Stress Analysis of Wings and Fuselage Questions and Answers

Answer: A

Correct answer: Positive margins for every member and case

The stress report is the document that demonstrates compliance with the strength requirements, and what it must show is that no member is overloaded in any of the design load cases - a positive margin everywhere. It does not have to show that the structure is optimal or that it has been tested to destruction, and performance is demonstrated by quite separate means.

Answer: B

A-basis and B-basis allowables carry defined statistical confidence, ninety-nine and ninety per cent survival respectively at ninety-five per cent confidence. Which applies depends on whether the member's failure would be critical or whether load paths are redundant.

Answer: C

Correct answer: The structure has redundant load paths

B-basis allowables are set at 90 per cent survival with 95 per cent confidence, against A-basis at 99 per cent survival. The lower figure is permitted where load paths are redundant, because failure of one member does not fail the structure - the load redistributes. A single load path member, whose failure is the structure’s failure, must be sized on A-basis.

Answer: D

The rib carries chordwise air load and, in a wet bay, fuel inertia pressure, delivering them into the spars and skins as shear. It is usually analysed as a shear web with flanges, often with substantial lightening holes to check.

Answer: A

Wherever a large concentrated load enters the shell, a heavy frame appears to diffuse it. The wing joint and the gear bay deliver the largest such loads, and the frames there are conspicuously deeper and thicker than their neighbours.

Answer: B

Correct answer: 0.040 m^4

For a thin circular shell,

The thickness must be converted to metres before substituting - using 1.6 directly would give an answer a thousand times too large. The cube on the radius is why fuselage stiffness rises so steeply with diameter.

Answer: C

Correct answer: 99 MPa

The extreme fibre is at the radius, so

Comfortable for aluminium alloy on its own - but this is only the bending contribution, and the pressurisation stress has still to be added, which is the point of the next item.

Answer: D

Correct answer: 133 MPa

Both act along the fuselage axis, so they add directly:

The combined figure is a third higher than bending alone, which is exactly why the two cases must be assessed together. The hoop stress, twice the longitudinal pressurisation stress, acts at right angles and is brought in through the failure criterion rather than by addition.

Answer: A

Evaluating \( \sqrt{\sigma_1^2 - \sigma_1\sigma_2 + \sigma_2^2} = \sqrt{17\,929 - 9\,212 + 4\,733} = \sqrt{13\,450} = 116 \) MPa. Note that the equivalent stress is below the larger principal stress, because the second one is of the same sign.

Answer: B

Correct answer: 1.59

Applying the definition,

A margin of 1.59 means the member could carry more than two and a half times the applied stress. On a weight-critical structure that is a great deal of surplus, and it usually means something else is sizing the member - buckling, stiffness, minimum gauge or a different load case.