Structural Sizing // V-n Diagram

Load factors, design airspeeds, and the flight envelope your structure has to survive.

Design

Pulls category, weight, wing geometry, and speeds from a design open in Aircraft Sizing (same browser tab) — or paste a share link. Available from any step — loading a design always brings you back to Step 1 first, so you see what changed before continuing.
No design loaded yet.

1 Load a Design

Use the Design panel above to pull in a design from Aircraft Sizing or a share link.

2 Load Factors & Design Speeds

Defaults below are starting points, not certified figures — see the note under each for where it comes from and how confident it is. Everything is editable.

2 V-n Diagram

The outer boundary — whichever of maneuvering or gust is more critical at each speed — is what the structure actually has to survive.
Maneuvering envelope — what the structure actually has to survive Gust load — separate from maneuvering; governs where it pokes outside the envelope Ultimate load (limit × safety factor) — structural failure boundary 1-G level flight / Vs1 stall speed Damage zone (between limit and ultimate) Failure zone (beyond ultimate)
Va — Maneuvering Speed
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regulatory minimum — real aircraft often certify higher
Vc — Cruise Speed
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Vd — Dive Speed
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Critical Load Factor
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Design Torque (limit, governing envelope case)
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Does not include aileron hinge-moment or maneuvering-asymmetry torque — real wing torsion is very often governed by aileron deflection, not the clean-wing pitching moment alone. A large PASS margin on the Step 6 Torsion Check does not mean the wing is torsion-safe with ailerons deflected. This tool has no aileron geometry data to compute that contribution from (Aircraft Sizing doesn't design ailerons) — if you've computed a real governing torque elsewhere, use the override below instead of trusting this derivation blind.

3 What You're Designing Against

Pulled live from Step 2 — if you go back and change load factors, everything downstream (this card, Trial Dimensions, Results) recomputes automatically the next time you view it, so you're always looking at current numbers, never a stale snapshot.

3 Spar Material & Form

Same material and cross-section form for both spars — dimensions are set separately for each in the next step.

3 Lift Shape & Wing Support

How the lift is spread along the wing and how the wing is held up. These set the bending moment and shear every spar is checked against; the spar material and form above only set what carries it.

3 Torsion Load Path

Construction Type is set on Step 2 — these inputs feed whichever path you picked there, not a choice made here. Spar positions are shared by both paths; everything below them is specific to one.

4 Spanwise Load Distribution

How much bending moment and shear are actually left at each span station, root to tip — using the same elliptical lift distribution as the root check below, just evaluated at more than one point. This is where a real spar would taper: loads drop off fast toward the tip, so a constant root-sized cross-section the whole way out is carrying far more material than it needs outboard. Split into Main/Rear the same way as the root numbers, by the Bending Share set on Step 3. Not affected by the Trial Dimensions below — this is load only, independent of what cross-section you choose. Hover either curve for the exact value at a station.

4 Trial Dimensions

Propose dimensions for each spar — Step 6 tells you if they're enough, same "propose, then check" pattern as the rest of this tool. Each diagram shows the selected cross-section shape and what each field name refers to, sized from that spar's own current values. Each spar's own "Solve for Minimum Size" scales that spar's currently-entered shape down (or up) to hit its own target margin — it keeps the proportions you've set, it doesn't invent a new shape, so it can't wander into a thinner-than-checked flange or web this tool wouldn't catch (this tool doesn't model buckling).

5 Section Transition

Optional — additional stations further out along the span, beyond the root. Loads drop off fast toward the tip (see Spanwise Load Distribution on Step 4), so a real spar usually tapers rather than carrying the root's cross-section the whole way out. These are separate from Step 4 Trial Dimensions / Step 6 Results — the root check isn't affected by anything here.
Thickness vs Span
Actual
Theoretical
Min Gauge
NoteNo taper stations yet
Not set up yet — the root-sized cross-section currently carries all the way to the tip.
FixAdd stations by hand, or use Auto-Generate above.

6 Spar Sizing Results

First-pass estimate only — simplified lift distribution (elliptical unless you chose otherwise in Step 3), simplified section properties. Not a substitute for a real structural engineer's sign-off before construction.

Summary

Torsion Check

Checked at the root (stressed-skin) / uniformly across every bay (fabric/truss), not swept along span — and this is provably sufficient, not a shortcut. Under this tool's constant-chord assumption, Am/skin thickness/bracing don't vary along span, so stress and bracing load scale directly with torque — which is highest at the root and decreases toward the tip (stressed-skin), or identical bay-to-bay (fabric/truss, since the distributed torque per unit span doesn't vary along span here). No outboard station or bay can show a worse result than what's already checked.

Main Spar

Rear Spar

Export & Report

Exports cover the option currently being edited. The print report is the curated record: results, torsion check and dimensions.