Airframes & Aircraft Systems #1 - Aircraft Structures - Loads Applied to the Airframe
Summary
TLDRThis lesson in the airframe structures series explores the various loads on aircraft structures, including tensile, compressive, and shear loads, and their effects on materials. It delves into the significance of stress, strain, and Young's modulus of elasticity in aircraft design. The concept of design limit and ultimate loads, along with safety factors, is discussed to ensure aircraft safety. The video also covers failsafe and damage-tolerant structures, explaining how they manage loads and prevent catastrophic failures. Additionally, it touches on the importance of aircraft's safe life and maintenance to mitigate fatigue and corrosion.
Takeaways
- ✈️ Aircraft structures experience various loads like tensile, compressive, and shear, which are critical in design and construction.
- 🔍 Tensile loads stretch structural members, while compressive loads shorten them; these are resisted by ties and struts respectively.
- 🔧 Riveted joints and adhesive bonding processes are used to resist shear forces, important in aircraft construction.
- 🔄 Bending, torsion, and buckling involve a combination of tensile, compressive, and shear loads, affecting the integrity of the structure.
- 📏 Stress is the internal force resisting an external load, and strain measures the deformation caused by this stress.
- 📐 Young's modulus of elasticity describes the relationship between stress and strain within the elastic limit of materials.
- 🚀 The design limit load for aircraft is based on the load factor, and structures are built to withstand design ultimate loads with a safety factor.
- 🛠️ Aircraft structures are designed to be failsafe or damage tolerant, with multiple load paths or damage-tolerant designs to ensure safety.
- ⏳ The safe life of aircraft components is monitored through cycles, such as flying hours or pressurization cycles, to prevent catastrophic failure.
- 📍 Aircraft components are identified by reference lines like fuselage station numbers, wing station numbers, and water lines, ensuring precise maintenance and repairs.
Q & A
What are the various loads applied to aircraft structures?
-Aircraft structures are subjected to tensile, compressive, and shear loads. Tensile loads stretch structural members, compressive loads shorten them, and shear forces slide one face of the material over another.
What is the function of stringers in modern aircraft fuselages?
-Stringers in modern aircraft fuselages are designed to resist tensile loads, particularly those produced by pressurization.
How do landing gear struts contribute to an aircraft's structural integrity?
-Landing gear struts, such as the oleo pneumatic strut, resist compressive loads during landing, ensuring the aircraft's structural integrity under impact forces.
What is the difference between a tensile stress and a compressive stress?
-Tensile stress is the internal force that resists a force that tends to stretch a structural member, while compressive stress resists a force that tends to shorten a structural member.
How is strain measured in aircraft structures?
-Strain is measured as the ratio of the change in a material's length to its original length, indicating the deformation of a loaded structure.
What is Young's Modulus of Elasticity and how is it discovered?
-Young's Modulus of Elasticity is a constant that describes the relationship between stress and strain for an elastic material within its elastic limit. It was discovered by the 18th-century English physicist Thomas Young.
What is the significance of the design limit load in aircraft design?
-The design limit load is the maximum load that a designer expects an airframe or component to experience in service, defined as a load factor, which is the ratio of the lift of an aircraft to its weight.
How does the safety factor relate to the design ultimate load?
-The design ultimate load is the design limit load multiplied by a safety factor, which is a minimum of 1.5 as specified in design requirements, ensuring the structure can withstand this load without collapse.
What are fail-safe and damage-tolerant structures in aircraft design?
-Fail-safe structures incorporate component redundancy and multiple load paths, allowing loads to be shared by adjacent members if one part fails. Damage-tolerant structures are designed to withstand a certain amount of damage before failure occurs, spreading the loading over a larger area.
How do pressurized aircraft fuselages handle stress from pressurization?
-Pressurized aircraft fuselages handle stress from pressurization through axial (longitudinal) and hoop (radial) stresses. Hoop stresses, which tend to expand the fuselage cross-section, are of greater significance than axial stresses.
What is the purpose of using reference lines in aircraft design for maintenance?
-Reference lines are used in aircraft design to locate components for maintenance and repairs. They identify positions fore-and-aft, left and right, and from top to bottom, using station numbers and datum lines.
