Lecture 1: Introduction
Structure —
An assemblage of materials which is
intended to sustain loads
Functions to protect people and things
Structures are so common and familiar to us,
that when we are informed of their use and
form we consider ourselves as knowledgeable
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Airplane structures are complex
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Structural layout
An aircraft is considered a system consisting of
structural subsystem
control subsystem
propulsion subsystem
cargo handling subsystem, etc.
A structural subsystem has multiple components
wing
fuselage
empennage
Components consist of many subcomponents or
structural members.
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Wing Fuselage Intersection
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AftFuselageStructure
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Typical structural subcomponents
spar: main load-bearing members in the wing.
wing skin: carries chordwise and spanwise
pressure distribution to the ribs and spars.
ribs: help the wing keep its airfoil shape, together
with the skin and spars form wing-box that resist
wing twist (torsion).
stringers (stiffeners): keep the skin from bending.
fuselage frames (bulkheads): maintain fuselage
shape, provide support to internal structure.
longerons: longitudinal fuselage beams.
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Typical (semimonococque) construction
Longitudinal
stringers
Cover Skin
Transverse
frames
(a)
Cover skin
Transverse
rib
Spar web
Spar cap
(b)
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Longitudinal
stringers
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The structural functions of the skin
1.It transmits aerodynamic forces to the
longitudinal and transverse supporting
members.
2.It develops shearing stresses which react the
applied torsional moments and shear forces.
3.It acts with the longitudinal members in
resisting the applied bending and axial loads.
4.It acts with the longitudinals in resisting the
axial load and with the transverse members
in reacting the hoop, or circumferential, load
when the structure is pressurized.
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The structural function of the longitudinals
1.They resist bending and axial loads along
with the skin.
2.They divide the skin into small panels and
thereby increase its buckling and failing
stresses.
3.They act with the skin in resisting axial loads
caused by pressurization.
Longitudinal members are sometimes referred
to as longitudinals, stringers, or stiffeners.
If they have a large cross-sectional area they
are called longerons. A strong underbody
beam for heavy loads is called a keel beam
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The function of transverse members
Frames, rings, bulkheads.
1.Maintain the cross-sectional shape.
2.Distribute concentrated loads into the
structure and redistribute stresses around
structural discontinuities.
3.Establish the column length and provide end
restraint for the longitudinals to increase their
column buckling stress.
4.Provide edge restraint for the skin panels
and thereby increase the plate buckling
stress of these elements.
5.Act with the skin in resisting the
circumferential loads due to pressurization.
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Most vehicle structures are thin shells
From the standpoint of structural weight, the
most efficient location for the structural material
is the outer surface.
monococque fuselage — unstiffened shell:
no keel, longerons, or stiffeners. It gets all of
its bending and torsional stiffness from the
tube/box formed by its skin and bulkheads.
semimonococque — stiffened shell with
some stringers and longerons
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Structures are made from materials
The history of structures follows the
development of materials and the development
of tools to fabricate materials.
Airframe
Early aircraft were constructed of wire-braced
frames with fabric covers.
Stressed-skin construction
Currently advanced composite materials are
attractive for weight-sensitive structures.
Material selection for structural efficiency:
Strength-to-weight ratio
Stiffness-to-weight ratio
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Evolution of wing structure
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Loads on structures
The first step in preliminary design is to
determine the external loads acting on the
structure.
Gravity forces, aerodynamic forces, and
inertial forces act on maneuvering flight
vehicles
Landing loads
Wind gust loads
Loads on a vehicle maybe classified as
Static or dynamic
Deterministic or probabilistic (wind gusts)
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Loads (continued)
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Loads (concluded)
In traditional design, these loads are not
affected by the structural configuration or
dimensions of the members. They are a
function of the wing shape, say, and other nonstructural factors.
Hence, determination of loads is essentially a
separate task typically performed by the
aerodynamicist.
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Lecture 2: How an Airplane is Built
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Lecture 2: Design Stages
Conceptual Design: Weight and center of
gravity analyses to ensure that the aircraft is
light enough and will balance properly.
Important criteria: producibilty, maintainability.
Preliminary Design: Materials for major
components are specified. Refined structure is
analyzed to ensure strength without excess
weight. Detailed physical and CAD/CAM
drawings are created.
Detail Design: Every component, fastener/
connector, linkage must be designed/specified
in exact detail. Structural test prototypes are
built and tested.
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Design guidelines
Overall dimensions of an airplane are usually
determined by more general requirements
rather than for structural considerations.
An important contribution to the overall vehicle
performance is to minimize weight in the
structural subsystem while maintaining integrity.
more payload
less fuel required
greater range
General guidelines:
1.Never attach anything to skin alone.
2.Structural members should not pass through
air inlets, passenger cabins, cargo bays, etc.
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Design guidelines (continued)
3.Major load-carrying members should carry
completely through a structural component.
4.Whenever possible, attach engines,
equipment, landing gear, seats, pylons, etc.,
to existing structural members. Plan the
position of major structural elements so that as
many systems as possible can be attached to
them.
5.Design redundancy into your structures to
have multiple load paths.
6.Mount control surfaces and high-lift devices to
a spar, not just the rear ends of ribs.
7.Innovate to come up with creative layouts....!
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Why do structures carry load?
Crux of the issue in design of complex
structures is know not only the use and form
but why a structure carries load.
By Newton’s law of action/reaction we know
that isolated forces do not exist in nature.
A force acting on an inanimate solid is
reacted by a force produced by the solid.
But how does the solid produce such a
reaction force?
Robert Hooke (1635 - 1702)
“Ut tensio sic vis” (the subtitle of Chapter 7)
Translation: as the stretch so the force
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