AGK: Systems

Basic Hydraulics

  • Bramah press (answer normally always 100)
    • P(PSI or N/m2) = F/A
  • Hydrostatic Press = Press exerted by a fluid due to gravity DEPENDENT ON HEIGHT - IRRESPECTIVE OF CONTAINER!!!
  • Pascal's Law "Pressure exerted ANYWHERE in a CONFINED INCOMPRESSIBLE FLUID will be felt instantaneously everywhere throughout the fluid and acts at 90° to the surface"
  • Using differential area (1 piston Area diff from other) = mechanical advantage = small piston moves LARGE
    -> Input F Input Dist. = Output F Output Dist.
    -> i.e. Work In = Work Out (where W = Fdist)
  • Passive Hy. Sys = no pump. Pressure only when force applied i.e. foot on brakes
  • Active Hy. Sys = Pressure pump (~3000psi)
  • EX: HYDRAULIC FLUID IS VIRTUALLY INCOMPRESSIBLE!!!!!
  • EX: What would happen if air gets into the system? A = feel spongy
  • EX: What would you do about it? A = Bleed it NORMALLY!!
  • EX: ACTIVE SYS Typical operating pressure ~3000psi.
  • Hydraulic Power
    • Only simple light AC use PASSIVE i.e. brakes
    • Large AC use ACTIVE powered by Eng or Elec Motor
    • The main advantage of HP is SIZE OF ACTUATORS ⬇️, ∴ NEED LESS FLUID = save weight of fluid & components & pipes can be smaller ∴ ⬇️weight
  • Seals
    • O-Ring(Square Section) - most common
      • Cheap but can leak @ HP
      • EXAM: What is the purpose of the backing rings?
        • A: Prevent extrusion of O-ring
    • Chevron (V shape) seal most common
      • Good in HP BUT ONLY SEALS IN 1 DIRECTN & Expensive
      • EXAM: What would we need if we were using V shape seals and would want to put pressure on either sides?
        • A: Use a pair of seals opposing each other!!
      • EX:When jack ram extended can pick dirt & damage seals ∴ WIPER RING used (i.e.wipes it clean)
  • Hy. Fluid Props.
    • Lubrication, virtually incompressible
    • Chemically inert
    • Low Freeze Pt, Hi Boil Pt
    • Non flammable with flash point >1000℃
    • LOW VISCOSITY
      • EXAM: Viscosity is a measure of a fluids resistance to flow
    • Non-foaming/sludging
    • Stable good storage properties - does not quickly deteriorate
    • Corrosion resistance
    • Reasonably Priced
  • Fluid Types
    • Mineral Oil: DTD 585 (aka OM15), Red, Synthetic Rubber seals
    • EX: Def Stan 91-48 super clean grade replaces DTD 585
      • Adv = Good Price / Disadv = flammable (∴ not in modern AC)
    • Synthetic: SKYDROL, Green or Purple, Butyl Rubber seals
      • EX: Skydrol = PHOSPHATE ESTER BASED SYNTH OIL, is an IRRITANT to skin & eyes. Used in modern AC cos FIRE RESISTANT, LESS PRONE TO CAVITATION & has HIGH BP. IT’s very expensive!!
    • CAVITATION = air drawn in & gets compressed due to suction of pump, can cause pipes to collapse (think of sucking in and cheeks collapsing). Think at hi alt, density ⬇️, pressure ⬇️ and BP ⬇️
    • Oil types must not be mixed under any circumstances
    • EX: Which hydraulic fluid do we put in? A= THE CORRECT ONE (Stated in Manual)! otherwise eats seals
    • EX: NEVER ID hy. fluid by colour alone!!!
    • EX: NEVER use hy. fluid from open container!!
  • Sys Components
    • Reservoir: with right fluid and CLEAN (or will destroy seals)
      • ALLOWS FOR SMALL LEAKS & THERMAL EXP.
      • Vented to prevent partial vacuum + For Hi Alt. AC pressurised to stop pump cavitation
      • Has de-aerator & baffles
    • Pump: Supplies pressure
    • Stack pipe used so if a leak fluid kept for essential services
      • EX: What is the purpose of the pump?
      • A= Produce a flow of fluid TO GIVE YOU THE RIGHT PRESSURE (wrong flo = wrong press...sys designed for a specific operating press.)
    • Actuator Jack: provides the output (i.e. does work)
    • Selector Valve: allows the supply return to switch over and never in the direction of the jack
    • Note: Every time actuator moves level of reservoir chgs…cos we can take out more than we put in (i.e. on return stroke) or vice versa
      • Reservoir could overflow…or we could get air in system = cavitation
      • Smaller side used for return cos we have smaller surface area compared with side that has the full area of the piston
      • We also have diff volumes

