Automatic flying control systems 7725

Question 1

with respect to F-35B what does “STOVL” stand for

Answer 1

short take off and vertical landing

Question 2

what are the 3 main components of the F-35B lift system

Answer 2

lift fan

roll post

3 bearing swivel system

Question 3

what are the specifications of the lift fan

Answer 3

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

Question 4

explain the 3 swivel bearing system

Answer 4

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

Question 5

what is the purpose of the roll posts on the F-35B

Answer 5

aircraft roll contol and stability,

roll post are mounted in the wings of the aircraft which provide 1,950lbf of thrust each

Question 6

how is the air from the turbo fan engine utilised during hover

Answer 6

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

Question 7

how many FADEC systems are there, and how are they used?

Answer 7

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

Question 8

turbulance

Answer 8

an irregular motion of air resulting from eddies, vertical and horizontal currents

Question 9

impact turbulance

Answer 9

caused by subsonic aircraft creating pressure waves outewards in all directions, which the aircraft then flies into

Question 10

mechanical turbulance

Answer 10

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

Question 11

therma (convective) turbulance

Answer 11

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

Question 12

frontal turbulance

Answer 12

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

Question 13

wind shear

Answer 13

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

Question 14

coriolos effect

Answer 14

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

Question 15

with reference to pressure gradient force how does air move

Answer 15

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

Question 16

what is the shape of the earth and how does it effect gravity

Answer 16

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

Question 17

EÖTVÖS phenomenom

Answer 17

• 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

Question 18

EÖTVÖS effect is week and strong

Answer 18

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

Question 19

static instability

Answer 19

an aircraft with static instability will depart from a Datum position in a uniform manner

Question 20

dynamic instability

Answer 20

this is when an aircraft oscillates about a Datum conditon, and the intensity of the oscillations increases until destruction

Question 21

expalin the referance frames

Answer 21

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

Question 22

Auto-stabs

Answer 22

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

Question 23

auto-pilot

Answer 23

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

Question 24

expalin the different types of damping

Answer 24

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

Question 25

short term errors

Answer 25

an aircraft may be temporarily displaced from it datum flight path by;

1. local varitaions of air pressure (turbulance)
2. 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

Question 26

long term errors

Answer 26

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

1. change in aircrafts centre of gravity due to fuel usage, release of stores and weapons
2. change in weight with fuel usage and stores release
3. 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

Question 27

micro-electro-mechanical (MEMs) accelerometer construction and operation

Answer 27

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

Question 28

hemispherical resonator gyro (HRG)

Answer 28

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

Question 29

micro electric mechanical (MEMs) gyro

Answer 29

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

Question 30

what is an algorithm

Answer 30

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

Question 31

what is redundancy and active redundancy

Answer 31

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

Question 32

tolerance

Answer 32

defined as the permissible range of variance in a dimension or measurement of an object or sensing device.

Question 33

what is kalman filtering (linear quadratic estimation (LQE))

Answer 33

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.

Question 34

how kalman algorithm works

Answer 34

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)

Question 35

electro pneumatic actuator

Answer 35

• 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

Question 36

electro-hydraulic actuator

Answer 36

• 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

Question 37

electro mechanical actuator

Answer 37

• 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

Question 38

electro-hydrostatic actuator

Answer 38

• 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

Question 39

series actuator

Answer 39

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

Question 40

what properties does a series actuator follow

Answer 40

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

Question 41

parallel actuator (auto-trims)

Answer 41

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

Question 42

trimming

Answer 42

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

Question 43

form of trim manual manoeuvres

Answer 43

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

Question 44

manual trim

Answer 44

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

Question 45

systems /autotrim

Answer 45

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

Question 46

hydrauliic shematic diagrams

Answer 46

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)

Question 47

feedback

Answer 47

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

Question 48

what is LVDT

Answer 48

linear variable differential transformer

Question 49

negative velocity feedback

Answer 49

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

Question 50

feedworward

Answer 50

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

Question 51

control law

Answer 51

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

Question 52

what is electronic noise

Answer 52

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