Outlines
🛫 Introduction to Aircraft Structures and Loads
This paragraph introduces the topic of airframe structures, focusing on the various loads that aircraft structures must withstand. It discusses the design and construction of critical components like the fuselage, wings, and tail unit, and the materials used in their construction. The effects of corrosion and fatigue on these materials are also examined. The paragraph further delves into the impact of a hard landing on the airframe and the significance of failure statistics in aircraft design. It explains different types of loads such as tensile, compressive, and shear loads, and how they affect aircraft structures. The concept of stress and strain is introduced, along with Young's modulus of elasticity, which describes the relationship between stress and strain within an elastic material's limit.
🔍 Deep Dive into Aircraft Load Management
The second paragraph expands on the concept of loads on aircraft, discussing how most loads are a combination of tensile, compressive, and shear forces. It explains the effects of bending, torsion, and buckling on aircraft structures. The paragraph also covers the definition and measurement of stress and strain, emphasizing their importance in aircraft design. It introduces the concept of Young's modulus of elasticity, which is a constant relationship between stress and strain for elastic materials. The discussion then moves to the design limit load, which is the maximum load an aircraft or component is expected to experience in service. The design ultimate load, which is a safety factor multiple of the design limit load, is also explained. The paragraph concludes with a discussion on the design philosophy of fail-safe and damage-tolerant structures, which are crucial for aircraft safety.
🛠️ Aircraft Construction Techniques and Maintenance
Paragraph three delves into the construction techniques of aircraft, particularly the use of stressed skin construction where each part of the airframe contributes to load distribution. It contrasts this with fail-safe structures that rely on component redundancy and multiple load paths. The paragraph also touches on the importance of maintenance programs to detect failures before they become critical. It discusses the concept of damage-tolerant structures that can withstand a certain amount of damage without catastrophic failure. The paragraph further explains the fatigue process, where materials fail at a lower load under repeated stress cycles compared to a steady load. It introduces SN curves, which graphically represent material performance under high cycle fatigue conditions. The paragraph concludes with a discussion on the methods used to locate components on an aircraft for maintenance and repair purposes, including the use of reference lines and station numbers.
🚁 Understanding Fuselage Design and Stresses
The final paragraph focuses on the fuselage, which is the main body of the aircraft. It details the roles of the fuselage, including carrying passengers, cargo, and crew, as well as housing controls and equipment. The paragraph explains how modern aircraft fuselages are pressurized and must withstand axial and hoop stresses caused by pressurization. It describes the significance of hoop stresses, which are greater than axial stresses, and the differential pressures that can be as high as 65.5 kN/m². The lesson concludes with a summary of key learnings, including the understanding of tensile and compressive loads, the meaning of stress and strain, Young's modulus of elasticity, the relationship between design limit load and design ultimate load, and the safety factor. It also highlights the importance of failsafe and damage-tolerant structures in aircraft design.
Mindmap
Keywords
💡Tensile Load
💡Compressive Load
💡Shear Force
💡Bending
💡Torsion
💡Buckling
💡Stress
💡Strain
💡Young's Modulus of Elasticity
💡Design Limit Load
💡Fail-Safe and Damage Tolerant Structures
Highlights
Various loads applied to aircraft structures are defined, including those during ground operations and flight.
Design and construction of the fuselage, wings, and tail unit are examined with a focus on materials and their resistance to corrosion and fatigue.
The effects of a hard landing on the airframe are discussed, highlighting the importance of structural integrity.
Failure statistics are reviewed, emphasizing their critical role in safe aircraft design.
Tensile loads and their resistance by ties, such as stringers in the fuselage, are explained.
Compressive loads and their resistance by struts, exemplified by the aircraft landing gear, are described.
Shear forces and their impact on riveted joints, with a shift towards adhesive bonding in modern aircraft, are discussed.
Bending, torsion, and buckling are introduced as complex loading conditions affecting aircraft structures.
Stress is defined as the internal force resisting external forces, with examples of tensile and compressive stresses.
Strain is introduced as a measure of deformation, with a clear example of its calculation.
Young's modulus of elasticity is explained as the constant relationship between stress and strain within an elastic material's limit.
Design limit load is defined in relation to load factors and its significance in aircraft safety.
The design ultimate load is described as a safety factor multiple of the design limit load.
Fail-safe and damage-tolerant structures are compared in terms of their design philosophies and practical applications.
The importance of maintenance programs in detecting failures before they progress in fail-safe structures is emphasized.
Damage-tolerant structures are explained as a method to spread loads and tolerate damage without extra structural members.
Fatigue is introduced as a failure mechanism due to continual loading reversals, with an explanation of SN curves.