Hydraulic Systems - Ch 5

  • AMC(Acceptable Means of Compliance) = CS legalease converted to english and used to ensure we are complying
  • Most AC have at least 2 individually power Hy. Sys (redundancy)
  • Actuators aka Hydraulic Jacks
    • EX: Convert pressure energy into linear motion
  • Single Acting - spring loaded 1 side
    • EX: Where are you likely to see a single acting actuator? A = U/C door locks
  • Dble Acting Balance
    • Area (RAM end) same both sides of piston = balanced
    • EX: We will see this in Nose Wheel Steering - equal forces required both sides
  • Dble Acting - UNBALANCED
    • EX: Used for landing gear and flaps. Large surface for landing gear up (counter gravity) or Flap extension (counter airflow), small side for down (cos gravity helps) or flap retraction
  • Selectors
    • Used to direct flow; dependent on the pilot selection
    • Closed Ctr:
    • Open Ctr:
    • Linear (Spool) Valve: Has neutral posn. When u make an input then pressure will go to 1 port & the opposite to will be open to return
  • Hydraulic Lock
    • EX: (Copy from sheet) Hydraulic lock - where jack/actuator cannot move…nothing in, nothing out…cos fluid is virtually incompressible. i.e. fluid is trapped
  • Electrical Selectors
    • Use solenoids and diff. area
    • Even if electrical failure still possible to move valve manually

