JNTU
JNTU B.Tech III AEROSPACE VEHICLE STRUCTURES II Previous Question Paper (August - 2022 - R13)
Answer any five questions. All questions carry equal marks.
Main
Write down the Euler stress equation for a panel stiffened by longitudinal members.
Answer
For a panel stiffened by longitudinal members (stringers), the Euler buckling stress is determined by considering the panel as a series of columns or as a plate with elastic restraint. The Euler stress equation for a simply supported panel of length L, thickness t, and width b is given by:
Where σ_cr is the critical buckling stress, E is Young's modulus, ν is Poisson's ratio, and k is the buckling coefficient which depends on the aspect ratio and boundary conditions. When longitudinal stiffeners are present, the panel is divided into smaller sub-panels, effectively reducing the width b and increasing the value of k, thereby significantly increasing the critical buckling stress of the overall structure.
What is the difference between Semi tension and complete tension field beams? Explain with neat sketch.
Answer
The distinction between semi-tension and complete tension field beams lies in the behavior of the web under shear loads beyond the initial buckling point.
Complete Tension Field Beam: In this idealized case, the web is assumed to have negligible compressive strength. Once the shear load exceeds the critical buckling load, the web buckles and carries no further compressive stress. The entire applied shear load is carried by pure diagonal tension acting at an angle of 45 degrees to the flanges. The flanges must be designed to resist the inward pull of the tension field.
Semi-Tension Field Beam: In practical aerospace structures, the web possesses some inherent bending stiffness and is not perfectly thin. Consequently, it carries a combination of diagonal tension and some diagonal compression. The actual stress state is a superposition of the initial shear stress and the tension field developed after buckling. The angle of the tension field is typically less than 45 degrees, and the web contributes to the overall shear capacity through both its post-buckling tension and its residual compressive strength.
Sketch description: A complete tension field beam shows diagonal lines representing tension members only, while a semi-tension field beam shows a web that retains some shear stiffness, with the tension field acting at an angle θ relative to the flange, where θ is less than 45 degrees.
A panel comprising a flat sheet and uniformly spaced Z-section stringers, a part of whose cross-section is shown in figure 1, is to be investigated for strength under uniform compressive loads in a structure in which it is to be stabilized by frames a distance l apart, l being appreciably greater than the spacing b.
Answer
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Derive an equation for the critical stress in a thin flat plate due to buckling.
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Write the different cross-sections used for stability analysis of longitudinal stringers with neat diagrams.
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What is shear centre? Calculate the shear flow distribution in the thin-walled open section shown in figure 2 produced by a vertical shear load Sy acting through its shear center.
Answer
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The wing section shown in figure 3 has been idealized such that the booms carry all the direct stresses. If the wing section is subjected to a bending moment of 300 kNm applied in a vertical plane, calculate the direct stresses in the booms.
Answer
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What are the classification of aircraft loads? Explain in brief about the stresses due to air loads.
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Discuss with the help of neat sketches about shear lag.
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Derive the relationship for shear force at any section of a tapered diagonal tension field beam, subjected to a load at its free end perpendicular to the axis in the plane of the beam.
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Write short notes on the following:
tapered wing and fuselage.
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the shear flow variation over the symmetrical wing and fuselage.
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