Why are jet aircraft so noisy? A resolution?

[timbeau]

Forgive me for such a basic question.

I live under an intermittent flight path. While I accept this as a natural consequence of where I live (West London, which has many other advantages!) I am amazed at the sheer volume of noise that civil airliners produce.

The aircraft in question appear to be flying no faster than a propeller plane is capable of, though they may well be gradually ascending. These are 747s, 777s, 767s, Airbuses...everything.

Surely there is something fundamentally flawed with a means of propulsion that produces so much noise. Would the noise be lower if the turbofans were actually ducted fans using internal combustion instead of jet engines?

I can understand that the brute power of a turbofan would require a monster of an engine, but from the lesson given by the Apple G5, many slow fans are quieter than one big high speed fan.

Thus, would it not be quieter to have, say, 8 small ducted fans instead of 2 turbofans? Are combustion and emissions more controlled?

Sorry, lots of questions.

Thanks in advance
Tim

 

[GregLocock]

Big jets are more efficient than small jets. Long ducts are inefficient.

Noisy as they are, jets don't actually radiate very much noise power compared with their power output. (1 or 2 %, at a guess)

This is all very depressing. The main reason that jets are used in the first place is that they work well at high altitudes where the drag is less.

Modern jets use high bypass ratios which are a bit quieter than the old engines, size for size, and are halfway between turbojets and turboprops in effect.





Cheers

Greg Locock

 

[IRstuff]

Also, you are a bit off on your estimate of landing speed.

A Cessna has a landing speed of 41 mph, while a 747 has a landing speed of 180 mph, which is a factor of 4 difference

TTFN

 

[corgum]

IRstuff,

The initial question seems to involve climbout noise signatures of jet transports, which are said to appear to be flying about the same speed as the propeller {turboprop) transports, though it is not clear why the propeller planes are mentioned.

 

[IRstuff]

I'll have to disagree with the notion that turboprops are so much quieter than jets.

I've ridden relatively small turboprops that were quite loud.

I've also been on the edge of the runway with E-2C's and F-18's landing and they're both noisy.

TTFN

 

[Miper]

As noisy as jet transports are today, they are much quieter than those produced forty years ago. Engineers have added inlet treatment, bypass fans, and exhaust mixers; and have refined inlet designs and fan blade details in order to substantially reduce noise.

For engines designed at the same point in time, a larger engine will give lower noise per pound of thrust than a small engine; so that dividing a large propulsor into many smaller ones will not help overall noise, because the total thrust required remains the same.

A modern gas turbine engine is a remarkable device, providing extraordinary power densities with unparalleled efficiency and reliability. It is no accident that turbine engines have become the primary movers in powerplants, ships and airplanes. Nothing else can do what they do.

The idea of replacing the turbine engine core with a piston engine is being pursued in the low power range (100 – 350 HP), where suitable turbofan engines do not yet exist. One of the main goals for this work is to reduce the propeller noise of light airplanes. While this is doable for small aircraft, scaling factors make it difficult to apply this approach to transport airplanes, where the engines produce the equivalent of several tens of thousands of horsepower. Piston engines of this size are an order of magnitude larger than an equivalent turbine power section, many times heavier, less efficient, and have a fraction of the reliability.

There is more that can be done to reduce jet engine noise, and improvements will continue. It is worth knowing, however, that airliners are optimized for minimum operating cost while meeting a particular noise regulation. Fortunately, a low cost of operation tends to drive a design toward smaller engines (less fuel), which also means less noise. It may seem desirable to optimize airplanes instead for low noise. In this case, there would be a significant improvement in sound levels in your community, not only because the airplanes would be as quiet as possible, but also because only a handful of people could afford to fly in them.

Consequently, noise regulations require the most that can be economically achieved, not the utmost, regardless of cost. Unfortunately, that means we will still be assaulted by unwelcome noise for many years.

 

[timbeau]

Firstly, many thanks to all who have contributed and assisting me in correcting misconceptions and righting wrongs!

