Automatic flying control systems 7725 Flashcards
with respect to F-35B what does “STOVL” stand for
short take off and vertical landing
what are the 3 main components of the F-35B lift system
lift fan
roll post
3 bearing swivel system
what are the specifications of the lift fan
the clutch and bevel gearbox provides the lift fan with 29,000 shaft horsepower
the lift fan has a 50-inch (127cm) diameter 2 stage counter rotating fan,
utilising hollow bladed disk technology, producing 20,000lbf of thrust
explain the 3 swivel bearing system
the 3 BSM is a swivelling jet pipe,
it redirects the main engine thrust downward to provide the rear vertical lift,
the jet pipe can rotate through 95 degrees in 2.5 seconds and produces 18,000lbf of thrust
what is the purpose of the roll posts on the F-35B
aircraft roll contol and stability,
roll post are mounted in the wings of the aircraft which provide 1,950lbf of thrust each
how is the air from the turbo fan engine utilised during hover
the F-35B main engine is a low-bypass turbofan engine
some of the air from the intake at the front is not mixed with the main engines core, for fuel and burned but bypasses it to flow outside the casing
during hover some of the bypass air is directed into the roll post ducts for roll control stability while performing vertical take off or landing
the F-35 needs a short runway for take off when fully loaded and provides enough thrust for a verticla takeoff when lightly loaded
FADEC varies the thrust through each roll post independently tyo allow the pilot full control over the aircraft while hovering
how many FADEC systems are there, and how are they used?
the F-35B uses uses four FADEC systems
two for the main engine to ensure complete redundency, of operation
two for the lift fan system software is the main technology that makes the STOVL work
turbulance
an irregular motion of air resulting from eddies, vertical and horizontal currents
impact turbulance
caused by subsonic aircraft creating pressure waves outewards in all directions, which the aircraft then flies into
mechanical turbulance
caused by friction between the air and ground amplified by the irregular terrain and man-made obstacles
- this eddy motion is affected by the wind speed
- the greater the wind speed the greater the turbulance
therma (convective) turbulance
turbulance can also be caused by uneven heating of the earths surface
certain surface such as barren ground, rocky and sandy areas are heated more rapidly than grass covered surfaces
isolated convective currents are set in motion by wam air rising and cool air descending
turbulance is extends from the base to the top of the convection layer with smooth conditions found above it
if cumulus, tower cumulus or cumulonimbus are present the turbulent layer extends from the surface to the cloud tops
turbulance increases as convective updraft intensity increases
frontal turbulance
the lifting warm air by the sloping frontal surface and friction between the opposing air masses produce turbulance in the frontal zone,
the turbulance is most marked when the warm air is moist and unstable and will be extremely severe if thunderstorms develop.
turbulance is commanly associated with cold fronts but can be present to a lesser degree, in warm front as well
wind shear
wind shear is the change in wind direction and/or wind speed over a specific horizontal or vertical distance
atmospheric conditions where wind shear exists include:
areas of temperature inversions
along troughs and lows
around jet streams
coriolos effect
the coriolis force is an inertial phenomenom, that acts on objects that are in motion relative to a rotating reference frame.
in a clockwise reference frame (southern hemisphere) the force acts left of motion of object
in anti-clockwise refernce frame (northern hemisphere) the force acts right of motion of object
with reference to pressure gradient force how does air move
air will move from areas of high pressure zones to low pressure zones
and due to bernoulli’s law will speed up as it goes
on a stationary planet, the air will flow in a straight line
but since the earth is rotating the coriolis effect causes the air to rotate
the coriolis force is greatest at the poles and zero at the equator
what is the shape of the earth and how does it effect gravity
the earth is an ‘ellipsoid of revolution’ the rotation of the earth causes it to flatten at the poles and bulge at the equator (oblate ellipsiod)
the nominal value for gravity is 9.8m/s²
EÖTVÖS phenomenom
- this is the perceived gravitational force, caused by the change in centrifugal acceleration
resulting from velocities either East or Westward.
