8/14/2019 Norton Bomb Sight.pdf

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T 8 O LI H T A U G U S T I 6 T H , 1945

The or den ombSightA System of Instruction

Between utopilot and Bom bSight An utomatic Speed

and Distance CalculatorThe semicircular gradu-ated scale beneaththetelescope eyepieceis forsetting trail lag, and thelargest of the threecentral knobs is forsetting falling speed.THE famous Nordeu bomb sight, which gained analmost legendary reputa tion in the ear l ier pai t ofth e war. wasac tually invented byCar l L Nordenas early as1928. Op eration of the ins t rumen t isl inked in-extr icably with the aut om atic pilot (officially kno wn asStab i l i sed Bomb Approach Equipment) which was also

developed byNorden in1935,sotha t full adva ntag e couldbe takenof his bom b s igh t . This equipm ent is the s tanda rdautomatic pilot fitted to Am erican a ircraft and, 111thissys tem, theNorden b om b s igh t is insta l led directly,o n thehousing of theaz im uth stabil is ing unit . In common wi thall foimsof au toma t ic p i lo t , S .B A .E . embodies th ree bas icgroups ot equ ipmen t compr is ing gyros , se rvo mo to is andfollowup system? to r ende r thedegree of contro l surfacedef lection proportional to the dev ia t ion of the a ircraf t .In thea u to p i l o t / b o m b s ig h t c o mb in a t i o n , the a z imu thstabilising unit (also called the directio nal stabiliser) cancontrol both the aircraft and theb o m b s igh t in a z imuthsimultaneously 01 separate ly . W hen the S.B .A .E. an d sightare used together thebom b s igh t knobs are rota ted toThis mock-upof thebombsight/autopilot installationw a s c o n s t r u c t e d toillustrate the relationshipand function of the majorcomponents.

po in t thes ighi at theta rget , which ac tion a lso turn s theaircraft in the requisite direction toensure a collision coursea t anyra t io theDom bardier d esires. Thef light gyro s con-trol the a ircraf t la tera lly and long i tud ina l ly th rough thefollow-up system by opera ting a ilerons and e levators asr equi red . Abankin g motor in t roduces bank in to tu rnsbymo v in g theaileron sector in the f light gyro by an amountpropor t iona l to r udde r m ovemen t , the reby ac tua t ing theappropr ia te se rvo motor to d isplace the a i le rons . Thefollow up system governs the a m o u n t of con trol surfacedeflection applied for any a ircraf t devia tion , and returnsthe control surface toneutr a l af ter thecorrection hasbeenm a d e ombing Problem

Thus, without going into deta ils ofthe comp osite mechan-ism, theau top i lo t is a me a n s w h e r e b y theaircraft can becontrolled in its f light a t t i tu de, and the bom b s igh t is agovern ing agent of theau tom a t ic p i lo t . Thebom bard ierlooks th rough hiss igh t ing te le scope and , hav ing pre se t onthe s igh t theva r ious eva lua t ions of thep roposed bombingcondi t ions , he takes over control of thea ircraf t . As thetwo basic e lementsof thecompo site system arecorre la ted,when thebom bard ie r tu rns theappropr ia te knobs on hisbomb s igh t , theautop ilot half of the installat ion ensuresth a t theaircraft is f lown according to the b o mb a r d i e r s re-quirementsin fact , he flies the ma c h in e .Fac tor s 01 the bombing problem are: aircraft speed,grav i ty ,airresistance , windag e and ta rget motion . Gravityan dairresistance factors are ob tain ed from a pre-calculatedtable f rom which thebom bardie r getshis firs t se t t ing. Since init ia l v e j : i tyot theb o m b ist h a t of theaircraft, andthis velocity is decreased by air re-sistancesotha t thebom b tra jectoryisa segment of a parabolic c urve , thesed a t a cama lso be pre-calcula ted andsupplied to the b o mb a r d i e r in ordert h a the can set the s ight tocompensatefor trail.Windage implies correction for driftwhich , unfor tuna te ly , is complex asthe bomb doesnotfall on the groundtrack of theaircraft butswerv es aside.The sight does net r equ i re a specialse tt ing for th i s as the correction isautomat ica l ly made by a mechanical

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AUGUST I6TH, 1945 LI H T I S I

THE NORDEN OM SICHTlinkage between the trailmechanism and d i r e c t i o ncontrol.

