PHX787
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Engine Noise Differences In Flight?

Mon Nov 26, 2012 5:50 am

Hey guys,

this may be a confusing question, as I am not a tech guy at all, but that's why i'm posting here...

I flew a domestic DL 763 back to PHX today and when I took off, the noise coming from the engine which was almost below me and to my left was much louder during takeoff and climb, but the whirr coming from the engine slowly faded to the normal sound of an aircraft plowing through wind-shear at 500MPH....I know the pilots probably don't use full power all the time, but they sure don't use anything lower than normal, in my opinion, so what causes the change in noise that is heard in the cabin? Is it pressurization?
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m1m2
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RE: Engine Noise Differences In Flight?

Mon Nov 26, 2012 6:04 am

Pressurization wouldn't change the sound coming from the engines. The power settings generally would be Take-off, climb, cruise, and descent (assuming the flight follows a "parabolic" flight path). Of course power settings can vary during flight as the aircraft climbs or descends during the flight to avoid turbulence or for ATC requirements.

The cabin also sounds different as flaps/slats are retracted and the wing is cleaned up as the aircraft climbs.

[Edited 2012-11-25 22:06:33]
 
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DocLightning
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RE: Engine Noise Differences In Flight?

Mon Nov 26, 2012 7:12 am

As you climb and pick up speed, the slipstream becomes the dominant noise. Also, if you listen carefully, you can hear when the engines are throttled back to cruise power from climb. However, AFAIK, climb power is not a constant setting all the way from climbout to initial cruise altitude. I think it's a gradual decrease.
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ZSOFN
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RE: Engine Noise Differences In Flight?

Mon Nov 26, 2012 10:28 am

I'm not so familiar with the 767's characteristics but certainly when I've sat in the same area of the cabin as you've described on 777s (particularly 77Ws) I've noticed the leading edge slats create a huge amount of wind noise as airflow gets disturbed and becomes non-laminar around the fuselage by wing's the leading edge. As soon as they're retracted the noise levels reduce significantly.
 
tdscanuck
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RE: Engine Noise Differences In Flight?

Mon Nov 26, 2012 3:19 pm

Quoting DocLightning (Reply 2):
However, AFAIK, climb power is not a constant setting all the way from climbout to initial cruise altitude. I think it's a gradual decrease.

It's something of a stepwise decrease in terms of fan speed. You're initially at takeoff power, then climb power from about 1500' to your initial clearance altitude (typically not your initial cruise altitude). ATC often has to make a few "steps" to get you up to initial cruise. At each level-off you're controlling on speed rather than thrust.

On top of all that, the actual engine thrust (amount of power being dumped into the air) is dropping with altitude due to the falling air density.

Quoting ZSOFN (Reply 3):
I've noticed the leading edge slats create a huge amount of wind noise as airflow gets disturbed and becomes non-laminar around the fuselage by wing's the leading edge.

It's almost always non-laminar around the fuselage unless you're in the flight deck. The slat noise is caused by separated flow, not turbulent (non-laminar) flow.

Quoting PHX787 (Thread starter):
I know the pilots probably don't use full power all the time, but they sure don't use anything lower than normal, in my opinion, so what causes the change in noise that is heard in the cabin?

Pilots will use as little power as necessary to get the job done. This applies to all flight phases, so the actual engine speed at any portion of the flight may differ considerably from flight to flight, even on the same aircraft. You will get a huge shift in engine noise when the fan tips drop below sonic speed. The only time you can be pretty confident that the engine is always at the same speed is on descent, when the engine is usually at flight idle. That speed may shift with whether anti-ice is on or not though.

Tom.
 
jetlife2
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RE: Engine Noise Differences In Flight?

Mon Nov 26, 2012 9:49 pm

A lot (maybe all) of this effect is due to fan tones, i.e. an acoustic resonance of the fan aerodynamics that occurs at certain corrected fan speed, that the engine passes through temporarily. Depending on the engine model this can persist for some minutes during climb. Then the aero-acoustics change, usually because the corrected fan speed increases and the fan passes out the other side of the tone.
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eaglepower83
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RE: Engine Noise Differences In Flight?

Wed Nov 28, 2012 4:30 pm

The Fan/engine noise is loudest at takeoff (obviously) due to it being at high throttle.
At this speed, the fan blade tips are rotating at super-sonic speed.
Tiny sonic booms are occuring at the blade tips, thereby giving off that "buzzsaw" noise often heard during takeoff/climb.

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