when moving eastward the objects angular velocity is increased (in addition to the earths rotation)
and thus the centrifugal force increases, causing a perceived reduction in gravitiational force
- when traveling eastbound the aircrafts velocity is added to the earths rotational speed landing to an increase in centrifugal force, and therefore a decrease in centripetal force, thus a perceived reduction in weight and subsequent reduction in perceived gravitiational force
when westbound the reverse is true, and the aircraft appears heavier for the reason detailed above
EÖTVÖS effect is week and strong
is greater at the equator and weekest at the poles, due to the earths rotaional speed being fastest atg the equator, 1040mph reducing through the higher latitudes
static instability
an aircraft with static instability will depart from a Datum position in a uniform manner
dynamic instability
this is when an aircraft oscillates about a Datum conditon, and the intensity of the oscillations increases until destruction
expalin the referance frames
ground reference frames can be further divided into two,
EAST, NORTH AND UP (ENU)
NORTH, EAST AND DOWN (NED)
ENU is used for ground vehicles and ships
NED is used for aircraft
Auto-stabs
auto-stabs provide stability in the pitch, roll, yaw axes,
by detecting aircraft motion within the body reference frame
via sensors and applies corrective control inputs
via the actuation system to the flying controls
auto-pilot
the auto-pilot provides a hands-off flight path control using the body reference frame in respect to the ground reference frame
subject to the limitations of design control laws
using the aircraft motion sensor inputs and signals from external systems sensors to the AFCS
such as steering commands from the navigation system, then applying corrective control commands through the auto-stabs function
expalin the different types of damping
overdamped
the system returns to equilibrium without oscillating, but slowly
critically damped
system returns to equilibrium as quickly as possible without oscillating
underdamped
the system oscilates (at a reduced frequencey compared to the undamped case) with the amplitude gradually decreasing to zero
undamped
the system oscillates at its natural resonant frequency without experiancing decay of its amplitude
short term errors
an aircraft may be temporarily displaced from it datum flight path by;
- local varitaions of air pressure (turbulance)
- gust of wind
short term errors are detected by sensors, this error signal, containing rate and displacement information is passed to the computation element which outputs a signal to deflect the relevent control surfaces by the appropriate amount to bring the aircraft back to flight datum. the autostabs detect, compute and return the aircraft to datum in typically, less than a second
long term errors
an aircraft may also be displaced from its datum flight path due to lasting deviations. this type of permanent displacement is called long term errors such as
- change in aircrafts centre of gravity due to fuel usage, release of stores and weapons
- change in weight with fuel usage and stores release
- prevailing winds
long term errors will be detected by the short term errors sensors. however for long term errors the control surface datum needs to be permanently biased by an appropriate amount
micro-electro-mechanical (MEMs) accelerometer construction and operation
green legs do not move
orange corners are fixed turning rest into springs
orange springs move against green legs and cause a change in capacitance
acceleration causes deflection of the proof mass from its centre position,
these fingures are positioned between plates that are fixed to the substrate.
each finger and pair of fixed plates make up a differential capacitor, the deflection of the proof mass determines the capacitance measured; the amplified output signals are proportional to acceleration.
this sensing method has the ability of sensing both dynamic acceleration and static acceleration (i.e. inclination or gravity). when mounted in the three planes, (orthogonally) the device can measure the acceleration along the pitch, roll and yaw axes
hemispherical resonator gyro (HRG)
the hemispherical resonator gyro has no moving parts and as such is not susceptible to wear that effects the mechanical gyro.
a standing wave is generated around the hemispherical resonator establishing a resonant vibration
the vibration is maintained by piezoelectric elements inside the high vacuum enclosure,
forms the node (low point) and antinode (high point) this primary vibration establishes a datum for the gyro.
if the gyro is disturbed from datum, the coriolis forces act on the vibration turning it by an angle proportional to the disturbance; and detected by the pickoff electrodes in the base
the phase shift of these secondary vibrations is continually monitored the HRG provides both dispaclement and rate information
micro electric mechanical (MEMs) gyro
isaac newton first law of motion (law of inertia)
an object at rest stays at rest and an objecvt in motion stays in motion with the samed speed and in the same direction unless acted upon by an unbalancing force
the oscillating bar is a peizo electric material, excited to move back and forth,
under newtons first law this movement continues in this plane,
there are normally 3 MEMs gyros mounted orthogonally (at 90º to each other.) to cover the pitch, roll and yaw planes.
with the unit just oscillating without an external disturbance the piezoelectric output from the oscillating bar is zero
if the reference frame is disturbed from the datum e.g. agust of wind, the coriolis force is felt between the oscillating bar and reference frame, the apparent distortion of the oscillating bar leads to a piezo electric output from the bar
what is an algorithm
defined as a set of rules or steps that precisely define a sequence of operation the computer carries out from the input to derive output
named after a 9th centery mathematician, Al-khwarizmi and the greek word arithmos
what is redundancy and active redundancy
it is having a second unit in case of a failure that can continue the work. i.e. if one light bulb fails the second is bright enough to light the room
active redundency is the second unit taking the work load of the first unit on as well as its own,
i.e. if the second light bulb doubled its brightness when the first failed
tolerance
defined as the permissible range of variance in a dimension or measurement of an object or sensing device.
what is kalman filtering (linear quadratic estimation (LQE))
it is an algorithm that uses a series of measurments observed over time, containing statistical noise and other inaccuracies, and produces estimates of unkown variables, that tend to be more accurate than those based on sensor measurments
A Kalman filter is an optimal estimation algorithm that estimate states of a system from indirect and uncertain measurements.
