Design Philosophy
By EjigayehuSystem Design Loads stresses and MaintenanceSystem Design PhilosophyThe design and construction of modern aircraft is controlled by the regulation detailed in JAR 23 for aircraft with a mass of 5700KG and less, and JAR 25 for Air Transport aircraft and aircraft of a mass greater than 5700Kg.This regulations classify aircraft structure in to Three GroupsPrimary structure:- stressed and could cause catastrophic in the event of failureSecondary structure: stressed but to a lesser degree. Tertiary structure: not stressed or nominally stressed and do not cause any catastrophic failure in the event of failure.PartsParts that make up a structure and systems are also categorized depending on the effect that failure would have on a unit or system.Critical parts:- must achieve and maintain a particularly high level of integrity if hazardous effects are not to occur at a rate in excess of extremely remote.The failing of a Major part might adversely affect the operational integrity of the unit in which it is installed.
Design Limit Load(DLL)The Maximum Load that the aircraft designer or component manufacturer expects the airframe or component to be subject to in operationDesign Ultimate Load(DUL)An aircraft could experience loads in excess of the DLL.A test is carried out where the minimum load applied to the structure must be 1.5xDLL for three seconds.There may be permanent deformation but it must not collapseThe difference between the DLL and DUL is the Safety Factor. It is expressed as the ratio of DUL to DLLCatastrophic FailureThe regulations specify that there will be no catastrophic failure due to FatigueCorrosionAccidental damageTo comply with this requirement designers must evaluate the materials they intend to use and the loads the aircraft would be subject to during their operational life.The result is Safe life philosophy. Safe LifeThe aircraft structure as a whole and components within the aircraft are given safe life. This is based on one, several or all of the followingCumulative flying HoursLandingsPressurization CyclesCalendar time.Fail Safe structure.No one item with in structure takes the entire load.
Damage TolerantSpreads the loads over a larger areaAny damage to the structure should be found and repaired during normal inspection before there is degradation of the structures integrity.Servicing CycleAircraft inspections are based on flying hour cycles, and a calendar date is used as back stop.Aircraft normally have a major servicing every five years.Loads and StressesAircraft structural members are designed to carry a load or to resist stressEvery part of the aircraft must be planned to carry the load to be imposed upon it.The determination of such loads is called stress analysisExternal loads or forces cause stress. Stress is a materials internal resistance, or counterforce, that opposes deformationThe degree of deformation of a material is strain. When a material is subjected to a load or force, that material is deformed, regardless of how strong the material is or how light the load is.Major stresses TensionCompressionTorsionShearBendingHoop stressAxial stress
Elasticity of MaterialsWhen the material is deformed beyond its elastic limit permanent deformation will occur.In aircraft deformation can take the form of bending, buckling, elongation, twisting, shearing or cracking which ultimately leads to fracture and creep in material. Where a material is deformed below its elastic limit but for a protracted period the material con deform this is termed as creep. The factors that affect creep areMaterial typeLoad appliedDuration of loadtemperature
Shock LoadsWhen a structure has sudden increase in the load that is being applied to it, this is termed as shock load.Bird strike, heavy landing are the causes of shock loads.It causes permanent deformation.
Fatigue in materialsInevitable in material that are subject to alternating load.
