AGK Systems - lesson 1 Flashcards
What’s the cause of most stress on a shorthaul aircraft?
Pressure cycles
What is the pressure on the fuselage created by pressurising the cabin?
8 - 9 psi
What is the time of useful consciousness at 36,000
30 - 60 seconds
What document sets the standards for normal, utility, aerobatic and commuter aeroplanes
CS 23
What document sets the standards for turbine powered large aeroplanes.
CS 25
What is the purpose of the certification standards?
“to ensure that an acceptable safety level for equipment and systems as installed on the aeroplane”
What is the principle behind certification?
The more catastrophic the consequences of failure would be, the less likely it must be that it can happen
From the certification standards:
Define:
Fail safe
In the event of a failure the component remains safe.
From the certification standards:
Define:
Safe life
Components are designed to survive a specific design life with a chosen reserve
Safe life can be measured in
Cycles
Landings
Calendar duration
Safe life requires components to have a good design life over a ___________ period.
predictable
Fail safe: Describe what is mean by multiple load paths.
A system that allows duplication of anything critical.
Describe damage tolerance
The ability of a structure to sustain defects safely until repair can be effected
Stress is the internal force per unit area inside a structural component as a result of
external loads
What is the equation for stress?
Stress = Load / original cross sectional area
Describe ‘Strain’
The deformation caused by stress on a material, given as a change in dimension percentage of original.
Forces imposed by maneuvering ark known as
dynamic loads
If you see ‘yaw damper’ in a question it refers to
Dutch roll
Rudder ratio control will refer to
the reducing requirement for the rudder at high speed
What type of load would build up very slowly over time or remain more or less constant over a period of time. For rxample, the loads when stationary on the ground
Static loads
What type of load would be described as loads which re-occur and rise and fall in magnitude. Vibration which occurs in rapidly changing frequencies, especially in turbine engines, is a good example of cyclic loading.
Cyclic loads
Name 5 types of loads and stress
Tension
Compression
Torsion
Shear
Bending
If a material returns to original shape is shows _______ properties
Elastic
If a material will permanently deform it shows _______ properties
Plastic (plasticity)
Aluminium possesses both elastic and ______ properites
plastic
In metals, plastic deformation leads to?
buckling component, permanent deformation.
Elastic range followed by plastic range. Which would be normal operating range for an aircraft?
Elastic range
If you see skin wrinkling or buckling then the structure is as likely to have experienced which force?
compression
If you see pulled rivets then you are likely to see the structure has experienced which force?
Tension
Where would you expect the largest bending moment?
Wing root
Wing bending relief in the air is countered by
Fuel and engine in/on the wings
When would maximum stress on the wing occur?
Empty wing tanks.
Why would you use centre tank fuel first?
Reduce the wing bending
You are reliant on fuel pumps for the centre tank
In flight top of the wing is being compressed while bottom is in tension however when on the ground:
top tension
bottom compression
Transport cat aircraft have a load factor derived by what multiplier on the load limit
“factor of safety”
1.5
Limit load value is
2.5g
Ultimate load value
3.75g
Two identical aircraft on the same mission. One has a 1% greater mass. What is the effect on airframe fatigue
5%
Each airframe as a fatigue life calculated by manufacturer based on
hours flown
number of load cycles
The reason for derated / reduced thrust take off is for
Direct operating costs of the engine
Failure is more likely under what type of loading?
tensile
Failures first start at points where stress is ____________
concentrated
Maximum stress is experienced at
sharp corners or ridges
Combination of corrosive condition and steady tensile load is known as
stress corrosion
Alloys must possess a number of qualities including:
Elasticity
Plasticity
Stiffness
Strength.
Common materials used for aircraft include:
Aluminium
titanium
magnesium alloys
steel
May include added lithium, magnesium, manganese, silicon, zinc
Alloy of copper and nickel with small amount of Magnesium and Iron. High resistance to corrosion, low coefficient to expansion and very strong. Used in Jet exhaust pipes
describes what?
Monel
In composite structures you can choose where to lay the fibres for what reason
adds strength to where you need it
A composite is?
Combination two or more organic / inorganic components
Advantages of composites:
Light, strong, stiff, durable, resistant to corrosion and fatigue
Excellent strength to weight ratio
Can be moulded into complex shapes
Disadvantages of Composites
Hard to inspect for flaws
May absorb moisture which freeze causing delamination
Expensive to produce
Difficult to repair
Not as tolerant to damage as aluminium
Not naturally electrically conductive
-Tendency to heat up and delaminate when struck by lightning
Quickly eroded by sand and hail – must be protected on leading edges.
