Chapter 1 : Introduction & LiteratureReview1.1 Background1.11 Aircraft NoiseThe aviation industry hasdrastically reduced the noise by a factor of 10 from commercial aircrafts inthe past 50 years from the de Havilland Comet 4 to the Boeing 777. This reductionis due to the progression of aircraft engine technology with a big push fromgovernments setting new legislation that put restrictions on noise.
Althoughaircraft noise has reduced the public perception has been exacerbated overtime, this is due to several reasons, two of which are from communities andairports moving closer to each other either by airport workers wanting to becloser to work or by airport expansions (e.g. new runways & terminals). Thesecond reason is due to globalisation, which means that more of the public wantto travel which of course encouraged the growth of the aviation industrythrough more flights. In 1950, 25 million passengers travelled whereas in 2014,there was over 3 billion passengers a year travelling which creates more than100,000 flights a day (Garfors, 2014).
So due to thePublic’s demands for quieter flights the policy makers and the aviationindustry are committed to reduce the noise produced from aviation.Noise pollution has been found tobe very disturbing to the communities near airports, aircrafts taking off andlanding have been found to cause depression, hypertension, cardiovasculardiseases and other small psychological problems (CAA, 2016).A Boeing 737 from 1.2 miles away can produces up to 90dB while landing, whichmeans the engines are on low thrust so the airframe is making the majority ofthe noise. Research has found that an annoying noise is 70dB, with the painthreshold being at 110dB. Due to the decibel (dB) scale being a logarithmicscale 90dB is in fact 4 times louder than 70dB. (IAC Acoustics, 2017) (Sengpie, 2018). Level Change Volume Loudness Voltage Sound pressure Acoustic Power Sound Intensity +60 dB 64 1000 1000000 +50 dB 32 316 100000 +40 dB 16 100 10000 +30 dB 8 31.
6 1000 +20 dB 4 10 100 +10 dB 2.0 = double 3.16 = ?10 10 +6 dB 1.52 times 2.0 = double 4.0 +3 dB 1.
23 times 1.414 times = ?2 2.0 = double – – – – ±0 dB – – – – – – – – 1.0 – – – – – – – – – – – 1.0 – – – – – – – – – – – 1.0 – – – – – ?3 dB 0.
816 times 0.707 times 0.5 = half ?6 dB 0.
660 times 0.5 = half 0.25 ?10 dB 0.5 = half 0.316 0.1 ?20 dB 1/4 = 0.
25 0.100 0.01 ?30 dB 0.
125 0.0316 0.001 ?40 dB 0.0625 0.0100 0.0001 ?50 dB 0.
0312 0.0032 0.00001 ?60 dB 0.0156 0.001 0.000001 Log. size Psycho size Field size Energy size dB change Loudness multiplier Amplitude multiplier Power multiplier (Sengpie, 2018)The original problem of noisepollution for aviation was the turbojet engines that were being used, the jetproduced a large amount of noise due to the engines movement of a small amountof air very fast, and this was more of a brute force engine.
Turbojet engineswork by fast moving compression blades squeezing air, the compressed air isthen sprayed with fuel which is ignited by an electric spark, this mixtureexpands as it burns through the turbine and blasts out the exhaust nozzle. Theseengines were also ridiculously inefficient because the thrust was simplyproduced by the high velocity of exhaust gases, the most efficient speed wasMach 2 (Loftin, 1985). Through the innovation of turbofans,jet flight became more fuel efficient by combining the ducted fan and the jet exhaustgases to create thrust. Turbofan engines draw air into a high and low pressurecompressor through large fan blades, the compressed air continues through thecombustion chamber where it is sprayed with fuel, the mixture of air and fuelis then ignited where is expands through the high and low pressure turbine thenout of the exhaust nozzle, this process is very similar to the turbojet, theturbofan however also draws the majority of the air through the large fanblades at the inlet and through the outer bypass duct. Turbofan engines move alarger amount of air slower than a turbojet which creates more thrust.
(Loftin, 1985) (Dankanich, 2017)Turbofan engines were originallyused in 1960s with a low air bypass however by the 1990s most engines wereusing high bypass ratio which were much more efficient and reduced the jetnoise, however the noise of the fan started to create more noise than the jetas seen in Figure 1. Consequently the reduction of aviation noise through usinga bypass on an engine has reached its limit, there will need to be atechnological breakthrough to reduce engine noise further therefore reductionsof aviation noise will need to be devised by improving noise production fromother areas of an aircraft. (Dobzynski, 2010) Figure 1Evolution of high to low by-pass ratio engines (T.