Fatigue CrackingWhen a structure is subjected to an average stress scores, scratches, fastner holes, sharp edges or sharp radial edges can build local stress levels 2 to 3 times greater than the average.Corrosion is the gradual deterioration of metal due to a chemical or electrochemical reaction with its environment.The reaction can be triggered by the atmosphere, moisture, or other agents. When inspecting the structure of an aircraft, it is important to watch for evidence of corrosion on both the outside and inside. Corrosion on the inside is most likely to occur in pockets and corners where moisture and salt spray may accumulate; therefore, drain holes must always be kept clean. Also inspect the surrounding members for evidence of corrosion.Forces that act on structures and materials in flightLift DragMassThrust AccelerationInertiaOther factors for considerationAirspeedTemperatureAltitude On the GroundDuring landingfrictionPressurizationThrust reversalBreakingStationaryTaxiingOn take off
Maintenance2. Airframe
Airframe major components
Attachment MethodsSkins, frames and formers must be joined together using rivets or bondingRiveting is time consuming and has disadvantage of requiring a row of holes drilled through the skin and the frame to which it is attached.Technological progress has allowed large billets of alloy to be machined(milled) to remove unnecessary material.This process has allowed the retention of material to give strength and rigidity.It is not necessary to attach frames with alloy skin.Chemical etching of material has further refined this process. Aircraft construction MaterialsMetalsHigh Tensile steelsStainless steelsTitanium alloysNickel AlloysLight AlloyPure AluminumAluminum Alloys
Composite MaterialsManufactured from reinforcing fibers embedded in bonding resinsThe main reinforcing materials areGlass (GFRP)Carbon(graphite)CFRPBoronAramid, known as Kevlar, KFRP, a synthetic materialLithium is being evaluated as materialSome bonding Materials areEpoxy ResinPTFE Advantages of Composite MaterialsThe ability to arrange the fibers to obtain directional properties consistent with the loadThe ability to make complex shapes, since the material is not homogeneousWeight savingsResistance to corrosionHigh specific strengthHigh specific stiffnessDisadvantages of Composite MaterialsThey are quickly eroded by hail, sand, etc, so leading edges must be sheathedDifficult to repairCan absorb moisture if the material is not correctly sealedWing, Tail surfaces and control surfacesThe WingPurposeProduces liftProvides attachment points for the landing gear, engines and the aileronAids in lateral and directional stabilityWing DesignDesignDepends on the intended use, size, weight and speed of the aircraftLocation is usually attached to the fuselageTypes of wing constructionBased on number of sparsMono sparTwo sparBox Beam
Box beam structure
Types of wing construction contdBased on how they are supported
Cantilever- doesnt need external supportSemi cantilever- needs external supportCantilever, semi-cantilever, wire braced
ContdBased on how stresses are transmittedTrussStressed skinTruss type The spars are separated by compression membersThe 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.Truss Type
Basic wood wing structure and components.
ContDrag wires (run from rear spar inboard to front spar outboard) Anti-drag wires (run from rear spar outboard to front spar inboard)The skin on the top surface of the wing has to be stiffer than the lower side. The upper surface is exposed to compressive stress, while the lower side is subjected to tensile stress.Former ribs attach to the spars to give shapeThe structure is covered with fabric Stressed skin
A metal skin is riveted to stringers and ribsThe stringers are also riveted to the skin and the ribsThe ribs transfer the stresses to the spars
Sandwich (bonded honeycomb)Metal bonded honeycombFiber glasscomposite
Structural members of the wingSpar- longitudinal members of the wingMain structural member of the wing Can be made of wood or metalTakes bending moment
Spar contdTypesSolid woodLaminated woodExtruded metalBuilt up metalSpar shapesBox BeamI beamCross sectional areas of wooden spars
Typical metal spar shapes
Spars
A plate web wing spar with vertical stiffenersA fail-safe spar with a riveted spar web.Stringers- Are also longitudinal membersGive the wing rigidityManufactured from aluminum alloys.Ribs are chord wise members Extend from the leading edge to the trailing edge or from front spar to the rear sparGive shape to the wingTransmit loads from the covering to the sparsPass concentrated loads from engines, landing gears and control surfaces in to the skin and spars.Can be made of wood or metal
Typical wing ribs
Types of wing RibsBuilt upStampedSkin carries primary aerodynamic loads
False ribs Extend from the front spar to the leading edgeGive streamlined shape to the leading edgeTransfer stresses to the front spar
Wing plan formsStraight wingTapered wingSwept wing (forward or back)Delta Wing
Wing ConfigurationLow wingHigh wingMid WingDihedral wingGull wingInverted gull wing
The empennageIncludes the tail boom, vertical stabilizer, and the horizontal stabilizer
Construction
The fuselage terminates at the tail cone with similar but more lightweight construction.Vertical stabilizer
The stabilizersHorizontal stabilizerVertical stabilizer
Horizontal stabilizerPurpose- provides longitudinal stability and control Provides attachment point for the elevatorConstruction- similar to the wingTrussStressed skinBonded honey comb
Vertical StabilizerPurpose provides directional stability and control Provides attachment point for the rudderConstruction- similar to the horizontal stabilizerLocation usually attached at the rear of the fuselage Control SurfaceAre hinged or moveable surfaces to control the attitude of the aircraft
Primary control surfacesThe elevator The ailerons The rudderThe ElevatorPurpose provides longitudinal control (pitch control)Location usually mounted on the trailing edge of the vertical stabilizer.
