landing gear Flashcards
functions
- to absorb kinetic energy of landing and reduce vibration
- provide means of controlling landing/descend and braking
- maneouvre on ground
- support the aircraft at a convenient height that gives clearance to propellor and flaps
- facilitate loading
Once airbone, landing gear is dead weight and unuseful.
Fixed landing gear
Found on:
light & slow and simple aircrafts/some larger ones
Fixed to wings or fuselage
Fixed means:
non-rectractable
Pros:
light weight
simple, low maintenance, low initial cost
cons
Lowered performance due to drag
3 Fixed landing gear types
Found on:
light & slow and simple aircrafts/some larger ones
Fixed to wings or fuselage
Fixed means:
non-rectractable
Pros:
light weight
simple, low maintenance, low initial cost
cons
Lowered performance due to drag
types of fixed landing gear
Cantiliver Spring Steel Leg
* Leg is a tube/strip of steel
* Employed at the mainundercarriage
* the upper end attached by bolts to the fuselage
* lower end terminating in an axle on which the wheel and brake are assembled.
Rubber Bungee / Cord
* absorbs shock and directs the shock to the rubber
* the undercarriage is usually in the form of tubular struts
* designed and installed so that the landing force is directed against a number of turns of rubber in the form of a grommet or loop.
Oleo-pneumatic strut
- Uses combination of gas and hydraulic fluid to absorb shock
- Found in advanced light aircrafts
- in some fixed main undercarriage
- in all nose undercarriage
- in all rectractable gear
- Struts come in differnent forms
- fitted with Spats
what is a spat
found in:
Oleo pneumatic struts
it’s an aerodynamic fairing required to minimize drag
con:
they capture mud from grass when landing/taking off = adds to the weight of the aircraft and affect it’s landing/take off peroformance
how often must a spat be cleaned
before every take off
Construction of the Oleo penumatic strut
- consist of one outer cylinder attatched to the airframe
- It houses the Piston assembly
- It houses the inner cyclinder
- Inner cyclinder is free to rotate & move up and down within the outher cyclinder
- the movement is limited/controlled by Torque link (which connects by cyclinders together on the outside)
- Gas and Fluid are spearated by free-flowing separator piston
- Above the separator= Hydraullic fluid
- Below the separator = Filled with compressed gas ( nitrogen gas or air)
- inner cylinder is attached to the axil
Torsion load on pneumatic struct
the torsion load is absorbed by the Torque link when maneuvering on ground
the smaller the turn the aircraft does, the more load is felt on the torque links
turns should be made as wide as operationally possible
role of compressed gas in the strut
supports weight of the aircraft on ground
cushions pumps during taxxing
absorbs shocks on landing
role of the fluid in the strut
to dampen oscillations
control rate of compression and extension of the cylinders
the piston connected to lower cyclinder
has holes in it to restrict the flow of fluid through the piston
therefore this dampens the movement between the 2 cyclinders
Oleo pneumati strut when static
When static:
* the weight of the aircraft is balanced by the strut gas pressure
* inner cylinder takes up a position approximately midway up its stroke.
Oleo pneumatic strut when taxing (moving on bumps)
Bumps are cushioned by the gas pressure in lower cyclinder
and dampened by the limited flow of fluid in the orfice
oleo pneumatic strut after take off
the gas pressure will cause the inner cyclinder to extend to its fullest extent
oleo pneumatic strut after when landing
The strut is shortened
fluid pushed thru the flutter valve (in outer cyclinder)
this restriction limites the speed at which the strut compresses
As the internal volume of the cylinders decrease, air pressure rises till it dampens the force of the landing aircraft
as the upward force decreases, the gas pressure acts as a spring and extends the inner cylinder. The speed of extension is limited by the restricted flow of fluid through the orifice
how to identify gas pressure leakage in the strut?
the strut will not expend to its fullest extent
uneven amounts of **fescalized metal **will show on each gear (shiny material forms)
the shiny part will indicate the portion that has the gas leak
Rectractable landing gear
in majority of modern aircrafts
in some light aircraft
Improve performance as they reduce/eliminate drag in high speeds
- use hyraulics for retraction and extension
*heavy weight due to mechanism and stowage
Types:
Nose gear
Main gear
Bogie gear (multiple wheels set up)
Retraction and extension
Rectraction:
usually hydraullic
But sometimes pneumatic or electrical system
Extension:
In some cases affected by gravity or slipstream (while rectraction is powered)
mechanical locks & gear devices
Locks:
* ensure rectractable gear is secure under each undercarriage
* Undercarriage wells are normally sealed by doors for aerodynamic reasons.
devices:
* indicate to the crew the position of each undercarriage;
* means by which the landing gear can be extended in the event of failure of the power source.
