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Fuel consumption

#1 User is offline   jwavey 

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Posted 08 August 2016 - 07:00 PM

I have been trying to predict fuel usage over the course of long sim flight in x-plane and I have a few questions.

First the facts:

I have studied real world specific fuel consumption (sfc) numbers for several planes -- currently working on a 747-400. I have the numbers where I want them in Planemaker. At top of climb (TOC) the computer that calculates range (both time and distance) shows 10 hours and about 5200 nautical miles. The flight performance grafts I have seen say the aircraft can fly about 7200 miles with 88k payload and full tanks on takeoff. As one would expect, as the fuel burns off during flight, the weight of the aircraft is reduced and the range increases. As one would expect, this is not linear. So the computer is looking at that moment and says "You can only fly 10 hours at this burn rate" and doesn't consider that in a couple of hours the weight will be less and actual consumption will be less.

This is so pronounced that between TOC and 1000 miles from the departure airport the range numbers only change a couple hundred miles. (On a recent test flight,at TOC the computer had 5200 mile range and 1000 miles the range was 4900). This makes calculating fuel usage...difficult. For me, at least. In fact, the way I have the plane set up now it will fly 7200 miles so I am confident in my Planemaker settings.

Does this happen in the "real world"? I have written down the burn rate for various weights and am I to average those to come up with the correct fuel load for an particular flight?

Any input would be helpful...

John
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#2 User is offline   jwavey 

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Posted 09 August 2016 - 10:05 AM

PS.

Last night I decided I was looking at this incorrectly. So, I did this test.

Starting at a waypoint just off the coast from LAX ~ FL340, no winds, 72f at surface, 100k payload and 365k fuel, no calculation for reserves, heading 200.

I started the timer and set the sim speed to X6. At each hour along the way I paused and noted the remaining fuel. At 14.5 hours and 7,204 miles from the waypoint, the engines spin down...out of fuel. (I stepped up to FL360 about 8 hours into the test flight.) BTW, I was well south of Australia.

I then took the notes I made along the way and inverted the "hour" notes -- "1 hour" became "14 hour" and it looks something like this:

14 hour: 340k
13 hour: 315k
12 hour: 295k
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