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 ...


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 ...


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 ...


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 ...


15

Engines could be made quieter by the simple solution of adding a muffler (or silencer as it's called in some places). They are available for many models and can be installed, at an astronomical cost. The parts and labor are actually pretty cheap, it's the certification costs that make it sting so badly. It costs a lot of money to develop and certify parts ...


13

This study on cabin noise with experiments in a Cessna 172s and a Piper PA-44 got the following results (noise comparisons are mine): Peak: Without headset: 101.3 dBA = rock concert (not front row though) With headset: ~88 dBA* = vacuum cleaner (old bag-style ones, not Dyson...) Average: Without headset: 86.26 dBA = heavy traffic (by the side of the road)...


12

It does help a little, yes. One source of noise is the fluctuation in dynamic pressure experienced by a blade of the rear propeller when it cuts through the wake of a blade of the forward propeller. Friction in the boundary layer on the first blade will reduce the dynamic pressure locally and create pressure fluctuations on the second blade, and noise. By ...


11

In a jet it's going to be pretty much all above 100hz. Most of the noise at cruise is broad spectrum "white noise" from the air flow and the engine core exhaust. The fan however, running at, typically 4000 rpm, with 60 blades say, will be producing a 4000 Hz noise, and this along with the exhaust white noise will be the dominant sound prior to cruise. In a ...


8

You are right, propeller noise is dominant in case of GA aircraft, but not all propellers make the same noise. Much depends on blade loading and tip speed - making the propeller larger and letting it turn more slowly will improve efficiency and reduce noise. This has worked very convincingly for model aircraft over the last 30 years. GA engines are normally ...


8

Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines. MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged: Engine Synchronizer System The engine synchronizer ...


7

Strictly speaking, its neither. Chevrons don't resemble either a sawtooth wave or a sine wave in mathematical terms. The closest is that it is a smoothed (modified) saw tooth form. Image from compositesworld.com Though the initial research used a serrated nozzle in saw tooth form, the final commercial application differs from it. However, the chevrons are ...


6

Heathrow has an extraordinarily elaborate runway alternation programme, designed to spread out noise as evenly between its different neighbours as possible. Daytime runway alternation follows a daily cycle and a fortnightly cycle. Alternation starts at 6am and continues till the last aircraft departs at the end of the day. In the morning we use one runway ...


6

To supplement Quentin's answer, the effects are accumulative and depend on the level of exposure. Most of the noise in a cockpit is low frequency sound of 75 to 100 hz with a 4 cyl engine/2 blade prop or 100 to 150 hz with a 6 cyl and 3 blade prop. Then there is the usual wide spectrum machinery noise and the white noise from the airflow. Long term ...


6

Generally speaking one is an add on and the other is integrated into the design. Over the years the FAA has implemented progressively more stringent noise regulations both for aircraft that are flying as well as to the full certification process. The changes came in stages and have been progressively altered for the past 30 or so years take a look at §36.103 ...


6

GE patented a synchrophaser system for turbofans in 1991 but I've never heard of the system being implemented. My experiences are with the CF34 family (both flying and on the technical side), both FADEC and non FADEC, and none of them had a dedicated synchrophasing system (except for N1 synching by the FADEc computers) and I can't find anything about ...


5

I'm no expert but the RAH-66 Comanche can help answer your question. It was a stealth attack helicopter prototype. The Wikipedia page lists two main design elements: A five bladed rotor, the more blades you have, the slower they'll turn, so less noise. Of course they are harder to balance, so it's not cheap. Canted blades, kind of like winglets. They ...


5

Apparently it's called Runway Alteration. The only hits I got were referring to Heathrow airport, so my guess is this is the only airport that uses this procedure. Actually, the alternation is in two ways: per part of the day, and per week. During week 1, from 06:00 to 15:00 runway 27L is used, and from 15:00 until last departure 27R is used. The week ...


