Course Name:- Aircraft structures I

By Ejigayehu Lemma(MSc.)

AIRCRAFT STRUCTURES

•INTRODUCTION •PRINCIPAL AIRCRAFT COMPONENTS

OF FIXED WING (Function, material and construction )

- Fuselage - Wing - Empennage - Flight control surface - Landing gear - Power plant structure - Doors and windows

Objective

• To understand the structural components of aircraft and the stresses imposed on those components

INTRODUCTION

• DEFINITION1. AIRCRAFT▫ Any structure, machine which is

designed to be supported in the air either by dynamic reaction with the air or by it’s own buoyancy.

▫ Eg. Aero planes, airships, gliders, balloons

•Airframe▫ basic assembled structure of any aircraft

(except that of lighter than air aircraft) or rocket necessary to support the aerodynamic and inertia loads imposed by the weight of the vehicle and its contents.

▫Includes the fuselage, wings, tail boom, nacelle, cowling, fairings, stabilizers, control surfaces and landing gear.

Brief History of Aircraft Construction

•Early dreamers▫Greek myth – Daedalus with his son Icarus,

flew with wings made of feathers and wax.

Corbis/Archivo Iconografico, S.A.Fall of IcarusIn Greek mythology Daedalus and Icarus escaped on wax-coated wings from the labyrinth on Crete where they were imprisoned by King Minos. Ignoring Daedalus’s warning, Icarus flew too close to the sun. His wings melted and he plunged into the sea. Fall of Icarus was painted in the 17th century by Jacob Peter Gowi. It is in the Prado Museum in Madrid, Spain.Microsoft ® Encarta ® Reference Library 2004. © 1993-2003 Microsoft Corporation. All rights reserved.

•Leonardo Da Vinci made suggestive drawings of the orinthopter, a parachute and a helicopter around 1500 a.d.

•The Montogolfier brothers made their hot air balloon from linen cloth lined with paper and flew their unmanned balloon in June, 1783

•Otto Lilienthal made about 2000 successful flights with gliders made of willow wands and waxed cotton in the 1890’s

•The Wright brothers made their successful flight in 1903, at Kitty hawk, north Carolina

•The early flying machines produced by the Wright brothers, and others had wings made of bent wooden ribs covered with fabric and a body of open framework made of strips of bamboo held together with piano wire.

•The next generation of airplanes before the first world war were built with a wood truss and had wings braced with struts and wires.▫ The occupants sat in open cockpits

•The Welded thin walled steel tubing truss came as a major breakthrough in the later years of the first world war replacing the wood.

•The stressed skin construction were the skin carries all of the structural loads was developed and widely used in the 1920’s and 1930’s

▫Thin sheets of wood veneer were molded in to a ply wood structure forming the fuselage

▫Laminated wooden rings were built at critical locations to provide attachment points for the wing, engine and landing gear

▫The wood was later replaced with aluminum alloy sheets which were riveted into thin sheet metal formers.

•The development of pressurized transport jet aircraft created new challenges in aircraft structure design▫In 1954, two de Havilland comets vanished

during flight suffering damages caused by pressurization loads around rectangular cutouts

▫A new system of fail safe construction was developed where doublers are installed at strategic locations and dual alternate load paths are provided.

CLASSIFICATION OF AIRCRAFT

1. LIGHTER THAN AIR2. HEAVIER THAN AIR

Lighter than Air

•Supported in the air by their own buoyancy

A.Balloons: - non-porous spherical bags filled with light air - Gas filled- Hot Air

B. Airships :- are engine driven and can be Steered.

Photo Researchers, Inc./Jerry WachterGoodyear BlimpThe Goodyear blimp, like other nonrigid airships, has no internal supporting structure; the pressure of the buoyant gas maintains the bag’s shape. Blimps are rarely used for their original purpose of transportation, but their hovering capabilities, high visibility, and enormous surface area make them attractive for advertising.Microsoft ® Encarta ® Reference Library 2004. © 1993-2003 Microsoft Corporation. All rights reserved.

Heavier than Air

•Supported in the air by the dynamic reaction of the aircraft structure with the air

• Can be classified as1. Fixed wing aircraft• Have pairs of fixed wings to generate lift

forward movement of the wing through the air generates lift

▫ Eg. Airplane, glider

•Monoplane - has a single pair of fixed wings

•Biplane – has two pairs of wings•Sea plane – lands on water surface•Amphibian – can land both on water

surface and on land.

