AGK
System design, load, stress & maintenance
• Damage/fault tolerant design:
- Capability to withstand a certain amount of weakening of a structure without catastrophic failure
- Takes cracking of the structure into account
• Fail safe design:
- More than one load carrying component, parallel structural parts, load sharing
- Based on redundancy of components
- Does not imply that the system will never fail despite having backups
• Safe life design:
- Replacement of part after a given number of cycles/flight hours in use
- One load carrying component is sufficient provided it is strong enough
- Does not imply that the system will never fail in the safe life period
There is no most favourable design method, as each component varies
• Maintenance:
1. Hard time: Component overhauled/removed after a set number of hours/cycles regardless of condition
2. On condition: Monitoring of critical parameters & replacement of parts if a limit value is exceeded
• Stress: Force/area
1. Tension: Force resisting from being pulled apart
2. Torsion: Caused by twisting
3. Compression: Push force
4. Torque: Axial rotation force
5. Shear: Force parallel to cross section
6. Buckling: Effect of more than one force
• Strain:
- Deformation due to stress, expressed as a % change of dimension of original dimension
• Elastic deformation
- Tendency of material to return to its original state
- Temporary & reverses when load is removed
• Corrosion: Incorrect metallic bonding
1. Stress: Continuous tensile load + corrosion
2. Intergranular: Grain boundaries inside metal
• Fatigue: Material is continually loaded & unloaded & will eventually break even though load is the same
• Aircraft flies beyond certified load factor: Subject to permanent deformation
Airframe
• Engine compartment decking & firewall: Stainless steel/titanium sheet
• Sandwich structure:
- Consists of two thin sheets separated with light core material
- Low mass high stiffness
- Stabilizes covering sheets
- Unsuitable for absorbing concentrated loads
- Does not use resin
• Composite structure:
- Consists of matrix & fibres
- Component strengths can be tailored to the direction of load, not the same in all directions
- Higher strength to weight ratio compared to other metal
• Truss type: Small light aircraft/training planes
• Monocoque:
- Takes all the load on a stressed skin
- Normally uses aluminium/magnesium alloy
• Semi-monocoque:
- Fuselage of transport airplanes
, - Consists of skin, frames & stringers
- Normally uses aluminium/magnesium alloy
• Cantilever:
- Attached to the aircraft at the wing root only (No struts/braces/wires)
- Vertical loads/bending moments highest at wing root
• Wings:
- Torsion box: Consists of spars, ribs, wing skin reinforced by stringers
- Ribs:
• Maintains aerodynamic shape
- Stringers:
• Assists skin to absorb longitudinal compressive loads
- Wing skin:
• When unable to bear load, it transfers them to the spar via ribs & stringers
• Bears cylindrical load during pressurization (TENSION)
- Spar:
• Bears most of the load
• Consists of web & girders (I-beam)
- In the air:
• Lift loads carried by upper/lower skin surfaces & spars
• Tension on lower surface & compression on upper surface
- On the ground:
• Tension in upper surface & compression on lower surface
- Wing bending moments:
• Reduced by installing “upfloat” ailerons, using fuselage fuel first while maintaining fuel in wings as long as
possible
• Torsion: Effect of aileron deflection or positive sweep (As the surfaces hit the air at non uniform levels a
twisting motion is induced)
• From wing root to the tip: First compression then tension
• Aerodynamic flutter:
- Caused by torsion & bending, COP ahead of COG
- Avoided by increasing torsional stiffness & adding balancing mass in front of control surface hinge
- Avoided by ensuring correct mass distribution within the control surface during design
- Wing bends downwards: Flutter may occur if the aileron deflects upwards as aileron COG is behind hinge line
- Wing bends upwards: Flutter may occur if the aileron deflects downwards as aileron COG is behind hinge line
• T – Tail aircraft:
- Vertical stabiliser Is not affected by influence of wing turbulence
• Fuselage:
- Consists of: skin, frames & stringers (No spars/girders)
- Pressurization load = Tension
- Shell structures transmit: Normal bending, tangent bending, tension & torsional stresses (NO SHEAR)
- Torque links: Most stress when making tight turns during taxiing, turning at a small radius
