Wing layout structure

To carry the distributed and concentrated loads prescribed by the

airwortheness requirement

• shear carried by the wing spars, the bending moment by the

wing 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 to

density ratio combined with good fracture toughness and fatigue life

Loads on wing members

life.

• The upper cover is loaded primarily in compression therefore it

should be designed in order to be stabilized or prevented from

buckling.

• Ribs carry the shear (and bending moment ) caused by the load

distributed chordwise

Manufacturing problems that exist

with the sweptback wing:

• Bending the spar caps is difficult

Rib arrangement in swept wing

• Bending the spar caps is difficult

• The skin gages required are

extremely thick.(needs multiple

brake operation)

• Angle of 90°in jigs, bulkheads,

and spar web are important to the

workman.Aerodynamical

accurate shape

• Lighter structure

• Easy to produce

Wing root triangle

A triangular section A is

indeterminate.

Single main beam for high

swept wing

Desirable preliminary studies 1. Draw platform of wing with necessary dimensions, to scale, to satisfy

aspect ratio, area & sweepback

2. 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%aileron. Spar cap width and control system gap need about 10% of the chordaileron. Spar cap width and control system gap need about 10% of the chord

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 also

used 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 constant

percentage of the wing chord.

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

Sometimes redesigning.

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 spar cap typeConcentrated spar cap type

Advantages

• Simplicity of construction

• It can be so design that spar buckling occurs near the ultimate stress

of 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 the airflow over the wing.(more drag)

• Fatigue failure due to the local bending stress in buckled sheet.

Wing bending 2

Distributed bending material type

• High number of stiffeners or multiple spar

• Different number of stiffeners in lower and upper surface (because

the negativ and positive load factors are different)

Skin is the only bending material

The skin outside the wingbox cannot take part in bendig

Safety considerations by the lower

surface

Federal Aviation Regulation (FAR): fail safe or safe life

This structure shall be able to carry 80%

of limit load times 1.15 dynamic factor of limit load times 1.15 dynamic factor

after a structural failure (fail safe)

There are five panels on the wing lower

surface as shown in figure. Each

spanwise splice between panels is a tear-

stopper which tends to stop the failed

panel to continuously crack to the next

panels. The carefully designed rivet

pattern and shear strength provide the fail

safe philosophy.

Considerations by compression panel

• Direct compression induced by bending of the entire section (+HAA

+LAA)

• Shear flows – Maximum panel shear

flows caused by wing box torsion

loads.

• Max shear flow with corresponding • Max shear flow with corresponding

local compression load to optimize the

least weight structure.

• Local bending effects caused by

surface aerodynamic pressure load.

• Local bending effects caused by wing

tank fuel(pressure, inertia) and by wing

bending crushing loads.

• Excentricity: stringer should end on

ribs to avoid change in cover centroid

Skin-Stringer panels

Skin-Stringer panels 2The machined (integral) skins combining with machined stringers are

the 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.

Skin-stringer area ratio

Optimum distribution of area between skin and stiffener for minimum

weight 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 because

of fatigueof fatigue

Integrally stiffened panels

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

Integrally stiffened panels 2

Advantages

• Reduction of sealing material for

pressurized fuel tank structure.

• Higher allowable stiffeners

compression loads by elimination

of attachments flanges.

• Increase joint efficiencies under • Increase joint efficiencies under

tension loads.

• Improved aerodynamics through

smoother exterior surfaces

• Light weight structure

The lightest cover panel design can be obtain with an integrally stiffened cover structure supported by sheet metal ribs with a

preference for a large spacing.

Cover panel splice design

Avoid complex extrusion forms (residual stress, crack)

Prefer double or stagger row of fasteners!

Stringer run-out

AvoidPrefer

Typical spar constructions

Non-buckling type: web never buckles

Buckling type: buckling criteria 1.0 – 1.5 g

Spar model for calculation

Spar caps

The beam (spar) cap should be design for strength/weight

efficiency. The cap sections for large cantilever beams which are

frequently used in wing design should be of such a shape as to

permit efficient tapering or reducing of the section as the beam

extends outboard. With cap additional stringer and skins are used

also to provide bending resistance.

Spar web

These cap sections are almost always used with a beam web

composed of flat sheet, which is stiffened by vertical stiffeners

riveted to the web.

Integrally stiffened spar

The cost is far less than the cost of a built-

up assembly of individual caps, web and

stiffeners riveted together.

General rules of spar design

1. Machine pads or add doublers to the web around spar web

cutout to reduce local stresses

General rules of spar design

2. To use double rows (or stagger rows) of fasteners between spar caps and webs, and also between spar caps and wing box skin.

General rules of spar design

3. 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 at

the spar cap where the tapered doubler along spanwise is

recommended.

General rules of spar design

4. 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 well

as skin should be made thicker to reduce local principle stresses.

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.

General rules of spar design6. In the area of the wing sweepback break, the spar cap horizontal flange and

local wing skin can be easily spliced by double shear splice plates. An

additional tension fitting should be provided to take care of the remaining part

such as spar cap vertical flange.

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.

General rules of spar design

8. 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 with diameter of one size bigger than adjacent attachments.