The significance of reference lines and datum lines in locating components on aircraft for maintenance is discussed.
The fuselage's role as the main structure of the aircraft, carrying payload and transferring loads, is highlighted.
Pressurized fuselages and their need to withstand axial and hoop stresses are explained.
Transcripts
in the airframe structures series of
lessons we will define the various loads
applied to aircraft structures looking
at the design and construction of the
fuselage wings and tail unit
we will examine the materials used in
construction and the effects of
corrosion and fatigue on these materials
we will discuss the effects of a hard
landing on the airframe and we will
review failure statistics and their
importance in safe aircraft design
in this first lesson we will discuss the
various loads applied to aircraft
structures both on the ground and in
flight
we'll examine the effect of fatigue on
structures and discuss the methods used
to mitigate these effects
a tensile load is one which tends to
stretch a structural member
the member is then said to be under
tension
components designed to resist tensile
loads are known as ties good examples of
ties are the stringers which in modern
aircraft take the tensile loads produced
in the fuselage structure by
pressurization
compressive loads are the opposite of
tensile loads and tend to shorten
structural members
members subject to compressive loads are
said to be under compression
components designed to resist
compressive loads are known as struts
you will find numerous examples of
struts in an aircraft landing gear the
primary one being the olio pneumatic
strut which takes the compressive loads
on landing this will be fully explained
in the landing gear lesson shear is a
force which tends to slide one face of
the material over an adjacent face
riveted joints are designed to resist
shear forces
rivets are being used in this picture to
fasten the fuselage skin to the frame in
modern aircraft they are being replaced
by adhesive bonding processes
to review tensile compressive or shear
loads click one of the three buttons
most loads to which aircraft components
are subjected are a combination of two
or all three of these basic loads
bending of the structure involves the
three basic loadings they are tension as
the outer edges stretch compression as
the inner edges squeeze together and
shear across the structure as the forces
are try to split it
torsion or twisting forces produce
tension at their outer edge compression
in the center and shear forces across
the structure
buckling occurs two thin sheet materials
when they are subjected to end loads and
two ties if subjected to compressive
forces
to review bending torsion or buckling
click one of three buttons
stress is the internal force inside a
structural member which resists an
externally applied force
and therefore a tensile load or force
will set up a tensile stress
and a compressive load compressive
stress
stress is defined as the force per unit
area and is measured in Newton's per
square millimeter or mega Newton's per
square meter
when an external force of sufficient
magnitude acts on a structure the
structural dimensions change this change
is known as strain
strain is measured as a ratio of the
change in a materials length to its
original length and is a measure of the
deformation of any loaded structure
our extreme example shown here the
original length of the material is 2
meters
load is applied its length increases to
2.5 meters this gives it an increase in
length of 0.5 meters
therefore the strain being felt by this
material is 0.5 divided by 2 which is
0.25
the 18th century English physicist
Thomas Young discovered that the
relationship between stress and strain
for an elastic material within the
elastic limit of the material is
generally a constant
this constant is therefore known as
young's modulus of elasticity
aircraft components are subjected to
some or all of these stresses and they
will tends to elongate compress bend
shear and twist the component
however provided the deformation is
within the elastic limit of the material
the component will return to its
original dimension once the stress has
been removed
if any load takes the structure beyond
its elastic limit then the deformation
will be permanent
the maximum load that a designer would
expect an airframe or component to
experience in service is termed the
design limit load
this load is defined as a load factor
load factor is the ratio of the lift of
an aircraft to the weight of the
aircraft
the load factor is expressed in
multiples of G in straight and level
flight lift is equal to weight so the
ratio of lift to weight is 1 and the
load factor is 1 G
the design limit load is 2.5 chief of
public transport aeroplanes
the design limit load is 3.4 23.8 G for
utility aircraft
and 6g for aerobatic aircraft
the design ultimate load is the design
limit load multiplied by a safety factor
the minimum safety factor specified in
design requirements is 1.5
the structure must withstand its design
ultimate load without collapse
the safety factor is the ratio of the
design ultimate load to the design limit
load
the aircraft manufacturer will attempt
to design an aircraft to take into
account all the loads that it may
experience in flight
our various guidelines formula and
experience to help them in the design of
good failsafe and damage tolerant
structures
the designer will take into account the
anticipated role of the aircraft so for
instance aircraft designed for long-haul
operations should not be used for short
haul as the extra takeoffs and landings
will not have been accounted for in the
