Hot answers tagged

55

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 ...


51

A sea plane was just going around it. It was very loud and got like 10 feet from the shore. It's just going in circles and won't take off and leave. As has been pointed out in other answers and comments, if the aircraft is taking off or landing, there are no regulations requiring distance from persons or structures, at least that I know of. What you may be ...


46

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 ...


46

In a rural area pilots can fly pretty low, and are permitted to fly even lower while carrying out a lawful activity such as crop-dusting. Even if the pilot flew at 500 feet above your house, which is probably the maximum that he needs to stay away, it would still be loud and disturbing to your livestock. You should try to work it out with the company and/...


43

If you have concerns over the safety of what is going on, here is what you should do: Record a video with your phone. Try to get the N number (it should be painted in large letters on the side of the aircraft). Record the date/time and your exact location where you shot the video from. File a report with the FAA, preferrably by calling the hotline number. ...


43

The sound you're hearing is the APU (Auxiliary Power Unit) running. Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to be bigger and heavier in order to produce enough torque to get the engine moving fast enough. The bigger the engine, the bigger the starter (and the batteries ...


43

Yes, actually you can only hear a supersonic aircraft after it has passed over you and is now flying away from you since it is moving faster than the sound moving towards you. The sound waves will still propagate in all directions and will eventually reach you: The frequency will be shifted according to the Doppler formula: $$ f = \frac{c \pm v_r}{c \pm ...


41

I did some Pawnee flying years ago, and have fielded a bunch of complaints about ag applicators from people. Here's my experience: The FSDO will probably not care. Ag application is a different set of rules, and there is wide latitude on min safe alt. You can talk to them, but in my experience, they yawn. New guys aren't always sensitive to the ...


36

First, some airliners do use it - the Bombardier Q400 uses a NVS (Noise and Vibration Suppression) system to reduce cabin noise. Basically, it uses devices called Active Tuned Vibration Absorbers (ATVAs) mounted on the fuselage frames to 'cancel' the vibrations from propellers and outside noise, thereby quieting the cabin. However, there are some issues ...


32

Source: wikimedia.org They're back. Above is the General Electric Passport. Entry into service was 2018. Its core cowling, exhaust cone and mixer are made in ox-ox composites, with inorganic high-temperature-tolerant resins and oxide ceramics CMCs to withstand 1,000°C without deformation, saving weight and allowing complex molding. The above hints at ...


31

Yes indeed, the fan blades are loose and rattle in their sockets when the fan is freewheeling in the wind. Once the engine is running, centrifugal forces will make sure they do not rattle any longer. The base of the fan blades have a shape like a fir tree, and they slide into sockets on the fan disk with a loose fit. When the fan is spinning slowly, they ...


31

The article states that it's not even clear the damage was caused by the helicopters, that there is no other known incidence of such damage being caused by helicopters, and that a scientific study has shown that while in theory there could be damage such has not actually been shown to be caused. In other words, it looks suspiciously like a group of people ...


29

In my 10 years of flying 747s on international flights, there was no way we would know of passenger reactions upon landing unless one of the cabin staff told us, and that was rare.


28

This PDF indicates an increase by ~10 dB for an F-8K in afterburner versus the same aircraft in 100% dry thrust. This PDF indicates smaller increases: +5 dB for an F-15 +4 dB for F-22 and F-35


27

Why don't modern jet engines use forced exhaust mixing? Because there isn't as much gain to be achieved from the jet exhaust any more, and because forced mixing impacts performance. Gains. From The Jet Engine by Rolls Royce. The text in the lower right corner reads: A comparison of the noise distribution of two generations of engines. The bubbles ...


26

Not feasible. A start of each engine can take 30-60 seconds or longer and would require the APU (noisy). In some airplanes you lose the packs during start so you would impact pressurization. Additionally many engines have a required time at idle after start before significant thrust is applied. Lastly, with all engines offline, the airplane will be in some ...


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 = ...


24

It has been around 20 years since I've been on a carrier deck, but I recall that it wasn't as dramatic of an increase as you might think. It may have gotten a little bit louder, but what I remember more is that the tone changed. The sound was more "full" when the afterburner was engaged. I realize this is a rather subjective answer.


23

If you ever get the chance, do try a winch start in a glider. It is, to say the least, an interesting experience. I do not believe it would be accepted by the general public. So, that is one more reason, it would scare the paying passengers.


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 ...


22

To answer the question: Yes, and even more, both would be able to notice the sonic boom of the other aircraft. Why To understand why, you have to understand how sonic booms are created. Sound is nothing more than waves of different air pressure travelling through the air. Once released by an aircraft or other object, they start to propagate in all ...


22

The only way to get anything that far that quickly is to send it into space, and that's exactly what Musk is suggesting. The BFR will launch a passenger carrying spacecraft out of the atmosphere and into a sub-orbital path, it will re-enter the atmosphere close to its destination. Rockets launch pretty much straight up and a sonic boom would have to compete ...


21

The "wop wop", usually known as blade slap, is heard when the tip of the blade passes through the vortex created by the previous one. It can be avoided. The most common flight regime when this happens is a shallow descent but still with quite a lot of power - e.g. fast and shallow. The vortex starts to move down as soon as it leaves the tip of the blade ...


21

Short Answer: The sound is caused by airflow across holes on the underside of the wings of certain aircraft. Long Answer: I too have noted the sound phenomenon that you describe. This video has a great example between 0:26-52 (link starts playback at 0:26 where the noise starts): I will readily admit that this answer is ...


21

I won't prove it's not possible, but I'd would be surprised it is. I'll develop the reasons why and provide the physical framework you can use to compute a solution to have, with some effort, a final answer. The glider case cannot be scaled up to an airliner It works with a glider for two reasons: The height to be reached before the glider is able to gain ...


21

You may be expecting the rotor to spin up in lock-sync with the engine. That's not quite what happens on a turbine helicopter. Turbine helicopter engines (turboshaft engines) have a turbojet engine inside them that makes thrust. This engine has a shaft of its own - so that its compressor blades are connected to its own turbine blades. The turbojet's ...


20

We can in my airplane: Quite frankly, I'm usually a little insulted when it happens. Were they actually so fearful for their lives and are now so happy to be alive that they need to applaud the heroic pilots? Sorry, but unless it's a true emergency and we did actually just save their lives, I'm not putting them in a situation where they are actually at ...


18

Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source. But there is more: The nacelle also ...


18

Is it possible to land them with the engines off routinely? Yes. Depending on the glide ratio of the aircraft, you most certainly can land them. And in flight training, we do practice this routinely, although we don't actually turn the engine off for safety reasons, merely reducing the power to idle. Technically, this is not the same thing, as the propeller ...


18

It depends on the type of operation: Approach (close to airport): Much aerodynamic broadband noise from the air-frame Modern turbofan engines are less noisy than the air frame during approach. But engines emit most of the tonal components in the sound. Depending on the type of aircraft and angle of descent the pilot might need to use air-brakes to reduce ...


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