Loads Flashcards
What are single loads / combined loads applied to an aircraft
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
Which forces affect aircraft structure
Stress & strain always present in an aircraft on ground or in flight
- Weight
- Lift
- Thrust
- Drag
Examples of loads
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*
Bending moment & relief
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
Bending forces are controlled by..
g’s
and
loading
To ensure Bending Relief exists..
Maximum All up Mass must be > Maximum Zero Fuel Mass
Amount of Bending Relief provided by the mass of wing fuel is dictated by….
quantity of fuel and it distribution (set by operating procedures)
this eleviates the effect of the lift
Results of loads
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
how does an object react to force
Depends on the magnitude of the force
If stress is low - nothing happens
if stress is high - deformation occurs (Strain)
Elastic limit
is the limit of a material in accepting deformation and returning to it’s original dimensions once deforming load is removed
Elastic Deformation
when strain exists but it’s light and does not cause permanent damage
can be irreversable
Plastic demormation
when elastic limit is exceed and the object is permanently deformed
strain is high and object cannot return to its original dimensions/damaged
Young Modulus Elsticity
the relationship between stress and strain in an elastic material is constant
Dynamic Loads
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
Static loads
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
What is the ultimate stress point
the stress in where an application fails (breaks) in a single application in a static load
In flight, what are examples of forces / loads it experiences
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
what ‘g’ is the aircraft flying when on constant speed and level?
1g
if an aircraft changes ‘ATTITUDE’
the weight of the aircraft structure changes and so does the loads applied to it
if an aircraft is in an emergency, what happends to the load?
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**.
- the aircraft might produce a yawning movement, which needs to be corrected, adding** load onto the fin/rudder and fuselage**
Design Limit Load - DLL
is the maximum load airframe or component can experience in services (set by the manufacturer during design)
measured in ‘g’
what is the DLL for Air transport, aerobatic, and Utility aircrafts
Ai transport: +2.5 and -1
Utility: 4.4
Aerobatic: 6
What is Design Ultimate Load - DUL
it’s DLL * safety factor
yielding the maximum limit a structure can withstand without collapse
what is the minimum safety factor accepted
1.5 or (150%)
what is safety factor
it’s the ration of Design Ultimate Load to Design Limit Load
Safe life and life count
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
What is the Elapsed life count also called?
fatigue cycle
what is cyclic loading
it’s the repeated application and removal of load on a structure
*Also called dynamic reversable load
Structural failure is caused by…
**high cycle metal fatigue **
or
exceeding ultimate load failure
what is fatigue
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 **
Fatigue life
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
To minimize chances of failure due to fatigue..designers apply these 2 principles
Fail safe
damage tolerance
Stressed skin/Semi monocoque
Fail safe multiple load path is
**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
distadvantage of the (load path)?
it can be heavy
need disassambely to be able to inspect
Damage tolerance
**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.
how are faults detected:
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.
Stressed skin’ or ‘Semi-monocoque’ style
‘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.
At the time of airworthiness certificate..the following are agree upon…
The criteria of:
1)inspection cycles
2)Design Limit Loads, and Design Ultimate Loads
What is stress concentration
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
how to increase fatigue strength?
by removing defects where the fatigue cracks occur
What are station numbers
method of locating components on an aircraft
based on the order maintainance and repairs are carried out
station numbers for fuselage
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
station numbers for Wings
Wing stations are measured from the centre line of the aircraft and are also given in inches left or right of the centre line.
Vertical & Horizontal station linnumberses
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.
which of these are prone to stress in Order?
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.
Techniques to reduce loads
- carry no more than required fuel
- minimize angle of banks and anticipate your turns
- do not land in excessive rate of descend
- aviod harsh handling of controls
5.reduce speed when in turbulence and ensure you are not over -controlling the aircraft when in turbulence
to avoid undetected structural weakness..crew should..
report any forces or g excursions experienced on flight that might be above the design load limit
There is also structural mass limitation to be set for an Aircraft..why?
because aircraft must be strong enough to carry a certain maximum mass and ..
how much this is depends on the phases of flight
What are the structural mass limits
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
