Yes, you indeed are missing something, which is the engine that provides the torque to turn the prop. If the engine cannot handle the larger prop, the smaller prop can, and probably will give you more static thrust. Disregarding the engine: if both props are at the same rpm, the larger prop (diameter OR pitch) will provide more static thrust. Small pitched props (less than 0.5D) tend to loose their static thrust advantage in flight very quickly. This is often explained like driving a car in first gear. This comparison is not valid though, due to very different basic principles.
ORIGINAL: houckj
I can see where wheel drag will affect simple thrust measurements I can also see the problems with setup drag. Is it fair to conclude that any thrust measurements will be on the low side due to these factors? Do you think it would be more than perhaps 10%?
I would think that changing thrust changes from different props or fuels would still provide a reasonably accurate difference between thrust measurements. Something you could make a decision on.
e.g. If a 12x6 give 6 lbs of thrust using the above method and a 13x6 gives 7 lbs of thrust on the same airplane. I cannot imagine an error so large that the 12x6 would actually give more thrust. Am I missing something?