Fuselage Design and Layout Questions and Answers

Answer: A

Correct answer: Seat pitch must fall to about 0.76 m

The cabin length is fixed, so more rows must come from a smaller repeat distance:

\[ \frac{24.3}{32} = 0.76\ \mathrm{m} \]

The objection is passenger comfort, and it is a commercial judgement rather than a technical one - though the exit requirement must also be checked, since twelve more seats may cross a capacity band. Widening or stretching the aircraft is precisely what the question rules out.

Answer: B

Correct answer: fineness; eight and twelve

Fineness ratio is length over maximum diameter, and transports sit between about 8 and 12 because that range balances the two drag penalties.

Below it the afterbody closes too abruptly and the flow separates; above it the extra length adds wetted area and skin friction with nothing to show for it. Aspect ratio and taper ratio are wing parameters, and slenderness ratio belongs to columns in structural work.

Answer: C

Correct answer: hoop; longitudinal

In a thin-walled cylinder the hoop stress is \( pr/t \) and the longitudinal stress \( pr/2t \), so the circumferential stress is twice the axial one.

A crack grows perpendicular to the greater stress, so it runs along the fuselage rather than around it. The consequence is designed for: tear straps and crack stoppers are fitted so that a crack turns and the failure becomes a controlled decompression rather than a rupture.

Answer: D

Correct answer: upsweep; separation

Upsweep raises the rear fuselage so the aircraft can rotate further before the tail strikes, and it is what allows a rear cargo ramp to meet the ground.

The cost is aerodynamic: an asymmetrically closing afterbody sheds a pair of vortices and, if the upsweep is severe, separates. That is why upsweep is used to the extent the rotation and loading requirements demand and no further, and why a ramp-equipped freighter pays a measurable drag penalty.

Answer: A

Correct answer: constant; plugs

A constant-section barrel means every frame in that region is identical, so a cylindrical plug of the same section can be inserted without redesigning anything.

Plugs are placed fore and aft of the wing so that the centre of gravity shifts as little as possible, and their lengths are chosen as whole multiples of the seat pitch. It is the cheapest way to grow an aircraft, which is why the constant section is designed in from the first variant.

Answer: B

Correct answer: below; above

On a low-wing transport the carry-through box passes below the cabin floor, intruding into the hold and dividing it into forward and aft compartments while leaving the cabin clear.

On a high-wing tactical freighter it passes above the cargo deck, so the hold floor is unobstructed for its full length and can sit low to the ground for vehicle loading. The wing must be carried through in both cases; only its height relative to the payload differs.

Answer: C

Layout Y sits at a fineness ratio of 8.6, comfortably inside the usual eight to twelve band, and six abreast with a single aisle is the established narrow-body arrangement. W is excessively long and Z would need a second aisle at seven abreast.

Answer: D

Correct answer: Thickening the skin to 2.0 mm

Hoop stress is inversely proportional to thickness, so the thicker the skin the lower the stress. From the 68.8 MPa baseline, 1.8 mm gives 61.1 and 2.0 mm gives 55.0 - the largest reduction of the options.

Raising the differential moves the wrong way, taking the stress to 72.2 MPa. The trade is weight: every increment of thickness adds skin mass over the whole fuselage, which is why the working stress is set against fatigue life rather than minimised.

Answer: A

Correct answer: 18

Read the seat column: 168 at 0.86 m and 186 at 0.76 m, a difference of 18.

The gain is three rows at six abreast. Working from the rows column gives the same answer, and it shows why the arithmetic must be done in whole rows: a 12 per cent reduction in pitch buys about 11 per cent more seats once the remainder is discarded. The eighteen seats are what the comfort reduction is being traded for.

Answer: B

Correct answer: Option C

Apply the constraint first. Option D has a fineness ratio of 11.1, above the 10.5 limit, so it is excluded whatever its capacity.

Among those remaining, C adds the most at eight rows with a fineness of 10.3, just inside the limit. The exercise is the standard shape of a stretch decision: capacity is wanted, and slenderness is the constraint that eventually stops it - along with the rotation angle, which C would also press upon.