I recall the Canadian National Research Council in Ottawa did some extensive experiments on helicopter icing in the 70's and early 80's. Perhaps you may wish to contact them.
https://www.nrc-cnrc.gc.ca/eng/index.html
EDIT
Just talked to Dean at the NRC .. I assume you are the person from Germany he is talking to.
He told me they are doing some research in their files to help you.
We had a great conversation about icing.
Great folks at the NRC.