Elevator constructionConstructionFabric covered trussStressed SkinBonded honeycombCan be actuated mechanically by control cables, electrically or hydraulicallyOperation is operated by moving the control yoke or wheel forward or backwardElevator movementMoving the control yoke forward, deflects the trailing edge of the elevator downwards, which moves the nose of the aircraft downwards.The opposite holds true when the wheel is moved backwards.
Transferring control surface inputs from the cockpit
2. The aileronPurpose provides lateral control (roll control)Location- usually mounted on the trailing edge of the wing
Ailerons contdFabric covered trussStressed skinBonded honey combOperation operated by moving the control yoke or wheel to the right or to the leftMoving the control yoke to the left deflects the trailing edge of the left aileron up and the trailing edge of the right aileron down, decreasing the lift on the left wing and banking the aircraft to the leftThe opposite holds true when the wheel is moved to the rightCan be actuated mechanically by control cables, electrically or hydraulically
3. The rudderPurpose provide directional control (yaw control)Location usually mounted on the trailing edge of the Vertical stabilizer
ContdConstruction Fabric covered trussStressed skinBonded honeycombCan be actuated mechanically, electrically or hydraulically Operation- operated by pushing rudder pedals in the cockpitPushing the right pedal deflects the trailing edge of the rudder to the right turning the nose of the aircraft to the right.
Control surface flutterRapid and uncontrolled oscillation of control surfaceOccurs as a result of an unbalanced surfaceCaused by the interaction of aerodynamic forces, inertia forces and the elastic properties of the surface or structure and can lead to catastrophic failure of the structure.Can be prevented by mass balancing control surface to alter the moment of inertia of control surfaces and its period vibration (moving C.G closer to hinge)Poorly maintained A/c (high flexibility or play of control surface ) may flutter below limit air speed.May be prevented by using engines as mass balance placing them on the wing pylon forward of wing leading edge. Combination control surfaceRuddervators (V-tail) functions as a rudder and elevatorElevons- serves the functions of the elevator and aileron Flaperons- functions as a flap and aileronStabilator- a hinged moveable horizontal stabilizer which can be used for pith control
Secondary control surfaces
TabsFunction- provides a means of trimming the aircraftAssists the pilot to move the main control surfaceLocation- hingled at the trailing edge of the main control surfaces
ContdConstructionCorrugated skinBonded honeycombStressed skinTypesTrim tabsServo tabsBalance tabsSpring tabsTrim tabs are used for trimming (fixed or adjustable)
Servo tabs- assist the pilot to move the main control surfaceOperation are operated by the cockpit control wheel are deflected to a direction that is opposite to the direction of movement of the main control surfaceAerodynamic forces act on the tab to move the main control surface
Balance tabsaid the pilot to move the main control surfaceAre deflected in the opposite direction when the main control surface is deflected
Spring tabsprovide control assistance at high speed
Balance panels
Flight control Tabs
Auxiliary control surfacesTrailing edge flapsLeading edge flapsLeading edge slatsspoilers
Trailing edge flapsPurpose increase wing camber and area toIncrease liftIncrease dragReduce airspeed for landingShorten the landing rollShorten the takeoff run
ContdTypesPlain flapFowler flapSplit flapSegmented flapConstructionFabric covered trussStressed skinBonded honeycomb
ContdLocation usually hinged or mounted on the trailing edge of the wingsCan be actuated mechanically, hydraulically or electrically2. Spoilers and Speed BrakesPurpose- to reduce lift to increase drag to aid the aileron in lateral control to reduce speed of the aircraft during decent and after landing
ContdLocation hinged at the upper surface of the wingsConstructionStressed SkinBonded honeycombSome a/c have speed brakes mounted on the fuselage
3. Leading edge flapsPurpose increase the camber of the wing and provide greater lift at lower airspeedsLocation 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 SlatsPurpose to reduce the stalling speed and increase lift at lower airspeedsLocation mounted on the leading edge of the wingConstruction similar to trailing edge flaps Operation normally flush with the wing leading edgeWhen extended move forward and open a slot to allow air flow and prevent stallingSome aircraft have fixed slots
