Fuselage Sizing

Fuselage Sizing (vehicle-design/sizing/fuselage-sizing)

Use when the task is fuselage sizing for a transport or civil aeroplane starting from the payload: cabin length from the seat count and seat pitch, cabin width and fuselage diameter from the cross-section layout, the length to diameter ratio sanity band, and the underfloor cargo volume check against the required baggage volume.

Domain quick reference

• Cabin length from the payload: L_cabin = rows * pitch, with rows the number of seat rows and pitch the seat pitch in m. Example: 30 rows at 0.81 m pitch (about 32 in) give 24.3 m. Typical economy pitch is 0.76 to 0.84 m (30 to 33 in).
• Cabin width from the cross-section layout: W_cabin = seats_abreast * seat_width + aisle_width, with seat width and aisle width in m. Example: 6 abreast with 0.48 m seats (about 19 in) and one 0.51 m aisle (about 20 in) give 3.39 m.
• Fuselage diameter: D = W_cabin + sidewall_allowance, the cabin width plus a total allowance for trim, insulation, and structure on both sides (typical 0.15 to 0.25 m, default 0.18 m). Example: the 3.39 m cabin above gives a 3.57 m diameter.
• Length to diameter sanity band: ratio = L_fuselage / D, with the OVERALL fuselage length (nose to tailcone, not just the cabin). A ratio from 6 to 12 is typical for transport jets; below 6 the layout is stubby, above 12 it is slender. The band is sizing guidance, not a certification requirement.
• Cargo volume check: required baggage volume V_req = passengers * 0.12 m^3 per passenger (typical range 0.10 to 0.15 m^3 per passenger); the available underfloor cargo volume must meet or exceed it.
• FAR-25 (14 CFR Part 25) and CS-25 set the certification context for transport-category aeroplane design (cabin layout, exits, emergency provisions); the geometric relations above are common conceptual sizing practice.

Workflow

1. Set the payload layout: seats abreast, seat width, aisle width, seat pitch, and row count.
2. Compute the cabin length with cabin_length(rows, pitch).
3. Compute the interior cabin width with cabin_width(seats_abreast, seat_width, aisle_width).
4. Compute the outer fuselage diameter with fuselage_diameter(seats_abreast, seat_width, aisle_width), adjusting sidewall_allowance for the structure and insulation concept.
5. Judge the length to diameter ratio with length_diameter_verdict(fuselage_length, diameter) using the overall fuselage length, and rework the layout until the verdict is within the typical band.
6. Estimate the required baggage volume with required_baggage_volume(passengers) and compare it with the available underfloor cargo volume using cargo_volume_verdict; rework the underfloor layout until the check passes.

Pitfalls

• Using the cabin length where the overall fuselage length belongs: the length to diameter ratio uses nose-to-tailcone length, which exceeds the cabin length by the cockpit, tailcone, and closures.
• Mixing units: pitch in inches, seat width in cm, or aisle width in ft with a row count in meters; keep everything SI (m).
• Forgetting the sidewall allowance: the fuselage diameter is larger than the cabin width; sizing the diameter equal to the cabin width leaves no room for trim, insulation, and structure.
• Treating the 6 to 12 length to diameter band as a hard limit: it is a conceptual sanity band that flags layouts for rework, not a certification requirement.
• Skipping the cargo check: a passenger-feasible cabin can still be cargo-limited, so the underfloor volume must be verified against the baggage volume per passenger.
• Counting the aisle once per side: the aisle width enters the cabin width once for a single-aisle layout, not multiplied by the number of seat columns.
• Passing zero or negative inputs; the module raises ValueError instead of returning a nonsense dimension.

Behavior contract (gate 3)

The cabin length, cabin width, fuselage diameter, length to diameter verdict, and cargo volume check relations are exercised by the gate 3 contract test: scripts/test_fuselage_sizing.py against scripts/fuselage_sizing_logic.py (stdlib unittest, offline). Run: python3 scripts/test_fuselage_sizing.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; the geometric fuselage sizing relations are common conceptual sizing methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.