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AGK: Systems (2 AC Constructn (3 Subassemblies FEW: Fuselage, Empennage,…
AGK: Systems
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
Basic Hydraulics
- Bramah press (answer normally always 100)
- 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
- 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
8 Primary Flight Ctrl
- Ailerons, Rudder, Elevator
- ALSO ROLL Ctrl spoilers
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