TOPIC 6 - WING LOAD AND STRUCTURAL DESIGN

WING LOADS

the forces, such as lift, drag, maneuver and turbulence forces, that pull and push on the wings of an airplane. Without knowing these loads, wings might be too light and collapse, or too heavy and use extra fuel. Proper load analysis is critical for safety, performance, and aircraft structure.

AERODYNAMIC LIFT

INERTIAL LOADS

FUEL LOADS

GUST LOADS

LANDING LOADS

CONTROL SURFACE LOADS

[enumerate] TYPES OF WING LOADS

AERODYNAMIC LIFT

Acts upward during flight; largest load on the wing.

INERTIAL LOADS

Occur during maneuvers or turbulence due to the mass of the aircraft

FUEL LOADS

Weight of fuel distributed in the wing tanks adds to bending moments.

GUST LOADS

Sudden, vertical wind gusts impose dynamic load on the wing structure

LANDING LOADS

Vertical and lateral forces during landing

CONTROL SURFACE LOADS

Forces due to deflection of ailerons or flaps

SPEED & ALTITUDE

AIRCRAFT SIZE & WEIGHT

MANEUVERS

ATMOSPHERIC CONDITIONS

FLAPS/SLATS

[enumerate] FACTORS AFFECTING WING LOADS

SPEED & ALTITUDE

Flying faster or at different altitudes changes the air pressure on the wing and thereby directly impacts loads.

AIRCRAFT SIZE & WEIGHT

Larger, heavier aircraft simply must create more lift, which corresponds to higher wing loads.

MANEUVERS

In turns, climb, or dive, forces on the wing become much greater, considering "G-forces"

ATMOSPHERIC CONDITIONS

Turbulence or unexpected wind gusts can generate unforeseen heavy loads.

FLAPS/SLATS

These are devices utilized for takeoff and landing that modify the shape of the wing and how loads are transmitted.

AEROSPACE STRUCTURAL DESIGN

This is concerned with aircraft components becoming strong, lightweight, and resilient enough to sustain these loads safely.

SPANWISE

Load distribution that refers to how aerodynamic loads are distributed along the length of the wing.

ELLIPTICAL

RECTANGULAR

TAPERED WING

[enumerate] SPANWISE LOAD DISTRIBUTION

ELLIPTICAL LOAD DISTRIBUTION

Achieved using elliptical planform wings or specific wing twist and taper designs.

RECTANGULAR WING LOAD DISTRIBUTION

Often seen in basic or uniform wing designs.

TAPERED WING LOAD DISTRIBUTION

Designed with a taper.

LIFT DISTRIBUTION IMPACT

The shape of the wing, wing twist (washout or washin), and airfoil shape changes along the span can modify the load distribution

CHORDWISE

Load distribution that refers to how aerodynamic loads are distributed along the width of the wing.

LEADING EDGE

MID-CHORD

TRAILING EDGE

[enumerate] CHORDWISE LOAD DISTRIBUTION

LEADING EDGE

Experiences high pressure and contributes significant to lift. Stall typically initiates here.

MID-CHORD

The distribution of lift pressure is generally smoother and more uniform. The maximum thickness point of the airfoil often lies around this point.

TRAILING EDGE

Lower pressure compared to the leading edge and mid-chord. Controls such as ailerons and flaps are located near this point, impacting the load distribution when deployed.

AIRFOIL SHAPE

ANGLE OF ATTACK

WING PLANFORM

WING TWIST

FLAPS AND AILERONS

FLIGHT CONDITIONS

[enumerate] FACTORS INFLUENCING LOAD DISTRIBUTION

AIRFOIL SHAPE

This affects how lift is generated

ANGLE OF ATTACK

This increases the overall lift but also affect the distribution.

WING PLANFORM

The shape of the wing significantly impacts load distribution.

WING TWIST

Helps in delaying tip stall.

FLAPS AND AILERONS

Deployment of these control surfaces changes the local angle of attack

FLIGHT CONDITIONS

These conditions change the load distribution due to varying speeds, altitudes, and aircraft configurations.

WING LOADING

This determines the magnitude of unstick speed during take-off, touchdown speed during landing, and the stalling speed. It is defined as the aeroplane mass divided by the wing area and is specified in Newtons per square meter.

STRESS

Defined as the strength of a material per unit area, it is also called the unit strength. It is the force on a member divided by area, which carries the force, expressed in psi, N/mm2 or MPa.

TENSILE

COMPRESSIVE

SHEAR

BENDING

[enumerate] TYPES OF STRESS IN WINGS

TENSILE STRESS

Type of stress on the upper wing surface during lift

COMPRESSIVE STRESS

Type of stress on lower wing surface or during ground operations.

SHEAR STRESS

Type of stress due to torsion from maneuvering or engine thrust

BENDING STRESS

Type of stress caused by aerodynamic lift and weight distribution.

STRAIN

Also known as unit deformation, it is the ratio of the change in length caused by the applied force, to the original length.

STRESS-STRAIN DIAGRAM

This diagram shows how a material reacts when force is applied. It helps identify key points where a material stays elastic, starts to deform permanently, and eventually fails, making it essential for understanding the strength and limits of wing components.

PROPORTIONAL LIMIT

From the origin O up to this point, the stress-strain curve is a straight line. Within this limit, the stress is directly proportional to strain.

ELASTIC LIMIT

Limit beyond which the material will no longer go back to its original shape when the load is removed

YIELD POINT

Find a twitch the material will have an appreciable elongation or yielding without any increase in load

ULTIMATE STRENGTH

The maximum ordinate in the stress strain diagram

RUPTURE STRENGTH

The strength of the material at rupture. This is also known as the breaking strength

BENDING

This occurs when a force is applied, perpendicular to the wings, longitude the no axis, causing it to deflect upwards or downwards. This is caused by lift force acting against the weight of the aircraft. Grade stencil stress on the upper wing surface and compressive stress on the lower surface.

TORSION

It is the twisting of the wing due to uneven forces like engine thrust, aileron movement, or asymmetric loading. It causes sheer stress within the wing structure.

YOUNG'S MODULUS

Also called the modulus of elasticity, is a measure of a material stiffness, and how much it resists stretching or compressing when a force is applied

SAFETY MARGIN

It is the difference between the actual strength or capacity of a structure in the maximum load or stress it is expected to experience. It is the extra capacity built into a structure to handle unforeseen load.

FACTOR OF SAFETY

It is a design margin used to ensure that a structure or component can withstand loads beyond what it is expected to experience during normal operation

1.5

For ultimate strength, Aerospace components use a factor of safety of ___.

1.25

For yield strength, aerospace components use a factor of safety of ___.

LIMIT LOAD

It is the maximum expected operational load under normal conditions. Can also be expressed as the Limit Load x FoS

ULTIMATE LOAD

It is the maximum load structure must survive without catastrophic failure