* means provided to prevent retraction with the aircraft on the ground
* means to guard against landing with the landing gear retracted.
Tricycle layout of the gear
Nose gear at front
- allows aircraft to stay level
- can be stirred for maneuvering on ground
- prevents aircraft from tipping over if there is a strong tailwind
- prevents from ground looping
- gives better visibility forward for the pilot for manoevering
the 2 main undercarriages
* are just ‘after centre of gravity’
* this supports 90% of aircraft’s weight and absorbs landing shock
in some, a tail might be used/or tail skid
to help with skidding or to prevent tipping
Tail dragger
Used on older planes to give more clearance to the propeller
helps aircraft not tip on its tail
tail gear at back
the 2 main undercarriages are just ‘after centre of gravity
needs extra training to land/take off
factors affection the layout of rectractable gears
Size of aircraft.
Weight of aircraft.
Role of aircraft.
High or low wing.
Performance.
Construction of aircraft and associated stowage problems.
cost
ability to be ‘multi-role’
height off the ground ( to ease loading of passengers and planes, rectractable is better, otherwise the aircraft doors are too high to reach and geears would be too long to be fixed)
Under the wing landing gear have:
- The leg (pin jointed to the aircraft)
- the wheel
- a means to absorb landing shock
- a means to control deaccleration
- a means to withstand turning and break stresses
in boeing, the main gear assist with steering during tight turns by reducing the turning radius. When the nose wheels are turned the main wheels turn in the opposite direction
Fuselage Mounted Landing Gear have
*same as wing landing gear….but..
1) there is no geometric lock, so provision is required to lock the undercarriage up and down
2)depending on the **wheels, they may need shock absorbers **or steering motors
3)can access the fuselage undercarriage to **lower the gear manually **in case of emergency.
types of load on landing gear
Torsional: ground movement
forward: during pushback
compression: landing or static state
side: during sidewind, taxing and take off
rearward bending
Nose undercarriage traits
Lighter than main unit
only withstands compression
carries less weight
has towing attachments
undergoes shear load (from towing)
Design aspects of nose gear
1. Castoring:
* Wheels must castor (move) freeliy through subjected compression and shear loads or else it creates bearing issues.
* Catoring is Ability for nose wheel to turn to either side in response to braking, or aerodynamic forces on the rudder.
Self centring
* Automatic self-centring so that it can align itself to be stowed properly in the stowage, otherwise the space won’t fit and will cause damage to the aircraft structure as the hydraulic system forces the gear upwards
* Need spring loaded cam or hydraullic dashpot
Shimmy
A vibration induced to the nose wheels because of:
* flexible tyre walls
* Worn or broken torque link
* wear in the wheel bearings
* uneven tyre pressures
To reduce shimmy:
* maintain good tyre pressure
* double nose wheel
* twin contact wheel
* Provision of a hydraulic lock across the steering jack piston.
* Fitting a hydraulic damper.
* Fitting heavy self-centring springs.
Withstanding sheer loads
Nose wheel steering
Nose wheel steering
Simple steering
* for light aircraft
* the nose wheel is mechanically linked to the rudder pedals
Powered steering
* Uses ‘powered steering’ - on large aircrafts
* Power steering uses a hydraullic system
Power Nose wheel steering
Nose wheel rotated by electric, pneumatic or hydraullic system (most common)
- Allowing the engines to be set at the minimum thrust for taxiing,
- saving fuel,
- reduces tyre and brake wear
- reduces noise pollution.
Steering is controlled, depending on the type of aircraft, by:
- A separate steering wheel.
- Operation of rudder pedals.
Incorporated in the steering system are:
* Self-centring jack.
