Inside a 747: wind noise or engine noise?

Factual Questions
1

Ever since I was a little kid I have ridden on jets. The 707 was the most impressive, but only because I was so little. The 747 just blew me away.

But even after all these years, when I settle down to try to sleep in a big jet, pressing my ear to the pathetic pillow against the hull, I wonder: is that thrumming, pulsing noise the result of the engines blasting, or the sheer pressure of the winds careening against the plane’s hull?

It does not seem to diminish, no matter what jet or how far I get from the actual thrust source, be it in the tail or the wings, nor does it get quieter with more altitude (thinner air? that means it’s all the engines and not the wind).

Sorry. I have spent at least a cumulative year of my life pressed up against an airplane hull thus, and wish to know the truth. I’ll sleep better next time.

Addendum: Oh, and what causes the characteristic “whining” sound of a jet engine?

2

It could be both wind noise and engine noise. One way to tell would be to see how it changes as you are under full thrust on takeoff versus when you are at cruise, but even that’s not a fair comparison.

The “whine” of a jet engine is part of the nature of their operation. I mean, it’s due to the sound of the air moving through the turbofan and the jet portion. Some people claim that it’s “bearing whine” of the engine, but I really don’t think that is the case, as anything that made that much noise in bearings seems likely to be on its last legs IMO.

3

IIRC, rolling element bearings are rarely, if ever used anymore on the turbine shaft I’d say your hunch is correct. Usually they’re plain bearings with no moving parts… not very dissimilar to the crank bearings on a car engine.

4

Turbine engine main bearings are either ball or roller types. Plain bearings like you see on a recip aren’t used because heat buildup would be prohibitive. You may see plain bearings or bushings in the accessory section, though.

5

That makes no sense to me. Plain betters are better suited for high heat than rolling as the active lubrication film itself is carrying the heat away.

I have never had the opportunity to dismantle a turbine, but from my past history (I am a bearing guy) and all that I have read has showed the bearings to be of a plain style because of heat and speed. I know that the relatively recent advent of good ceramics has made some inroads in “smaller” applications, but I was not aware of them in larger (IE - aircraft) applications.

In your experience and seeing rolling element bearings, can you tell me what they’ve used? I’d be genuinely curious to know!

6

If there is an aircraft turbine out there with plain bearings, I have never seen or heard of it. Course, I ain’t seen everything, by a long shot.

I have some documentation right here for several popular aircraft turbines, if you are interested:

Pratt and Whitney JT8D:
Probably the most popular engine ever for airliner use. Used in the 727, 737, DC-9 and MD-80. The engine has seven main bearings.
#1 is a rollerbearing for the N1 compressor, mist and vapor lubricated.
#2 is a duplex-ball bearing for the rear of the N1 compressor, directly lubed by an oil stream.
#3 is a single ball type for the front of the N2 compressor, direct oil jet lube.
#4 is a duplex-ball type for the rear of the N2 compressor, direct oil jet lube.
#4 1/2 is a single-roller for the N1 shaft, direct oil jet lube.
#5 is a single-roller type for the front of the N2 shaft, mist lubricated.
#6 is a single roller which supports the N1 turbine, direct lubed.

Allison 250:
Popular for helicopter use.
8 main bearings, all of the single ball type except for the rear power turbine and front gas producer bearings, which look to be single roller. All I have right here is a cutaway diagram, so it’s hard to tell. All direct oil jet lubed.

Pratt and Whitney PT-6:
Verrrrry popular for medium to large turboprop airplanes, such as the King Air.
Has roller bearings in the prop reduction gearbox.
Front thrust bearing is ball type.
2nd stage carrrier bearing is single roller type.
#1 and #4 main bearings are identical ball type.
#2 and #3 main bearings are identical single roller type.

That’s all I have sitting here with me. If you would like the bearing types for another model of engine, let me know and I would be happy to look it up for you. Or, since you are in the bearing biz, if you want some part numbers or specifications, I will see what I can come up with.

