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 Fuel Consumption Based On Incremental Drag
 Seyah From United States of America, joined Mar 2008, 3 posts, RR: 0Posted Tue Mar 4 2008 09:44:35 UTC (8 years 2 months 3 weeks 2 days 15 hours ago) and read 6864 times:

 I'm trying to determine how much additional fuel would be consumed if an aircraft were to add a specific amount of drag. For instance, if an airline were to install an antenna on a B737-800, how much additiona fuel would the aircraft consume due to the incremental drag associated with the antenna. I know the amount of drag quoted by the manufacturer (~50 lbs) and I'm assuming that's at 0.8 mach and 35k feet. I understand it will be different depending on the operating characteristics of the aircraft, flight phase, etc. so I'm really just focusing on cruise and would use some industry averages for block hours, flights, etc. Thanks.
 JetMech From Australia, joined Mar 2006, 2724 posts, RR: 52 Reply 1, posted Tue Mar 4 2008 10:24:18 UTC (8 years 2 months 3 weeks 2 days 14 hours ago) and read 6859 times:

It's a tricky question as you noted. One reason is that a given increment of additional thrust from a jet engine does not necessarily equate to a given incremental change in fuel consumption. As you can see from the following chart, the change in the amount of fuel you need to get a particular additonal thrust increment depends on the operating point of the engine at the time.

http://www.aircraftenginedesign.com/pictures/fig_14e.jpg

To answer your question, you would need to know the thrust the engine is puitting out at your cruise scenario. You would than need to find a similar thrust vs. SFC chart for the CFM56-7B engine to find out how an additonal 50 lbs of thrust will affect the SFC.

Regards, JetMech

 JetMech split the back of his pants. He can feel the wind in his hair.
 Seyah From United States of America, joined Mar 2008, 3 posts, RR: 0 Reply 2, posted Tue Mar 4 2008 10:46:35 UTC (8 years 2 months 3 weeks 2 days 14 hours ago) and read 6853 times:

 JetMech, Thanks for the quick response. I've found some of the info you're refering to for different engines and have run some numbers but due to the size of my results, I was thinking I might need to ask someone more "learned" than I. For example, for a CFM56-7B engine on a B737NG, I found the values of TSFC = 0.55 and Thrust = 5,960 lbs at cruise (0.8 mach and 35k feet). My calculations resulted in additional fuel burn of ~27 lb/hr for the 50 lb drag antenna; which seemed excessive. With the Thrust and TSFC values above, do you know how to generate a fuel number? Thanks.
 JetMech From Australia, joined Mar 2006, 2724 posts, RR: 52 Reply 3, posted Tue Mar 4 2008 12:54:39 UTC (8 years 2 months 3 weeks 2 days 12 hours ago) and read 6831 times:

 Quoting Seyah (Reply 2): TSFC = 0.55 and Thrust = 5,960 lbs

Assuming the SFC has the units of lb's fuel per hour per pound of thrust, the absolute magnitude of your fuel consumption is about 3,278 lb's / hr. It thus looks to me that your additional 27lb's / hr due to the antennae will bring this to 3,305 lb's / hr, which is an increase of about 0.824%.

This seems reasonable to me, as thrust equals drag in steady state cruise conditions. The aircraft drag would thus be 5,960 lbf not including the antenna. The additonal antenna drag is approximately 0.839% of the aircraft drag figure. Not sure if this helps!

Regards, JetMech

[Edited 2008-03-04 13:26:08]

 JetMech split the back of his pants. He can feel the wind in his hair.
 Jetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25 Reply 4, posted Tue Mar 4 2008 14:19:54 UTC (8 years 2 months 3 weeks 2 days 11 hours ago) and read 6807 times:

 It's a very simple sum, no need to make it complicated. If TSFC is 0.55 then the additional fuel flow for 50 lb of drag is: 0.55 * 50 = 27.5 lb/hr The intermediate step of working out fuel flow from an assumed thrust is not necessary.
 The glass isn't half empty, or half full, it's twice as big as it needs to be.
 Seyah From United States of America, joined Mar 2008, 3 posts, RR: 0 Reply 5, posted Wed Mar 5 2008 05:26:56 UTC (8 years 2 months 3 weeks 1 day 19 hours ago) and read 6767 times:

 Jetlagged, Thanks for your input as well. I'd hoped it was as simple as you stated but when I factored in annual block hours (typical ~3,000), conversion of pounds to gallons (6.7 lbs/gal) and a fuel price of \$2.50/gal, the annual incremental fuel cost of \$30,784 seemed abnormally high.
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