Question: Engine settings in cruise config.

[cieloitaliano]

When a jet transport takes to the air the engines sound like they're being flogged to within an inch of their design capabilities. :eek: I know they are throttled back a bit once established in the climb out but when cruising altitude is achieved what settings, compared with climb out , are the engines set to. Can a comparison be made with another form of propulsion? Also, what would the engines sound like if I were positioned at FL370 and it flew by at close proximity? My question is: would they still sound like they do at take off?

[chiglet]

You wouldn't hear a thing...you'd be dead through lack of oxygen...:E
watp,iktch

[cieloitaliano]

Thanks for that Chiglet. I was speaking hypothetically not hypothermally. :hmm:

[BOAC]

No - jets run at around roughly 80-90% of max rpm in the cruise, and that reduction represents a significant and non-linear reduction in stress on the engine. You will get a rough idea if you sit near an engine in the cabin and listen to the change in noise when top of climb is reached.

[SNS3Guppy]

Part of what you're hearing on takeoff, especially with large, high bypass turbofans, is the sound of the airflow through the engine at low speed...not the jet effluence or jet noise, but the intake. As the airplane accelerates, the angle at which the incoming air meets the blades changes and the sound of the engine will change. The airplane will sound different when it's moving through the air quickly, as compared to the same power setting on takeoff.

The vantage point from which you listen to the engine makes a very big difference, too.

[cieloitaliano]

I notice that on a cockpit photo of a 737 taken at criuse altitude there was a lever below the throttles that appeared to be set to 'idle.'
Are the engines just 'ticking over' after TOC is achieved or, does 'idle' mean something else here?

[TopBunk]

Those 'levers' you mention are the fuel control switches. They can be at 'idle' or at 'cut off'. In the former the engines are working, in the latter they are shut down. It has no bearing on idling as in producing idle thrust, do not get confused over this. Generally, engines in the cruise will produce sufficient thrust to maintain the desired speed, this will be significantly more than idle and less than maximum, but in rpm terms closer to the latter.