Airframe Design

Question 1

Minor failure

Answer 1

Conditions may be probable

<10x-3

Question 2

Major failure

Answer 2

Conditions no more freq than remote

<10x-5

Question 3

Hazardous

Answer 3

Extremely remote <10x-7

Question 4

Catastrophic

Answer 4

Extremely improbable

<10x-9

Question 5

Safe life

Answer 5

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

Question 6

Fail safe

Answer 6

If something fails, the structure can carry something for a limited time ( until next inspection)

Question 7

Damage tolerant

Answer 7

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

Question 8

Three basic bending loadings

Answer 8

Tension - outer edge stretches (pulling load)

Compression - inner edge squeezes together

Shear - across the structure as the forces try to split it

Question 9

Aileron upfloat/upset

Answer 9

At a certain speed ailerons are put slightly upwards to reduce wing bending

Question 10

Stress

Answer 10

Internal force per unit area

Any type of load

Question 11

Strain

Answer 11

Quantifiable effect of stress

Deformation caused by the action of stress on a material

Question 12

Elasticity

Answer 12

Material returns to its own shape

Question 13

Plasticity

Answer 13

Material to permantly deform - goes past ts elastic limit

Question 14

Aluminium

Answer 14

Posses both plasticity and elasticity

Question 15

Stringer

Answer 15

Inverted L and prevents buckling

Question 16

Dynamic loads

Answer 16

Tend to build up quickly due to changes in the flight conditions

Often quite severe

Could effect other parts

Turbulence could cause this

Question 17

Cyclic loads

Answer 17

Loads alternate according to the flight path

Goes in cycles

Load on, load off

Question 18

Fatigue

Answer 18

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

Question 19

S-N Woher curve

Answer 19

Diagram showing cycles to failure

Question 20

CS 25 and CS 23

Answer 20

Certification standards

23 = normal, utility and commuter a/c

25 =.applies to turbine powered large a/c

Question 21

Design limit load

Answer 21

Max load the designer expects the a/c to experience in service

Based on 2.5G for a transport a/c

Question 22

Design ultimate load

Answer 22

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

Question 23

Main types of a/c construction

Answer 23

Framework

Monocoque

Semi monocoque

Question 24

Framework

Answer 24

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

Question 25

Monocoque

Answer 25

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

Question 26

Semi monocoque

Answer 26

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

Question 27

Stringers and stiffeners

Answer 27

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

Question 28

Pressurised a/c max difference

Answer 28

9 psi

However the normal difference is around 7 psi

Question 29

3 ways the wing can be supported

Answer 29

Externally braced - old a/c, biplanes

Semi cantilever - mostly light, high wing a/c, C152

Cantilever - self supporting with no external bracing

Question 30

Floor venting

Answer 30

Blow out panels, open automatically to equalise the pressure across the floor structure

Prevents distortion during rapid decompression

Question 31

Type of doors on a/c

Answer 31

Plug type

Question 32

Typical construction of large transport a/c windscreens

Answer 32

Glass, heating element, vinyl (polycarbonate), glass

The heating element increases strength and durability

Question 33

Direct vision windows and how they open

Answer 33

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

Question 34

Nose wheel l/d causes structural damage to

Answer 34

Front pressure bulkhead

Nose-wheel and shock struts

Possibility of nose wheel collapse

Question 35

Tail strike could damage

Answer 35

Empennage structure

Rear pressurisation bulkhead

Higher rick on flapless l/d

Question 36

Difference between Heavy l/d and overweight l/d

Answer 36

Heavy = To do with FPM

Overweight = too much weight causes a bad l/d

Question 37

Freight doors

Answer 37

Opened by means of hydraulic power pack - self contained unit away from th hydraulic system

Question 38

Wing construction

Answer 38

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

Question 39

Front spar

Answer 39

Fabricated -formed of multiple pieces

Middle piece is called a web

Question 40

Torsion box

Answer 40

Made up of a front, rear spar, 2 ribs and 2 skins

Allows for small bending but resistive to torsion

Question 41

Empennage consists of

Answer 41

All the supporting structure below the control surfaces and all the control surfaces

Question 42

Wing twist can be prevented by

Answer 42

Using the engines as a mass balance

Engine weight is brought forward of the wings CG