cpayne(at)joimail.com
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 Posted: Wed Aug 02, 2006 12:04 pm Post subject: Cruise Efficiency |
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Getting good gas mileage over a 1000 mile x-country is something this po' boy cares about. Longer legs also mean fewer stops, saving ground time. But ya still gotta stop and thaw yer butt out after serious time on the 'chute.
So, I added a 15 gallon aux tank to help out some and all went well on my Tuesday flight home. Surface temps were hovering around 100 degrees but winds aloft were calm. Typical of gaint summertime highs in the South and Midwest. This gave me an opportunity to do a little testing. With JPI fuel flow coupled to the GPS I ran through various power settings at density altitudes from 4500 to 15000 feet. Quite a bit of the flight was in smooth air above the clouds.
The results were unexpected, Yes, best economy was at slow speed and worst at high power settings but what got my attention was that the mileage "curve" was fairly flat in the mid-range.
Published M-14P cruise settings calc'd to the following actual percent of Power:
Nominal I - 80%
Nominal II - 67%
Cruise I - 50%
Cruise II - 40%
At Cruise II showed 11.5 NM per Gallon (120KIAS) but at Cruise I yielded 8.7 GPH while Nominal II got 8.2 GPH. A "flat" spot in the curve. Thus, (with my airplane) savings would be small for running at Cruise I rather than Nominal II.
This is gonna take some more testing, under good flight conditions using multiple 2-way runs. Of course, differences exist between airplanes, based on W&B, type of prop and carb settings. The object of a designer is to fly the airplane at design CL of the wing at the most efficient power settings that yield the desired cruise speeds.
A old saying is flying the airplane "on the step", which really means at flying at minimum profile drag of the wing at "design lift" while the engine is running in it's sweet spot. Hard to achieve sometimes. More testing ahead.
Craig Payne
(cpayne(at)joimail.com)
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