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#73 Wrong units on {I|B}SFC
Unless there is some new math going on, the units are wrong on all references to Specific Fuel Consumption. It is all consistent, so the final answers are correct, but it probably should be cleaned up. Specific fuel consumption is (fuel flow rate)/(power), so the units are something like (LBS/HR)/HP. (See http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption) The current units in FGPiston.{cpp|h} and FGXMLElement.{cpp|h} are LBS/HPHR (or KG/KWHR). (LBS/HR)/HP = LBS/(HPHR) <> LBS/HPHR = (LBS*HR)/HP. Various comments in the code where specific fuel consumption is declared or used also carry this same error.
The conversion factor for English to SI matches what is listed in Wikipedia (to four decimal places) for the correct units.
Corrected the summary.
LBS/(HPHR) or KG/(KWHR) is correct. Note that Wikipedia shows implied multiplication without parenthesis, e.g.: kg/kWh. However, this gets messy with multi-letter variables, e.g: KG/KWHR.
Anders and I discussed this on and believe we should switch to LBS/HR/HP and KG/HR/KW. The property, propulsion/engine[*]/bsfc-lbs_hphr uses implied multiplication so we will leave that as is.
As far as I can tell no aircraft are currently using the units so a change shouldn't present a problem there.
I will prepare a patch next week if there are no further comments.
Ron
LBS/HR/HP and KG/HR/KW look excellent to me. As I mentioned there are some comments that use the bad units as well, e.g. FGPiston.h comment on ISFC.
Note, FGXMLElement comments the conversions as "Fuel Consumption" rather than "Specific Fuel Consumption"
Interestingly, the engtm601.xml file has this: <psfc unit="LBS/HR/HP"> 0.669 </psfc>
The Wikipedia unit of kWh is a "standard" SI combined unit for energy listed on electric bills. Clearly it is a goofy unit since it is kJ/s*hr. But you're right that there is an implied multiplication that must take precedence to be mathematically correct in the listed formula.
One last comment on rereading what I wrote. Specific fuel consumption is fuel flow rate normalized by some factor. Brake specific fuel consumption is fuel flow rate divided by brake horsepower. There are other specific fuel consumptions created using other normalizing factors that have different units. The obvious example is thrust specific fuel consumption that is normalized by the thrust force, so the units are (LBS/HR)/LBS, but that is deceptive is it is LBM/HR/LBF using the fun-with-English-units units. (kg/hr)/N in SI units. Here's the kids version:
http://www.grc.nasa.gov/WWW/K-12/airplane/sfc.html