressure
4. Include a forced-transition or roughness view before crowning
5. Adjust weights to match the review board’s priorities
6. Read why-#1 out loud — if you cannot defend the sentence, change the inputs

Honesty reminder

Selection is a documented screen. It is not a certified airfoil choice. Escalate to tunnel / higher-fidelity CFD when the programme gate requires it.

In simple words: what this page is for

How to fill geometry filters, Re, cruise CL and ranking inputs — and what scored candidates, why lists and confidence mean.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Selection & compare fields: filters, cruise CL, weights and why-#1 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 Selection & compare fields: filters, cruise CL, weights and why-#1 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 Selection & compare fields: filters, cruise CL, weights and why-#1 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

• Airfoil selection with a documented why-#1
• The HENFOIL analysis ladder: when to escalate
• Create airfoil fields: NACA, analytic and CST — what each digit means
• Digitize field-by-field: every control, why fill it, what you get
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
• Login · Register now