* Shimmy damper.
power steering design
In hydraullic system:
- a cockpit steering wheel or tiller
- a control valve
- steering cylinders to turn the nose gear
- a mechanical feedback device to hold the steering at the selected angle
- a power source, normally the aircraft hydraulic supply fed from the engine driven pumps
Undercarriage considerations
the size of aircraft and the ALL UP WEIGHT lead to the increase in wheel loading
This is the load on each wheel at take off weight
high wheel loading can affect the runway surface (ie it can damage a low strength runway)
solution - to spread the load:
replace large single wheel with high pressure tyres with small wheels with lower pressure
configuration of undercarriage relies on
the stowage when retracted
the load spreading consideration
Muti wheel benefit
Weight: small wheels weigh less
Ease of service
safety factor if one bursts, there are others to carry load
easy of stowage
Multi wheel disadvantage
small tight turns can cause:
- Larger foot print causing crabbing while turning
- tyre wear due to scrubbing (treads become worn off)
runway contamination affecting gears
Might be cause from slush which is deposited during take off, and freezing during climb making it difficult to retract or extend the gear.
to solve:
* cycle the gear after take off
* select gear up , down and up
* the shocks inflicted on the gear during this cycle should be sufficient to remove any deposits from it.
Hydraullic gear retraction system
- Hydraulling system takes its power from engine powered pumps + alterntive system
on small aircraft, there is a self contained ‘power pack’ which houses:
* a reservoir and selector valves for the landing gear and flap systems;
* an electrically driven pump may also be included,
* or the system may be powered by engine driven pumps.
- This type of system normally provides for powered retraction of the landing gear, extension being by ‘free-fall’, with the assistance of spring struts.
restrictor valves
Benefitial for main gears:
helps limit the speed (rate) of lowering of the main undercarriage units, which are influenced in this direction by gravity.
Not beneficial for the nose gear
The nose undercarriage often lowers against the slipstream and does not need the protection of a restrictor valve.
Pneumatic retraction system
Operation of a pneumatic retraction system, is similar to that of a hydraulic system
, except that pressure in the return lines is exhausted to atmosphere through the selector valve.
electrical gear retraction system
light aircraft
do not require the use of a high pressure fluid system.
The main and nose undercarriage units push and pull forces on the retraction mechanism are obtained by an electric motor and suitable gearing
Gear position indication
1)An electrical indicating system used to provide a positive indication to the crew of the operation of the locks and of the position of the landing gear.
The system usually consists of microswitches on the uplocks and downlocks,
there is also
2) magnetic indication
3)light indication
electrical indication colours
Green: locked down
Red: In transit
No colour: Locked In
*bulbs are duplicated to avoid confusion or false indications in case of bulb failure
in some aircraft:
Green: lock down
Amber: Locked up
Some aircrafts also show ‘Gear Door” position
Gear safety features
Nose wheel centring
* always be aligned in a fore (front) and aft (rear) direction during retraction
* hydraulic nose wheel centring on aircraft with powered steering.
Gear selector lock
device that prevents movement of the selector lever to not accidentally retract whilst gear is on the ground
Ground locks
pins or metal sleeves that interfer with unpowered aircraft
intended to prevent collapse of the gear when the aircraft is unpowered on the ground.
They are fitted with warning flags which should prevent the crew from getting airborne with them still in position on the gear.
warning devices
* a horn connected to a throttle operated switch.
* If one or more throttle levers are less than approximately one third open, as would be the case during approach to land, the horn sounds if the landing gear is in any position other than down and locked.
A horn isolation switch
*often provided to allow certain flight exercises and ground servicing operations to be carried out without hindrance, but
*can be used and forgotton by accident
- an airspeed switch
- is a definite advantage, since unlike an isolation switch, it cannot be first used, and then forgotten, with perhaps disastrous consequences.
- An airspeed switch can also be used to prevent the horn sounding during initial descent from high altitude.
**GPWS - ground proximity warning system
**will be inhibted belelow 500ft and flaps are in landing position
Emergency lowering system
Is having another means of extending the landing gear and locking it in case the main system fails.
two types:
Uplocks are released mechanically or electrically by manual selection.
* **The landing gear ‘free falls’ **under its own weight (gravity) and the downlocks are engaged mechanically.