Target motionhas a similareffect to that of wind of thesame velocity in the oppositedirection. In actual practiceitmakes no difference whetherthe wind or the target is inmotion,nordoesacombinationof windand target motionre-quire additional setting. Aftersettingsfortraillag and fallingtimearemade,theonly criticalfactors are those of resultant clos ing speedand directionof the aircraft relative to thegarget. These elementsareirre-spectiveof thefact th atthe air-craft itself is capable of bothlinear and angular movementin three dimensions. Ho wev er,this difficulty is taken care ofbythe electrically driven gyroswhich governthestabilised lineof sight.The essential criterion ofbombing is, naturally, the de-terminationof thecorrect p ointat whichtoreleasethebombs,andthe Norden bomb sightisbasically an automatic speed-ar.d-distance calculator whichresolvesthecritical factors in theformof anangle. As thebombisreleased, its momentum movesit forward, gravitypullsitdown,and air resistance retards both actions. Thefirst calculationis thelengthof time requiredfor thebombto reachthetarget; thenthesight determinesthehorizontalspeed relative to the target and multiplies that speed bythe timeof fall. The quotient is the horizonal distance,without retardation, which the bomb would travel duringthe timeof fall. After subtracting the trail lag from thatdistance,the bdmb sight producesa range angle which isformedby the intersection between the bombardier's lineof sightto the targetand a vertical line from the aircrafttothegroundat the exact instantof release.This evokesa riglit-angle triangle formed by the verticalline from theaircraft to theground, the ground line fromthat pointto the target, withthehypotenuse formedby thelineof sight from the bomb sightto target. In the bomb

IRCR FTHE DING TR CK

Aft viewofsight. Leftof the eyepiece is thegyroscope window inwhich bubble levelsindicate precessioncor-rected by the knobabove. Thescale belowand left of the windowis for pre-setting drift.

sight itself thereis atinyreplica of this trianglewhich is created by thesame vertical line be-neath the aircraft andthe same hypotenuse lineof sight, but is boundedonthebottomby ahori-zontal wire. The ratioof thesidesof thesetwotrianglesis, otcourse, equal.In the timing typeof bomb sight the target is t r a c k e d by moving a cross-wire back alongthis horizontal wire for a number of seconds(againstan index) correspondingto theestimatedtime it will take the bomb to fall. Thi s crosswireiskeptinline betweenthebomb ardier'seyeandthe target, whilstat the same timea secondcross-wire is moving forward from the verticalpoint directly beneath the bombardier's eye.The speedof the first cross-wire corresponds toground speedand, of course,the distance whichit moves alongthe l ine of si gh t wire corres-pondsto thedistanceon the ground over whichthe aircraft travels during the timeof tracking.Iftherear wire travels forwardat thesame speedthat the front wire moves rearward, and if thetracking timeisequalto thetimeof fall, thentherear wirewill move ahead of the vertical a distance correspondingtothe actual distance whichthe aircraft will travel duringthe falling timeof thebomb.

However, this willnotgive the correct range until tiailhas been subtracted. The rearmost wire is initially set tothe rearot the verticalby an amount corresponding to thetrail distance. It thus starts forward with a handicapsothat it will travel beyond the verticalfor a distance corre-spondingto thecorrect range. Withallthese settings m ade,the bombardier's lice of sight through the rear cross-wireisthe lineof sight, which should pass through the targetat the instant of release. By virtueof the autopilotsyn-chronisation, the bombardier directs the aircraft on therun-upto thetargetand,whenthe rear cross-wire comesinline, the bombs are automatically and instantaneouslydropped. Thelineofsightisactuallytheaxisof thesighting telescope, which isswungby a variable-speed drivesothat the telescope automatically keepson the target. Thedrive mechanism setsup within itself a release lineof sightangle corresponding to that comprised by the rear cross-wireaspreviously described. The lateral cross-wireiskeptonthetargetas thetelescopeisdriven,and,whenthetelescope reaches the correct angle, an electrical contact ismade which actuates the bomb releases.

j^iagramof relation between aircraft and target throughoutbombing operation, indicating linearandangular drift value s.

SE FIRE XV OFF TH SECRET LISTSOME details of the Supermarine Seafire XV, which is thefastest naval interceptor fighter built in Britain, werereleased last week-end by the Admiralty.Fitted with a 1,890 h.p. Merlin engine, it has a top speedin the neighbourhood of 400 m.p .h . , a ceiling of more than35,000ft., and a faster rate of climb than any other British-built naval fighter.Armament consistsof two 20 mm. cannon and four 0.303m.machine guns,and one 500 lb.bombcanalsobe carried. Provision is also made for the use of rocket-assisted take-off wheuoperational circumstances demand it.Flying weightof the Supermarine Seafire XV with full radioand other equipment, including deck-arrester hook, is roundabou t 8,000 lb. Dimensions are: Span 36ft. ioi n.; length31ft. ioin.; wing area, 242 sq. ft.