8 Primary Flight Ctrl

  • Ailerons, Rudder, Elevator
  • ALSO ROLL Ctrl spoilers

1 Airframe Design & Materials

  • CS23 vs CS25 (CAT)
  • Repeated stressing leads t fatigue
  • Relationship Prob. vs Severity
    • Minor Failure = Probable (10-3) = Slight crew workload
    • Major Failure = Remote (10-5) = Significant crew workload
    • Hazardous Failures = Extremely Remote (10-7) = Excessive crew workload
    • Catastrophic Failure = Extremely Improbable (10-9) = Crew Fatalities
  • Design Philosophies
    • EX:Safe Life: life given to component where it will NOT suffer a CATASTROPHIC FAILURE. Must be replace at end of safe-life! Life based on flying hrs, cycles, TO/LND,..
    • EX: Fail Safe: failure of part compensated for by alternative load path provided by an adjacent part that is able to carry the load for A LIMITED TIME PERIOD
      • EX: Any single failure in any one structural member can be safely carried UNTIL THE NEXT PERIODIC INSPECTION
      • Main disadvantage of Fail Safe = Weight/HEAVY
    • Damage tolerant: compromise btwn fail safe/safe life. Made stronger than needs to be so if they fail they have enough strength still. Eliminates the extra structure by SPREADING THE LOAD OVER A LARGER AREA
  • We use a combo of Fail Safe and Damage Tolerant in reality
  • Maintenance
    • Hard Time: Item removed for inspectn/repair
    • On Conditn: Inspectn (visual, measurements, tests) W/O DISASSEMBLY OR OVERHAUL. Only removed if check failed.
  • Loads & Stresses
  • Stress = internal force per unit area inside a structural component as a result of external loads. 4 types
    • Stress/Tension/Traction
    • Compression
    • Torsion (Twist)
    • Shear (Scissors)
  • Strain = deformation as a result of stress. Given as a chg in dimensions cf original
  • Buckling = tendency to distort under compressive loads. Skin wrinkling or pulled rivets
    • EX: Stringers used to prevent buckling
  • Bending = combination load i.e. tension+compression+shear
    • A Wing with lift = Compression on top (skin wrinkling) + Tension underside (pulled rivets)
    • EX: Commensurate with flap retraction we lock the outboard ailerons out. We use the inboard ailerons assisted by spoilers when necessary. To avoid excessive torsion on the wing due to the large moments that would be generated
  • Elastic & Plastic Deformation
    • Elasticity = material returns to original shape
    • Plasticity = deformation is permanent
    • Aluminium possess both elasticity and plasticity
    • Metals - plastic deformation leads to buckling component, permanent deformation
  • Stress concentrations - we round everything off (i.e. round not square windows) to avoid stress concentrations. Especially important for pressurised AC
  • Bending loads
    • THE WING ROOT EXPERIENCES THE LARGEST BENDING MOMENT
    • EX: Wing Bending Relief = Airborne bending counteracted by WEIGHT of fuel + Wing mounted Eng
    • Aileron upfloat/upset also provides bending relief!!!!
    • Max stress = empty wing fuel tanks
  • Structural Limitations
    • Max Ramp Mass = includes TAXI
    • MTOM no taxi included
    • MLM
    • EX: MZFM = w/o USEABLE FUEL!! Ensure max wing loading at ROOT not exceeded at designed max LF (g)

  • Load Limits


    • DLL = 2.5g = max load designer expects in normal service
    • DUL = 1.5 * DLL (AC must withstand DUL for 3secs w/o fail)

  • Cyclic loads


    • Fatigue based on amplitude of stressors + nbr of cycles
    • Long haul vs Short haul have different fatigue indexes due to diff. usages
    • Fuselage stress amplitude based on pressurisation cycles
    • EX: AMPLITUDE of the load will dictate the number of cycles to failure
    • Ex: S-N Wohler Shows relationship between amplitude of load & number of cycles to failure
  • DURALUMIN = Most common alum. alloy (copper based + manganese, silicon, magnesium)
    • V.Good thermal & electrical conductivity w/ excellent strgh/weight ratio
    • More susceptible to corrosion & hard to weld
    • Used for skins & stringers
  • ALCALD = Duralumin w/ Aluminium coating = corrosion protection
  • Corrosion etc
    • Corrosn can be starting pt for fatigue. Destroys metal smooth surface exposing underlying sharp crystalline structure
    • Oxidation = reaction with O2 producing oxide i.e. rust
    • Electrolytic Corrosn= 2 metals w/ diff electric potentials. A current builds up btwn, the weaker one eaten away. Prevalent in hot/humid/by sea
    • Stress corrosn = Interaction btwn fatigue & corrosion. Moisture enters thro microfractures. Water expands/freezes @ alt = wider fractures

Composites

  • Composite
    • At least 2 elements (matrix + Fibres) to produce new matl having properties different from original
      • Matrix = holds it all together, supports & protects fibres
      • Fibres = Load bearing i.e. Carbon, Kevlar (Aramid), Glass
      • Sometimes Honeycomb core replaces heavier matrix = even lighter but at expense of strength
  • Process =
  • Prepreg Tow = Pre-impregnated with matrix…when heated the chemical reaction takes place and job done
  • Composite strength adjusted by orientation of layers of fibres or loose or tight wound
    • Advantages
      • Light, strong, stiff, durable, resistant to corrosion & fatigue
      • Excellent strength to weight ratio
      • Moulds into complex shapes

