Thanks for your answer.
So I ll use the APC as a benchmark.

Do you know any site (or any published reports) where i can find low reynolds number aerofoils and their polars. I ve found various sites having aerofoils, mostly designed by professors that want some fame , but in order to get the data from testing you have to pay. So are they any lift and drag data on the net for aerofoils designed for 30,000 to 200,000 Re?

Anyway, thanks again for your answer. Hopefully tommorow i will scan the mould (from the Master Airscrew) and confirm if they use the Clark Y.