Airframe Design Flashcards
Minor failure
Conditions may be probable
<10x-3
Major failure
Conditions no more freq than remote
<10x-5
Hazardous
Extremely remote <10x-7
Catastrophic
Extremely improbable
<10x-9
Safe life
The life we give the a/c that operates to its specifications in which it will not suffer from a catastrophic failure
Based on flying hours, cycles, l/d, t/o and pressurisations
Fail safe
If something fails, the structure can carry something for a limited time ( until next inspection)
Damage tolerant
Eliminates the need for extra structure like fail safe by spreading the load over a wider surface area
Reduces weight compared to a fail safe
Three basic bending loadings
Tension - outer edge stretches (pulling load)
Compression - inner edge squeezes together
Shear - across the structure as the forces try to split it
Aileron upfloat/upset
At a certain speed ailerons are put slightly upwards to reduce wing bending
Stress
Internal force per unit area
Any type of load
Strain
Quantifiable effect of stress
Deformation caused by the action of stress on a material
Elasticity
Material returns to its own shape
Plasticity
Material to permantly deform - goes past ts elastic limit
Aluminium
Posses both plasticity and elasticity
Stringer
Inverted L and prevents buckling
Dynamic loads
Tend to build up quickly due to changes in the flight conditions
Often quite severe
Could effect other parts
Turbulence could cause this
Cyclic loads
Loads alternate according to the flight path
Goes in cycles
Load on, load off
Fatigue
Repeated cycles of stress
Cycles of pressure and tension will make metal fatigue rapidly
Larger the load (amplitude), the smaller number of cycles req to failure
S-N Woher curve
Diagram showing cycles to failure
CS 25 and CS 23
Certification standards
23 = normal, utility and commuter a/c
25 =.applies to turbine powered large a/c
Design limit load
Max load the designer expects the a/c to experience in service
Based on 2.5G for a transport a/c
Design ultimate load
A safety factor is applied to the limit load to calc ultimate load
1.5 x DLL
Structure must be able to withstand the DUL without collapse
Main types of a/c construction
Framework
Monocoque
Semi monocoque
Framework
A frame where longerons are connected in triangles via diagonal and vertical web members (tubular steel)
Frame takes all the loads
Warren truss of tubular steel
Monocoque
Skin laid over light formers
Skin takes all the loads - can deform due to lack of support between formers
Heavy - poor strength to weight ratio
Normally used in light a/c
Semi monocoque
Skin takes all the major loads but is reinforced by frames, longerons and stringers
Good strength to weight ratio
Deformation is prevented by longerons and stringers - load is spread over a wider surface area
Stringers and stiffeners
Stringers - used to give the fuselage its shape in between formers and frames - attached to the skin and the frame
Stiffeners - provides additional support to the skin between the frames
Pressurised a/c max difference
9 psi
However the normal difference is around 7 psi
3 ways the wing can be supported
Externally braced - old a/c, biplanes
Semi cantilever - mostly light, high wing a/c, C152
Cantilever - self supporting with no external bracing
Floor venting
Blow out panels, open automatically to equalise the pressure across the floor structure
Prevents distortion during rapid decompression
Type of doors on a/c
Plug type
Typical construction of large transport a/c windscreens
Glass, heating element, vinyl (polycarbonate), glass
The heating element increases strength and durability
Direct vision windows and how they open
Can be opened when not pressurised to provide a sufficiently extensive, clear view of the a/c and the surrounding areas
Open on a track which first lets the aft end of the window tilt inwards, then it slides along a track until its open
Nose wheel l/d causes structural damage to
Front pressure bulkhead
Nose-wheel and shock struts
Possibility of nose wheel collapse
Tail strike could damage
Empennage structure
Rear pressurisation bulkhead
Higher rick on flapless l/d
Difference between Heavy l/d and overweight l/d
Heavy = To do with FPM
Overweight = too much weight causes a bad l/d
Freight doors
Opened by means of hydraulic power pack - self contained unit away from th hydraulic system
Wing construction
Built in boxes (torsion box)
2 spars - front spar is taking the twisting and bending loads
The 2 spars are connected by ribs which are aerofoil shape and contain holes for pipes and wires
Front spar
Fabricated -formed of multiple pieces
Middle piece is called a web
Torsion box
Made up of a front, rear spar, 2 ribs and 2 skins
Allows for small bending but resistive to torsion
Empennage consists of
All the supporting structure below the control surfaces and all the control surfaces
Wing twist can be prevented by
Using the engines as a mass balance
Engine weight is brought forward of the wings CG
