Structural Analysis of an Aircraft

The structural analysis of an aircraft is a specialty that will never go out of fashion. It will need engineers who are trained in specialized software along with an understanding of the functioning and underlying structural framework of aircraft. This blog was inspired by the following webinars - Part 1 Part 2

 

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In this article, we will be looking at a detailed analysis of aircraft structures, you will learn about:

• The basics of an aircraft structure

• Structural elements of an aircraft 

• Forces that act on an aircraft structure

• Materials used in an aircraft structure 

• Defining fasteners and their different types 

 

Function of Aircraft Structures

 

In the simplest of terms, a structure works as the basic skeleton of any equipment or automobile, supporting systems, and elements. For example, the tubular structure of a motor-bike acts as a bearing to fit in other components such as the fuel tank, the suspension system, and digital meter. A structure helps different parts of an automobile integrate with each other to function efficiently as a whole.

 

Structure of an aircraft

 

 

• Nose: The nose is the tapered portion of an aircraft located at the front. The structure of the nose is designed to alleviate the stress placed upon the plane during take-off, movement at stabilized speed, and during the descent and landing of the plane.

• Fuselage: The fuselage of an aircraft continues from the nose and is where the passengers are seated, the fuselage of the aircraft is also the storage compartment where the luggage of the passengers and crew members are stored.

• Wings: The wings of an aircraft enable the aircraft to fly and are made of aerofoil sections that are designed to create an aerodynamic lift.

• Winglets: These are up-turned tips at the end of the wings that manipulate the movement of the plane, in modern models of aircrafts these winglets are used to reduce the fuel consumption.

• Flaps: The flaps on a plane are mechanical parts of the wings, facing away from the nose, and are used to manipulate stall speed, increase lift, and increase the drag of the plane.

• Vertical & Horizontal Stabilizers: The stabilizers are located on at the tail of the plane and help the equilibrate the movement of the plane during turbulence. 

 

Elements in an Aircraft’s Fuselage Structure 

• Longerons: Long indirect loads carrying members along the body of the fuselage to provide a basic framework. 

• Struts: Bracing present between longerons to reinforce the structure of a fuselage.

• Bulkheads: Intermediate frame structures placed at an appropriate distance in the fuselage to support the structure as a whole.

• Stringers: Members that connect across all bulkheads to form an integrated structure.

 

Elements in an Aircraft’s Wing Structure

 

Structural Elements 

• Ribs: Members that are placed in different precise places of the wing to form its structure.

• Stringers: These are members that connect the ribs together, strengthening them in their respective positions.

• Spars: Plate like structures placed at specific points in a wings design to carry flight loads and the weight of the wing while on the ground. They are placed very close to the flaps.

• Skin: This covers all other structural members and is riveted to it firmly such that it cannot deform while the plane is airborne. 

 

Controlled surfaces

• Wing Flap: The wing flap moves back and forth and can be moved up and down as per the requirement of the pilot.

• Ailerons: These are quite similar to the wing flaps but can be used to roll the airplane around its longitudinal axis. It is typically used to bank the aircraft for turning and usually works in opposition to each other.
  For example, if the right aileron is pointed upward, the left will be pointed downward and vice versa.

 

Elements in an Aircraft’s Tail Structure

 

Structural Elements

• Horizontal and Vertical Stabilizers: The stabilizers on a plane are made of ribs, spars and stringers and are integrated and sized depending on the design requirement.

 

Controlled surfaces 

• Trim Tabs & Elevators: Trim tabs and elevators are used to balance out aerodynamic forces acting on the plane.

• Rudder: The rudder is used by the pilot to maneuver the plane.

Forces that Act on an Aircraft During Takeoff and Landing

The structures at the top of the plane undergo compression while the lower portions experience tension during the takeoff and landing. The compression at the top causes the plane to fold onto itself towards the center, while the lower portion of the plane remains under high tension. This can occur for several reasons, for example, the weight of the plane, its own design, etc, which is why the material that is chosen to construct the body of the aircraft should have low modulus. 

Forces that Act on an Aircraft when Airborne

 

An aircraft is stabilized when the lift of the plane is equal to its weight. During the lift, the engines of the aircraft push the air down towards the ground pushing the aircraft up. The engines are the primary drivers of force that allow the aircraft to take off. These counteract against the wind resistance (drag) to move the plane forward(thrust).

Air pressure is also an important fact to consider, the pressure difference between the cabin of the aircraft and the pressure outside the aircraft will also induce stress-force unto the entirety of the plane’s different structural elements. The passenger cabin of the plane operates at a different pressure to the pressures outside the aircraft and this must be taken into account during the designing phase of an aircraft.

 

Fundamental Movements of an Aircraft 

• Pitch: Rotation around a planes lateral axis

• Roll: Rotation around a planes longitudinal axis

• Yaw: Rotation around a planes vertical axis 

 

The movements of an aircraft are different from that of a car or a boat as a plane can be maneuvered in three dimensions instead of two. These movements places stress on the aircraft’s structural integrity.

 

The Basic Forces that Act on an Airplane

 

 

 

These forces are constantly acting on the body of the aircraft, for example, the skin surface of the plane is always being subjected to shearing which leads to tears. But no significant damage is done to the plane as it is protected due to its design and the strength of the materials that it is made of.

Desired Material Properties of Aircraft Structures 

 

Physical properties 

• Toughness to withstand tearing or shearing 

• Strength in the material to resist stress and deformation 

• Low-density material for a lighter structure 

• High strength to mass ratio 

• Elasticity to enable it to return to its original size and shape when force is removed

• Thermal expansion to retain the designed aircraft parts to their original shape and size

 

  Chemical properties 

• Chemically stable to be less reactive to particles in the air 

• Resistance to corrosion to avoid the deterioration of parts

• Thermal stability to withstand unstable temperatures while functioning 

 

These are common requirements for materials used in vehicles, but aircraft carry a slight difference as aircraft fly at high altitudes and thus have a need for stronger yet low-density materials to consume less fuel while being protected from the harsh turbulent environment.

 

Common Materials used in Aircrafts

 

AI 2024 T3: This is commonly used on the wings leading edge due to its high resistance to corrosion. Depending on the design they can be found on the ribs, spurs, and stringers of the plane. 

T3 Condition: Solution heat treated, cold worked and naturally aged

 

AI 7075 T6: This alloy combination is another common material that is used on the wing skins, panels, and covers.

T6 Condition: Solution heat treated and artificially aged 

 

Composite materials: Fiber-reinforced plastic (FRP), especially those made from glass fiber, carbon fibers, and kevlar- are widely used in the aircraft industry.

 

Fasteners

 

Fasteners are tools that join different structures together, there are many structures that do this and are designed to safeguard and keep the different structures intact while preventing any interference with the functionality of the surfaces of the aircraft. This is extremely crucial as one faulty bolt in an aircraft could be catastrophic.

 

Uses 

• Joining skins to structure 

• Structural joints between members

• Assembling Flap track beams 

 

Types

• Temporary fastener 

• Hi-locks

• Hi-Shear locks

• Rivets, depending on the type required.

It is important to study the different components that are integrated into the making of an aircraft to better understand the precautions that are taken to ensure the vehicle performs safely and in the most fuel-efficient manner. Studying the structural analysis of an aircraft will also help in solving technical issues in aeronautics and can be used in the design process to guarantee optimum performance.

 

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