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Originally Posted by SteveC  The IO-360 runs pretty close to your 1"/100 RPM rule of thumb, and that is based on a 300 RPM change. The O-320 runs about 1/2 that figure for a 100 change. If prop efficiency were a factor wouldn't those ratios be reversed? |
I have no idea, what I do know is if you changes a blades angle you change its effeciency and what you are measuring doesn't take that into account. Take aircraft A operating at 22" and 2200 RPM would be say 140 horsepower and say aircraft B is operatting at 23" and 2100 RPM. As you stated the horsepower in the second situation would be less so we have less thrust right? Well we have less drag too since the prop is now more effecient, whether or ont they go hand and hand 1 and 1 for every aircraft I am not sure. The person who taught this to our flight dynamics class was a navy engineer, he did the performance calculations for aircraft in the navy.
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What I suspect is happening is that the lack of accuracy in the resultant measurement equipment (ASI, VSI) that you've used for your tests overshadow the power differences so that casual observation would fit your hypothesis. The change in knots or FPM may be too small to be noticeable maybe? Anyway, if that's true I still think it might be disingenuous to actually take those casual rule-of-thumb observations and turn them into statements of fact like "23/2300 = 24/2200 = 25/2100 = 26/2000."
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As for my tests, they were an hour in the seminole and in the arrow and each included a full work up from 18"/1800 to full power. I did + 4 and - 4 for MP/RPM across the board and I gave the aircraft between 30 seconds and a minute to stabalize on each approach. This was done by the engineers at the college too, I will see if I can get the spreadsheet.
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I don't know, I'm just looking at a small amount of data and not coming up with the same conclusions that you are. I haven't flown a piston aircraft in years and I don't have an easy way to do field tests so I have to use what little logic and knowledge I have available to me to try to follow along. 
I'd be interested if taylor or fish have any insight into this question as well.
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No big deal, we are all here to learn and if this is wrong I certainly would like to know that. For all my tests to this point (only in an arrow, seminole, and RG) the results have been I would guestimate within 20 percent of accurate.
I would like to hear from them too please! I would also like if the topic poster could test this out in his aircraft and tell us what the numbers are. Fly it at 23/23 then 24/22, 25/21 and finally 26/20 and give the numbers here for what the resultant speed was after say 5 minutes or so.
I think for the calculations side of it very few people will know what it takes to get an accurate result. We are pilots we test things, or just make it squared.