- Floor proximity emergency escape lights: Gives additional guidance during evacuation in reduced visibility
• Cockpit window:
- De-icing provided by electrical heating
- Some aircraft have speed restrictions related to bird impact when window heating inoperative
- Window heating improves strength of cockpit windows
- Cockpit side windows not provided with de-icing, only defoggers
- Made of: Glass & Inner surface made of soft polycarbonate laminate
- Green system “On” information light and an amber failure warning light
• Airplanes designed for long haul cannot be used for short haul flights as lifetime of fatigue sensitive parts have been
determined on a load spectrum
• MZFM: Maximum zero fuel mass – Total maximum permissible mass of the aircraft without usable fuel
Hydraulics
• Pascal’s law: Pressure exerted on hydraulic fluid within an enclosed system the pressure will increase equally throughout
the fluid, and act at right angles to the container walls. Force/area\
• Area(A) x distance(A) = Area(B) x distance(B)
• Most common:
- Phosphate ether based fluids (Skydrol) is purple
- Synthetic oil (Maybe mineral). Synthetic = Highest resistance against cavitation
, - Operated at 3000psi
• Hydraulic power is a function of system pressure & volume flow
• Monitoring parameters: Pressure, fluid temperature & quantity
• Flight deck indicator for hydraulic pressure: Transducer connected to an indirect indicator
• Max power output & low mass: Achieved by having high pressure system & low volume flow
• Hydraulic circuits:
- Open centre: Has capabilities for idle flow
• Hydraulic braking system:
- Uses nitrogen
• Valves:
- Shuttle valves: (Switches between 2 sources of pressure)
• Switch hydraulically operated units to the most appropriate pressure supply
• Enables an alternate supply to an actuator
- Selector valve: Direct system pressure to either side of the piston of an actuator
- Check/non return valve: Works the same as an electronic diode allowing fluid to flow in one direction only
- Pressure relieve valves: Protects against excessive system pressure
- Cut-out valve: Used in a fixed volume pressure control hydraulic system
- High pressure relief valve: Failure of normal method of system pressure limiting control
- Relief valve: Makes sure the pressure does not exceed permitted pressure in the system
• Actuator/jack:
- Converts hydraulic pressure into linear motion
- Hydraulic lock is when no movement of piston takes place
- Single actuator: Is powered in one direction only by hydraulic power, the return movement being another force
• Pumps
- Variable displacement vs. constant pump: Variable adjusts the fluid pumped to the fluid required, moves fluid only
when necessary
- Separate pressure regulator: Used in hydraulic system in conjunction with constant delivery type pump
- Hand pumps: Connected to the bottom of the reservoir
- Axial piston pumps: Produces high pressure when required but can be off loaded to reduce power consumption
- Over heat detectors: Installed at the pumps
- Low pressure alert: Located at pump outlet, indicates insufficient pump output
- Pump failure: Quill drive will shear to offload & protect gearbox
• Filters:
- In both pressure & return lines
- Pop-out indicators: Warn of impending clogging/by-pass
• Reservoir:
- When powering up, fluid in reservoir will decrease slightly
- Discounting leaks, it fluctuates with jack displacement & accumulator pressure
- Pressurized to prevent cavitation in the pump inlet to the EDP, using bleed air
- Pressurized to ensure inlet is provided with continuous supply of fluid free from foaming
- Hydraulic fluid temperature is measured here
• Accumulator:
- Store fluid under pressure (energy storage)
- Provide a limited alternate supply of pressure in an emergency
- Dampen out fluid pressure fluctuations/variations
- Allow for thermal expansion
- Cater for small internal leaks
- Hydraulic fluid found in gas container = an internal leak in accumulator
• Hydraulic fluid properties:
- Thermal stability
- Low emulsifying characteristic
- Anti-corrosive
- High flash point
- Irritating to eyes & skin
- Incompressible
- Ideally low viscosity to minimise power consumption and resistance to flow
• Internal leaks will cause fluid temperature increase (As the pistons/accumulators have to work more, becomes hotter)
• The security of the hydraulic system comprises:
- Filters
- Pressure Relief Valves
- By-pass valves
- Fire Shut-off valves
- Hydraulic fuse: Prevent total system loss in the event of hydraulic leak/rupture
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