I would like to say, however, that I still question if turboprops are as loud as heard from the ground - and yes I am talking about climbout from approx 25 minutes drive from the airport. I have often seen turboprop craft at similar altitudes...but not heard them.

I have travelled in shuttle aircraft and they are indeed noisy, but then the airframe is smaller and the engines are very much closer!

Maybe this is due to the frequencies they give off or the nature of the noise. It is the groaning whine of the turbine, not the rumble of exhaust that appears more intrusive. If it were a Merlin or a Double Wasp I might be more tolerant!

I tend to agree with Miper in regard to aircraft manufacturer motivation. It must surely be so as I would have thought that the issue would have made more progress. Of course piston engines have their problems, but it was the thrust mechanism (ducted, prop or turbofan) I was more concerned with.

Thanks again!

 

[corgum]

IRstuff

Umm, I think the topic is external noise during climbout here.

Of course turboprops are noisier inside, but that wasn't the question. As timbeau has mentioned, even though interior noise in a turboprop is high, the external noise of a turboprop is noticeably less to a ground observer under the climbout flight path.

On the ground near either the E-2 or the F-18, noise levels can be beyond the ability of the human ear to discriminate differences. Besides, different airplanes carry different amounts of power on approach, so landing sound levels are not a meaningful comparison of climbout levels, which are almost always at maximum climb power settings. The original question has to do with climbout noise levels.

For those not familiar with the E-2, it is a turboprop-powered airplane with 5250 HP engines. Turbine noise from the gas generator sections at max power would be very high, though not as high as the F-18 at max thrust.

 

[FredGarvin]

"Maybe this is due to the frequencies they give off or the nature of the noise."

You hit on the head what the issue is...the nature of the noise.

A turboprop does not derive it's output from a high velocity jet exiting from it. A turboprop gets as much useful work as it can in the power turbine section to rotate the prop. That is the main reason for the noise level differences in most cases.

A turbofan will develop enough power in the turbine section to keep the engine running. The lion's share of the thrust is due to the acceleration of the jet stream out the back of the engine, which is a HUGE amount of air.

I don't know if you've ever had the opportunity to hear the Concorde take off, but that is a turbojet. If you could do a comparison between the turbojet and turbofan, you'd see that we have come a long way when it comes to noise just by moving to turbofans. It's been my experience that what you usually pick up on is the fan noise/whine, not the turbine noise.

I definitely agree that market conditions will push the issue of noise. Until it becomes mandated and economical for engines to be quieter the progress will come in baby steps, not leaps and bounds. We can see that in pretty much any industry though, not just aviation.

 

[aviat]

There have been rules out for several years concerning noise around airports. It is under IACO Annex 16 and enforced by participating states by varying degrees. Because aircraft last several years there are several chapter 1 aircraft still flying.
There is noise sensors placed around major airports and there are several airports that landing or takeoffs are prohibited at night. Pilots are required to depart along certain routes and if they don’t they could be subject to fines.

http://www.icao.int/icao/en/env/noise.htm

Also the new generation turboprops have a noise & vibration suppression (NVS) system that generates a frequency that opposes the aircraft engine and prop noise and cancels it somewhat. It gives an interior noise level in cruise of about 75 db, which is close to a turbojet.

 

[AeroE]

It's all about the mass of air being moved, and about your perception of the noise. In fact, perception of noise is the major hurdle in regulating and controlling noise - this is why there are so many dB scales in use, in an attempt to capture a means of mathmatically describing a nuisance that affects each individual differently.

You would be amazed at the amount of noise from a bare, unpowered airframe, and in many airplanes, this is a source of high frequency whistles, too.

Part of the reason turbo props sound less noisy is that their relatively large diameter propellors move a larger volume of air at low speeds, compared to the small volume/high speed air pumping of turbojets. Turbofan engines fall in between. The tips of the turboprop propellors moving at high speeds produce additional noise that is mostly captured in jet engine nacelles. The othe reason turboprops are less noisy is that they tend to be producing less power than the jets.

It's a complicated problem with uncommon (in aircraft engineering) social factors coupled with the engineering, and we can't just make it go away.