airframes anticipated fatigue life
calculations
the safe life of an aircraft structure
is defined as the minimum life during
which it is known that no catastrophic
failure will occur
all critical components have a safe life
which may be measured in flying hours
landings pressurization cycles or even
on a calendar basis
after the permitted safe life count has
been reached the relevant item is
replaced or overhauled
during the operational life of the
aircraft to minimize the effects of
mental fatigue aircraft designers apply
the principles of failsafe or damage
tolerant construction
a fail-safe structure is based on the
principle of component redundancy it has
multiple load paths in parallel which
means that the loads are shared by
adjacent members
therefore if one part fails the load
will be carried by the adjacent member
for a limited period
this must be until at least the next
periodic inspection
the disadvantage of dueling the load
paths is that it is very expensive in
terms of weight as each of the members
has to be strong enough to do the work
for both
it is now only normally used for wing
and stabilizer attachment points
the bracing struts between the wings and
the fuselage of the shorts sky van for
instance are each made up of three
members
this design philosophy must be
accompanied by a maintenance program to
ensure that failures are detected before
they progress too far
to gain access to vulnerable areas a
certain amount of dismantling is usually
required although non-destructive
testing may be used in less critical
areas
modern concepts of aircraft design
employ the stressed skin style of
construction where each piece of the
airframe including the stress skin plays
its part in spreading loads throughout
the entire airframe and is tolerant to a
certain amount of damage
these damage tolerant structures
eliminate the extra structural members
needed in a fail-safe design by
spreading the loading of a particular
structure over a larger area
damage tolerant structures are designed
to withstand a certain amount of damage
which again should be detected during
the normal inspection cycle before a
failure occurs
a structure which is subject to
continual reversals of loading such as
the landing gear or the fuselage of a
pressurized aircraft
will fail at a load of less than would
be the case for a steadily applied load
this is known as fatigue
the failure load level will depend on
the number of reversals experienced
in high cycle fatigue situations a
material performance can be graphically
characterized by an SN curve also known
as a volar curve
this is a graph of the magnitude of the
cycle stress s against a logarithmic
scale or cycles to failure
n
it can be seen that after 100 cycles at
80% of the ultimate stress the specimen
will fail
but if the stress is reduced to 30% of
the ultimate
the component will fail after 10,000
cycles
a method of locating components on the
aircraft must be established in order
that maintenance and repairs can be
carried out
this is achieved by using reference
lines to identify positions fore-and-aft
left and right and from top to bottom
fuselage station numbers are determined
by reference to a zero datum line which
is at or near the nose of the aircraft
on some aircraft such as the one shown
here it is Ford of the nose station
numbers are identified in inches or
millimeters forward or aft of the zero
datum
stations forward of the datum are
identified with a minus sign and aft
with a plus sign or no sign at all
in this installation they are all after
the datum so no sign is used
wing station numbers are measured in
inches left or right of the aircraft
centerline
vertical position is identified from the
horizontal datum
these positions are known as water lines
or buttock lines and are measured in
inches from the datum
plus is used for locations above and
minus for locations below the datum
this example the datum is at the bottom
of the extended landing gear
the fuselage is the main structure of
the aircraft it carries the aircraft
payload be it passengers or Freight as
well as the crew in safe comfortable
conditions
it also provides the crew with an
effective position for operating the
aircraft and space for controls
accessories and other equipment
the fuselage transfers loads true and
from the wings tail plane fin landing
gear and in some configurations the
engines
modern aircraft fuselages are
pressurized and these pressurized
aircraft structures must also be capable
of withstanding axial and hoop stresses
imposed by the pressurization forces
axial or longitudinal stresses are set
up in the fuselage of aircraft when
pressurized and tend to elongate the
fuselage between the front and rear
pressure bulkhead
hoop or radial stresses also caused by
pressurizing the fuselage tend to expand
the fuselage cross-section area
the hoop all radial stresses are of
greater significance than the axial
stresses
the differential pressures that set up
these stresses can be as high as 65 and
a half kilonewtons per square meter or
nine and a half pounds per square inch
this is the end of the lesson
you should now know that tensile loads
are resisted by ties
that compressive loads are resisted by
struts
and the meaning of the terms stress and
strain
you should know the meaning of the term
young's modulus of elasticity
and you should also understand the
relationship between design limit load
and design ultimate load and the safety
factor
you should also know what the design
limit load is for various types of
aircraft
and finally you should understand the
properties of failsafe and damage
tolerant structures
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