Noise in the Jet Age
By OLIVER ST.EWART AI RCRAFT size is a measure of aircraft noise. An aircraft of one hundred tons will be twice as noisy as one of fifty tons. Arrange the new jet liners in an ascending order of size: Caravelle; Tu-104A; Cornet 4; Boeing 707; Douglas DC-8; and they fall into an ascending order of noisiness. For the same performance the thrust must be proportional to the weight and the noise will be proportional to the thrust. It is as simple as that and as inevitable.
Jet engines cannot be silenced. Motorists know that a motor-car must sacrifice power to go quietly. It can afford to do so. But an aircraft, if it is to carry a big load fast, cannot afford the increased drag and lowered thrust imposed by silencers. Jet-engine noise attenuators, such as those devised by Mr. F. B. Greatrex and used in the Comet 4, are remarkably effective. They quieten the enginei without much loss of power or increase in drag. But to anyone standing near the engines when they are running at full power the noise will still be within five decibels of the Pain level which, we are told, is 140 decibels. In this logarithmic scale a full orchestra playing fortissimo gives eighty decibels, while the point at which noise becomes an unbearable nuisance is usually put at 105 decibels.
It is - the opinion of many engineers that a regularity of 80 per cent. for the non-stop London-New York run with a sufficient payload can be attained only by aircraft with a take-off weight of 130 metric tons or more. So the modern air transport dilemma is this : large, fast aircraft are necessary if the London-New York non-stop service is to be run regularly and eco- nomically. But large, fast aircraft are horribly noisy.
At forty-three metric tons the Caravelle is the quietest of the new jet liners; but it is a medium- and short-range machine. (Metric tons are used in these comparisons because American tons and British Imperial tons differ and are not standard- ised.) The Cornet 1, at forty-eight and a half tons, Was a short-range machine. The present Comet 4, at seventy-one tons, can do the Atlantic run non- stop, as it has shown; but it will not be able to achieve high regularity in non-stop services against the prevailing winds and will often make intermediate landings. The Tu-104 and the Tu-I04A come between the Comet 1 and the Cornet 4, but I do not know their exact all-up weights. The Boeing 707 and the DC-8, at maxi- mum permissible all-up weights, will take off at 125 to 130 tons. At this weight they will be able to work the North Atlantic non-stop with good regularity. The Boeing was first limited to eighty- six tons at London Airport. On future visits it Will be allowed to go to one hundred tons.
If London is to be effective as an international airport it must allow big, heavy aircraft to work there, and that means that it Must allow noise. It does not mean, however, that the amenities of the residential areas around the airport need to be neglected. It means that they must be protected, not by banning noisy aircraft, but by trying to prevent the noise from going beyond the con- litles of the aerodrome. With respect, I would say that the Ministry of Transport and Civil Aviation's 'abatement' policy on aircraft noise has been muddled and irrational. The object should be to check the dissemination of noise rather than to tackle the more difficult task of reducing the noise at source.
When taking-off, an aircraft opens its engines at one end of the runway and then trails its noise along the runway and 'beyond it. People living near the ends of the main runway thus obtain a full dose of noise. The quicker the take-off and the steeper the climb, the shorter the time during which noise reaches those on the ground and the smaller the area over which it is spread. A vertical take-off aircraft might rise from the centre of a big aerodrome and attain height without causing annoyance to anyone living around. If the aerodrome were surrounded by a band of thickly grown trees and shrubs, people on the outskirts would receive less noise from a VTOL airliner. than from a clip-on phut-phut passing along the road outside their houses.
At the other extreme, the greatest noise nuisance is caused by the aircraft which runs for along distance before gathering flying speed and which makes its initial climb at a flat angle. So vertical or short take-off and a steep climb are required. They are given by high power-to-weight ratio, low wing-loading and by special means such as energised circulation (which is used in the NA-39). If the Ministry of Transport and Civil Aviation must exercise its authority in this matter, Would be better if it turned its attention to power-to-weight ratios and wing loadings and to extending the tree and shrub planting pro- grammes at its aerodromes so as to surround the runway areas with wide belts of noise-absorbent growth. If it acted rationally in its noise-reduction attempts the Ministry would restrict the move- ments of heavy, low power-to-weight aircraft with high wing-loadings. Politically this might seem awkward to the Minister, because it would admit the Russian aircraft and exclude the American!
A quarter of a century ago the four-engined Handley Page 42 airliners used to leap off from a grass runway at Croydon and other small aero- dromes and to balloon high over the houses on the outskirts,, causing almost no noise nuisance. Safety and comparative quiet went together, for not one fare-paying passenger was so much as scratched during the whole period of service of these aircraft. But they weighed only thirteen tons! What is now needed is this kind of take-off performance with the heavier and faster machines.
There is a second nuisance which may be asso- ciated with the big jet aircraft. It is smoke. During take-off these machines may make smoke in large quantities and, although this is not likely seriously to affect the public, it might cause diffi- culties for air traffic control.
In order to lift their immense bulks with en- gines which barely give enough power some of the new jet liners use water injection. This enables a greater total thrust to be taken from the engines, although it lowers combustion efficiency. The result is smoke and soot. An aircraft taking-off in one of the high-weight categories may inject nearly three tons of water during three Minutes of acceleration and initial climb. The smoke is copious. A strong wind quickly disperses it; but in certain weather it hangs about over the runway.
Many difficulties face the makers and operators of the new generation of jet airliners, and it is partly because of them that air transport can be said with truth to be entering the most interesting period of its history.
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