Fuselage Design and Layout Questions and Answers

Answer: C

Correct answer: Cabin cross-sectional area

Cross-sectional area is a property of the diameter, which the question holds constant, so lengthening changes it not at all.

The other three all grow: wetted area in proportion to length, fineness ratio by definition since it is length over diameter, and the structural bending span as the tail loads act through a longer arm. That is the reason a stretch is limited - three penalties grow and only the capacity grows with them.

Answer: D

Correct answer: Poor, since pressure is then carried in bending

An ellipse has varying curvature, and only uniform curvature allows a shell to carry internal pressure as pure membrane tension. The flatter regions must resist the load in bending, which requires much heavier structure for the same strength.

Where a wider, shallower section is genuinely wanted, the double-bubble achieves it properly: two circular arcs, each still in tension, tied at the floor by beams that carry the outward pull at the junction.

Answer: A

Holding the cabin closer to sea level means a bigger gap from the thin air outside, and the shell carries that difference every flight. Composite fuselages tolerate the extra fatigue duty, which is why they offer a more comfortable cabin.

Answer: B

Correct answer: Exit provision depends on the fuselage length

Exit requirements are set by the number of occupants, in defined capacity bands, not by how long the fuselage is. A long aircraft carrying few passengers needs no more exits than a short one carrying the same number.

The other three are true: a stretch can push an aircraft over a band boundary and force an extra exit pair, provision follows the occupant count, and the ninety-second evacuation demonstration is the requirement the bands are calibrated against.

Answer: C

Correct answer: Increasing the differential

Hoop stress is proportional to the differential, so raising it demands a thicker skin and heavier frames throughout the pressurised section. Weight rises.

The other three all reduce it: a shorter fuselage has less structure and a smaller bending span, a lower differential allows a thinner shell, and less wetted area means less skin and stiffening. The differential is a design choice, traded against cabin altitude and passenger comfort.

Answer: D

Correct answer: Not feasible, since two need over 3.06 m

Two containers of 1.53 m need 3.06 m before any clearance for handling and restraint, and the hold offers 3.00 m. It does not fit, and careful handling cannot recover six centimetres that do not exist.

Rotating them changes the dimension presented but not the arithmetic, and an empty hold is no wider than a full one. This is the step-change the cross-section must be designed to clear: a hold just short of the width for two carries one.

Answer: A

Correct answer: A clear cabin with the box below the floor

That describes a low wing. On a high wing the carry-through passes above the cabin, so the intrusion is overhead rather than underfoot - a different constraint, not an absence of one.

The other three are genuine consequences of choosing a high wing: a low flat cargo floor for vehicle loading, a heavier carry-through and fuselage reinforcement, and engines held clear of debris on unprepared surfaces.

Answer: B

Correct answer: Both extremes cost drag

The two penalties act at opposite ends. A low fineness ratio forces an abrupt afterbody closure and the flow separates, giving pressure drag; a high one adds wetted area and skin friction with no pressure benefit left to gain.

The total therefore has a minimum, which is why transports cluster between about 8 and 12 rather than pushing either way. Neither "higher is better" nor "lower is better" survives the trade.

Answer: C

Correct answer: An expensive centre section redesign

The carry-through box carries the wing bending moment, and a stretched variant is heavier and generates more of it. A box sized for the baseline alone will not take the growth.

Redesigning it later means changing the most heavily loaded structure in the aircraft, with new tooling and a fresh certification effort. Building in the margin at the outset costs a little weight on the first variant and saves a great deal on every one after - which is why families are planned from the start.

Answer: D

Correct answer: It lowers the pressure differential the shell must carry

The differential is set by the cabin altitude and the cruise altitude, and no choice of section changes it. A double-bubble carries exactly the same differential as a circle would.

The other three are the real reasons for adopting it: a wider cabin without a very deep section, each lobe still working in membrane tension rather than bending, and the floor beam tying the lobes together to carry the outward pull where they meet.