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DTC MSc Course - Systems Study

Helicopter Avionics


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9/13/2013 Department of Aerospace, Power &

Sensors

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Website

More detailed information on this topic

(and many others) may be found on:

www.rmcs.cranfield.ac.uk/aeroxtra


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Sensors

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Overview

Flight Instruments

Using pressure measurements & gyroscopes Attitude Indicator, Turn/Slip Indicator, Directional Indicator,

Altimeter, AirSpeed Indicator & Vertical Speed Indicator.

Displays CRT & LCD displays, Head-Up & Helmet-Mounted displays.

Navigational Aids NDBs & ADFs, VOR & DME, TACAN, Hyperbolic Area Navigation,

Doppler Radar, Inertial Navigation, GPS, JTIDS.

Sensors

Flight Control Systems

Databus Connection


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Flight Instruments

Requirement grew as aircraft performance increased with need

to operate in dark and in poor weather conditions. Many helicopter instruments are similar to those used on FW

aircraft.

Several important systems rely on principles of the gyroscope(constant reference position and precession) while others use

pressure measurements.


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Flight Instruments - Attitude Indicator

Attitude Indicator

This is one of the most important instruments for the pilot. It incorporates a spinning gyroscope; this has a bar which

remains permanently fixed in the horizontal position (the artificial

horizon).

Any vertical movement shows the pitch attitude while any tiltindicates the degree of bank angle being flown.

The top half is normally coloured blue with the lower half black,representing the sky and ground respectively.


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Flight Instruments - Turn & Slip Indicator

Turn & Slip Indicator

This comprises two separate instruments to show roll and yawmovement.

The turn indicator uses a rate gyro connected to a needle togive the rate of turn.

The slip indicator usually uses a simple pendulous weightsystem. The movement of a bubble between two white lines tellsthe pilot if he is using his flight controls efficiently to make a co-

ordinated turn or not.

In this case the pilot is making a turn to the

left; the bubble of the slip indicator is in the

centre, indicating a co-ordinated turn with

no slip.


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Flight Instruments - Altimeter

Altimeter

The basic altimeter is an aneroid barometer which uses anatmospheric pressure measurement and compares it with the

ISA values for an equivalent altitude.

A more sophisticated device is a radio or radar altimeter whichrelies on radio or radar wave reflection principles. These are

independent of atmospheric fluctuations and also give altitudes

relative to the land mass below.



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Flight Instruments - Airspeed Indicator

Airspeed Indicator

This uses a combination of static (p) and total pressure (po)measurements and converts the difference (dynamic pressure)

into a velocity, using:

V = [2(po-p)/ ]

If the sea-level ISA value for is used (1.225 kg/m3) then theairspeed is known as the indicated airspeed, which is similar to

the equivalent airspeed.

The true airspeed is obtained by multiplying by the (o/)



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Flight Instruments - Vertical Speed Indicator

Vertical Speed Indicator

This shows the rate of climb/descent. Particularly useful when landing through cloud and in poor

visibility.

Uses pressure difference measurement to give vertical speeddata.

Devices also contain devices to compensate against variationsin atmospheric pressure & temperature.


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Flight Instruments - Miscellaneous

The pilot also requires detailed information about many other

aspects of the aircraft, e.g. powerplants, transmission drives,weapons, lights, etc.

This can often result in a plethora of dials, switches and leversand a very cluttered cockpit area.

AH-64 Apache Bell XV-15


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Displays

Displays

The pilot usually has all the relevant flight information presentedon a large panel in front of him.

The amount of information has grown tremendously over theyears and has forced a rethink on display technology.

Nowadays the number of screens has been reduced with each

capable of presenting more than one item of information (i.e.multi-function displays orMFDs).

This is commonly known as a glass cockpit.

RAH-66 Comanche using only2 MFDs for most navigational

and instrumentation needs.


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Displays - CRT

Cathode Ray Tube (CRT) Displays

These operate in the same way as a conventional TV or PCmonitor.

A CRT is dependent upon the critical alignment of the systemsguns & screens and so is quite fragile in a high vibration

environment, as on a helicopter.

It is also rather bulky (requires a lot of depth behind the screen)and uses up a lot of power.

A Cathode

B Conductive coating

C AnodeD Phospor Coated Screen

E Electron Beams

F Shadow Mask


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Displays - LCD

Liquid Crystal Displays (LCDs)

These work by reflecting ambient light from the back of a panelback to the observer.

A liquid crystal layer at the front of the panel is divided intopixels. A voltage is applied to each which alters the polarisation

of the light passing through it - if three separate layers are used

(red, green & blue), a coloured image is produced. Compared with CRTs they are more compact (flatter), lighter

and consume less power.

However, they are more costly, have a smaller viewing angleand are difficult to manufacture to a large scale with adequate

resolution (each pixel requires a separate transistor). A colour 1024x768 pixel screen therefore has

1024x768x3=2359296 transistors embedded into the screen.