    • Disadv


      • Intolerant to impacts, hard to repair, expensive
      • Bad conductor, hard to inspect, may absorb moisture

    • Fibre Glass: Fairings & Radome (EX: Radio wave transparent)

    • Carbon Fibre: Can build whole AC with. Intolerant to impact + expensiv
    • Kevlar(Aramid): V.fine fibre. Used areas of impact.
  • Hybrid of Carbon & Kevlar used. Carbon FB = strength, Kevlar = impact resistance
    • Sandwich:
      • Honeycomb core bonded by structural adhesive
      • No deformation, hi compression stiffness & shear strength
      • EX: Sandwich has MOST STRENGTH DOWN THRU OPEN ENDS OF CELLS!!
      • Ideal for engine cowling, flying controls, floors, walls, thermal/noise/vibration insulation, empennage skin, flooring
      • Disadv = Doesn't like a concentrated load! Soln = LOAD SPREADERS

2 AC Constructn

  • 3 Subassemblies FEW: Fuselage, Empennage, Wing
    • Wings & empennage = aerodynamic surfaces
    • Fuselage forms a pressure hull btwn fwd & rear pressure bulkheads
  • Attachmnt Methods
    • Rivets: most common, quick, cheap but weaken structure
    • Weld: not all matls can be welded
    • Bolts: Labour intensive, heavy
    • Bonding:
    • Fastening: ~'rivets 4 composites' Ti or Stainless steel
    • *Pinning: Str8/Tapered fasteners to attach structures together
  • Alu Fuselage Constructn
    • Truss (Framework)
      • Internal framework. Longerons main component 4 stifness + Xmembers for extra bracubg
      • FRAME TAKES ALL LOADS
    • Monocoque (Stressed skin)
      • Skin laid over formers, no frame. Formers define shape
      • SKIN TAKE ALL LOADS
      • Weakened whenever aperture cut. Doublers used to strengthen aperture
      • Poor Strength/Weight & deforms underload due to lack of support members. Only 4 small AC
    • Semi-Monocoque
      • Skin still takes major loads but reinforced w/ frames (= xtra strength + stiffness), longerons (main longitudinal load carriers - withstand bending loads) and stringers
      • Good Strength/weight ratio
  • Stringer: thin, run longitudinally and add stiffness to skin
  • EXAM: What’s the purpose of a stringer.
    • To take the TRACTION & COMPRESSION load
    • Prevent buckling under COMPRESSION
  • Components
    • Frames = load bearing and give AC shape
    • Formers = give AC shape, not load bearing
    • Stiffners = only go between the frames…attached to the outside skin…riveted to outside skin only. They give the skin extra stiffness/ strength.
    • Machined/Integral Structure = 'stringer-like'. Cos no holes for rivets or bolts risk of fatigue induced cracking alleviated.
    • Bulkheads: structural partitions & Support
    • Firewall: fire-resistant bulkhead, btwn engine and rest of AC. Made from heat resistant stainless steel or TI alloy
  • Pressure Hull: bwtn fwd & rear bulkhead
    • Subject to HOOP STRESS (radial, expands out) & LONGITUDINAL STRESS (Axial, elongates)
    • Cyclic stress experienced every time cabin pressurised.Fatigue life measured in flight cycles
    • EX: MAX PRESS. DIFF = 9PSI
  • EX: Fuselage
    • Rectangular: Non-pressurised AC only!! Corners = load concentrations if pressurised. Easy to build, Hi Strength/Weight
    • Circular: Ideal for pressurised. Aerodynamic. Hoop stresses spread evenly. Can waste space in some pax/cargo configs.
    • Oval: Cheap, Greater capacity, Better options for cargo loading/unloading
    • Double Bubble: Less wasted space
    • Dbl Bbble side-side: more pax and 70% fuel saving BUT 10% slower
  • Wings - stressed skin
    • Main, front, & rear spars.
      • EX: SPAR = main load carrying structure in wing
        • Spar can be 1 piece (GIRDER) or multiple. Bit in middle called WEB
      • Gear attached to main spar. Control surface to rear spar
      • Ribs across spars for support and shape. Can be baffles in wet wing
      • Stringer + skin
    • EX: Torsion Box: Spar-spar + rib-rib + skin-skin(maybe stringers)**
      • Allows wing to bend but also resist some of the torsion in the +ve SWEPTBACK WING!!
  • Wing designs
    • Externally braced: Old AC i.e. Wright Flyer
    • Semi-Cantilever:* Some support strucs i.e. Cessna
    • Cantilever: Supported 1 end with no external supports
  • Empennage
    • Empennage = all tail surface AS WELL AS ALL the SUPPORTING structure below it!!!
      • Longitudinal stability & control: given by the horizontal stabilizer and rudder
      • Directional stability and control: given by the vertical stabiliser and the rudder
    • T-Tail: Out of dirty air BUT DEEP STALL
  • Aerodynamic Flutter = Divergent oscillations
  • Floor venting = Blow out panels, that open automatically to equalise pressure across floor structure, may be installed to prevent distortion of the flooring during a rapid decompression.
  • AC Doors: **EX: Modern AC use PLUG TYPE DOORS that open IN.
  • Cargo doors: Open OUT. Hydraulics or hydraulic power pack
  • Windows (Cockpit):
    • EX: Windscreens, must withstand forces caused by the airflow, precipitation, insects and pressurisation and temperature
    • EX: Triplex: Glass, heating element, vinyl (poly carbonate), Glass
    • EX: We Heat windows to better resist impact & anti-ice
    • EX: If screen fails, What actions might be necessary? A: Reduce speed to weaken impact of an impact.
  • EX: Direct Vision (DV)
    • Uses = Direct viz (look on grnd) & emergency escape & clear cabin of smoke/fumes
  • Eye Reference Position: standardises visual attitude especially on the approach & landing

