Which of these is NOT a valid conclusion from a successful static test to ultimate load?
Answer: C
Static strength and fatigue are entirely different questions: a structure can pass ultimate load comfortably and still crack after twenty thousand flights. Fatigue is demonstrated by a separate, far longer, cyclic test.
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Taken together, the activities in this topic exist to demonstrate that
Answer: D
Correct answer: Every member has a positive margin
That is the whole object of structural substantiation. Idealisation, section properties, stress calculation, failure criteria, joint and fitting checks, allowable selection and static testing all serve one conclusion: no member is overloaded in any design case. Range, lift and minimum weight are demonstrated by other disciplines and by other means entirely.
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A wing spar cap is sized by compression rather than tension because
Answer: A
Aluminium's compressive and tensile strengths are similar, but the compression cap must also survive local and overall buckling, which sets a lower allowable. The tension side is instead usually governed by fatigue and damage tolerance.
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The tension surface of a wing is typically governed by
Answer: B
Static tension margins are usually comfortable, but the lower surface sees the largest tensile stress cycles and every fastener hole is a potential crack origin. Its allowable is therefore set by crack growth and inspection rather than by ultimate strength.
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This asymmetry means a wing's upper and lower surfaces are usually
Answer: C
The upper surface is stability-critical and the lower fatigue-critical, so different alloys and different thickness distributions are common. Alloys with high compression strength go on top and damage-tolerant ones underneath.
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A spar cap has a static compressive allowable of \( 300 \) MPa but a crippling stress of \( 210 \) MPa. Its usable allowable is
Answer: D
Correct answer: 210 MPa
A member fails at whichever stress it reaches first, and the cap will cripple - its thin flanges buckling locally and shedding load - at 210 MPa, long before the material’s 300 MPa compressive allowable is approached. The usable allowable is therefore the crippling stress. Averaging the two, or adding them, would credit the member with strength it cannot deliver; the governing figure is always the lowest of the applicable allowables.
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Assertion (A): A wing's upper and lower surfaces are often made from different alloys.
Reason (R): The upper surface is stability-critical while the lower is fatigue-critical.
Select the correct answer from the options below.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the design logic. In normal flight the upper surface is in compression, so it is sized by buckling and crippling and wants a high compressive yield and modulus - the 7000 series alloys. The lower surface is in tension and spends its life cycling under gust and manoeuvre loads, so it is sized by fatigue and damage tolerance and wants toughness and slow crack growth - the 2000 series. Two different requirements, two different alloys.
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Consider the following statements about wing surface design:
1. The upper surface is usually sized by buckling or crippling.
2. The lower surface is usually sized by fatigue and damage tolerance.
3. Both surfaces are sized by static tension.
Which of the statements given above is or are correct?
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct and reflect the different loading of the two surfaces. Statement 3 is wrong: static tension rarely sizes either. The upper surface is in compression and gives way by instability well below its tensile capability, and the lower surface, though in tension, is limited by crack growth and inspection intervals rather than by the load at which it would break in one pull.
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Which of these is NOT a reason the compression surface of a wing has a lower allowable than the material's strength?
Answer: C
The material's compressive and tensile strengths are broadly similar; it is the three stability modes listed that cut the usable allowable well below it. Attributing the difference to the material misses where the limitation actually lies.
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The single most important idea carried across this whole topic is that
Answer: D
Static strength, stability, fatigue and joint modes each have their own allowable, and the lowest governs. Identifying which one that is for each member, and demonstrating a positive margin against it, is the whole substance of airframe stress work.
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