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AGK > Loads > Flashcards

Loads Flashcards

1
Q

What are single loads / combined loads applied to an aircraft

A

Tention (tensil load)
Tends to stretch a structure
Resited by ‘Ties’

Compression (compressive load)
Tends to shorten a structure
Resited by struts

Shear
tends to slide one face of the structure over an adjacent face.
Resited by ‘Riveted joints’

Buckling
* Effect of more than one force
* Occurs to thin sheets of metal
* structure essential bends
* occurs when subjected to end loads and ties subjected to compressive forces
* excessive compression can cause object to ‘wrinkle’

Bending
* Involves Tension, compression and shear.
* Tension as the other edges stretch
* compression as the inner edges squeeze
* Shear across the structure as force tries to split it

Torsion
* Twisting forces that produce:
* tention to outer edges
* compression in the middle
* shear across the structure

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2
Q

Which forces affect aircraft structure

A

Stress & strain always present in an aircraft on ground or in flight

  • Weight
  • Lift
  • Thrust
  • Drag
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3
Q

Examples of loads

A

Tention (tensil load)
When thrust force is pulling forward (propellor), and Drag force acting backwards (tail)

Compression (compressive load)
when aircraft lands with wheels on ground - there is the force applied from AC to ground, and ground to AC

Shear
Rivets holding the wing to the fuselage experience forces from opposing directions

Bending
Wings affected by turbulence or gust of wind
Aircraft is fully loaded but wing tanks are empty
Lift bending the wing upwards
Landing gear bends wings upwards when stationary

Torsion
Rotation of the shafts in an engine, creates a torsional force on the engline and the wings

Buckling
excessive compression load on the wing spar could lead to metal ‘wrinkling/buckilng*

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4
Q

Bending moment & relief

A

The wing suffers the greatest stress at the root = large bending moment

When stationary:
* Static force from the landing gear move upwards and acts on the wings

In the air
* Lift produces upwards dynamic forces on the wing
* To counteract - weight downward force gives a degree of bend relief

*if wing tanks are empty, there is no bending relief and the wing has to bear the full bend force caused by lift

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5
Q

Bending forces are controlled by..

A

g’s

and

loading

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6
Q

To ensure Bending Relief exists..

A

Maximum All up Mass must be > Maximum Zero Fuel Mass

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7
Q

Amount of Bending Relief provided by the mass of wing fuel is dictated by….

A

quantity of fuel and it distribution (set by operating procedures)

this eleviates the effect of the lift

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8
Q

Results of loads

A

Stress
* internal force per unit of area inside the structure
* as a result of external forces, therefore
* a tensile force will set up a tensil stress
* or a compression force will set up a compression stress
* N/mm2 OR MN/m2

Strain
* Deformation caused to the structure dimensions because of Stress applied to the material.
* Ratio of Change of dimension/size of demformed object expressed in % from the original object dimension

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9
Q

how does an object react to force

A

Depends on the magnitude of the force

If stress is low - nothing happens

if stress is high - deformation occurs (Strain)

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10
Q

Elastic limit

A

is the limit of a material in accepting deformation and returning to it’s original dimensions once deforming load is removed

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11
Q

Elastic Deformation

A

when strain exists but it’s light and does not cause permanent damage

can be irreversable

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12
Q

Plastic demormation

A

when elastic limit is exceed and the object is permanently deformed

strain is high and object cannot return to its original dimensions/damaged

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13
Q

Young Modulus Elsticity

A

the relationship between stress and strain in an elastic material is constant

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14
Q

Dynamic Loads

A

Forces imposed by normal or excessive manoevering

load changes dynamically depending on flight conditions

Loads that tend to build up & subside quickly

**do not happen continously **

usually happen in flight(when AC is manoevering) but also can happen on ground (exposure to wind when parked)

Can induce load on other aircraft parts

can be severe

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15
Q

Static loads

A

loads that happen consistently & build up slowly and remain constant over time.

usually happen when AC experiences gravitational forces (thus can happen on ground or in flight)

ex: stationary aircraft weight imposes static load on wing spar through the upward forces excerted by the landing gear

