I'm trying to figure out which size engine to put on a large plane I am building. I have noticed some major descrepancies between published thrust figures and the calculated thrust using the same parameters that were in the published review. I'm using the formula: prop diameter squared times pitch times RPM times a constant of .000011127. I got this formula from an article in Model Aviation. I have an MDS .68 that I'm thinking of using. In an RC Report review of this engine, they claimed 176 oz of thrust using a 14x5 prop at 10,400 RPM. According to my calculations, 14x14x5x10,400x.0000111127 equals 113.4 oz of thrust. Why is there such a dramatic difference in the two numbers? I'm sure the constant, .000011127, takes prop inefficiency, etc. HHow can I tell what the real thrust is? Any help on this would be greatly appreciated.