In my design/idea gathering phase of a slowflying long range Solar powered motor-glider, I came upon he idea of having a 4 or even 6 bladed folding prop. If i understand it correctlly, the more blades, the slower the prop has to turn in order to produce the same amount of thrust. That's why some commuter turboprops, especially in europe, have 6 or even 7 bladed props. This reduces noise.It would only be solar powered - no batteries so it would be turning pretty slow by comparision. Are there any benifits to using these multi-bladed props, besides noise reduction?

In their case noise is THE overriding issue. For models it's been shown that the less blades the better from an efficiency standpoint. In fact Frank Zaic in one of his yearbooks actually showed that the single bladed props are the MOST efficient. But trying to make them run smooth is more trouble than it's worth. So 2 blades is the best compromise.

That's why all the human powered stuff, the low orbit electric wing and other craft like that all use two blades.

PS: almost forgot the other reason. With the hugely powerful turboprop engines a two blade prop would be so large to soak up the power that the landing gear would be ostrich like. The same went for the late war prop fighters too.