krisyyz
Topic Author
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DRAG/FF Percentages For New Aircraft

Thu Jun 16, 2011 5:21 pm

I have a question regarding fuel flow and drag computations that appear on the FMC of most aircraft.
I've learned that FF deteriorates as an aircraft gets older due to engine performance, condition of the skin etc. But what is the usual DRAG/FF specifications for a brand new, “out of the box” airplane? Is it usual to have a 1-2% higher FF than the plane was designed for? Is there any sort of compensation that airlines get when Boeing or Airbus delivers a plane with a 1.0 or 2.0 % FF?

Is having an increased Drag performance less common than an increased FF rate?

Thanks,

KrisYYZ
 
vikkyvik
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RE: DRAG/FF Percentages For New Aircraft

Thu Jun 16, 2011 6:26 pm

Quoting krisyyz (Thread starter):
Is having an increased Drag performance less common than an increased FF rate?

Well, to be honest, I'm not entirely sure what you're asking here. But fuel flow is directly related to drag. Thrust force has to be equal to drag force in unaccelerated flight. And the energy for thrust comes straight from the fuel.

So if your new airliner has higher-than-predicted drag, it will also have higher-than-predicted fuel flow (keeping everything else equal, like engine efficiency).
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LAXintl
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RE: DRAG/FF Percentages For New Aircraft

Thu Jun 16, 2011 10:24 pm

A new bird should ideally be at zero, if not even a fraction negative in values (perform better than book values) when delivered.

Somewhat related you also have growing issues were the FMC software does not match the operated plane configuration. For instance a US 757 operator needs to use a -4% bias on its 757 winglet fleet for the FMC to compute right numbers.
Other similar problems exist with 772s that have gone through Boeing PIP packages with lacking software to match. The MD-11 in its day was similar.
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krisyyz
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RE: DRAG/FF Percentages For New Aircraft

Fri Jun 17, 2011 9:21 pm

Quoting vikkyvik (Reply 1):
So if your new airliner has higher-than-predicted drag, it will also have higher-than-predicted fuel flow (keeping everything else equal, like engine efficiency).
Quoting LAXintl (Reply 2):
A new bird should ideally be at zero, if not even a fraction negative in values (perform better than book values) when delivered.

Somewhat related you also have growing issues were the FMC software does not match the operated plane configuration. For instance a US 757 operator needs to use a -4% bias on its 757 winglet fleet for the FMC to compute right numbers.
Other similar problems exist with 772s that have gone through Boeing PIP packages with lacking software to match. The MD-11 in its day was similar.




Thanks for the info! Much appreciated.

The reason I was asking was I saw a 1 year old B77W on an aviaiton video that had a 1.0% FF.

KrisYYZ
 
tdscanuck
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RE: DRAG/FF Percentages For New Aircraft

Thu Jun 23, 2011 4:50 am

Quoting vikkyvik (Reply 1):
So if your new airliner has higher-than-predicted drag, it will also have higher-than-predicted fuel flow (keeping everything else equal, like engine efficiency).

This is true, but the FMC has separate values because the drag factor will always be there regardless of thrust (e.g. an idle descent) while the fuel flow factor only has meaningful impact when the engines are off-idle. The total performance split between off-drag and off-fuel flow is important to get the whole flight profile properly calculated.

Tom.
 
OldAeroGuy
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RE: DRAG/FF Percentages For New Aircraft

Thu Jun 23, 2011 7:35 pm

Quoting LAXintl (Reply 2):
A new bird should ideally be at zero, if not even a fraction negative in values (perform better than book values) when delivered.

This may be true on a fleet average basis but it would be rare if it were true on an individual airplane and engine.

Fuel flow and drag are a result of the industrial processes used to build the engine and airframe respectively. Since the build process for either one is not 100% repeatable, variations creep in resulting in fuel flow higher or lower than nominal and drag lower or higher than nominal. Combining the two introduces additional uncertainty.

In short, actual airplane fuel mileage performance is a statitical process, with individual airplane performance distributed normally about a fleet mean value.
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vikkyvik
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RE: DRAG/FF Percentages For New Aircraft

Sun Jun 26, 2011 4:08 pm

Quoting tdscanuck (Reply 4):
This is true, but the FMC has separate values because the drag factor will always be there regardless of thrust (e.g. an idle descent) while the fuel flow factor only has meaningful impact when the engines are off-idle. The total performance split between off-drag and off-fuel flow is important to get the whole flight profile properly calculated.

Gotcha, thanks for the info.
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