How does a Plane TaxiWhat does an airplane taxi do?
What do planes do?
Those who live near an airport and on the route of arriving or leaving aircraft obviously have enough of the noises. Therefore, overnight flying is completely prohibited in democracies and a lot of technology is used to reduce aircraft noises on the floor when taxiing, taking off, climbing and landing.
Airborne sounds on the ground: take-off, first ascent, approach: are treated by means of specific processes, so-called "noise reduction processes". This is mandatory at many aerodromes and determines how you control your damper positions and speeds so that your floor level annoyance is kept to a minimum. Aerodrome administrations carry out periodic on-site inspections and large penalties are imposed for breaches of the regulations.
We' ll focus on the local noises, and only on the motors. Throughout the years, thrusters have experienced several enhancements to reduce noises. Today's high-bypass thrusters don't make much fuss on the floor when idling or in a low-power taxi, which is the outcome of several years of constructive work.
Due to air traffic control legislation and aviation sound certificate standards, the reduction of engines has become one of the most important research areas. They regulate the maximal sound levels that airplanes are permitted to generate. Even though the cabin generates a lot of air traffic disturbance, it is a major cause of the overall sound signatures of an airplane, the main cause of it.
Essential sound originates from the ventilator or blower, the turbines and the flue gas jet(s). Generating sound from these constituents rises with increasing flow speed. Flue gas stream sound differs by a greater magnitude than that of the supercharger or turbines, so a decrease in flue gas speed has a greater impact than corresponding decreases in the others.
Blast emission noises are induced by the strong and turbulent mixture of the flue gas with the atmospheric air and by the shear effect due to the relatively high velocities between the blast and the atmospheric air. Turbulences occurring near the flue gas outlet cause high frequent noises ( "small vortices") and further down the flue gas, turbulences cause low frequent noises ("large vortices").
Additionally, a surge is generated when the flue gas velocities exceed the acoustic velocities. Reducing the acoustic emission levels can be obtained by accelerating the mix rates or by reducing the flue gas velocities in relation to the atmospheric conditions. You can do this by modifying the shape of the tailpipe stream as shown.
Increasingly higher engine by-pass conditions led to the development of other noisy areas. Ventilation and its environment were now important candidate for sound reduction. The research has shown a good grasp of the development of noise and there are extensive regulations for sound engineering. They are based on the need to minimise turbine wake effects, mitigate the severity of interaction between rotary vane and fixed vane, and optimise the use of acoustic absorbing liners.
The sound absorbent cladding fabric transforms sound power into warmth. As a rule, these claddings are made of a pigmented layer of foam carried by a comb and creating a gap between the top layer and the motor channel. In order to achieve optimal attenuation, the acoustical characteristics of the hide and liners are meticulously tuned to the background noises.
They offer, however, a very efficient signal-to-noise ratio. Today, an aircraft silencer named Chivron (here a Boeing 787) is used in passenger aircraft and is a good example of a proprietary NASA development. Chivrons are the sawtooth patterns that can be seen at the rear edge of some engines' dies.
Then, as heated exhaust blows out of the motor cores with colder exhaust blows through the motor blower, the serrated corners help smoothen the mixture, reducing whirling that makes noises. Boeing's new 787 is one of the most advanced aircraft to use a chevron to lower the sound level of the engines, or a sporty chevron on the nacelle or catch housing.
Boeing 747-8 has chevron engines at the gondolas as well as at the inner jets. You can see we're a long way from eliminating nozzle noises!