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rjsampson
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Fuel flow: Same N1/EPR at different altitudes?

Fri Feb 26, 2021 10:59 am

Gents,

On takeoff: If say, an 350 is pushing out (say, 85% N1), ~80 pounds of thrust per engine...

...and is at 85% N1 at cruise: Is the fuel flow appreciably different, while pushing back far less thrust?

[Disclaim the multitude of variables. Stick with ISA at TO, FL370 at cruise, etc. etc.]... just curious as to the overall picture?
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Flow2706
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Re: Fuel flow: Same N1/EPR at different altitudes?

Fri Feb 26, 2021 11:37 am

Fuel flow at higher altitudes is significantly lower than at lower altitudes due to density and other factors. Don't know the values for the 350 but on an A320 the fuel flow will be around 4000kg/h per engine during takeoff, during cruise it is around 1000-1200kg/h per engine.
 
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Starlionblue
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Re: Fuel flow: Same N1/EPR at different altitudes?

Fri Feb 26, 2021 12:48 pm

Similar differences on the A350. In the cruise 3000kg/h per engine. At takeoff 8000k+kg/h. N1 not that different.
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mxaxai
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Re: Fuel flow: Same N1/EPR at different altitudes?

Fri Feb 26, 2021 3:33 pm

Of course thrust at cruise altitude is significantly less than at sea level as well.
 
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rjsampson
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sat Feb 27, 2021 5:37 pm

Makes total sense. On aircraft with moving thrust levers (ie, not-airbus): Would the thrust lever be in the same position during a T/O run at 85% N1, as it would be for 85% at FL370? Would it be lower?
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GalaxyFlyer
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sat Feb 27, 2021 6:47 pm

Basically, same position, but throttles are just rheostats relaying lever angle to the FADEC.
 
thepinkmachine
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sat Feb 27, 2021 7:01 pm

What is worth noting, fuel flow is more or less directly proportional to the thrust produced by engine, regardless of altitude.

i.e. the engine will produce (roughly) the same thrust for given fuel flow, regardless of altitude - although at different N1

This is not entirely correct, especially for turbofans, but close enough...
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Lemmy
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sat Feb 27, 2021 11:43 pm

GalaxyFlyer wrote:
Basically, same position, but throttles are just rheostats relaying lever angle to the FADEC.


Prior to automated engine controls, how was this managed? Did the pilot or FE have to adjust for air density?

Which brings up another question: What, exactly, did the throttle on early jet engines actually DO? On a carbureted piston engine, I can visualize the throttle opening and closing and imagine how that controls the fuel/air flow into the piston. Did jet throttles simply control the amount of fuel into the burner cans?
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Starlionblue
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sun Feb 28, 2021 12:44 am

Lemmy wrote:
GalaxyFlyer wrote:
Basically, same position, but throttles are just rheostats relaying lever angle to the FADEC.


Prior to automated engine controls, how was this managed? Did the pilot or FE have to adjust for air density?

Which brings up another question: What, exactly, did the throttle on early jet engines actually DO? On a carbureted piston engine, I can visualize the throttle opening and closing and imagine how that controls the fuel/air flow into the piston. Did jet throttles simply control the amount of fuel into the burner cans?


AFAIK only the earliest jet engines metered fuel purely on the basis of thrust lever position. Jet engines react badly to the wrong air/fuel proportion, so any roughness on the lever(s) or sudden change in air density could rapidly quickly lead to issues like overheating or the flame going out.

A middleman between the lever and the engine was needed, and hydromechanical fuel controls were introduced. I don't know how early these came about but I'm guessing the early fifties. These were followed by hydromechanical units with electronic controls. And now we have FADEC.

Even today, if your autothrust/autothrottle is inop, you have to adjust the lever positions.

There's some interesting information here on fuel metering systems: https://www.aircraftsystemstech.com/p/t ... neral.html
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
e38
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Re: Fuel flow: Same N1/EPR at different altitudes?

Sun Feb 28, 2021 2:18 am

Lemmy wrote:
Did the pilot or FE have to adjust for air density?


No, prior to automatic engine controls such as FADEC, we referenced aircraft performance manuals to obtain either N1 or EPR values for specified phases of flight--particularly takeoff, climb, and cruise. Once the throttles were set, adjustments were minimal; i.e., we did not need to make adjustments for air density.

For descent, we normally used idle power, unless a higher setting was required to maintain pressurization, and then for approach and maneuvering in the traffic pattern, we used approximated N1 or EPR values based on configuration, desired airspeed, and flight parameters (level flight, descent on 3 degree glidepath, etc.).

It was pretty simple. And it still is, with automation.

e38

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