how kalman algorithm works
kalman gains
number between 0 and 1 that indicates the accuracy of either the estimaste value or the measured value,
a number closer to 1 shows that the measured value is more accurate,
closer to 0 shows that the estimate value is more accurate,
calculated by using error in estimates (Eest) and errors in measurement (EMeas)
current estimate
used to give the best possible actual value of position, velocity or other required value by using calculated previous estimates (EST time - 1) kalman gain (KG) and measurements (Meas)
electro pneumatic actuator
- uses compressed air or gas to move a piston,
- gas/air is produced by a small pump or is fed by a boittle or canister
- the main drawback of this actuator is thermal expansion of gas/air as it compressable
- the expansion causes the mechanical movement to be innacurate with wide tolerances
- electro pneumatic actuators are cheap to produce and used mainly in missile guidance systems
electro-hydraulic actuator
- construction /operation is similar to electro-pneumatic system but uses incompressable hydrualic fluid
- hydraulic fluid increase accuracy/predictability of operation
- usually fed from a reserviour and pump with associated pipework
- system is heavy, usually used in older aircraft
electro mechanical actuator
- using high power brushless DC motors and efficient gearing,
- enables forces of up to 2500lbf to be applied directly to control surfaces
- no requirement for gas or fluid as it uses powerful motors and gearing to produce movement
- due to lack of fluid there is no contamination/leak risk
- found on modern aircraft
electro-hydrostatic actuator
- this system uses both electric and hydrauilic fluid in a self contained package
- electric pump, control manifold are self contained in a single unit, making it line replaceable unit
series actuator
because the operation of the auto stabs is normallyl invisible to the pilot
the movement of the series actuator does not move the cockpit controls to achieve this
as it operates against the resistance of the stick position and centre device
the actuator is installed in series within the flying control
the actuator forms part of the control run
when not operation, or in failure, the series actuator reverts to a rigid link (fixed length) part of the flying controls
being installed within the aircraft control run their operation is linear
what properties does a series actuator follow
fitted in series with the flying control
fast acting in operation
authority is limited to minimise aircraft risk in the event of a system malfunction
actuator reverts to a rigit link when not operating
actuator does not oppose pilot inputs
parallel actuator (auto-trims)
the config of the actuator means its effect is the same as the pilot control column movement
the actuator moves the pilots controls and control surfaces
slower in operation
full authority, the same amount of control as the pilot
to minimise the risk in the event of a system malfunction, an electrical clutch is fitted, normally within actuator
through the action of the clutch or stick position and centring device signal the actuator does not oppose pilots inputs
enables an electro-mechanical method of trimming the aircraft position
trimming
its necessary for the pilot to amend datum position of the A/C to compensate for long term errors or, such as change in CofG or speed
form of trim manual manoeuvres
aircraft with an artificial feel, helicopters, the pilot can move the control column tightening the spring of the artificial feel to adjust the aircraft,
the spring force will return the aircraft controls, and therefore, the control surfaces back to datum after column is released
manual trim
used to make large, fast changes to datum
by moving the control column (manual manoeuvre) pressing the trim release switch will disengage the motors within the parallel actuator and allow the spring to unwind the the new datum,
or you can release the trim switch before moving the pilots control column
systems /autotrim
the auto-stabs systems corrects for short term errors using the fast series actuatorand slow parallel actuators, but due to the limited operating range of the series actuator, could easily run out of authority (tolerance range)
to prevent this, the systems/autotrim drives the parallel actuator motor to a position that maintains the series actuator at the mid-point of travel, therefore keeping them operating at their full range
hydrauliic shematic diagrams
working line, return line and feed line
pilot control line (long dashes)
drain or bleed line (short dasshes)
hydraulic flow (triangle)
spring (zigzag)
hydraulic pump with one direction of flow
ome flow path (box with arrow)
two closed ports (box with two T’s)
servo/pilot valve (3 boxes, cross, double T, opposite arrows, srping either end
non return valve (circle, sping two lines)
pressure relief valve, (box with arrow and spring)
accumulator (oblong)
filter or strain (diamond with dotted line)
feedback
feedback exist between two parts when each affects the other and occurs when the outputs of a system are routed back as inputs to effect or control the system, as part of a chain of cause-and-effect that forms a circuit or loop
what is LVDT
linear variable differential transformer
negative velocity feedback
this feebback provides damping to the aircraft prevent over-shooting, signal supplied by a tacho generator
negative velocity feedback is a convergent signal, where as positive velocity feebback is a divergent signal
feedworward
the problem with purely feedback systems is that they are reactive systems with the associated time lag between disturbance, computation and adjustment movements
if the system could be anticipatory it would be far quicker, feedforward is one method of achieving this anticipation
control law
the general objective of the aircraft control laws, is to improve the natural flying qualites of the aircraft, stability, control and flight envelope protection
flight envelope protection prevents the pilot or AFCS from exceeding the aircraft design parameters.
they also allow the aircraft to maintain flight after a component or system failure
control laws are computational laws stored within the computer
what is electronic noise
electronic noise is a characteristic of all circuits
caused by random fluctuations in electrical signals, noise generated by electrical devices and defects in semiconductor construction,
thermal noise is inevitable with circuits above temperature of absolute zero