Ways to protect from corrosion
Painting – Aesthetics, branding, good corrosive and UV resistance – periodical repaint required
Anodising – suited to alloys, provides a hard wearing coloured coating
Powder coating – suited to alloys, hard wearing zinc coating, may be coloured
Polishing – Useful for Aluminium, Chromium and Stainless Steels – encourages natural hardened surface layer
Bedding compounds – used between dissimilar materials to prevent Galvanic corrosion – typically a Zinc Chromate paste
Sources of corrosion
Any fluid leaks.
There are two types of maintenance method:
Hard time maintenance - fixed
On condition maintenance - variable
Define:
Redundancy
duplicating critical parts so should one fail the other can still perform the task
Define:
Stress and Strain
always present within the structure – static and dynamic
Define:
Static loads
on the ground – weight and gravity
Define:
Dynamic loads
in flight – vary in magnitude according to manoeuvres and atmospheric conditions
Define:
Greatest stress
wings and wing roots – inspect during pre-flight for cracks, popped rivets buckled skin or structural distortion
Define:
Corrosion
weakens a structure and creates a point where stress fractures can form
Rust – ferrous based materials
Oxidation – Aluminium and alloys (white powdery residue)
Marine and humid environments pose greatest threat of ?
Corrosion
Define:
Fatigue
increased risk where corrosion is present, high number of cycles, hard manoeuvring and/or low maintenance
Define:
Hard Time Maintenance
fixed intervals – Time, cycles – cannot be exceeded – A Check or D Check
Define:
On condition Maintenance
as required – brakes, tyres, wipers, lights
Define:
Composite structure
combination of 2 or more organic / inorganic materials
Define:
Fibre Reinforced Plastic (FRP)
Typically Carbon or Kevlar construction with polymer resin
Define:
Glass Reinforced Plastic (GRP)
Fibreglass – glass strands impregnated with resin – Epoxy or Poly Vinyl
Limits on airframe:
Maximum Structural Ramp Mass – the maximum permissible mass before the aircraft commences taxi
Maximum Structural Take-Off Mass – the maximum permissible mass at the start of the take-off run
Maximum Zero-Fuel Mass – Maximum mass of the aircraft without usable fuel
Maximum Structural Landing Mass – the maximum mass on landing, in normal circumstances.
Aircraft designs utilise either:
High wing, low wing, high or mid-set tail plane and T-tail
Typically the following apply:
Arc welding – ferrous metals of high grade (thickness)
Mig welding (Metal Inert Gas) – lower grade metals and uses a gas to protect the weld – Argon usually
Tig welding (Tungsten Inert Gas) – Aluminium and alloys (including Stainless Steel) – uses Argon as an Inert gas shroud
Principal components of the semi-monocoque design are:
Aircraft skin - provides aerodynamic shape, carrying a portion of the load
Formers/frames - define the shape
Stringers - run longitudinally and help to withstand skin buckling
Longerons - the main longitudinal load-carrying members
Bulkheads - provide structural partitions inside the fuselage
Firewall - a fire resistant bulkhead
What semi-monocoque construction element run longitudinally to Withstand buckling?
Stringers
Define:
Aircraft skin
provides aerodynamic shape, carrying a portion of the load
Define:
Formers/frames
define the shape
Define:
Stringers
run longitudinally and help to withstand skin buckling
Define:
Longerons
the main longitudinal load-carrying members
Define:
Bulkheads
provide structural partitions inside the fuselage
Define:
Firewall
a fire resistant bulkhead
A single machined piece of aluminium forms skin and stringer-like structure, what are the advantages?
No need for rivets and other attachment methods saving a great deal of weight
Ideal for wet wings where structure of wing itself used to form a fuel tank cell – no rivet holes!
Most fuselage cross-sections are circular or almost circular in shape This is done for two reasons:
Ensures that air flow will not separate at moderate angles of attack or sideslip
More easily withstands the loads imposed by pressurisation.
The oval type cross section fuselage, results in:
Lower manufacturing cost
Greater capacity resulting in increased revenue
Better options for cargo loading and unloading
Define:
The Pressure Hull
Section of fuselage between forward and rear pressure bulkheads
The pressure hull experiences:
Axial stress acts along the longitudinal axis
Hoop stress acts radially across the fuselage cross section
Cyclic stress experienced every time cabin is pressurised so the pressure hull has a fatigue life, measured in ______ ______
flight cycles
There are three types of wing design:
The cantilever wing
The semi-cantilever wing
The externally braced wing
Describe a cantilever wing
All high speed jet transport aircraft have cantilever wings because struts create too much drag.
737
Describe a semi-cantilever wing
A semi-cantilevered wing has some supporting external struts
eg. C172
Describe an externally braced wing?
