VIC polar fields: what to fill, why it matters, what the sweep returns

Goal of a polar in HENFOIL

A VIC polar answers: under these assumptions (Re, Mach, transition, α or CL), what are CL, CD and Cm? It is the production ranking engine for compare and select — not a wind-tunnel campaign or a flight-Re RANS certificate. Every curve carries method IDs and gate language.

Pass Geometry Doctor first. Bad geometry invents lift and drag before the solver deserves the blame.

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Sweep (α / CL / Re family / Mach family)

Why choose carefully. Wrong sweep answers the wrong question with a smooth-looking plot.

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Re (Reynolds number)

What it is. Chord Reynolds number for the flight or tunnel condition. Why fill it from mission data. Re sets boundary-layer regime; a “pretty” polar at the wrong Re misranks foils. How to get it. Use the built-in Reynolds calculator (speed, chord, viscosity/altitude) or enter Re directly. Re list (family sweep). Comma-separated values, e.g. climb and cruise Re. Result. Forces and moments consistent with that Re assumption — labelled as such.

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Mach

What it is. Freestream Mach (0 = incompressible assumption). Why. Even low-Mach UAV work should state Mach; higher Mach needs the Mach family for drag-rise awareness. Mach list. For Mach family sweeps (e.g. 0, 0.3, 0.5, 0.6). Keep values below 1 for this polar path. Result. Compressibility-aware screen at the stated Mach — not a full Euler/RANS certificate.

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Ncrit (eⁿ critical amplification)

What it is. Transition sensitivity for the eⁿ model. Typical guidance (also in the UI tooltip):

• ~9 — average wind-tunnel air
• 4–6 — turbulent / dirty air
• 11–14 — sailplane / quiet air

Transition environment select. Maps named environments to Ncrit so you do not invent a number blindly. Why it matters. Lower Ncrit → earlier transition → usually higher drag / different CL_max behaviour. Result. Polars that shift with transition assumption — use for robustness, not as “the one true polar.”

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Trip upper / lower (forced transition x/c)

What it is. Chord fraction where you force transition on upper and lower surfaces (1 = free transition only). Purpose. Model trips, bugs, or manufacturing trips; compare free vs forced. Rule. Transition happens at whichever comes first (free eⁿ vs trip). Result. Robustness view of the same geometry under dirtier assumptions.

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Roughness k/c at x/c

What it is. Discrete roughness height / chord at a chord station. Physics screen. Braslow–Knox style: at each polar’s Re, Re_k > ~600 trips; < ~200 may stay submerged. In a Re sweep the same bump can trip at high Re and not at low Re. Why fill. Leading-edge contamination or known roughness studies. Leave at 0 for clean screens. Result. Annotated effect on the polar when the roughness is active.

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AoA start / end / step

Purpose. α window for α sweeps (and supporting brackets). Why choose mission-relevant bounds. −4°…12° with 1° steps is a common early screen; stall studies need a higher end and finer step. Wrong window. Misses stall break or wastes solves far outside the envelope. Result. Discrete α points; denser step → smoother curve and more monthly polar usage.

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CL from / to / step (CL-target sweep)

Purpose. Solve for α that hits each target CL. When. Cruise / climb design points where you care about equal-lift drag, not α grids. Result. Trimmed operating points — ideal input to selection and optimize.

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Opt-in recovery (≤25 α steps)

Purpose. If a point’s viscous interaction does not settle, recovery can warm-start from a neighbour, half-α continue, or repanel — and the result says so. Why optional. Recovery can change geometry representation; leave off for strict repeatability, on when you need a filled curve. Result. Fewer holes in the polar, with honesty notes when recovery ran.

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What you get after Sweep

Expect:

• CL, CD, Cm vs α (or vs the family variable)
• Derived metrics such as L/D peaks, α at CL_max, stall-related markers when available
• Method IDs and gate / recovery notes
• Optional Keep as overlay to compare conditions on one chart
• Downloadable polar package (PDF, images, JSON, CSV, Excel) when the run is good enough to archive

Jobs that leave the interactive loop appear under Dashboard → Jobs.

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Field checklist before you hit Sweep

1. Geometry gate PASS or understood WARNING
2. Re from real chord × speed × viscosity
3. Mach stated (even if 0)
4. Ncrit matched to environment
5. α or CL window matched to the decision
6. Trips / roughness only if the question needs them

Honesty reminder

A dense, pretty polar can still answer the wrong Re. HENFOIL labels VIC as VIC. Take the selection package into tunnel or RANS when the program requires it — do not pretend the polar already was that evidence.

In simple words: what this page is for

Sweep type, Re, Mach, Ncrit, trip stations, roughness, AoA range — purpose of each Polars field and the CL/CD/Cm results you should expect.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. VIC polar fields: what to fill, why it matters, what the sweep returns 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 VIC polar fields: what to fill, why it matters, what the sweep returns 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 VIC polar fields: what to fill, why it matters, what the sweep returns 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
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