aircraft noise reduction
Noise Reduction Program on the hangar apron at NASA's Lewis Research
Centre (now Glenn Research Centre) in Cleveland, Ohio.
Noise from aircraft engines presents problems for wildlife and people, and
NASA has undertaken various programs to reduce aircraft engine noise.
Aircraft noise has always been a
problem, and even though some piston-engine planes produced noise that
many found annoying, it was the arrival of jet engines that increased the
level of noise on many aircraft. The operation of the Boeing 707, first
delivered in 1958, immediately presented problems to the airports where it
landed and took off. Their noisy turbojet engines prompted complaints
about "noise pollution" from surrounding communities. Airport authorities
at London Heathrow and New York Idlewild (now Kennedy) airports instituted
noise limits in the 1960s. They required long-range aircraft to fly at
lighter weights (less fuel and passengers) so that they could climb faster
and get farther from populated areas quickly. Some aircraft took off from
Heathrow and landed at another airport in England for refuelling before
travelling across the Atlantic not because they lacked the range but
because they were not allowed to take off at full weight and full engine
power. This was obviously very inconvenient.
In 1966, after a number of lawsuits in
the United States and a public outcry in Europe, the major commercial
aviation authorities called an international conference on aircraft noise
to establish rules for aircraft. After much politicking and delay, the
U.S. Federal Aviation Administration (FAA) decided to implement its own
rules in Federal Aviation Regulations (FAR) Part 36, in 1971. FAR Part 36
established limits on the maximum noise that could be produced at an
airport at three points—two on either end of the runway beneath takeoff
and landing paths and one at the middle and sides of the runway. It also
established a sliding scale for allowable noise versus takeoff weight for
large aircraft (in other words, bigger aircraft could be noisier).
The FAA regulations on aircraft noise
were tightened several times after the initial rules were set. Naturally,
this has prompted aircraft manufacturers to try to develop quieter
aircraft. The first attempts to reduce aircraft noise preceded government
regulation and were made in the 1950s. Although there was a lot of
theoretical research by engine designers on the causes of aircraft noise
and a number of proposed theoretical solutions, these theoretical
solutions often did not solve the problem, and aircraft designers resorted
to trial and error methods.
The biggest source of aircraft noise is
the engines (although the air rushing over the airframe also creates
noise). Designers of jet engines suspected that the major source of engine
noise came from the region behind the engine where the high-velocity
exhaust (or "efflux") mixes with the lower velocity surrounding air.
Engine designers in Europe concentrated on changing the nozzle designs of
engines, primarily by corrugating the outer edge of the exhaust nozzle.
Rather than smooth and round, they made it warped or angled, often looking
like a flower. This better mixed the high-velocity efflux with the air
behind the engine. In the United States, designers used this approach and
also another method involving venting the exhaust from several tubes. But
both of these methods increased drag and reduced engine performance, and
the multi-tube approach also increased weight, sometimes substantially.
Some commercial aircraft in the 1960s were fitted with truly
bizarre-looking multi-lobe and multi-tube nozzles to reduce noise.
Another solution that emerged in the
1960s was the "ejector-suppressor." This was essentially a large tube
fitted aft of the engine around the exhaust nozzle that allowed air from
outside the engine to mix with the exhaust, reducing the final efflux
velocity. The inside of the ejector was also lined with a noise-absorbing
material. Whereas most early noise suppressors reduced low-frequency
noise, the lined ejectors reduced high-frequency noise as well. But work
on fully lined ejector-suppressors was halted as high-bypass ratio
subsonic engines became available for commercial aircraft.
The establishment of the FAA noise
regulations in the early 1970s called for a 25 to 50 percent reduction in
maximum noise generated by the existing long-range Boeing 707, Douglas
DC-8 and Vickers VC-10 aircraft. Larger aircraft then entering service or
in development, such as the Lockheed L-1011, Douglas DC-10 and Airbus A300
were designed with the more stringent requirements in mind. The Boeing
747, a monster commercial aircraft, did not meet the new regulations and
required some modification.
The development of high-bypass turbofans
that powered these newer aircraft was prompted by the need for greater
thrust and fuel efficiency, but also resulted in a beneficial reduction in
noise. In a high-bypass turbofan, the central turbine drives a large fan
in front of it that passes a lot of air around the turbine (this is what
is meant by high-bypass—a lot of air bypasses the turbine). Not only does
the fan produce less noise per pound of thrust, but the cooler air mixing
with the hot jet exhaust also insulates the engine, acting as a muffler
(it "muffles" the noise). This reduction can be directly observed at a
major international airport, particularly one that also includes flights
of older turbojet-powered aircraft operated by poorer nations. For
instance, a modern Airbus A340 jet is noticeably quieter than the rare
707. These older aircraft have to gain special permission to violate the
Many aircraft built during the 1970s and
1980s, such as the 737, have engines that do not appear perfectly round
when seen from the front. This is because the bottom of the engine cowling
(or covering) has to have clearance above the runway so that it does not
scrape along the ground. When the 737 first appeared in the 1960s, it had
fairly small turbojet engines that were suspended high above the ground.
As the engines became larger with the introduction of turbofans, the
fuselage and wing remained the same height above the ground, so the bottom
of the engine cowling was flattened somewhat to still achieve the maximum
air intake without scraping the ground.
Airports prepare noise exposure maps that show existing noise conditions
at an airport.
They are used to identify areas that may have too much noise.
Although it is easier to design quieter
engines from scratch than to try and quiet an existing design, there is a
substantial market for aircraft "hushkits" to reduce the noise on current
aircraft. These hushkits include some of the same technological approaches
first explored in the 1960s by aircraft designers. As the number of
commercial airplanes flying increases, local communities around airports
complain more and this leads to calls for even greater regulation of
airplane noise. As a result, aircraft designers are constantly looking for
ways to make their aircraft quieter.
During the late 1960s, several countries
were trying to develop a supersonic transport, or SST, a plane that would
fly faster than the speed of sound. There were numerous hurdles for the
designers to overcome, including both engine noise and sonic booms.
Ultimately, the United States abandoned its effort for various reasons,
including concerns over excessive noise pollution. The British and French
jointly developed the Concorde, but its use was restricted to only a few
airports due to noise concerns. Noise remains a major challenge for any
future large, fast aircraft.