Fuselage Design and Layout Questions and Answers

Answer: A

Any departure from circular means the shell must carry pressure partly in bending, which is far heavier than membrane tension. Where a wider cabin is wanted the answer is usually two arcs meeting at the floor rather than an ellipse.

Answer: B

Correct answer: Two circular arcs tied at the floor

A double-bubble section is two circular arcs of different radii meeting at the floor, with the floor beams acting as a tie to carry the outward pull where the arcs join.

The arrangement keeps each arc in membrane tension, so pressure is carried without bending, while giving a taller section than a single circle of the same width - useful where a hold must sit under a wide cabin. It is one pressure vessel, not two, and the floor tie is what makes it work.

Answer: C

Correct answer: Seats abreast and aisle

The cabin cross-section is built up from the inside: the number of seats abreast, their width, the aisle width and the sidewall clearances give the internal width, and structure and insulation add the rest.

That is why fuselage diameters cluster around a few values - six abreast for a narrow-body, eight to ten for a widebody. Engine diameter, tail span and wing area are all sized separately and none constrains the cabin.

Answer: D

Standard unit load devices come in fixed dimensions, and a hold that cannot take two containers side by side wastes a great deal of volume. The section is therefore laid out around the containers rather than the other way about.

Answer: A

Correct answer: Length divided by maximum diameter

Fineness ratio is \( L/D \), a dimensionless measure of slenderness, and transports typically fall between about 8 and 12.

It expresses a drag trade. Too low and the afterbody must close quickly, so the flow separates and pressure drag rises; too high and the length brings wetted area with no pressure benefit, so skin friction dominates. The optimum sits between, which is why the range is as narrow as it is.

Answer: B

Correct answer: Pressure drag from separation

A short, fat fuselage must close its afterbody over a short distance, which means a steep adverse pressure gradient. The boundary layer cannot follow it, the flow separates, and a wide low-pressure wake forms behind.

Pressure drag then dominates. The remedy is to lengthen the closure, which is why fineness ratios below about 6 are avoided on aircraft that cruise fast. Skin friction is the penalty at the other end of the range.

Answer: C

Correct answer: Skin friction from wetted area

Beyond the optimum slenderness the afterbody already closes gently, so there is no further pressure drag to save. Every additional metre adds surface for the boundary layer to shear against.

Skin friction therefore grows with no compensating gain, which is what sets the upper end of the useful fineness range. Structure and weight grow too, and the rotation angle falls - but of the drag terms, friction is the one that rises.

Answer: D

The tail must clear the runway as the aircraft rotates, and sweeping the underside up achieves that far more cheaply than lengthening the gear. Excessive upsweep separates the flow and adds drag, so it is kept to what the rotation angle demands.

Answer: A

Correct answer: The available rotation angle falls

The rotation angle available is set by the geometry between the main gear and the lowest point of the rear fuselage. Lengthening the fuselage behind the gear brings the tail lower for the same rotation.

The tail therefore strikes at a smaller angle, and since rotation angle sets the lift coefficient achievable at unstick, the takeoff performance suffers. The remedies are a longer undercarriage, more rear upsweep, or a tail skid to make a strike survivable.

Answer: B

Holding the cabin near eight thousand feet keeps occupants comfortable without demanding an impractical pressure differential. Newer composite fuselages hold a lower cabin altitude because the structure tolerates the greater differential more readily.