Airframes & Aircraft Systems #1 - Aircraft Structures - Loads Applied to the Airframe

Summary

This 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

00:00

🛫 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.

05:02

🔍 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.

10:03

🛠️ 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.

15:03

🚁 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.

Keywords

💡Tensile Load

A tensile load is a force that tends to stretch or elongate a structural member. In the context of the video, ties are components designed to resist tensile loads. An example given is the stringers in modern aircraft that take the tensile loads produced in the fuselage structure by pressurization. Tensile loads are crucial in aircraft design as they affect the structural integrity, especially in areas that experience stretching due to various forces like pressurization or aerodynamic loads.

💡Compressive Load

Compressive loads are forces that tend to shorten or compress structural members. The video mentions that members under compression are known as struts, with the example of the oleo pneumatic strut in aircraft landing gear that takes compressive loads upon landing. Compressive loads are significant in aircraft design as they influence how structures like landing gear are engineered to withstand the forces experienced during takeoff and landing.

💡Shear Force

Shear force is a force that causes one layer of a material to slide over another. Riveted joints in aircraft are designed to resist shear forces, as depicted in the video where rivets fasten the fuselage skin to the frame. Understanding shear forces is vital for aircraft design as it ensures the joints and connections within the airframe can withstand the sliding forces that can lead to structural failure.

💡Bending

Bending refers to the deformation of a structure when subjected to loads that cause it to curve. The video explains that bending involves tension, compression, and shear forces. Bending is a critical concept in aircraft design as it affects the structural response of wings and fuselage to aerodynamic and other loads, which must be carefully managed to prevent failure.

💡Torsion

Torsion is the twisting of a structure due to applied forces. The video mentions that torsion or twisting forces produce tension at the outer edge, compression in the center, and shear forces across the structure. Torsional loads are important in aircraft design as they can cause significant stress, particularly in parts like wings and tail sections that are subject to aerodynamic forces.

💡Buckling

Buckling is the deformation or failure of a structure subjected to compressive loads, often occurring in thin sheet materials. The video explains that buckling can occur in ties when subjected to compressive forces. Buckling is a key consideration in aircraft design, especially in the context of lightweight materials that must maintain structural integrity under high compressive loads.

💡Stress

Stress is defined in the video as the internal force within a structural member that resists an externally applied force. It is measured in Newtons per square millimeter or mega Newtons per square meter. Stress is a fundamental concept in aircraft design as it helps engineers determine the strength required in various components to ensure they can withstand operational loads without failure.

💡Strain

Strain is the measure of deformation of a material in response to an applied force, defined as the ratio of the change in a material's length to its original length. The video provides an example of a material that increases in length from 2 to 2.5 meters under load, resulting in a strain of 0.25. Strain is important in aircraft design as it helps engineers understand how materials will deform under load, which is crucial for predicting and preventing structural failure.

💡Young's Modulus of Elasticity

Young's Modulus of Elasticity is a constant that describes the relationship between stress and strain for an elastic material within its elastic limit. The video explains that this constant is used to predict how materials will behave under load. Young's Modulus is a key parameter in aircraft design as it informs material selection and structural analysis, ensuring that components can return to their original dimensions after loading.

💡Design Limit Load

The design limit load is the maximum load that a designer expects an airframe or component to experience in service. The video specifies this load as a load factor, which is the ratio of the lift of an aircraft to its weight, expressed in multiples of G. Understanding design limit loads is essential for aircraft design as it sets the standard for the strength and durability that aircraft components must possess to ensure safety.

💡Fail-Safe and Damage Tolerant Structures

Fail-safe structures are designed with redundancy, allowing loads to be shared among multiple members so that if one part fails, the load is carried by adjacent members until the next inspection. Damage tolerant structures, on the other hand, are designed to withstand a certain amount of damage without catastrophic failure. The video explains these concepts as part of aircraft design philosophy, aiming to ensure safety and longevity of the airframe by accounting for potential failures and damages.

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