ORIGINAL: cubluver
Guys, thanks for the responses, but I still don't understand why the calculated and measured thrust of a single engine and a single prop can be so different. I'm not worrying about any complexities, like angle of attack, air density, prop efficiency, or stuff like that. The formula used in the calculations was published in Model Aviation magazine and written by a knowledgeable person. I'm sure the formula is somewhat simplified and thats OK. The measured thrust figures were from an engine review in RC Report magazine. There is just too much difference between 113 oz and 176 oz. The engine under review was mounted on an engine test stand, so there shouldn't be any fuselage interference, air density altitude, etc, to worry about. Thanks.
The reason that the calculated and actual are different is the complexities you are not worried about. Pe Reivers has it correct. Your formula is simplified and does not take into account the different performance and geometry numbers for your prop versus the test prop in the article. The thrust calculation is sensitive to ALL of the variables you want to ignore. As long as you do not correctly and accurately quantify all the variables involved, you will not get close in your calculations.
BTW, Pe has built the best thrust calculator going and he has made it available free to all who want to download it. Use it correctly and you will get very good thrust predictions for your engine.
Mark