The Fuselage The main body of the aircraft to which the wings, tail, and landing gear attachPurpose Provides space for cargo, controls, accessories, passengers and other equipmentProvides attachment points for the engines
RequirementsMust be strong and light in weightMust be streamlinedMust be air conditioned or ventilatedMust be pressurized if the aircraft flies at high altitudes must be provided with emergency exist
Types of ConstructionThree general types :- depending upon the method by which stresses transmitted to the structure TrussMonocoqueSemi-monocoqueTrussA rigid frame work of bars, beams, rods, tubes and wires. The members are joined together by riveting or welding
ContdLongitudinal longerons are the primary load carrying membersLateral bracing is placed at regular intervals. The frame work is covered with fabric, wood, aluminum or fiberglassThere are two types of truss constructionPratt trussVertical and diagonal members connect the longeronsThe diagonal members can be wires (carry only tension) or rigid tubing (can carry both tension and compression)
Contd2. Warren truss
The longerons are connected only with diagonal membersMaterialSteel and aluminum alloy
Stressed Skin Construction
All the loads are carried in the outside skinCan be built in a clean, smooth and efficient aerodynamic shapeFull MonocoqueIs a metal tube or cone without internal structural members Formers can be used to give shapeRelies on the strength of the skin to carry stressesAn airframe using monocoque construction
Semi MonocoqueHas additional longitudinal members (Longerons and stringers) to reinforce the skinThe skin is riveted to stringers which in turn are riveted to the formersSemi Monocoque
The Structure includesSkin (plating)- aluminum alloy, titanium, and stainless steel Longitudinal MembersStringersLongeronsVertical membersFrames or formersbulkheads
PressurizationMany aircraft are pressurized. air is pumped into the cabin after takeoff and a difference in pressure between the air inside the cabin and the air outside the cabin is established. This differential is regulated and maintained. In this manner, enough oxygen is made available for passengers to breathe normally and move around the cabin without special equipment at high altitudes.ContdPressurization causes significant stress on the fuselage structure and adds to the complexity of design. In addition to withstanding the difference in pressure between the air inside and outside the cabin, cycling from unpressurized to pressurized and back again each flight causes metal fatigue.Pressurized fuselage structures undergo extensive periodic inspections to ensure that any damage is discovered and repaired.
Stringers Length wise membersNumerous in number and smaller in size than longeronsRun from the nose of the aircraft to the tail of the aircraftProvide space for fastening the skinTypesExtrudedCastFormedContdCross Sectional ShapesL- anglesBulb angleHat SectionU- channelZ- SectionMaterialAluminum alloy
ContdLongerons- are also length wise membersHeavier than stringersCarry primary bending loadsSimilar to Stringers in shape and construction
Contd3. Vertical Members
Bulkheads are the heaviest vertical membersCarry concentrated loads2. Formers Give shape to the fuselage Transfer stresses from the fuselage to the bulkheadsContdC. Reinforced Shell The structure is reinforced with a framework of structural membersCabin Floors
ContdManufactured from a series of Panels attached to supporting beams and cross membersAircraft having pressurized cabin and hold are manufactured from honey comb structure.This allows light weight structure to withstand compression loads and heavy rigidity without incurring weight penalty.
Blow out Bungs Press fittedTo equalize the pressure in the event of loss of pressure in one of the compartments
WINDOWSModern subsonic and transonic air transport aircraft use stepped nose profile, this allows theNose to be aerodynamically profiledWindscreens to be located optically acceptableThe pilot required vision for both ground and flight operationsReduction in the physical size of the windscreens
Contd
Direct vision windowTo comply with the regulations on maintaining clear vision in the event of failure of the demisting system one of the side panel windows on each side can be removed from within flight deck to allow clear view.
Passenger Cabin Window
Aircraft Doors certified under JAR 23 and JAR 25
ContdPlug type doorClose from insideLocking pins that engage in the door frameCabin pressure which is greater than ambient pushes the door more tightly in to the frame.Seat mounting support structure
Spar AttachmentThe strongest part of the fuselage is where the wings are attached, as this structure is the point where all flight loads will be felt.