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16
Q

What is the ultimate stress point

A

the stress in where an application fails (breaks) in a single application in a static load

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17
Q

In flight, what are examples of forces / loads it experiences

A

Wings during a Lift or normal g experience:
1. Bending upwards of the wing
2. Causes tention on bottm surface by edges,
3. Causes compression down surface in the top middle
4. Torsional forces that twist the wint

Wings during a Lift or excessive positive g
1.buckling happens at top surface of the wing
2.Bottom of the wing experiences Tension by the rivets being pulled
3., which causes the rivets to Sheer and rivets become lose

Drag causes the gearto:
1. Bend backwards
2. While the mass of the aircraft pull it downwards

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18
Q

what ‘g’ is the aircraft flying when on constant speed and level?

A

1g

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19
Q

if an aircraft changes ‘ATTITUDE’

A

the weight of the aircraft structure changes and so does the loads applied to it

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20
Q

if an aircraft is in an emergency, what happends to the load?

A

the loads it experiences will change drastically

ex: in engine failure, **
1. only one engine might work on one side, so thurst is produced on one side of the aircraft, thus also adding load onto the wing**.

  1. the aircraft might produce a yawning movement, which needs to be corrected, adding** load onto the fin/rudder and fuselage**
21
Q

Design Limit Load - DLL

A

is the maximum load airframe or component can experience in services (set by the manufacturer during design)

measured in ‘g’

22
Q

what is the DLL for Air transport, aerobatic, and Utility aircrafts

A

Ai transport: +2.5 and -1
Utility: 4.4
Aerobatic: 6

23
Q

What is Design Ultimate Load - DUL

A

it’s DLL * safety factor

yielding the maximum limit a structure can withstand without collapse

24
Q

what is the minimum safety factor accepted

A

1.5 or (150%)

25
Q

what is safety factor

A

it’s the ration of Design Ultimate Load to Design Limit Load

26
Q

Safe life and life count

A

is the **minimum ‘life’ **during which it is knowns that no catastrophic damage should occur

the life count can be in:
* calendar basis
* accelaration
* cycles of landing
* pressurization events
* flight hours
*

Once the life count is reached, the aircraft component is replaced or overhauled

27
Q

What is the Elapsed life count also called?

A

fatigue cycle

28
Q

what is cyclic loading

A

it’s the repeated application and removal of load on a structure

*Also called dynamic reversable load

29
Q

Structural failure is caused by…

A

**high cycle metal fatigue **

or

exceeding ultimate load failure

30
Q

what is fatigue

A

Definition:
application of cyclic loading till a point where the structure is fatigued/degregaded - which happens a much less rate than if it occured from steady load

if a material has load applied / removed and reapplied again

it will eventually break even if the load was consistent throughout

microscoping cracks in the metal’s crystaline’s structure grow to a larger fracture that is noticeable.

*damange spreads to the corners of the metal

**Note:
Metal fatigue can occur rapidly if the stress is large (weight) even if cycles are not frequent **

31
Q

Fatigue life

A

related to the number of hours flown and the fatigque cycle (ex: flight sectors)

the heavier the aircraft, the more the fatigue life is consumed

RULE OF THUMB
1% increase in weight = 5% increase in fatigue life consumption

32
Q

To minimize chances of failure due to fatigue..designers apply these 2 principles

A

Fail safe

damage tolerance

Stressed skin/Semi monocoque

33
Q

Fail safe multiple load path is

A

**every area of the aircraft structure in which a failure could lead to loss of the aircraft must have a ‘fail safe multiple load paths’ set up. **

Must be inspected rigidly before the alternate load path fails

This is where:
- components of safety critical ares must be duplicated = redudancy

this allows for **multple load paths **

if one path fails, alternative load path can carry all loads via duplicated structure/redudant component

** for a LIMITED time period**

Note:
a structure which, after any single failure or crack in any one structural member can safely carry the normal operating loads until the next periodic inspection

Found in:
wing and tail attachment areas, cabin windows, engine fittings

34
Q

distadvantage of the (load path)?