5

Firstly, I'd like to make clear that the particular reasons for the placement of the taxiway are not all publicly disclosed, and there are undoubtably multiple factors involved. After looking at the airport in Google Maps, I notice a couple things. (Excuse the large picture, explanation below!) 1) Physical & Geographical Limitations There is a pre-...


5

Edited for clarification due to the useful comments below In general most light airplanes aren't dramatically louder than most cars (see Estimated Airplane Noise Circular on page say, 140 and Car Interior Noises ). What makes a difference though is how humans perceive these noises. If for example a person stands next to an airplane engine and a car engine ...


4

Yes indeed - the higher the bypass ratio, the lower the noise: Noise is a function of the velocity of the accelerated air stream. For a given thrust, accelerate more air to a lower velocity and noise goes down (and efficiency goes up). The bypass air is much slower than the turbine exhaust air, and surrounds it like a tube. The hush kits were for previous ...


3

I measured levels in the cockpit area of a twin Cessna where the propellers were inline with the pilot to the left and right and levels were 110dBA where I would be sitting. Levels were 100 in the back in the cabin. Without headsets, it would be impossible to communicate with crew/passengers and I'm sure hearing loss would result.


3

Prop powered general aviation aircraft are far louder than automobiles. No pilot flies without a headset because the noise makes it otherwise impossible to communicate with the ground or other pilots. Peak levels exceed 100db and the continuous average is around 85db, which is damaging to hearing without protection. The above charts showing arrival and ...


3

Why Mach 10? Would Mach 3 or 4 not be enough? See here and here and here for the complications which arise at higher speeds. The linked article is full of misrepresentations - of course will the Busemann biplane create lift at sub-supersonic speed (what is that, anyway? I read it as subsonic speed). Give it a proper subsonic airfoil and it will do so just ...


3

The biggest noise reduction is achieved by closing most of the landing gear doors again after extension is complete. Of the factors that influence drag and noise, one is the distance between wheel and fuselage. The picture below is taken from S. Hoerner's book "Fluid Dynamic Drag", chapter XIII.5 of which deals with the drag of landing gears. Another ...


2

Generally, it is most fuel efficient - when considering fuel for the whole flight as opposed to individual flight phases - to climb to cruise altitude as quickly as possible (i.e. using maximum thrust), since the aircraft flies much more efficiently at cruise altitude than at lower levels. This is in direct contradiction to anything to do with close-in noise ...


2

The idea is certainly valid. Busemann's original design could not generate lift, but modern variations can. Here are just three recent papers on how it can be done: Kusunose, Matsushima and Maruyama. (2011). "Supersonic biplane — A review". Progress in Aerospace Sciences 47. pp.53–87. Wu, Jamieson and Wang. (2012). "Adjoint based aerodynamic optimization ...


2

If you're crew, you probably want to be wearing a headset for communication purposes. For passengers, these airplanes were flown for many decades before headsets became common (even for crew, using the handheld mic and overhead speaker) and long-time instructors had slight hearing loss, but for the random person taking a scenic flight to look at leaves or ...


2

The answer to all of our questions is broadly yes, but to break it down: This company makes mic's for the Bose ANR audio headsets (quiet comfort line) that may fit your use case. You should be able to use a step up adapter to plug any standard aviation headset into 3.5 mm. It might be worth noting that many of the headsets out there now have bluetooth ...


2

You are basically describing an ejector pump. This would not be very efficient, I'm afraid (I'll try to dig up some comparative figures). The arrangement in this case would be a jet engine used to accelerate an amount of air inside a duct. With a long enough duct, the end result would be a single airmass exiting at (somewhat) uniform speed. Unfortunately, ...


1

Here's a site comparing many of the headsets that are available https://www.sportys.com/pilotshop/aviation-headsets/guide.html I don't know if laptops support audio-out via bluetooth (I've not seen that, my HP Probook and my Lenovo W530 don't), so you might want to look at units which accept a wired Aux-in, and then plug in a wire from the laptop ...


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