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# Fuel burn-off percentage

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# Fuel burn-off percentage

29th Jan 2010, 17:18

Join Date: Aug 2000
Location: UK
Posts: 3,644
Fuel burn-off percentage

I'm defining fuel-burn off percentage as the proportion of fuel burnt carrying its own weight. Forgive me if there's a more standard term -- that seems to be what Boeing calls it. In considering transport category aircraft, typical figures quoted are 4-5%/hour.

I'd like to estimate the fuel-burn off percentage for a light aircraft. Many light aircraft POHs don't give weight-dependent cruise speed figures. Naively, I would have thought that a 1% increase in weight causes about a 1% increase in drag at best L/D, hence if I fly around at best L/D the whole time the fuel burn-off % is fuel-burn/aircraft-weight. e.g. if I burn 36 kg/hr to fly a 1800 kg aircraft, the burn-off % is 2%/hr. Presumably at typical light-aircraft cruising speeds well above best L/D the proportion goes down.

Does that sound right? Is there a better way to estimate this?
29th Jan 2010, 18:34

Join Date: Aug 2001
Location: Dorset
Posts: 775
Fuel flow in a piston/prop is proportional to power not thrust.

So minimum fuel flow will be at Vmp not Vmd.

A 10% increase in weight gives approximayely a 5% increase in Vmp.

Power required is proportional to TAS cubed.

So a 10% increase in weight, will increase Vmp by a factor of 1.05.

1.05 cubed is approximately 1.15.

So our 10% increase in weight costs us about a 15% increase in fluel flow.

All very approximate of course.....
30th Jan 2010, 07:46

Join Date: Aug 2000
Location: UK
Posts: 3,644
Fuel flow in a piston/prop is proportional to power not thrust.
So minimum fuel flow will be at Vmp not Vmd.
I guess I was assuming that I had places to go. In which case specific range is important, and you want to maximise v/(D*v), in other words minimize D, not D*v.

But I think you illustrate well that the fuel flow increment depends on the airspeed chosen.
30th Jan 2010, 09:27

Join Date: Aug 2001
Location: Dorset
Posts: 775
If we accept that the power required curve is representative of the fuel flow curve then we can use it to test how fuel flow will vary with weight. But we can also test the effects of speed.

If we increase weight, the power required curve moves up and to the right, but it remains nested within the previous (lower weight) curve. This means that for a given weight increase, the % fuel flow penalty is less at higher speeds.

Using one of the charts in the CAP 697 Flight Planning Manual (for the JAR exams) I have found that (in the climb) a 5% increase in weight causes a 16% increase in fuel flow. But the scale of the chart is very small so the figures are obviously very sketchy. But all of the cruise charts are for a single aircraft weight, so they are of no help.
31st Jan 2010, 14:15

Join Date: Oct 2007
Location: Hong Kong
Posts: 23
Not much in it.

Operaters of turbo props usually like their planes flown as fast as possible as maintainence is much more expensive than fuel. So the increase in specific fuel consumption is only as a percentage of the drop in TAS at higher weights. One can work this out from the Max cruise power tables in the performance section.

Enjoy!
31st Jan 2010, 22:29

Join Date: Feb 2005
Location: flyover country USA
Age: 77
Posts: 4,580
Operaters of turbo props usually like their planes flown as fast as possible as maintainence is much more expensive than fuel.
I'd think airframe maintenance cost is primarily a function of CYCLES (TO & landing) flown, not hours. Thus, independent of speed.

On the other hand, turbine engines often have a "sweet spot" where total propulsion cost (fuel + maintenance) can be minimized. The sweet spot may be somewhat less than max cruise.

And fuel cost varies from time to time, so the "sweet spot" will shift.

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