Avgas shrinkage at low temps
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Avgas shrinkage at low temps
This is about 0.1% per degC.
So, let's say I fill right up with avgas from a bowser which has been sitting around all day in Greece, and is say +25C.
Now, I have a flow totaliser, verified accurate at every fillup, to within ~ 0.5% against UK airport pumps (checked by Weights & Measures fairly regularly).
The transducer for this is a Floscan 201 turbine transducer which like just about every liquid flowmeter (short of Coriolis meters) is measuring volume flow, not mass flow.
At FL180, the fuel will eventually cool down to say -25C.
That is a 5% shrinkage in volume. (Not mass, obviously).
So the flowmeter should be reading 5% lower for the same mass flow, yes?
And the engine power derives from mass flow of fuel.
Does any of this actually matter??
The one thing which should matter is that there is a significant difference between uploading fuel at say +5C and +25C - a 2% short-change on the latter and a 2% aircraft range reduction. Why is this not reflected in fuel pricing?
What do they do on big jets? They must work on mass too, not volume flow, even though it is volume that is measured by the flow totalisers (unless they use coriolis meters, or do some kind of fuel temperature compensation).
So, let's say I fill right up with avgas from a bowser which has been sitting around all day in Greece, and is say +25C.
Now, I have a flow totaliser, verified accurate at every fillup, to within ~ 0.5% against UK airport pumps (checked by Weights & Measures fairly regularly).
The transducer for this is a Floscan 201 turbine transducer which like just about every liquid flowmeter (short of Coriolis meters) is measuring volume flow, not mass flow.
At FL180, the fuel will eventually cool down to say -25C.
That is a 5% shrinkage in volume. (Not mass, obviously).
So the flowmeter should be reading 5% lower for the same mass flow, yes?
And the engine power derives from mass flow of fuel.
Does any of this actually matter??
The one thing which should matter is that there is a significant difference between uploading fuel at say +5C and +25C - a 2% short-change on the latter and a 2% aircraft range reduction. Why is this not reflected in fuel pricing?
What do they do on big jets? They must work on mass too, not volume flow, even though it is volume that is measured by the flow totalisers (unless they use coriolis meters, or do some kind of fuel temperature compensation).
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Only on light airplanes is the flowmeter calibrated to read only volume. Most larger or more complex aircraft use temperature-adjusted mass flow based on weight. I want to know how many pounds per hour, or Kg per hour, of fuel I'm burning. Same for fuel in the tank. The number of gallons is really irrelevant, because the energy per unit of fuel is by weight not by volume. Additionally, for weight and balance purposes, volume isn't relevant...but the weight is.
We fuel by weight, keep track of burn by weight, and plan our reserves by weight.
This is usually accomplished using sensors in one or more fuel tanks, which are then used to apply a correction factor to fuel information in the tanks and in the system.
We fuel by weight, keep track of burn by weight, and plan our reserves by weight.
This is usually accomplished using sensors in one or more fuel tanks, which are then used to apply a correction factor to fuel information in the tanks and in the system.
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With large piston airliners (and I have flown several, including...Lockheed 1649 Constellation, DC6/7, Boeing 377) tank capacity was always indicated in US gallons, not weight.
Fuel uplift by US gallons, fuel consumed in US gallons.
Fuel temperature effect made little difference, with these aircraft.
Jets...a whole different scenario.
Been there, done that.
Them's the facts.
Fuel uplift by US gallons, fuel consumed in US gallons.
Fuel temperature effect made little difference, with these aircraft.
Jets...a whole different scenario.
Been there, done that.
Them's the facts.
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Turbine fuel v's Avgas
Both have different calorific values, however they both produce a given amount of energy for a given weight of fuel, not volume. As 411A said, most piston engine aircraft displayed tank contents in gallons (US or IMP) or in litres, uplift in gallons or litres, measure fuel burn in gallons or litres per hour. To a point I agree with 411A in that fuel temperature did not play a big role in the piston airliners mentioned, however they were not subjected to the same extreme variations as modern jet aircraft. It does not matter whether you are flying a Cessna 150 or an Airbus A380, you will derive a given amount of energy (relative to engine type) from each pound or kilo of fuel you burn and this may or may not be the same volume depending upon temperature variation encountered. One method used to get maximum fuel on board is to uplift refrigerated fuel, i.e. same volume but more weight.
