Hello Pierre:
You can get away with running small, thin-profile, injection-molded, toy propellers backwards.
A propeller of normal thickness is terrible when running backwards as a turbine rotor.
I once did a serious, documented, experimental project to develop a wind turbine rotor that could work in either direction for a non-aiming turbine in a bi-directional wind resource. Talked an architectural firm into paying for it. Tried a few versions. Turned out not to work very well - unacceptable performance.
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