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Effect of Fuel Specific Gravity on Range

  • hazzafx
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hazzafx created the topic: Effect of Fuel Specific Gravity on Range

Hey there, I am recently studying ATPL Systems and Aerody and came across this question (pictured in attatchment).

The answer is (a) and I was wondering how a fuel with a lower specific gravity would reduce range and not actually increase it?

Any help is welcome,

Thanks!
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  • John.Heddles
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John.Heddles replied the topic: Effect of Fuel Specific Gravity on Range

Absolutely and positively (a). The same applies to avgas powered cycles and is totally relevant to the CPL/PPL/RPL folks working hard to get their licences via Bob's books. Generally, though, not something we think too much about on little aeroplanes because we have little fuel tanks and the difference isn't all that great.

Think a bit about it .... what are you doing with the fuel ?

Answer - burning it.

Now, if you hark back to your engines stuff, you will recall something about optimum fuel/air ratios and the maximum/minimum ratios which can support reasonable combustion. Now, these numbers are talking about ratios (fractions) by (or considering) weight (strictly mass) of the fuel and the air (oxygen). You can find plenty of links to confirm this eg x-engineer.org/automotive-engineering/in...-engine-performance/

So, it follows that, to burn some fuel (doesn't matter whether we are into kero or petrol), we need to balance the mass (weight) of the fuel stuff to the mass (weight) of the air (oxygen) stuff. That is, we can burn bigger and better if we have more stuff in total to burn. If we are burning a wood fire at home in winter, does it work better with lots of wood or a couple of bits of wood ? I just stoked up our fire with lots of wood and it is burning just fine. (Bob and Stuart, of course, wouldn't know about this, living in the near tropics).

Now, what does SG mean ?

SG is the ratio of the mass of something to the mass of the same volume of water. Avtur, typically around 0.79-0.80, Avgas 0.69-0.72. So, if we have fuel with a lower SG, it weighs LESS than fuel with a higher SG considering volume comparisons. The fuel with the higher SG allows us to burn bigger, better, and for longer ie we can go further.

As a for instance, some years ago, QF went for a jolly in a 744 direct from the UK to Australia. See www.airwaysmuseum.com/Qantas%201st%20Eng...0non-stop%201989.htm

They went to the trouble of sourcing high SG fuel and then made sure that it was real cold when they loaded it onto the aircraft (colder fuel has a slightly higher SG), even going to the trouble of towing the aircraft out to the runway and topping up prior to departure ... and, even then, only just made it by the skin of their teeth. A great achievement and, I am sure, kept the crew on the edge of their Jeppesens running lots and lots of howgozits on the way over.

I've only ever had one trip (much shorter) in like vein on the Electra (years ago) and, let me assure you, really minimum fuel operations are not any fun at all ....

But, if you want to hear a tale about real minimum fuel, Harold's little adventure, 40-odd years ago, probably takes some beating.



Some more details are in a PPRuNe thread at www.pprune.org/pacific-general-aviation-...fitzroy+crossing+F28

Engineering specialist in aircraft performance and weight control.
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  • hazzafx
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hazzafx replied the topic: Effect of Fuel Specific Gravity on Range

Awesome! Thanks John!

Would it work the same for endurance too?

As in, the fuel with the larger SG will have greater endurance?

Thanks :)
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John.Heddles replied the topic: Effect of Fuel Specific Gravity on Range

It doesn't really matter whether we are looking at range or endurance - just varies the derivation sums. Either we are seeking to minimise weight of fuel burned per unit time (endurance) or unit distance (range). Still driven by the available fuel weight.

Think of it this way - if we have 100 litres of a particular SG fuel, then we will be able to keep making noise for about half the time which would apply with 200 litres of fuel. That's a boringly obvious situation. Now, if we drill down a little and look at the case where we are comparing the same volumes, but with differing SGs, the situation is much less dramatic but still valid - the weight of fuel is a little greater for the fuel with the higher SG. It's pretty trivial if you only have, say, 200 litres of fuel in a little machine, but if you are driving a 744 with something like a squillion litres, it can really add up to a significant weight delta.

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