- If the main source of power to the gear has failed, there is no power to retract them after they have been released,
- the doors will remain open.
- The doors may contact the ground upon touchdown
- Some aircraft have doors fitted with a frangible portion at their lowest extent so that replacement problems are minimized.
aircraft the landing gear is extended by an emergency pressure system
Pressure for the emergency system may be supplied by a hydraulic accumulator, a hand pump, a pneumatic storage cylinder, or an electrically powered pump.
A Mechanical Indicator will be provided to indicate gear locked down.
Landing Gear operating speeds
VLO: Max Velocity for Landing Gear Operation
VLE: Max Velocity for Landing Gear Extended
When the **landing gear is fully retracted or fully extended **it is locked into position and is **more resistant to damage from high airspeeds. **
When the landing gear is in the process of extending or retracting (operating) there is no locking mechanism, and the only thing resisting the airflow is the extension/retraction mechanisms.
Additionally, on some aircraft, the landing gear may swing or swivel in odd directions in order to tuck into their recesses, this can cause odd aerodynamic behaviour in the rest of the aircraft if done at high speeds.
Once the landing gear is extended, it is rare that a pilot would then exceed V LO . Most of the time the landing gear is lowered shortly before landing and the pilot is doing everything he can to slow the aircraft further.
If aircraft had to be flown a long distance with the landing gear extended (such as a ferry flight to a repair facility) the pilot would go ahead and fly V LE .
Air/Ground logic system
microswitches
position will be changed when the weight of the aircraft compresses the oleo, or alternatively, on take-off, when the weight of the wheel and bogie assembly extends the oleo.
On more modern aircraft,- proximity sensing devices
deduce the extension or retraction of the oleo by capacitive or inductive sensing equipment fitted to the oleo.
Whichever system is used, a controlling signal will be sent to a relay or bank of relays, which in themselves are capable of switching the affected circuits on or off as required.
Some aircraft use sensors on just one main landing gear oleo, but it is common to find the sensors duplicated on both main oleos to provide a degree of redundancy in the system.
Shock absorbers
Main gear absorbs shock and transfers it to fuselage/airframe via the tyres but mainly by the gear’s shock absorbers
kicking off
main gear handles vertical load but it can also handle, but not well, some side-load created by crosswind
great care need to minimize side load by
- turning at slow speed
- removing/kicking off any drift before touchdown
nose gear doesn’t absorb?
doesn’t absorb landing shock..
since it’s not in the centre of gravity, you **cannot land on it **as it will experience great bending load
oleo strut system
Outer cylinder
- contains hydraulic fluid
- attached to airframe
- actis as a dampner to spring action
between the two:
-Orfice allow flow of fluid between the two cylinders
- Rate of strut can compress dependso on the speed of flow of fluid throug the orific
**Inner cyclinder: **
- contains nitrogen
- attached to wheel axis
- acts as a spring to absorb load
what’s the gas pessure like when aircraft is stationary in the Oleo
gas pressure = weight force of the aircraft
any additional force on aircraft, puts pressure on the gas
oleo extention is adjusted by:
gas pressure in the leg
excessively compressed oleo indicates:
low gas pressure + fluid leak
check for fescalized metal (shiny metal) on the strut.
Shiny part indicates low pressure
Landing gear locks
Up lock:
-operaterd by hydraulic pressure/actuator
locks up the gear
Down lock
Pins fitting to Prevents gear from retracting when on ground accidentally
geometric
locks gear down.
doesn’t need hydraullic power - its mechanical asssted by weight of gear
Note: to break this, you need hydraulic actuator
Landing gear operation
1) Gear lever:
when gear is downlocked and aircraft has its weight on the ground or there are sensors that determine that it’s close to the ground - then there are** safety latches/selenoid switches/Weight on Wheels’ that prevent the level to be pulled up**
it can only be overwritten by pulling a safety latch in certain situations
2)Sounding:
when aircraft is configured for landing, there are warning horns that ‘sound’ ** if gear is not down **
gear doors may show light signal on the dahsboard
3)Lights
Red - gear is unlocked & traveling
Green - locked down
No lights - gear up/locked
RED + GREEN. = if emergency is carried out
DOOR UNLOCKED (light after
