# Tag Info

56

Well, it does look pretty, doesn't it? Boeing wanted to reduce the noise generated because of jet blast of the engines. Many airports around the world are implementing new noise regulations. As mentioned by Boeing: To combat the sound of jet-blast from the rear of the engine, Boeing, General Electric, and NASA developed serrated edges called chevrons ...

48

I think both answers are correct, but I would like to spend some time describing the phenomena that chevrons try to reduce so that I can complement the answers already provided. So, what is happening in an engine? We have a hot gas at high speed leaving the engine core, and another gas at higher speed than the external air but much slower than the core, on ...

25

There are two factors reducing the volume of a sound when travelling through the air: The pressure wave expands as the surface of a sphere, which will reduce pressure as a function of distance $r$: $$p(r) \sim \frac{1}{r^2}$$ Since human hearing is logarithmic, we typically use Decibels (dB) as a unit for volume, rather than pressure directly:  L_p = ...

22

Actually, there were similar tricks used on Boeing aircraft before to reduce jet noise. The Rolls-Royce Conway (as used on the Boeing 707) had a scalloped exhaust which improved jet mixing and reduced exhaust noise. Since the Conway was also the first operational bypass engine, the lower exhaust speed of this design helped to reduce noise already. At the ...

9

The main purpose of the chevrons in the engines is to reduce the engine noise. Most of the civil airliners use high bypass turbofan engines, which produce significant amount of noise especially at high thrust conditions. Aircraft (engine) noise is especially critical during the takeoff and approach phases as it affects the people in the area around the ...

9

Yes, you can definitely hear planes from that altitude. Twin turboprop regionals flying over my city at altitudes between 21000 to 25000 ft are clearly audible at pretty much any weather, so a C-130 being much larger and having two more engines would have no trouble transmitting sound down to the ground from 27000 ft.

6

As DeltaLima states, the 787 is a Chapter 4 aircraft. Directly under the table in your question is another table listing the requirements for each category. So the categories are based on the certified Chapter, and the margin to Chapter 3 standards. Based on these values, the 787 would be categorized as Chapter 4 Low. You can look up aircraft in the ...

5

The B787 is a chapter 4 aircraft. I don't know whether that would be chapter 4 'High', 'Base' or 'Minus'. These qualifications seem to be specifically used at Heathrow. I'd expect the B787 to be in the Base or Minus category since its noise levels are well below the ICAO Chapter 4 standard.

1

I am going to work in SI units for all engineering work, anything else would cause instant insanity on my side. I'll leave the altitude in feet (isn't used) and convert speed from knots to m/s so you can follow the flow easily. Since $C_L$ is unitless it should not matter what units you use in the calculation as long as you are consistent. Altitude: 34000 [...

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