A

it can be heavy

need disassambely to be able to inspect

35
Q

Damage tolerance

A

**Unlike fail safe **which requires an extra structure to save the integrity of the main structure causing heaviness..

Damage tolerant structure eliminates the extra structure..

Instead:
the ** loading of a particular structure over a larger area. **

This means that the structure is designed so that damage can be detected during the normal inspection cycles before a failure occurs.

36
Q

how are faults detected:

A

BY planned inspection programme:

  • to gain access to the vulnerable areas a certain amount of dismantling is necessary
  • use of **non-destructive testing **(NDT) may be employed in less critical areas.
37
Q

Stressed skin’ or ‘Semi-monocoque’ style

A

‘of construction where each piece of the aircraft has its part to play in spreading loads throughout the airframe and is tolerant to certain amount of damage.

Ex: if a crack of detectable length has been missed at the first inspection, the structure will allow this crack to develop until a subsequent inspection before it becomes critical.

38
Q

At the time of airworthiness certificate..the following are agree upon…

A

The criteria of:

1)inspection cycles
2)Design Limit Loads, and Design Ultimate Loads

39
Q

What is stress concentration

A

is the point on an object where stress is applied

object is strong = if force is applied evenly over the object

object is weak = if area is reduced and force is localized ==>fatigue crack

40
Q

how to increase fatigue strength?

A

by removing defects where the fatigue cracks occur

41
Q

What are station numbers

A

method of locating components on an aircraft

based on the order maintainance and repairs are carried out

42
Q

station numbers for fuselage

A

Fuselage station lines are determined by :

Reference to a zero datum line (fuselage station 0.00) at or near the forward portion of the aircraft as defined by the manufacturer

. Station numbers are given in inches forward (negative and given a - sign) or

aft (positive and with a + sign) of the zero datum

43
Q

station numbers for Wings

A

Wing stations are measured from the centre line of the aircraft and are also given in inches left or right of the centre line.

44
Q

Vertical & Horizontal station linnumberses

A

Vertical position from a ground line

horizontal datum can be known as a water Line (WL), given as a dimension in inches from the horizontal datum.

45
Q

which of these are prone to stress in Order?

A

1)Wing spar and skin near root of the wing - experiences the most tension and compression

fuselage skin near bulk head - Bulk head experiences a lot of stress due to the pressure difference between outside and inside the aircraft. PLUS ..Aerodynamic dynamic forces on the tail

skin at the top and bottom of fuselage - tenstion and compression caused by landing gear

aircraft skin around the windows experiences tension due to differential pressures inside/out

WHAT DOES NOT
**Wing spar and skin near the tip **of the wing does experience force but not much

Engine nacelles (skin around the engine) does not experience stress.

46
Q

Techniques to reduce loads

A
  1. carry no more than required fuel
  2. minimize angle of banks and anticipate your turns
  3. do not land in excessive rate of descend
  4. aviod harsh handling of controls

5.reduce speed when in turbulence and ensure you are not over -controlling the aircraft when in turbulence

47
Q

to avoid undetected structural weakness..crew should..

A

report any forces or g excursions experienced on flight that might be above the design load limit

48
Q

There is also structural mass limitation to be set for an Aircraft..why?

A

because aircraft must be strong enough to carry a certain maximum mass and ..

how much this is depends on the phases of flight

49
Q

What are the structural mass limits

A

Max Structural Taxi Mass / Mas Ramp up Mass (MRM):
max permissible weight when aircrafit starts to taxi

Max Structure Take off Mass (MSTM)
max permissible weight at the start of the take off run

Max Zero Fuel Mass (MZFM)
Max permissible mass when there is no useable fuel in the tanks

Max Structural Landing Mass (MSLM)
Max permissible mass on landing in normal cirumstances

NOTE:
MRM is > MSTM Because aircraft consumes fule while taxing and before take off

MSLM is < MSTM because aircraft has to withstand the dynamic forces upon landing so needs to be lighter

AGK (15 decks)

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