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.