  • Hvy landing = vertical descnt too great
  • Overweight Landing = exceed landing mass
  • Both can = damage wheels, tires, axel, Oleo, struts
  • Nosewheel Landing
    • Danger of structural damage: Front press. bulkhead, Nose wheel drag and shock struts, nosewheel collapse
  • Tail strike
    • Higher risk if oer-rotation on flare or flapless landing
    • Danger of structural damage: Empennage, rear press. bulkhead
  • Non-Rtn Valve (NRV)
    • EX: What is purpose of arrow embossed on NRV casing? A = Indicates directn of flow
  • Shuttle Valve: automatically allows most appropriate src of press. to operate a service i.e. essential srvs
  • Press. Reducing Valve: ⬇️ sys. press. to that reqd by subsystem
  • Press. Maintain/Priority Valve: maintain falling press. for essential srvcs by closing supply to less essential srvcs
  • Restrictor Valve: Restricts fuid flow to control rate of travel of actuator. Found in gear UP line to ctrl rate of lowering & Flap DOWN line to ctrl rate of retraction
  • Seq. Valves: enable correct seq of different ops eg. gear retraction/extension
  • HYDRAULIC FUSE: prevent fluid loss if big leak. Often found in brake systems
  • Hydraulic Circuits
    • Light AC may use a self-contained POWER PACK OR basic open system for GEAR
    • EX: Hydraulic Power Pack its 100% self contained!!!! not connected to engine etc
    • May not be the same hydraulic fluid as the AC system
  • Open centre or LP system
    • EX: Operates at 2000psi. Press. only generated when srv needs it
    • Sys. press. = low till srv selected. Then acutator wont move til press. builds up to 2000psi. After actuator done then press. relief valve sends back to reservoit and selector back to neutral.
    • EX: Disadv = Only1 SERVICE AT A TIME!! Cos the selectors are open centred and they are in series
    • EX: Open centred hydraulic system = light AC