Chong, 2017) As flight paths increased there wasa development of new and expansion of existing airports, this became a massiveissue for communities around these environments which is why plans to reducenoise pollution started to be developed by governments. Concerns and issuesbrought forward to the council about aircraft noise can have a detrimentaleffect on the growth and prosperity of an airport, with councils stoppingairport expansions. Airport such as Birmingham, London City, London Heathrow.In the case of Heathrow airport, Hillingdon council have rejected the expansionthrough a third runway, the council have blocked this runway expansion due tofears of destroying communities, adding noise and air pollution. The firstserious consideration of the building of a third runway was brought forward inJune 2001, however Hillingdon council have battled hard to block this newrunway through petitions. After 16 years it looks like Heathrow will finally begetting their desired third runway as the expansion is building momentum withthe launch of the 10 week consultation for the public to have their say.Restrictions such as night flight curfews also heavily cap the total capacityof many of the airports worldwide, with Heathrow airport only allowed 5,800take-offs and landings a year, which on average is only 16 aircraft movements anight where 80% of these happen between 4:30am and 6:00am. (Hillingdon Council, 2017)The FAA developed the CLEEN programin 2010, this program was created to accelerate the innovation of new aircraftsand technologies that will reduce noise, emissions and fuel burn.
The FAAshared costs with industries to aid the integration of the technologies intoexisting and new aircrafts, the FAAinvested $125 million, which when added to the companies included (Boeing, GE,Honeywell, Pratt & Whitney, Rolls-Royce) brought the total investment to just over $250 million (FAA, 2014). Due to the successof bringing new technology into aircrafts by 2016 by the first phase of theCLEEN program the FAA and the companies involved have invested $200 million inthe second phase the CLEEN II in 2015 which hopes to bring more new technologiesinto new aircrafts by 2026. (FAA, 2016) Major Projects CLEEN CLEEN II Aurora N/A Unconventional aircraft configurations Boeing Adaptable trailing edges & ceramic matrix composite acoustic nozzle Structurally Efficient Wing (SEW) & Short engine inlet GE Fuel burn reduction using integrated flight-propulsion control Flight management systems & Alternative fuels Honeywell Alternate 100% bio fuels, new compressors, advanced material and Air-Air sealed turbines Compact combustor & Advanced turbine system Pratt & Whitney Geared turbofan propulsion efficiency Geared turbofan Motor efficiency Rolls-Royce Fuel burn reduction through dual-wall turbine airfoils and CMC turbine blade tracks Advanced combustion system UTAS N/A Short thrust reverser & advanced tailored acoustics Flightpath 2050 is how the AdvisoryCouncil for Aeronautics Research in Europe (ACARE) backed by the Europeancommission in 2011, envision the aviation industry to develop and improve by2050. This publication discusses ways to improve, financial systems, scarcityof resources, the challenges of globalisation and climate change. The Europeancommission hope to reduce the perceived noise emissions of a flying aircraft by65%, Perceived noise is that of which is measured by comparison to the soundpressure level at minimum perceivable noise of a human (European Commision, 2011).In 2001 ACARE initially set up the programme Vision 2020 which planned todecrease perceived noise levels of 2001 by 50% by 2020.
However this projectwas found to be too ambitious that is why the Flightpath 2050 was created. (ACARE, 2001) ENGINE NOISE REDUCTION FIGURE 21.2 Literature Review1.21 Aerodynamic Sound The study of aerodynamic noise andthe science of aeroacoustics began around 65 years ago with Sir James Lighthillwhile he was at the University of Manchester. Lighthill was the first person toidentify the source of sound from aerodynamic turbulence (Lighthill, On sound generated aerodynamically, 1951) (Lighthill, On sound generated aerodynamically, 1954). This work laid thefoundation for future aerodynamic noise and aeroacoustics, which have been usedto solve both civil and military aerodynamic noise issues over the following 65years.
Present day research into aerodynamic noise and aeroacoustics isresearch into reducing aircraft self-noise from high lift devices such aswings, flaps and slats, also the reduction of the aircraft turbofan jetengines.1.22 Airfoil Self-NoiseThe Aerodynamic noise produced from an airfoil is called trailingedge noise or airfoil self-noise. This noise is caused by the effect of aturbulent air flow within the boundary layer of the airfoil interacting withthe sharp trailing edge of the wing.