Aircraft Structures I Chapter-1

Course Cou rse Name Name::- Air Aircra craft ft struc structur tures es I By Ejigay Ejigayehu ehu Lemm Lemma(MSc a(MSc.) .)

AIRCRAFT STRUCTURES  INTRODUCTION  PRINCIPA L  AIRCRAFT COMPONENTS OF FIXED W ING ING (Function, material and

construction ) - Fuselage - Wi n g - Empe Empenn nnag agee - Flight control surface - Land Landin ing g gear gear - Power plant structure - Doors and windows

AIRCRAFT STRUCTURES  INTRODUCTION  PRINCIPA L  AIRCRAFT COMPONENTS OF FIXED W ING ING (Function, material and

construction ) - Fuselage - Wi n g - Empe Empenn nnag agee - Flight control surface - Land Landin ing g gear gear - Power plant structure - Doors and windows

Objective  To understand the structural components of 

aircraft and the stresses imposed on those components

INTRODUCTION 

DEFINITION 1. AIRCRAFT  A ny 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.  A ero planes, airships, gliders, balloons

 A irframe   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.

inth 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 I ca

oration. 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 TH AN AIR  2. HE A  V IER TH AN 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  A ir B.  A irships :- are engine driven and can be

Steered.

ng 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 v

rosoft Corporation. All ri ghts reserved.

Heavier than Air  Supported in the air by the dynamic reaction of 

the aircraft structure with the air

Can be classified as Fixed wing aircraft

 1. 

Have pairs of fixed wings to generate lift forward movement of the wing through the air generates lift 

Eg.  A irplane, glider

   

Monoplane - has a single pair of fixed wings Biplane ± has two pairs of wings Sea plane ± lands on water surface A mphibian ± can land both on water surface and on land.

2. R otary Wing  A  rotating airfoil shaped wing generates lift E.g. Helicopter,  A utogiro

 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 ² is the stress that resists a force that tends to pull a material apart

Compression ² is the stress that resists a crushing force

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

2. Combination stresses Bending ± 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

R equirements  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.

Truss

2. 3.

Monocoque Semi-monocoque

Truss  A  rigid frame work of bars, beams, rods, tubes and wires. The members are joined together by 

riveting or welding

longerons are the primary load  Longitudinal longerons 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 construction 1. 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  A ll 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  R elies 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 Members Stringers       Longerons      

3. Vertical members Frames      

or formers        bulkheads

The Wing Purpose  Produces lift  Provides attachment points for the landing gear,

engines and the aileron  A ids 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  S wept wing :  If the leading edge of a wing meets the ai rflow  at an angle, it is called a swept wing

Straight wing

S wept

wing

TYPICAL WING FORMS 9.8¶

Rectangle (Wright brothers)

Compound (Space shuttle) 7.2¶

S=40¶ C=6¶

Trapezoid (F18) S=13¶ Cr =15¶ Ct=6¶

30.5¶

Trapezoid (Boeing 747)

35.7¶ 60¶

S=81.3¶ Cr =54.3¶ Ct=13.3¶

Triangle (Concorde) S=42.5¶ Cr =90.75¶

PARTS OF A WING   Wing box  Fixed leading

edge  Fixed trailing edge   Ailerons  Spoilers  Flaps  Slats

WING TERMINOLOGY

Root

      

Leading edge SPAN (b)

Tip Trailing edge

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

Types of wing construction Based on number of spars  Mono spar  T wo 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 carr y the compressive stresses,  while the drag and anti-drag wires carr y  the tensile forces. The structure carries the entire load. The skin is usually not a stress- carr ying 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 Configuration Low wing  High wing  Mid Wing  Dihedral wing  Gull wing  Inverted gull wing 

Tail unit  The empennage  Includes the tail boom, vertical stabilizer, and the horizontal stabilizer

The stabilizers  Horizontal stabilizer   Vertical stabilizer

Horizontal stab stabilizer Purpose- provides longitudinal stability and control attachment point for the elevator Provides attachment Const onstructi ructionon- simi similar lar to the the wing wing Truss x

x

x

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 f uselage



Control Surface  A re 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 surface R uddervators (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  A ssists the pilot to move the main control surface

Location- hinged at the trailing edge of the main control surfaces

 Construction  Corrugated skin  Bonded honeycomb  Stressed skin

Types    

Trim

tabs Ser vo tabs Balance tabs Spring tabs

uxiliary control surfaces 1.Trailing

edge flaps 2.Leading edge flaps 3.Leading edge slats 4.spoilers

 T y pes  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 A ir ir 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 landi landing ng gear  T wo main wheels (aft of the CG) and a

nose wheel   Widely used on modern airplanes   Advantages   Allows more forcef ul application of the

 brakes with out nosing over  Offers better visibility   Tends to prevent ground looping

Nacelles

or Pods

  A re 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 f rame that supports the engine and

attaches it to the f uselage or the wing  Can made f rom welded steel tu bing or formed sheet metal

Main Structural

Components of A

Helicopter    

The

f uselage The main rotor The tail rotor The landing gear

The fuselage 

Has similar features as the fuselage of fixed wing aircraft

The main rotor  is the component that produces lift  It is also used for control