2. Rotary Wing

•A rotating airfoil shaped wing generates lift

E.g. Helicopter, Autogiro

•Main Structural Components of fixed wing aircraft▫The fuselage▫The wing▫The landing gear▫The stabilizers▫Flight control Surfaces

•These structural components are an assembly of structural members

•The structural members are designed to carry loads or resist stress

Stress- The internal force of a material to resist stress

Basic Stresses

•Tension •Compression •shear

Tension –

is the stress that resists a force that tends to pull a material apart

Compression –

is the stress that resists a crushing force

Shear-

is a stress that resists the force tending to slide one layer of material over an adjacent layer

2. Combination stressesBending – is a combination of tension and

compression Torsion- is a stress that produces twisting

Basic requirements to be met by aircraft structures•High aerodynamic cleanness•Low weight of construction with

necessary strength and stiffness•Simplicity of operation•Ensuring high reliability combat

survivability and flight safety•Production effectiveness.

The Fuselage •The main body of the aircraft on which

the wings, tail, and landing gear are attached.

Purpose• Provides space for cargo, controls,

accessories, passengers and other equipment

•Provides attachment points for the engines

Requirements

•Convenience for accommodating the crew, passenger, equipment and cargoes,

•Sufficient bending and torsional stiffness.•Must be strong and light in weight•Must be streamlined•Must be air conditioned or ventilated•Must be pressurized if the aircraft flies at

high altitudes •must be provided with emergency exist

Types of Construction

• Three general types :- depending upon the method by which stresses transmitted to the structure 1. Truss2. Monocoque3. Semi-monocoque

Truss• A rigid frame work of bars, beams, rods,

tubes and wires. The members are joined together by riveting or welding

•Longitudinal longerons are the primary load carrying members

•Lateral bracing is placed at regular intervals. The frame work is covered with fabric, wood, aluminum or fiberglass

There are two types of truss construction1. Pratt truss• Vertical and diagonal members connect

the longerons• The diagonal members can be wires

(carry only tension) or rigid tubing (can carry both tension and compression)

2. Warren truss

•The longerons are connected only with diagonal members

Material•Steel and aluminum alloy

Stressed Skin Construction

•All the loads are carried in the outside skin

•Can be built in a clean, smooth and efficient aerodynamic shape

Full Monocoque

•Is a metal tube or cone without internal structural members

•Formers can be used to give shape•Relies on the strength of the skin to carry

stresses

Semi Monocoque

•Has additional longitudinal members (Longerons and stringers) to reinforce the skin

•The skin is riveted to stringers which in turn are riveted to the formers

The Structure includes

1.Skin (plating)- aluminum alloy, titanium, and stainless steel

2. Longitudinal MembersStringersLongerons

3.Vertical membersFrames or formersbulkheads

The Wing

Purpose Produces lift Provides attachment points for the

landing gear, engines and the aileron Aids in lateral and directional stability

WINGS• Wing is essentially a beam which gathers and

transmits all the aerodynamic loads to the central fuselage attachment

Requirements

•Minimum possible drag•Minimum value of product (Cl-S)•Maximum aerodynamic quality•Presence of free volume.

Design•Depends on the intended use, size, weight

and speed of the aircraft•Location – is usually attached to the

fuselage

WING TYPES• Straight wing :

▫If the leading edge of a wing is perpendicular to the airflow, it is called a straight wing

• Swept wing :▫If the leading edge of a wing meets the

airflow at an angle, it is called a swept wing

Straight wing Swept wing

TYPICAL WING FORMSRectangle

(Wright brothers)

Triangle(Concorde)

S=40’C=6’

S=42.5’Cr=90.75’

Trapezoid(F18)

Trapezoid(Boeing 747)

Compound(Space shuttle)

S=13’Cr=15’Ct=6’

S=81.3’Cr=54.3’Ct=13.3’

7.2’60’

35.7’

9.8’

30.5’

PARTS OF A WING

•Wing box•Fixed leading

edge•Fixed trailing

edge•Ailerons•Spoilers•Flaps•Slats

WING TERMINOLOGY

• Leading edge is the portion of the wing front of the front spar • Trailing edge is the portion of the wing back of rear spar• The chord is the distance between the leading edge and trailing edge• Wing box is portion of the wing between the front spar and rear spar• Ribs are the airfoil shaped members from leading edge to trailing edge• Span is the distance between the root and tip of the wing• Aspect ratio AR = B2/A

Leading edgeTrailing edge

TipRoot

SPAN (b)

Types of wing construction

Based on number of spars•Mono spar•Two spar•Box Beam

Based on how they are supported

1. Cantilever- doesn’t need external support

2. Semi cantilever- needs external support

Based on how stresses are transmitted•Truss•Stressed skin

Truss type

•The spars are separated by compression members

•The truss is held together with high strength steel wires. The compression members carry the compressive stresses, while the drag and anti-drag wires carry the tensile forces. The structure carries the entire load. The skin is usually not a stress- carrying member.

