Spoilers are often used for roll control at high speed in place of ailerons because
Answer: A
A spoiler dumps lift locally by separating the flow and produces very little pitching moment on the section, so it does not suffer reversal. Many transports lock the outboard ailerons at high speed and roll on spoilers and inboard ailerons for exactly this reason.
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Torsional divergence of a wing occurs when
Answer: B
Above the divergence speed a small twist raises the incidence, which raises the lift, which twists the wing further, with no equilibrium available. It is a static aeroelastic instability and it is avoided by keeping the torsion box stiff and the aerodynamic centre near the flexural axis.
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The flexural axis of a wing is the locus of
Answer: C
Loads applied along the flexural axis bend the wing without twisting it, since each section is loaded through its own shear centre. The relative positions of the flexural axis and the aerodynamic centre determine whether an increase in lift twists the wing nose-up or nose-down.
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A wing box carries \( 300 \) kN m of torque with an enclosed area of \( 1.5 \) m\( ^2 \) and a critical wall \( 2 \) mm thick. The shear stress there is
Answer: D
The flow is \( 300\,000/(2 \times 1.5) = 100 \) kN/m, and dividing by the thickness gives \( 100\,000/0.002 = 50 \) MPa. Checking the thinnest wall is the essential step, since it is always the critical one under torsion.
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If that same box's critical wall were thickened to \( 3 \) mm, the shear stress would become
Answer: A
The flow is unchanged at 100 kN/m, so the stress becomes \( 100\,000/0.003 = 33 \) MPa. Thickening the wall reduces the stress in proportion but does nothing to the flow, which is fixed by the torque and the enclosed area.
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Assertion (A): Transports often lock their outboard ailerons at high speed.
Reason (R): A torsionally flexible outer wing can suffer aileron reversal.
Select the correct answer from the options below.
Answer: B
Both are true and the second explains the first. The outboard aileron has the longest moment arm but acts on the most flexible part of the wing, so it reverses first. Rolling on spoilers and inboard ailerons at speed avoids the problem entirely.
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Assertion (A): Wing torsional stiffness is defended carefully during design.
Reason (R): It sets both the divergence speed and the aileron reversal speed.
Select the correct answer from the options below.
Answer: C
Both statements are true and the second is the reason for the first. Both speeds must lie comfortably outside the flight envelope, and since both rise with torsional stiffness, the closed box's integrity is an aeroelastic requirement as much as a strength one.
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Consider the following statements about aeroelastic consequences of torsional stiffness:
1. Divergence speed rises with torsional stiffness.
2. Aileron reversal speed rises with torsional stiffness.
3. Aileron reversal occurs above the divergence speed.
Which of the statements given above is or are correct?
Answer: D
The first two are correct. The third has the order wrong: reversal typically occurs at a lower speed than divergence, which is why it is the practical limit designers meet first and why spoilers are adopted for high-speed roll control.
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Consider the following statements about the flexural axis:
1. It is the locus of the section shear centres.
2. A load applied along it bends the wing without twisting it.
3. It always coincides with the line of aerodynamic centres.
Which of the statements given above is or are correct?
Answer: A
The first two are correct. The third is false and structurally important: the offset between the flexural axis and the aerodynamic centre is precisely what determines whether increasing lift twists the wing nose-up, which is the mechanism of divergence.
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Match each aeroelastic phenomenon in List I with its cause in List II.
List I
A. Torsional divergence
B. Aileron reversal
C. Flutter
D. Control surface buzz
List II
1. Aileron deflection twisting the wing against itself
2. Self-reinforcing twist above a critical speed
3. Shock-induced oscillation of a control surface
4. Coupling of bending and torsion modes with the airflow
Select the correct match.
Answer: B
Correct answer: A-2, B-1, C-4, D-3
Four distinct phenomena. Divergence is a static instability in which aerodynamic twist reinforces itself above a critical speed; aileron reversal is the aileron twisting the wing so far against itself that the roll response inverts; flutter is a dynamic coupling of bending and torsion modes with the airflow; and buzz is a shock-induced oscillation of a control surface. The first two are direct consequences of torsional flexibility.
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