I agree. Correct me if I'm wrong, majortomski: Be careful using that 20% figure, stek79. I don't believe majortomski's analysis was intended to be detailed enough to quantify the difference under any particular set of conditions. Tom's analysis made several assumptions, including the CL of the prop airfoil and only considered the 9 and 3 O'clock positions (the points at which the resultant velocity difference is greatest), and only at the tips of the blades, and the effect of vortices was not considered. To meaningfully quantify the difference would also require analysis at several different stations. Every change in angle of attack, airspeed and prop RPM or pitch will result a change in the "center of thrust". Majortomski's analysis does splendidly prove that there is a significant difference however, which I believe was his intent.