BSFC must be calculated based on OBSERVED hp, not corrected hp.

I disagree.  If BSFC was based upon observed HP, then no two BSFC curves could be compared.  It would always be apples to oranges.  Standard day corrections are used for everything and are not "junk science" (as for "snap HP," well, that's a different matter).  Bottom line is that you would like for every engine to be dyno-evaluated in exactly the same manner for making valid comparisons.  FAA engine tests are one way in which that is achieved.  Car manufacturers dyno-test to specifications published by SAE.  The local speed shop or aircraft engine rebuilder . . . who knows?

Moral of the story is to rely upon manufacturer curves where available.  Second moral of the story?

BSFC is useful for comparing engines and not much else.  If you want to know how efficient your airplane is with a given engine (i.e., "system efficiency" composed of airplane aerodynamics, engine efficiency and popeller efficiency components), the only realisting way is to run a closed course and measure the fuel use and then correct to standard atmosphere (ala CAFE) if you want to use the data for comparison purposes.  As we all know, a "standard atmosphere" is an engineering fiction.  I doubt there's ever been a standard atmosphere here in Houston.

Rick Price

OK, two things. First, I didn't mean to imply that corrected HP is "junk science," only that one must be sure of what the correction factors were. My point was that some correction factors lead to publication of unrealistically high HP numbers, so that when you calculate 75% hp based on observations of speed and map, you may be starting from an artifically high "100%" value. No argument that corrections must be applied to compare engines that were tested under varying conditions. Unfortunately, despite engineer's best efforts, correction factors may not apply equally to different engines, and they also may loose their linearity under some more extreme test conditions.

Now, as for using corrected HP to determine BSFC, it cannot be done! Why? Because when HP is measured on the dyno, the engine is producing a certain actual (observed) hp. It is consuming a certain actual amount of fuel at that moment in time. The amount of fuel being metered by the carb or FI system is being affected by the ambient conditions in the test cell (and affected differently if it is a carb or FI system -- another source of error for the correction factor). When the correction factor is applied, it is intended to provide a good estimate as to how much power the engine would be producing at some standard conditions, based on the observed power at the actual ambient test conditions. This corrected power can be considerably higher (or lower) than the observed power, The correction factors DO NOT ATTEMPT TO CORRECT THE FUEL CONSUMPTION. So, if you use an observed fuel rate divided by a corrected HP value, you get an erroneous BSFC number. Lets say you test an engine in a test cell with 120 deg air, 100% relative humidity, and a barometric pressure of 28 in Hg (a hurricane in Cancun). You may observe 100 hp, and you may be burning 45 lbs/hr of fuel. Observed BSFC is .45 lb/hp-hr. Now, apply the correction factor to the hp, and you may get 120 corrected hp (yes, it can be that different, especially with the OLD sae corrections to 60 deg dry air). Calculate the BSFC now, and it is .375 lb/hp hr. If you were to actually run the engine at the same ambient conditions as in the correction factor, you would probably have a BSFC somewhere between the two numbers. Of course, testing under such extreme conditions cannot provide very good data for apples to apples comparisons, but that is just how some dyno results are obtained.

Jon Lauter
Rotary Power Marine Corporation

 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org