I am not sure how widely the software is used, but I am using Piano-X to investigate the influence of total aircraft drag on block fuel consumption and noticed something odd. It is my hope that someone familiar with the software can assist me.

If I decrease the total aircraft drag both the block time and fuel consumed increases, which is the opposite of what I would expect. As expected however, the block fuel/min does decrease.

Shown below is the results from the investigation for the drag systematically decreased : enter image description here

I expect that the block fuel/time would decrease but don't understand how the total time and fuel will increase for decreased drag.

Is there a rational explanation for this?

The software documentation only states the following concerning drag calculation:

"Aerodynamic drag is calculated in detail (as a function of lift coefficient, Mach number, and Reynolds number) and tuned with actual lift/drag data (‘polars’) whenever these are known. You can either factor the drag uniformly using one overall value or you can adjust certain items individually."

My approach was to vary total drag, i.e. induced and parasite drag simultaneously.

I will add that the software calculated total block fuel including estimates for both taxi's and the climb-out etc. Now in taxi drag doesn't have an influence, but I doubt that this will make such a large contribution.

  • $\begingroup$ How do you / does Piano X change the drag? By changing the aircraft's speed or by changing the aircraft's design? From what I can see it looks like the aircraft is flying slower with the lower drag numbers. If you are using a jet / turbofan engine it would make sense that the block fuel goes up. $\endgroup$ – DeltaLima May 15 '15 at 0:06
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    $\begingroup$ Why is it measuring time in meters? $\endgroup$ – raptortech97 May 15 '15 at 0:55
  • $\begingroup$ That chart seems to show the time taken going up by almost a factor of 2 when the drag is cut in half. If drag is changed only because speed changes, then your COEFFICIENT of drag didn't change, just the speed did. If the program were truly changing the coefficient of drag (simulating a cleaner, less draggy aircraft), then it is not at all obvious why speed would go down like that, and it is counterintuitive that fuel burn would go UP with the cleaner aircraft. Something else is clearly at work here. Program bugs or misunderstood/misrepresented parameters would be my top 2 guesses. $\endgroup$ – Ralph J May 15 '15 at 0:57
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    $\begingroup$ @raptortech97, if you are clever enough to notice that , I am sure you are also clever enough to deduce that I meant 'm' for 'minutes' (-; $\endgroup$ – Jonny May 21 '15 at 7:28
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    $\begingroup$ Reason for question being asked was due to a bug in a software package $\endgroup$ – Jonny May 22 '15 at 9:16

I would suspect that reducing drag in this case would be result of reducing airspeed. If your drag coefficient 1 is for a certain speed, slowing down would cause smaller fuel flow but increase flight time and hence increase total consumption.

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    $\begingroup$ That is not necessarily the case. Slowing down from typical cruise speeds generally gives greater fuel economy, not less, since you're usually cruising faster than max range & max endurance speeds. Of course, if you are starting AT the speed for max range, slowing below that WILL increase fuel burned per mile traveled, but that's usually pointless. $\endgroup$ – Ralph J May 15 '15 at 0:48
  • $\begingroup$ @Ralph J Correct. But assuming any speed for any coefficient on the given graph is a little hard. $\endgroup$ – Sami May 15 '15 at 7:04
  • $\begingroup$ Totally agree. Unless an expert on that particular software can address what's going on, there are far too many unknown assumptions here to make sense of the data provided. On its face, the data makes no sense whatsoever as far as I can tell. $\endgroup$ – Ralph J May 15 '15 at 15:53
  • $\begingroup$ I contacted the makers of the software and it turns out that range was not kept constant and that was the reason for the behavior. Sort of a bug in the program. If you remove this answer I will delete the question. I think it will be better. $\endgroup$ – Jonny May 22 '15 at 5:52

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