Upload
appanna
View
61
Download
4
Tags:
Embed Size (px)
Citation preview
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.