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OldFella
Does this mean that not only are we paying more when Fuel is Hot, but we need to use more to produce similar energy as Cool Fuel? I think it does. Double Whammy. Safer to fly Turbine relative to Piston, eh?
My PT-6 burns Gasoline, my IO540 won't do anything on Jet. Not that I would do such a thing knowingly.
My PT-6 burns Gasoline, my IO540 won't do anything on Jet. Not that I would do such a thing knowingly.
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Pay more and Use more
Airfoilmod, if the fuel is dispensed by volume rather than weight you will pay more per pound if the fuel is hot and, yes, you will burn more in volume for a given output in thrust or HP. Turbine engines are more reliable than piston engines, you have only to look at relative TBO's to see that. Don't run your PT-6 for any longer on gasoline than you have to either. Apart from unwanted deposits on the turbine, if memory serves me correctly your range is reduced.
Last edited by Old Fella; 7th Jun 2008 at 10:27.
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Jet fuel is "sold" by volume
As has been pointed out, most aircraft buring Jet fuel do all the fuel calculations by mass, not volume - our aircraft fuel gauges are calibrated in pounds, the flow meters indicate pounds per hour, the FMS calculates pounds required, all our flight planning calculations are done in pounds, and when you work out what uplift you need, you calculate that in pounds.
But, when you get the invoice, it is always priced by volume. (For those not familiar, the international standard pricing measure is usually US cents per US gallon). So, when comparing prices, and deciding which leg to tanker fuel on, you have to take into account the fact that if the posted price is identical, the fuel at the hotter airport is actually more expensive!
But, when you get the invoice, it is always priced by volume. (For those not familiar, the international standard pricing measure is usually US cents per US gallon). So, when comparing prices, and deciding which leg to tanker fuel on, you have to take into account the fact that if the posted price is identical, the fuel at the hotter airport is actually more expensive!
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Following the discussion on this particular racket with interest.
Even Concorde (remember?) measured fuel burn in kilograms (fuel volume sensors plus fuel specific density sensors).
Were they billed in litres or kilograms, though? I don't know.
Even Concorde (remember?) measured fuel burn in kilograms (fuel volume sensors plus fuel specific density sensors).
Were they billed in litres or kilograms, though? I don't know.
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OK, lets look slightly deeper in the subject of avgas...and high(er) altitudes.
My previous statement that it doesn't matter about enroute temperature is correct, for most 4-engine piston airliners, simply because they most often did not operate above FL210.
Except for the B377 (Stratocruiser, and yes, I've personally flown this airplane, a joy to fly) as it went to FL230, on occasion.
Not all that high.
However...
The Consolidated-Vultee B-36 USAF bomber often cruised at /above FL400, simply because it had, in addition to an engine-driven supercharger, two General Electric quite large turbosuperchargers, which allowed high altitude operation.
The fuel?
115/145 octane avgas, delivered (oftentimes, refrigerated), to pack in more BTU's.
How do I know?
My neighbor down the street, that's who, and he was (now retired) a wing commander of 'ole B36's, long ago.
A bird Colonel.
I suspect he knows his 'stuff'.
My previous statement that it doesn't matter about enroute temperature is correct, for most 4-engine piston airliners, simply because they most often did not operate above FL210.
Except for the B377 (Stratocruiser, and yes, I've personally flown this airplane, a joy to fly) as it went to FL230, on occasion.
Not all that high.
However...
The Consolidated-Vultee B-36 USAF bomber often cruised at /above FL400, simply because it had, in addition to an engine-driven supercharger, two General Electric quite large turbosuperchargers, which allowed high altitude operation.
The fuel?
115/145 octane avgas, delivered (oftentimes, refrigerated), to pack in more BTU's.
How do I know?
My neighbor down the street, that's who, and he was (now retired) a wing commander of 'ole B36's, long ago.
A bird Colonel.
I suspect he knows his 'stuff'.
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411A,
Thanks, interesting....
So even then they tried to load as much "bang for their buck" as they could?
Or rather as many BTU as possible for the volume being uploaded...
I doubt that in those days anybody worried much about the actual fuel bill...
Thanks, interesting....
So even then they tried to load as much "bang for their buck" as they could?
Or rather as many BTU as possible for the volume being uploaded...
I doubt that in those days anybody worried much about the actual fuel bill...