An externally braced wing has external supporting struts and bracing wires
.eg. Cessna 152
Describe a wing spar
Is the main span-wise structural member of the wing
Withstands bending loads and supports the weight of the wing on the ground
Large or very long wings may have more than one spar
Typically constructed either as box section or as an I beam
Describe a wing rib
Provide aerodynamic shape
Allow the stressed skin to be attached to the wing structure and transfer the loads into the spar
Describe a stringer
Combine with ribs to share loads and transfer load to spars
Stressed Skin
Provides the smooth aerodynamic outer cover and withstands tension and compression stresses
Chord line vs chord
Chord line - the line
Chord - a length
Define:
Wet wing:
Internal wing spaces can form an integral fuel tank
If it’s using existing structure then it’s ‘integral’
Define:
Torsion box
With multiple spar designs the spars are linked together to form a torsion box
The torsion box is very rigid, resistant to twisting and greatly adds to wing strength
Define:
Wingbox
Section between the wings. It connects to wings and fuselage
The wing experiences torsional stresses for two reasons:
When the engines are mounted on the wing their thrust line tends to cause twisting forces on the wing structure
-Therefore, engines are positioned forward of the wing
Stresses caused by the difference between the positions of the centre of gravity (CG), and the centre of pressure (CP)
What does moving the CofG forward on the wing help to prevent?
Flutter
Because the CG and CP don’t act through the same point on the wing, they form a couple which tends to twist the wing around its ________ ____
flexural axis
Some torsional stress can be relived by carefully designing the position the engines and the fuel tanks to move the ________ ____ closer to the centre of pressure.
flexural axis
Manufacturers employ a range of methods to prevent flutter on aircraft wings:
Chord wise and span wise positioning of masses
Engines are pylon mounted ahead of the wing CG
Fuel is stored in integral wing tanks that are towards the front of the wing
The wing is designed with good torsional stiffness
An addition to torsional stiffness the wing displays bending flexibility
The ability to absorb vertical loads
What can the crew do to reduce chances of flutter?
carry out correct fuel balancing procedures in flight
The Empennage
Provide longitudinal stability from horizontal stabiliser and longitudinal control from the rudder
Directional stability from the vertical stabiliser and directional control from the rudder
Caution must be applied during take-off and landing to prevent a ‘tail strike’
This will cause detrimental damage to the airframe – accidents have been attributed to damage caused to the aft pressure bulkhead by a tail strike.
T-Tail
2 good
2 bad
Move the tail out of the downwash from the main wing
Provide a good glide ratio, and are more efficient on low speed aircraft
More likely to enter a deep stall and are more difficult to recover from a spin
Must be of a stronger construction and therefore heavier than conventional tails.
What is control surface flutter
Flutter is the rapid oscillation of a flight control surface due to imbalances
Control surface Flutter is primarily caused by an interaction of
aerodynamic forces
inertia forces
elastic properties of the control surface
How do we prevent control surface flutter?
mass balancing
What material is often used for aircraft floors?
ALUHEX – provides an extremely light, yet strong material, which is used on aircraft floor panels, walls, doors, hatches and skins
Explosive blow-out bung in the cabin floor
Prevents the cabin floor from collapsing in the event of cargo area decompression.
If a door has to open outwards what can be employed to keep it closed.
Outward opening doors require an inflatable seal or rubber cuff to make them air-tight.
Flight deck windows must (CS25):
Have excellent optical qualities
Be sealed securely to withstand cabin pressurisation forces
Withstand the stresses imposed by temperature difference
Withstand significant impacts without breaking
Have a de-ice / anti-ice facility
A windscreen is made up of
and why
Windscreen panels are made up of two transparent plies
Interlayer absorbs impact loads and prevents the glass from fragmenting.
CS 25 requires aircraft equipped with direct vision window to have what properties
Designed either to slide back or to open inwards
Large enough to be used as a means of emergency escape.
Hydraulic systems transmit power to ______ locations
remote
Typical uses for hydraulics in large aircraft include:
Landing gear and wheel brakes
Nose wheel steering
Primary flight controls
Spoilers
Flaps
Speed brakes
Engine thrust reversers.
In a hydraulic system is important to understand that Pressure is generated though what?
a resistance to flow!
A hydraulic pump generates what?
flow (not pressure)
In a hydraulic system we don’t really care about ______ pressure.
static
Oil is incompressible.
true/false
true
According to Pascal’s law the pressure induced by a 500lb load on oil enclosed in a cylinder is felt as ____lb of static pressure at every point in the oil.
500lb
Force =
force = pressure x area