7

I’m guessing it’s both engine and windstream, the proportion varying but always some of each.

Although you might expect the sound of the windstream to dimish with thinner air at higher altitude, the thinner air also reduces drag, which allows the plane to travel faster, which results in an increase in wind noise which basically balances everything out to the same level of noise.

Having flown in an aircraft without an engine, I can say that there is a noticable amout of noise from going through the air. In powered aircraft, the sound of the windstream increases with increasing speed - but then, so does engine noise.

In the planes I fly I can distinguish the noise coming from the engine vs. the noise from the windstream. I would presume jet pilots can do something simillar. If you tried, you yourself could probably learn to distinguish the two sounds, if you haven’t learned to do so already without realizing that’s what you’re doing.

8

I’d love a couple of examples if you could give me a couple part #s.

Thanks! I appreciate it!

9

I will look up the mains for a JT8D tomorrow at work and get back to you.

10

As for the whining I´d put my bet on the stator and turbine blades of the engine; there´s a lot of air and hot gasses moving and changing directions trhu them, and being thin metal pieces I would venture that they resonate a quite a bit; and the sum of all those small parts resonating produces the whining.

The roar, of course is caused mostly by the exhaust.

As for the overall noise, I´ll get Salomonic and say that it´s both cause by the engine and the air stream. :smiley:

11

The whine comes mostly from the compressor section of the engine. The first stage of compression on a high bypass 777 engine is that huge fan on the front of the engine, with a relatively low pitched sound. Compare that noise to the more whiney 727, which has a low bypass turbofan.

The roaring noise you hear is from the shearing of the hot high-velocity exhaust and the outside air.

Modern high bypass engines tend to have more noise coming from the inlet side. They get a great deal of their thrust from the bypass air, and use most of the exhaust energy to drive the compressor.

12

Wow! I didn’t expect to get into the physics of ball bearings in turbine engines :slight_smile:

But now that I come to think of it, the people we should ask are the passengers and crew of Air Transat Flight 236 to Lisbon (an Airbus A330) which ran out of fuel for various reasons and glided to a landing in the Azores.

Somehow, though, I don’t think they would really want to discuss the non-engine-noise portion of the story.

13

I’ve wondered this question several times myself, and being a pilot, I decided to find it out myself. This is what I learned (sorry, no cites, mainly just personal experience and various notes of wisdom from many sources over the years)

On an airliner, most (99%) of what you hear is the engines. Especially at cruise and takeoff… I highly doubt any of what you hear is wind noise.

They’re soundproofed quite well, and my understanding is that most of what you hear is coming through the metal structure of teh plane (which is why when an aircraft goes supersonic, the engines can still be heard in the cockpit)

The sound of drag would be near-nonexistant in a large pressurized and soundproofed aircraft. They try to keep drag to a minimum (more drag = more fuel = more $$$) While it does get louder as you increase speed, at the high altitudes that airliners fly at, airspeed is indicated much lower than the groundspeed. Therefore, as their speed and altitude increases, wind noise should stay around the same level.

As an interesting aside to that point; some large airliners may be flying at as low as tens of knots (indicated air speed) above their indicated stall speed (which does not change in straight and level flight) when they’re crusing at altitude.

Stand under an airliner when it’s landing, it will be extremely quiet… About as quiet as they are when idling on the ground, you probably won’t hear any wind noise at all. (and that’s with flaps and gear down) Likewise, in the aircraft, when they reduce their power before landing, you usually cannot hear the engines (compared to cruise anyways) Although you can usually hear the drag when the gear and flaps are lowered. (although it’s usually, in my experience, just a slight raise in the background noise.) But at cruise, you’re hearing engine noise.

Actually quite interesting. I’d love to go in to aircraft design if my math was stronger.

Oh, and sorry if this is a little scattered (or completely unreadable due to bad grammar) It’s been a long day. (Wasn’t I supposed to go flying today?)

14

Just letting you know, I haven’t forgotten, just been busy. I will get you the info.