Charlie said: Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negative slippage'. :-)
I don't think the analogy quite applies here. For those types of crafts it is the wind that is doing the powering. While it is true these types of craft can sail faster than the wind, but not while pointing straight into it! By definition to cruise in an airplane, the prop has to be generating some kind of thrust and therefore could never go faster than "the wind" - the only wind it sees is the relative wind that is generated due to its own thrust (in cruise). In a descent, sure negative slippage is a fact. And slippage has to increase greatly in a climb. I guess I am arguing that the only way you could see zero or negative slippage in cruise is if either your blade cross section is asymmetrical (it usually is right?) or if the pitch number used in the calculation is not really right based on the kinds of factors you outlined Charlie.
Supposing that we have a typical non-symmetric blade cross section, an accurate pitch based on the chord line of the cross section, and I suppose a twist that is correct for the cruise RPM. What then would be considered a good or reasonable slippage in cruise? I saw 3% thrown out there. And if your prop selection is good for all those conditions (in other words as efficient as possible), is this the slippage you expect? I am just wondering if you can use a slippage calculation to judge efficiency (roughly).