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Old 02-05-2011, 02:43 PM
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hevansrv7a hevansrv7a is offline
Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,549

Originally Posted by plehrke View Post

A prop is like a screw so what is the “threads per inch” so to speak of the propeller and how is that related to the pitch? Can I calculate the prop efficiency from this plot? ...
2700 rpm, 174 kts (my best effort at reading the chart and exactly what my 7A does).

174 kts = 200 mph or close enough for this excercise.

200 mph = 200/60 = 3.333 miles/minute = 3.333 x 5280 = 17,600 feet/minute x12 = 211,200 inches/minute.

211,200 inches/minute divided by 2700 revolutions per minute gives 78.22" inches per revolution. I call this the "effective pitch". I have seen other labels, too. This is what your prop is actually doing on your airplane. The 85" pitch specification is probably based on a geometric measurement at the 75% radius off the flat of the blade. That's good for comparing two Sensenich props and unlikely to be accurate between two props of different design and/or on two different airplanes.

As for prop efficiency, the short answer is "no". Your data do not tell you the efficiency. The prop efficiency is the thrust HP divided by the BHP. The thrust HP is best inferred from the drag curve in level flight. The BHP is very hard to know but there are several ways to estimate it. Please refer to the various articles and spreadsheets on my website (see signature). Contrary to a popular rule of thumb, you cannot even estimate relative BHP from RPM with much accuracy. The cube rule does not hold up well.

Thanks for the data. It seems very well done!
H. Evan's RV-7A N17HH 215 hours
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