Noise levels dictate future aircraft design!
While studying physiological effects of noise on human beings, it came to light that sales of tranquilizers, sleeping pills, antacids and cardio-vascular, anti-hypertensive drugs increased after an exposure to aircraft noise in the vicinity of airport s. Sales of these drugs, on the other hand, remained unchanged in non-exposed areas. Do you need any other proof to say that aircraft noise levels in the vicinity of airports is high? It is not known how many people are getting affected due to noise-induced-hearing damage. Noise has become a silent hazard.
If it is so, what will happen in the near future! The aircraft industry is poised to expand unprecedentedly. The No. of aircraft flying is likely to double in next 20 years and the No. of passengers are going to be increased at least 3 times in the same period. At London Luton Airport, there has been 400% increase in demand for flying, in last decade. The question one would ask: "Where are the noise regulation bodies and what are they doing? What will happen in future with increase in the number of aircraft?"
Though there are many regulating bodies such as Federal Aviation Administration in the US and Joint Aviation Authority in Europe and several such bodies elsewhere too, they cannot do much to mitigate the people's sufferings. No amount of regulation will help. But what can improve the situation is the re-design of aircraft for reducing noise levels. Engineers observe that the overall shape of passenger aircraft has not changed much over the last 5 decades. The aircraft design has never received that much attention as to decrease noise as a primary requirement from the beginning. Experts say that the configuration of silent aircraft has to be radically different from current designs.
Incidentally NASA has identified aero-acoustics as one of the ten critical areas of science and engineering and started laying thrust on revolutionary concepts in aerodynamic design. NASA has rightly set a goal of achieving a reduction of aerodynamic noise by as much as 30EPN (Effective Perceived Noise) dB relative to 1977 levels by the year 2022. This definitely shows the concern of NASA. NASA knows that increasingly stringent Civil Aviation noise regulations will require design and manufacture of extremely quiet commercial aircraft.
Brisk research programme from Cambridge University and Massachusetts Institute of Technology (MIT) unveiled a major silent aircraft design initiative based on revolutionary concepts. This team is aiming to design a quiet aircraft which will produce 63 dB(A) outside airport perimeter which is nearly 25 dB lesser than current aircraft. They also predict such aircraft when realized will give 149 passenger-miles per UK Gallon of fuel compared to 120 for the best current aircraft in this range and size. Not only quiet, but fuel-efficient too!
Is 25dB noise reduction a big challenge?
Sound power is measured on a logarithmic scale - the decibel. If one can reduce sound by 10 dB it means sound energy is reduced to 1/10th. In the same vein, reduction of 25dB corresponds reducing sound energy to 500 of its original value. A big task indeed! Our ears respond to sound non-linearly. Suppose we have 2 grinders working producing certain noise level. If we switch off one grinder out of two, we will be producing half the sound energy, but we barely hear a reduction in noise. That's why we have to reduce aircraft noise by as much as 25dB to have reasonable difference in noise level.
What makes a silent aircraft?
There are 3 major noise sources in an aircraft: the engines, the under carriage, and the airframe - the physical structure of the plane. The engines are the obvious sources of noise with their huge fans at the front, fast moving machinery and jet exhaust.
Unlike current aircraft, the engines will be above the wings so that shielding occurs from listeners on the ground. At present, all aircraft have engines underneath the wings and hence sound tends to be reflected downwards towards ground. Extensive acoustic liners are proposed in the inlet and exit engines to absorb engine noise in addition.
The silent aircraft will have continuous descent (CDS) approach compared to current step-down approach. This CDS keeps aircraft at high altitude and at lower thrust for longer duration, which eliminates level segments. The aim is to develop efficient "Continuous Descent Approach" procedures for the airport which will produce noise reductions for surrounding communities by keeping aircraft higher and at lower thrust for longer than conventional approaches, which also burns less fuel, and produce fewer emissions. We need greater integration of engine and mainframe. The technology challenges are so many to be overcome that this silent aircraft will be a reality in 2030.