The Landing Gear (Under Carriage)PurposeCushions the landing impactsupports the aircraft during ground operationsDampens vibrations while towing and taxingPosses a low coefficient of dragProvide minimum friction between the aircraft and groundLocation- is attached to the fuselage or the wingCan be fixed or retractableExtending and retracting systemsMechanicalElectricalHydraulic
ContdHas shock absorbers to cushion the landing impact and dampen vibrationsShock chordSpring gearSpring oleoAir oleo
Spring gear
ContdSkis 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
FloatA completely enclosed water tight structure attached to an aircraft to provide buoyancy and stability while landing on water surfaces.Floats
skis
Landing gear arrangementConventional has two main wheels and one tail wheelTricycle- two main wheels and a nose wheel
Conventional landing gear Consists ofTwo main wheels ( forward of the CG) and one tail wheelSeldom used on modern aircraftAdvantagesProvides good propeller-ground clearance Advantageous if aircraft operates on rough fieldDisadvantagesRestricted visibilityNosing overGround looping
Tricycle landing gearTwo main wheels (aft of the CG) and a nose wheelWidely used on modern airplanesAdvantagesAllows more forceful application of the brakes with out nosing overOffers better visibilityTends to prevent ground loopingFlight Controls
IntroductionThe movement of flying control surfaces in response to the movement of cockpit controls can be achievedMechanically:- the control surfaces are connected directly to the cockpit controls by a system of cables, rods, levers,and chainsHydraulically:-the control surfaces are moved by hydraulic power, the control valve may still be operated mechanicallyElectrically:- movement of the cockpit control sends an electrical signal to the control surface, the movement of the control may be achieved hydraulicallyManual controls
ContdIt is manually controlledReversible:- the force applied to the cockpit control will move the control surface and the force applied on control surface will move cockpit control.Control system ChecksIn some situations it is necessary for the pilot to check part of thisCable tensionSafety and locking of controlsRange of movements of control(freedom and operation in the correct sense)Friction in the systemBacklash of the system
Cable TensionIf too low the cable will be loose permitting excessive cable movementToo high the control will be stiffIt can be adjusted by turn bucklesThe tension can be measured by tensiometer
Tensiometer
Temperature compensationIf aircraft structural material is different from cable material change in length will occurWhen checking for tension allowance should be made for temperature
Safety And LockingAfter tension has been correctly set turn buckles has to be checked for safetySufficient thread must be engaged between the end fittingsTo enable this inspection holes are provided in the turn buckle.Inspection hole must be blocked to ensure safetyCheck by inserting hardened pinRange of control movementThe limit of control movement is controlled by mechanical stopPrimary stop: stopping control surface secondary stop: stopping control sticks and pedals
Control system FrictionIn ground operation it is normalDuring flight stick force increases due to air loadsIf friction force is too high the feel of the controls with changing airspeed will be destroyed.It is caused by over tension cables or un lubricated bearings.BacklashFree or ineffective movement of cockpit control when the cockpit control is reversed.Control system should be free of backlashMay indicate worn or incorrect components in the system
Control LocksWhen an aircraft is parked in open strong or gusty winds can blow the controls against their stops to cause mechanical damageIt has to be cocked.Duplicate Inspection of Control syatemIf control system is disturbed in any way it has to be checked by two separate persons(one of them may be a pilot)Powered Flying ControlsOn some modern aircraft controls are subjected to heavy loads due to Heavy loads orHigh speedTo reduce stick forces created by high speed or load hydraulic or electric power is needed.Power operated controls
Essential components of simple power operated controlsA hydraulic ActuatorA servo or control valveAn artificial feel unit.Irreversible power control
Reversible powered controlsTo prevent over controlling due to lack of feel about the control action artificial feel unit is installedDesigned to give pilot artificial feel proportional to speed of aircraft and to the amount of control surface movement made.This unit varies from simple spring loaded to box to Q pot operating systemSpring Box artificial feel
Q pot feel mechanism
ContdTo be effective Q spot would have be very large and nowadays this units are used with a hydraulic spool valve selector
Artificial feel system
Artificial feel system
Artificial Feel system
Secondary Flight ControlsLift augmenting devices high lift devicesLeading edge and trailing edge devices
Leading edge flap typesPlain Split SlottedBlownFowlerSlotted fowler
Leading edge Devices areKruger flapsSlotsFixed slatsRetractable slatsDroop nosePlain Flap
Zap flap
Basic flap operation in light aircraft
Flap selector valve for other than light aircraft
Fly By wire (FBW)Power flying control systemUses electronic inputs to solenoid operated servo valve rather than mechanical in putThe pilot operates flight deck control via control stick or pedalThis in turn operates transducers which convert mechanical input to electrical output which is amplified processed by computers with a processed command out put to servo valve which controls the movement of hydraulic actuators.
FBW block diagram
Redundancy Safeguards to eliminate the possibility of loss of control in the event of hydraulic or electrical failure must be provided in modern aircraft controlThis is achieved by building some form of redundancy in to the control systemSplitting the system in to two or three separate sections, each powered by separate actuators and hydraulic system is the usual method.Computer system redundancy is also provided in Airbus Flight control