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David
Joined: 20 Oct 2000
Posts: 1381
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For Duncan-----Noise Level?
 Sat Jul 07, 2001 1:41 pm
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Mr. Kunz, Good morning. If I understand correctly, you are involved in jet engine maintainace or repair? If so I would like to ask a question. In another thread I asked the question, How can low flying jet aircraft, (Multi-engine)leaving a chem/contrail, make no noise what so ever.
Is there a device to quiet them to the extreme?
I am very interested in hearing a reply from someone with knowledge in this field.
Thank you,David |
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Duncan Kunz
Joined: 19 Oct 2000
Posts: 582
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Sat Jul 14, 2001 1:25 am
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Dear Mr. David,
My apologies for not answering your post earlier; I just got back from a week-long trip out of town. I did want to tell you that I'll pull together some basics, and post some inputs for you within 24 hours.
Regards,
Duncan Kunz
duncan.kunz@prodigy.net
Mesa AZ
480-891-2525 |
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Duncan Kunz
Joined: 19 Oct 2000
Posts: 582
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Sat Jul 14, 2001 10:39 am
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Dear David,
I am not engaged in jet engine maintenance or repair per se, although my company builds aircraft, we do not build the engines for them. Typically, when a customer selects a Boeing product, they choose from several available engines. On large fixed-wing aircraft, their choices are typically Pratt and Whitney (P&W), General Electric (GE), and Rolls-Royce (RTM).
In order for each of these engines to fit equally well on the aircraft and provide the same capabilities, the different choices for, say, a Boeing 747-400 will all be about the same size, have the same interface characteristics, and produce about the same power. In other words, a customer can’t order am engine from a different manufacturer and expect to get any special capabilities, like reheat (afterburner) or a “quiet” mode.
That said, the FAA, who has been the target of angry homeowners near airports, has recently (within the past ten years or so) imposed stringent noise limits on the airlines operating domestically. As a result, engines are much quieter today than they were five or ten years ago; but of course, they’re still pretty loud.
Now, after my typical engineering rambling, let me respond to your question: “Is there a device to quiet them to the extreme?”
No.
When a jet engine mixes lots of air with lots of kerosene and heats it lots of degrees, you will get lots of noise. The expanding air expansion from the resultant heat makes shock waves to which you eardrum reacts, sends an electrical signal to your brain, and --- ouch!
Stopping the sound (shock) waves mean not heating any air, which means shutting the jet engine off.
There are baffles available to aircraft engines which lower both the aural (sound) signature and IR (heat) signature of an aircraft engine, which is why it’s not all that easy, in a battle situation, to hear an aircraft coming or to shoot it down with an IR-seeking missile. But Delta or Southwest Airlines (both of whose aircraft I a passenger on this past week), like all airlines, are operating on pretty thin profit margins, and won’t pay a dime for anything they’re not required to have by the FAA’s sound/safety standards – and besides, they’re not engineered to go on commercial aircraft.
So the real question might be “How quiet can they get?” Well, as I mentioned earlier, modern aircraft engines are quieter, especially at optimized power settings. Also, differences in atmospheric conditions, such as humidity, wind velocity and vector, layers of air at different temperatures (“inversions”, etc.) can all decrease or increase the perceived sound made by an aircraft’s engines. Given the innate complexity of our atmosphere from the surface to 15,000 meters, you could have a multi-engine aircraft like a Boeing 737-500 as little as 1,500 meters (less than a mile) above you – and not hear it.
Under most conditions, aircraft (engines or control surfaces) make contrails when the temperature is about -40 deg (F or C) and the relative humidity approaches saturation (100 percent). This is usually at altitudes of around 8,000-10,000 meters and higher. Typically, at those contrail-forming altitudes, sound would take from between 25 and 35 seconds to get to you -- and that’s if it’s directly overhead. If the plane is at that altitude at a perceived azimuth of, say, 30 degrees from the horizon, the distance (and resulting time-delays) would be twice that. Often the plane will be soundless when you first see it and gone by the time you hear a faint rumble.
It’s VERY difficult for us – for several reasons -- to determine that altitude of an aircraft (we don’t have Doppler radar built in). But I can’t think of a way that a jet aircraft at altitudes lower than 1,500 meters won’t get your attention in a HURRY; and under almost every condition commonly found, an aircraft at that altitude will not be producing contrails.
Regards,
Duncan Kunz
duncan.kunz@prodigy.net
Mesa, AZ
480-891-2525
[p.s. Interestingly enough, I flew both last Sunday and yesterday (Friday) on ~10-hour trans-Pacific flights at altitudes from 8,500 to 10,000 meters. The little pop-up TV screens in Business Class seats have a flight recorder program that tells you where you are on a moving map, and include air speed, distance/time to the destination, altitude, headwinds, and outside temperature. What I found interesting was that (once we reached cruising altitude) from off the coast of Los Angeles through the Aleutian Islands, Sea of Okhotsk, and down to Tokyo-Narita Airport, the outside ambient temperature was within four degrees either way of -40 deg (F or C). This would’ve meant that we were probably producing “on-and off” contrails ourselves.]
[Edited 1 times, lastly by Duncan Kunz on 07-14-2001] |
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RidesTheWind
Joined: 27 Feb 2001
Posts: 1255
Location: The Void |
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Sat Jul 14, 2001 12:07 pm
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DK....Just wondering...The other day I witnessed a plane go over that looked like the back edge of the wings had a scalloped look as if you could visably see a row of objects neatly spaced along the edge of the wing...I note this,because this plane was spraying wingtip to wingtip.Is this possible?
My eyes don't lie usually. |
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Duncan Kunz
Joined: 19 Oct 2000
Posts: 582
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Sat Jul 14, 2001 4:51 pm
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Dear Mr. Wind:
I am not familiar with any modern large commercial aircraft with scalloped trailing edges to its wings; although the Dornier DO-X, which flew in the 1920's and 1930's, did have such wings, they are all gone now.
Some agricultural spraying aircraft can be fitted with spray nozzles which abut the lower portion of the trailing edges, but most of them have a separate nozzle and tube assembly. In either case, you would have the capability of spray from the entire wingspan, but i have only seen such spraying actually take place at altitudes of about ten feet.
Sorry I couldn't provide any more help, but if you have any further information (e.g., number of engines, tail number, estimated altitude, any markings, etc.) or a photograph, pass it on -- either on this forum or directly to me -- and I'll ask around.
Regards,
Duncan Kunz
duncan.kunz@prodigy.net
Mesa, AZ
480-891-2525 |
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David
Joined: 20 Oct 2000
Posts: 1381
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Sat Jul 14, 2001 5:58 pm
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Duncan, What a great reply, thank you.
I will try to be more aware of altitude in respects to noise level, con/chemtrail.
However, some of the planes I have seen are low, too low to contrail, and very quiet if noisy at all, leaving persistant trails.
David |
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