Stressed skin

▫A metal skin is riveted to stringers and ribs

▫The stringers are also riveted to the skin and the ribs

▫The ribs transfer the stresses to the spars

Sandwich (bonded honeycomb)

•Metal bonded honeycomb•Fiber glass•composite

Typical wing shapes

Wing ConfigurationLow wingHigh wingMid WingDihedral wingGull wingInverted gull wing

Tail unit

•The empennage▫Includes the tail boom, vertical stabilizer,

and the horizontal stabilizer

The stabilizers

•Horizontal stabilizer•Vertical stabilizer

Horizontal stabilizer

Purpose- provides longitudinal stability and control

Provides attachment point for the elevatorConstruction- similar to the wing

Truss Stressed skin Bonded honey comb

Vertical Stabilizer

•Purpose ▫provides directional stability and

control - Provides attachment point for the

rudder- Construction

- similar to the horizontal stabilizer- Location –

- usually attached at the rear of the fuselage

• Control Surface▫ Are hinged or moveable surfaces to

control the attitude of the aircraft

Primary control surfaces

1.The elevator 2.The ailerons 3.The rudder

•Combination control surfaceRuddervators (V-tail) – functions as a rudder and elevator

Elevons- serves the functions of the elevator and aileron

Flaperons- functions as a flap and aileron

Stabilator- a hinged moveable horizontal stabilizer which can be used for pith control

Secondary control surfaces

Tabs

Function- ▫provides a means of trimming the aircraft▫Assists the pilot to move the main control

surfaceLocation- hinged at the trailing edge of

the main control surfaces

•Construction• Corrugated skin• Bonded honeycomb• Stressed skin

Types

•Trim tabs•Servo tabs•Balance tabs•Spring tabs

Auxiliary control surfaces

1.Trailing edge flaps2.Leading edge flaps3.Leading edge slats4.spoilers

•Types▫Plain flap▫Fowler flap▫Split flap▫Segmented flap

•Construction▫Fabric covered truss▫Stressed skin▫Bonded honeycomb

•Location ▫ usually hinged or mounted on the

trailing edge of the wings•Can be actuated mechanically,

hydraulically or electrically

Spoilers and Speed Brakes

•Purpose▫ to reduce lift▫ to increase drag▫ to aid the aileron in lateral control▫ to reduce speed of the aircraft during

decent and after landing

Leading edge flaps

• Purpose • increase the camber of the wing and provide

greater lift at lower airspeeds• Location

• usually hinged on the leading edge• normally flush with the lower surface of the wing

• Can be actuated mechanically, electrically or hydraulically

4. Leading Edge Slats• Purpose

▫ to reduce the stalling speed and increase lift at lower airspeeds

• Location▫ mounted on the leading edge of the wing

• Construction▫ similar to trailing edge flaps

• Operation▫ normally flush with the wing leading edge

• When extended move forward and open a slot to allow air flow and prevent stalling

• Some aircraft have fixed slots

The Landing Gear (Under Carriage)•Purpose

▫ supports the aircraft during ground operations▫Dampens vibrations while towing and taxing▫Cushions the landing impact

Location- is attached to the fuselage or the wing• Can be fixed or retractable

Extending and retracting systems• Mechanical• Electrical• Hydraulic

•Has shock absorbers to cushion the landing impact and dampen vibrations▫Shock chord▫Spring gear▫Spring oleo▫Air oleo

Spring gear

•Skis are used for take off and landing on snow or ice

•Floats are used for those aircraft which can take off and land on water surfaces

Float

•A completely enclosed water tight structure attached to an aircraft to provide buoyancy and stability while landing on water surfaces.

Floats

skis

Landing gear arrangement- Conventional has two main wheels and

one tail wheel- Tricycle- two main wheels and a nose

wheel

Tricycle landing gear

•Two main wheels (aft of the CG) and a nose wheel

•Widely used on modern airplanes•Advantages

▫Allows more forceful application of the brakes with out nosing over

▫Offers better visibility▫Tends to prevent ground looping

Nacelles or Pods

•Are streamlined enclosures used to cover the engines

•The structure consists of skin, cowling, structural members, the fire wall and engine mounts

The cowling

•Is the removable covering of the engines found on areas, which need regular access.

The engine mount

•Is the frame that supports the engine and attaches it to the fuselage or the wing

•Can made from welded steel tubing or formed sheet metal

Main Structural Components of A Helicopter

▫The fuselage▫The main rotor▫The tail rotor▫The landing gear

The fuselageHas similar features as the fuselage of

fixed wing aircraft

The main rotor

• is the component that produces lift•It is also used for control