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Displays - HUD

Head-Up Displays (HUDs)

Used extensively on FW fighters and consist of sheet of glassmounted and reflecting a CRT display from instrument panel

into pilots field-of-view.

This can significantly reduce distractions.

Overall system also includes sophisticated optical elements for

collimating the projection (I.e. presenting it at infinity so that pilotdoes not have to change his focus).

This figure shows the HUD used on a

Cobra (AH-1F).

Nowadays increasingly

commonplace on helicopters.


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Displays - HMD

Helmet-Mounted Displays (HMDs)

A miniaturised form of the HUD - entire system is mounted onpilots helmet so that view is always in pilots vision.

First used on FW US Navy Phantomfighters in 1973.

Accuracy, weight, CG and separation (for

safe ejection) issues are key technology

areas.

Major companies involved in development include: KaiserElectronics, Elbit, Honeywell & Sextant Avionique.


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Displays - HMD

Helmet-Mounted Displays (HMDs) (2)

Major application is for target acquisition of AAM - these

can then be aimed by mere movements of the pilots head.

Used effectively on AH-64D Apache where external

turret-mounted low-light and IR sensors transmitinformation back to HMDs of pilot & gunner.


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Navigational Aids - NDB/ADF

Non-Directional RadioBeacons (NDBs) & Automatic

Direction Finders (ADFs) This is the oldest type of radio navigational equipment still in

service today, operating in the 190-535 kHz frequency range.

The transmitters are cheap to install and operate and

consequently often found at small airports.

The airborne receiving equipment is anADF which uses a

needle to continuously point in the direction of the NDB antenna

signal it is tuned to receive.

The number of available NDBs is in steady decline.


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Navigational Aids - VOR/DME

VHF Omni-Directional Radio (VOR) & Distance

Measuring Equipment (DME) This is the main navigational system adopted by the civilian

sector, working in the 108 to 118 MHz band.

Two signals are sent out by the transmitter - a reference signal

in all directions and a variable one rotating through 360o.

Equipment then detects which radial line aircraft is flying on.

Restricted to line-of-sight operation and there is also a range

limitation (about 40 km at low altitude).

Many VOR also have UHF DME - this sends a signal to theground station which then replies. Time delay is converted into a

slant-range distance and then into a horizontal distance using

known altitude information.


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Navigational Aids - TACAN

Tactical Area Navigation (TACAN)

This is the main navigational system adopted by the military,with similar operating principles to the civilian VOR-DME.

TACAN ground equipment consists of either a fixed or mobiletransmitting unit.

The airborne unit in conjunction with the ground unit reduces thetransmitted signal to a visual presentation of both azimuth and

distance (bearing and slant-range) information.

TACAN is a pulse system and operates in the UHF band offrequencies.

Its use requires TACAN airborne equipment and does notoperate through conventional VOR equipment.


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Navigational Aids - Hyperbolic Area Navigation

Hyperbolic Area Navigation

This uses low-frequency radio technology.

A pair of transmitters is positioned several hundreds of miles

apart and emit identical pulsed signals. These are received on

the aircraft and on-board equipment converts the data into a

hyperbola.

This is repeated with another pair of stations and the

intersection of the two curves gives a direct positional fix.

The main system in use is known as LORAN-C.

It is now gradually being replaced by GPS, though is still usually

retained as a back-up system.


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Navigational Aids - Doppler Radar

Doppler Radar

Gives an accurate measurement of the helicopter speed overthe ground.

Principle of operation involves emitting an array of radar beamsfrom beneath the fuselage and receiving their reflections.

Any relative movement between the emitter and reflector willcause a change in frequency due to the Doppler effect and this

can then be converted into the flight speed.

Provided that the heading is accurately known then the aircraftposition may be tracked.

This type of system is employed on the Lynx Tactical AreaNavigation System (TANS). The system also offers other

navigational aids such as the calculation of routes to targets,

etc.


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Navigational Aids - IN

Inertial Navigation (IN)

Uses acceleration measurement and then numerical integrationmethods to derive values for velocity and displacement.

Three independent accelerometers are required to provideacceleration values along the three axes.

Knowledge is also required of the attitude (yaw, pitch and rollangles) in order to calculate displacements in terms of earth-

centred latitude and longitude (done with gyroscopes).

Overall system is generally fixed relative to the airframe andthen known as strap-down.

The accelerometers and gyros used need to be extremelyprecise otherwise significant calculation errors will result.

Because of high accuracy and precision requirements, INsystems tend to be very expensive.


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Navigational Aids - GPS

Global Positioning Systems (GPS) GPS is now globally accepted for many civil navigational applications

involving land, sea and air use. The US DoD originally developed it for military use but it has since

been made freely available on a widespread and low-cost basis.

The operating principle is similar to that of hyperbolic navigation inwhich a position fix is made from four or more transmitters.

