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DRAG/FF Percentages For New Aircraft  
User currently offlinekrisyyz From Canada, joined Nov 2004, 1593 posts, RR: 0
Posted (3 years 6 months 1 week 2 days 22 hours ago) and read 6548 times:

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

6 replies: All unread, jump to last
 
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10340 posts, RR: 26
Reply 1, posted (3 years 6 months 1 week 2 days 21 hours ago) and read 6537 times:

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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User currently offlineLAXintl From United States of America, joined May 2000, 26150 posts, RR: 50
Reply 2, posted (3 years 6 months 1 week 2 days 17 hours ago) and read 6505 times:

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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User currently offlinekrisyyz From Canada, joined Nov 2004, 1593 posts, RR: 0
Reply 3, posted (3 years 6 months 1 week 1 day 18 hours ago) and read 6429 times:

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


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (3 years 6 months 3 days 10 hours ago) and read 6319 times:

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.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3598 posts, RR: 66
Reply 5, posted (3 years 6 months 2 days 20 hours ago) and read 6287 times:

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.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10340 posts, RR: 26
Reply 6, posted (3 years 5 months 4 weeks 1 day 23 hours ago) and read 6204 times:

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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