I agree that cox rubber duckies are much more durable than the stiff props. I agree that cox rubber duckies are more efficient than most offerings. A cox rubber duckie 5x3 would not even keep my J3 in the air. My Herr planes flew best with the 6x3 cox grays (.049) and 6x4 (.061) If speed were the criteria for plane performance a low drag small plane with a small prop spinning ultra high rpm would be optimum. For most BEGINNER planes lower speed and slower reaction with enough thrust to pull you out of problems is my idea of optimum. I agree that cox rubber duckies may be the best compromise. (btw..My thrust tests were done at speed not on a static bench)