8/12/2019 1969 - 0416

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ensu re that the aircraf t is no t left out of trim even in theunlikely event of a four-engine failure and a violent decelera

tion to subsonic speed.

S O N I C B O O M

There are about as many people who think that the sonicboom will be worse than 'we thin k as there are wh o thinkit will be better than we think—which is another way ofsaying that nob ody k now s what i t is going to be like.

There is no mystery about the physical nature of the boom.It is caused by the fact th at when an aero plan e is travellingsupersonically it is moving faster than the pressure disturbanceswhich it is propagating. These coalesce into conical shockfronts inclined to the aircraft path at an angle which becomesmore acute wi th Mach number.

Around the aircraft there are a series of shock wavesemanat ing f rom the fuselage, the wing, and the ta i l . Awayfrom the aircraft these merge into two main shock waves.

On reaching the ground they pass over the human ear as aboom or, i f the two shocks are more than 50 mil l i secondsapar t , a double boom. The actual pressure on the groundand the width of the boo m carpet depend on the height ,weight and Mach number of the a i rcraf t , and many othervariables such as cloud cover, terrain, buildings, and so on.Th e boom zone could be up to 50 miles wide, and typicalpressures recorded on the ground from the Concorde mightbe between 1 and 21b/ft , the peak being durin g the clim bwhen supersonic transition is made below 35,000ft. Someexperts say that a boom of 0.751b is as much as the publicwill tolerate; others say that 0.2lb is as much as could besuffered day in day out.

One of the leading experts on the sonic boom, ProfessorE. J . Richards , of Southampton Univers i ty, has sa id in Science

ournal tha t there is quit e definitely not going to be one

critical value of the sonic boom below which flying is acceptable and above which i t i s in tolerable . As a very rou ghguide as to what the public might have to suffer, or mightbe expected to tolerate, a 21b boom may have about the sameannoy ance value as 110 PNd B*— which is the m aximu mallowed airport daytime take-off l imit. At night this l imit is102 PN dB . So a 21b boom might br ing as man y complaintsfrom the public as an airc raft tak ing off in the day andgeneratin g 110 PN dB or takin g off at night and g enerati ng102 PN dB . W ith, of cour se, one big difference: l ike the ladyof Banbury Cross with bells on her toes, she shall have musicwhere'er she goes.

Th e t rue solut ion, in Professor Rfchards ' s opinio n, appear sto lie not in designing the SST in a particular way but inchoosing its routes with extreme care; and this should bethe real point of d iscuss ion. Acceptable t ransat lant ic routesare relatively simple to find. We must miss Bristol [the homeof the Con corde ] , South Wa les , Dub l in and Belfast , and routethe aircraft as much as possible over the sea. On the otherhand, flights to New York and Los Angeles from Paris,F rankfu r t , Co logne , Bonn , Hamburg , Copenhagen and S tockholm need lengthy consideration. Many of these routes passdirectly along the London airways and supersonic traffic wouldquickly, and r ight ly, g ive r ise to a massive outburs t of complaints. Only by choosing the least built-up areas can suchflights be made acceptable. The situation in respect of domesticroutes in the United States is, of course, sti l l more severe.The ultimate judgment must be on the basis of whether or

* A perceived noise decibel is a measure of noise which takesinto account the pitch as well as the pressure (db) of a sound.An equivalent perceived noise decibel (EPNdB) is a PNdB corrected

for tone and duration of the noise.

light International • ON ORDE

not supersonic flying provides an overall advantage to communi ty and i t s economy.

It is just possible that the Concorde boom will be negligibWe shall have some idea towards the end of the year whthe prototype starts to fly at Mach 2. If i t is bad then shall have to do what we have had to do at air po rts : schedthe operation of the aircraft in such a way that i t is least nuisance, and lay down international criteria which agreed and enforced—though in this case the enforcauthor i t ies wi l l have to be nat ional ra ther than the a i rpow ners, and the prob lem of identifying transgressor s will more difficult . One cheerful thought for the operators the fact that Concorde miles per gallon are almost the sasubsonic as supersonic , so that the economic penal t ies—thouserious in terms of time and seat-mile costs—will not d isas t rous . W e are a lso to ld that the manufactu rer ' s marestimate of 250 aircraft is based on the assumption that supsonic flight over land will be banned.

How about a i rpor t noise? The Concorde wi l l have to mthe requirements which limit perceived noise decibels atpoint four miles from the start of take-off roll to 110 PNby day and 102 PNdB by night. Limits on airport boundnoise (sideline noise) and on approach noise—which in opinion of many people who l ive near a i rpor ts i s mdistressing than take-off noise—are also to be laid domoni tored and enforced. Proposed new American requiremeare 102-108 EP Nd b on the approach , 93-108 on take-off an102-108 EPN dB sideline. Fu tur e aircraft will have to certified for their noise wo rthin ess.

R A D I AT I O N A N D O Z O N ETw o other new pro blem s will have to be carefully watc h

though they do not fil l the Anti-Concorde League with sdelight as does noise. Both are a function of the higcruising alti tude, of from 55,000ft to 63,000ft. One is cosradiation and its effect on skin tissues, and the other is higher ozone content of the upper atmosphere and its effon passengers.

There are three sources of radiation: atomic particles frouter space, most ly protons; solar f lare radia t ion; and nuclexplosion debris drifting in the upper air. Radiation measured in roentgens , and the amount absorbed in one grof hum an t issue is know n as a rep . Th e dam age caudepends on the type of radia t ion, a lpha type being wothan X type, and the measure of the damaging effect ofradia t ion dose is kno wn as a rem . Th e l imi t for radia tworkers is laid down as five rems a year. The worst case the Concorde is l ikely to be suffered at high latitudes alti tudes during a period of intense sunspot activity like twhich occurred on February 23, 1956.

I t has been es t imated that the occupants of a Concor

flying on that day at high polar latitude (the magnetic poattract atomic particles) at 65,Q00ft would have had a din one flight equal to the five rem allowed for radiatworkers in one year. The 1956 sunspot was the wors t recordfor 22 years; but i t is clear that Concorde crews and passengwill have to be protected against such occurrences.

Th e Concor de wi l l be f i tted w i th a radia t ion meter ca l ibrato warn the pilot to descend to a safe alti tude should solar- f lare radia t ion level reach 0 .2 rems per hour. Tpenalties of taking evasive action are unlikely to be serioand the chances of Concorde exper iencing such radia t ion statistically very small, with a sunspo t cycle of 11 year s amost Concorde schedules a t lower la t i tudes .

Cosmic radiation and nuclear debris can safely be ignoras their effects will be no wor se than that of a lumin ous wwatch.

Ozo ne, or O3, is pro duc ed by the action of ultra-vio let