In this case, however, the transmitters are located onboard a numberof US (Navstar) and Russian (Glonass) satellites orbiting the earth at ageostationary altitude of 21300 km.

Communication is made via direct line-of-sight, allowing increasedaccuracy using a high frequency signal of 1.5 GHz.


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Navigational Aids - GPS

Global Positioning Systems (GPS) (2)

A coarse acquisition code is made available to users,giving a typical accuracy of 100 m, while a high precision

code is used by the US military which is accurate to just a

few metres.

A hand-held GPS receiver can now easily be bought for

less than 200.

Differential GPS uses a fixed ground station of knownlatitude and longitude.

This receives the GPS position fix and calculates the error.

This is then transmitted via VHF radio to any other users inthe vicinity.


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JTIDS & MLS

Instrument Landing System (ILS)

This provides precision vertical & lateral navigation information. The ILS consists of a localizer operating in the 108-112 MHz

band, a glideslope operating in the 328.6-335.4 MHz band, andassociated marker beacons operating at 75 MHz.

Microwave Landing System (MLS) This improved system is being designed to replace ILS; it is lessaffected by terrain, structures & weather.

It operates in the 5000-5150 MHz range with associated DME in960-1215 MHz band.

Joint Tactical Information DistributionSystem (JTIDS)This is a combined US/NATO navigation/communication

system and operates on 51 frequencies in the 960-125 MHz

frequency range.


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Sensors - FLIR

Forward Looking Infra-Red (FLIR)

These use sensors similar to a common video camera but whichare also sensitive to the low frequency infra-red light invisible to

the bare human eye.

All matter emits infra-red energy at a frequency which dependsupon the surface temperature.

An IR camera therefore detects surface temperature.

Such systems are severely restricted by the presence of waterparticles in the atmosphere, i.e. in rain and fog conditions.

FLIR sensors are designed to operate at one of two

wavelengths - 3 to 5 or 8 to12 microns. These are "atmosphericwindows" where water vapour in the air is much less of a

problem.


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Sensors - II

Image Intensification (II)

II works by amplifying the visible received light up to observablelevels.

They are most commonly found in the form ofnight visiongoggles (NVGs), though larger and more sensitive devices are

also often fitted to the main helicopter structure


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Sensors - Longbow Apache

AH-64D Longbow Apache Sensors

The sensors suite fitted to the Longbow Apache is highlyimpressive.

Firstly there is the distinctive Longbow radar dome fitted on topof the mast, used to detect surrounding ground forces, aircraft

and buildings. This uses millimetric radio waves that can make

out the shape of anything in range. The radar signal processor then compares these shapes to a

database of tanks, trucks, aircraft, etc to classify each potential

target.

The computer also pinpoints these targets on the pilot's and

gunner's display panels. This data is complemented by a mixture of day and night image

enhanced information from the sensors.


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Sensors - Longbow Apache

AH-64D Longbow Apache Sensors


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Flight Control Systems - SASs

Stability Augmentation

Many helicopters are fitted with stability augmentationsystems (SASs).

These improve the helicopter's stability without adverselyimpacting upon its control characteristics.

The pilot's control inputs are supplemented with inputs

proportional to the helicopter's motion. The complete system therefore comprises:

a sensor (rate gyro or accelerometer) to measure thehelicopter's motion,

an actuator to provide the control input, and

signal processing equipment.

The pilot can usually manually disengage the system via acockpit control panel.


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Flight Control Systems - Autopilots

Autopilots

These allow the helicopter pilot to fly automaticallyorhands-offand are designed to maintain a

specific flight variable, such as flight speed,

heading or height.

More sophisticated versions are now availablewith features such as:

automatic tracking of waypoints/beacons,

hover,

hover to forward-flight transition, etc.


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Flight Control Systems - ACT

Active Control Technology (ACT)

This is effectively an extended SAS in which the system has fullauthority over the controls.

A simple ACT system could emulate the mechanical control

system but more advanced forms are generally used to

significantly improve handling qualities and reduce pilotworkload.

They are, however, highly expensive with all components

demanding the highest reliability rates through the use of triplex

and quadruplex redundancy.


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Databus Connection

Databus Connection

The modern solution to the complex wiring requirementinevitable due to the plethora of devices used nowadays is to

use digital databus architecture.

All avionics components are thus connected to a single wire

traversing the entire helicopter. The current military architecture standard is known as Mil-

Std 1553, which was implemented in the 80's and operates

at a data transfer rate of only1 megabit per second.

This will be replaced in the very near future by fibre-opticcabling, offering significantly increased data transfer rates

and also reduced susceptibility to electromagnetic

disturbances.


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Databus Connection

Databus Connection (2)

The main control system being incorporated into the RAH-66Comanche, for example, uses a single pair of optical fibres

with a data transfer rate of 80 Mb/s. The image data is

transferred at an even faster rate of 1 Gb/s.

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