vip_wing layout structure_short introduction

8/4/2019 VIP_Wing Layout Structure_short Introduction

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Wing layout structure

To carry the distributed and concentrated loads prescribed by the

airwortheness requirement


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shear carried by the wing spars, the bending moment by thewing covers (skin, spar caps and stringers) and torsion by the

wing skin only

The lower cover is loaded primarily in tension therefore it requires

careful material in order to assure fairy high tensile strength todensity ratio combined with good fracture toughness and fatigue

Loads on wing members

life. The upper cover is loaded primarily in compression therefore itshould be designed in order to be stabilized or prevented frombuckling.

Ribs carry the shear (and bending moment ) caused by the loaddistributed chordwise


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Manufacturing problems that existwith the sweptback wing:

Bending the spar caps is difficult

Rib arrangement in swept wing

The skin gages required areextremely thick.(needs multiplebrake operation) Angle of 90in jigs, bulkheads,and spar web are important to the

workman.

Aerodynamical

accurate shape

Lighter structure

Easy to produce


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Wing root triangle

A triangular section A is

indeterminate.

Single main beam for highswept wing


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Desirable preliminary studies1. Draw platform of wing with necessary dimensions, to scale, to satisfy

aspect ratio, area & sweepback2. Determine the mean geometric chord and check if the CG lies in plane

perpendicular to CG chord at the mean aerodynamic center.3. Locate the front spar at the constant percentage of the chord (12-17%),

from root to tip.4. Locate the rear spar similarly of the chord (60%) to accommodate a 30%

.

5. If flaps chord less then aileron, auxiliary spar is needed to support flaps.Sometimes

6. Ribs are located at each aileron and flap hinge. Reinforces ribs are alsoused for engine-mount, landing gear attachments and fuel-tank supports. Rib

spacing determined from panel size considerations.

7. Spanwise stringers are located parallel to each other or at constantpercentage of the wing chord.

8. Adding other detail like the wheel well for the retraction of the landing gear.Sometimes redesigning.


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Wing bendingClassification of wing structure according to the disposition of the bending material: All bending material is concentrated in the spar caps. The bending material is distributed around the periphery of the profile Skin is primarily bending material

Concentrated s ar ca t e

Advantages Simplicity of construction It can be so design that spar buckling occurs near the ultimate stressof the material (higher allowable stress)

Disadvantages

Skin buckling at a very low load. Skin can be in a wave state having large amplitude which disturbs theairflow over the wing.(more drag) Fatigue failure due to the local bending stress in buckled sheet.


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Wing bending 2Distributed bending material type High number of stiffeners or multiple spar

Different number of stiffeners in lower and upper surface (becausethe negativ and positive load factors are different)

Skin is the only bending materialThe skin outside the wingbox cannot take part in bendig


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Safety considerations by the lowersurface

Federal Aviation Regulation (FAR): failsafe or safe life

This structure shall be able to carry 80%.

after a structural failure (fail safe)

There are five panels on the wing lowersurface as shown in figure. Eachspanwise splice between panels is a tear-

stopper which tends to stop the failedpanel to continuously crack to the nextpanels. The carefully designed rivetpattern and shear strength provide the failsafe philosophy.


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Considerations by compression panel Direct compression induced bybending of the entire section (+HAA+LAA)

Shear flows Maximum panel shearflows caused by wing box torsionloads.

local compression load to optimize theleast weight structure.

Local bending effects caused by

surface aerodynamic pressure load.

Local bending effects caused by wingtank fuel(pressure, inertia) and by wingbending crushing loads.

Excentricity: stringer should end onribs to avoid change in cover centroid


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Skin-Stringer panels


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Skin-Stringer panels 2The machined (integral) skins combining with machined stringers arethe most efficient structures to save weight.

Advantages the skin can be tapered spanwise and chordwise, can thickened around holes can produce rib lands as shown in fig.


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Skin-stringer area ratioOptimum distribution of area between skin and stiffener for minimumweight exist:

k=1,4 assuming equal buckling stress in skin and stiffeners k=1,7 in case of unflanged, integrally stiffened panels k=1,5 for Z section stiffeners (thickness ratio = 1.05)

In practical design the total weight fraction of skin is higher becauseof fatigue


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Integrally stiffened panels

A weight reduction of 10-15% can be realized compared to the assembled structure


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Integrally stiffened panels 2Advantages Reduction of sealing material for

pressurized fuel tank structure. Higher allowable stiffeners

compression loads by eliminationof attachments flanges.

tension loads. Improved aerodynamics throughsmoother exterior surfaces Light weight structure

The lightest cover panel design can be obtain with an integrallystiffened cover structure supported by sheet metal ribs with apreference for a large spacing.


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Cover panel splice design

Avoid complex extrusion forms (residual stress, crack)

Prefer double or stagger row of fasteners!


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Stringer run-out

AvoidPrefer


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Typical spar constructions

Non-buckling type: web never buckles

Buckling type: buckling criteria 1.0 1.5 g


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Spar model for calculation


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Spar caps

The beam (spar) cap should be design for strength/weightefficiency. The cap sections for large cantilever beams which arefrequently used in wing design should be of such a shape as to

permit efficient tapering or reducing of the section as the beamextends outboard. With cap additional stringer and skins are usedalso to provide bending resistance.


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Spar web

These cap sections are almost always used with a beam web

composed of flat sheet, which is stiffened by vertical stiffenersriveted to the web.


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Integrally stiffened spar

The cost is far less than the cost of a built-up assembly of individual caps, web andstiffeners riveted together.


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General rules of spar design1. Machine pads or add doublers to the web around spar web

cutout to reduce local stresses


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General rules of spar design2. To use double rows (or stagger rows) of fasteners between spar caps

and webs, and also between spar caps and wing box skin.


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General rules of spar design3. Spar web splice doublers should be designed such it is strong enough to

carry not only the vertical shear force but also the spar axial force atthe spar cap where the tapered doubler along spanwise isrecommended.


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General rules of spar design4. The tension fitting is required wherever appreciable concentrated loads exit,

such as engine pylon, main landing gear support, aileron and flap track fitting,

etc. at these locations, the local material thicknesses of spar cap, web as wellas skin should be made thicker to reduce local principle stresses.


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General rules of spar design5. Do not allow any fixed leading or trailing edge panel to be directly

riveted to the spar cap to avoid potential fatigue cracks.


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General rules of spar design6. In the area of the wing sweepback break, the spar cap horizontal flange andlocal wing skin can be easily spliced by double shear splice plates. Anadditional tension fitting should be provided to take care of the remaining part

such as spar cap vertical flange.


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General rules of spar design7. Clips, provided for the support of wires, hydraulic tubes, control rods,

ducts, etc, should be fastened to spar vertical stiffener only.


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General rules of spar design8. Fasteners spacing along vertical stiffeners should not be too close to

make the local web net area shear critical. In addition, the fasteners going

through the spar cap and stiffeners should be at least two fasteners withdiameter of one size bigger than adjacent attachments.

vip_wing layout structure_short introduction

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