Thank you, serb1389. My experience has been with the Russian Church Abroad and OCA.
As for this specific issue...my personal guess is that the Fan was an necessary liturgical implement in the years before air-conditioning. These days, it's rare to have flies hovering over the chalice. But if you had the windows/doors of the church open to keep air circulating in the summertime...you would need a fan constantly over the chalice to protect the Gifts.
IIRC, the fans date from ancient civilisations, where they were used to keep insects away from the rulers and their families, and to provide shade from the sunshine to keep them cool. I think they were used in ancient Ehypt. Because of this association, in time they came to be seen as signs of honour given to revered persons and things. I don't know when they came into use in Christianity or whether they were ever used practically to keep flies away. Probably they were but certainly their use had long been estalished by that point as honorific as well. Certainly, their importation into Chrictian use was early enough that their use is common to both east and west. In the west, they still resemble something much closer to their original form, made from feathers
, while in the east the feathers disappeared and they took on a circular shape, often with icons of Saints or seraphim on them.
As for their being needed constantly over the chalice to protect the gifts, I'm not sure that this is necessarily the case as the chalice covered for most of the Liturgy, either with veils or by a small paten at various times. It is only at certain points that it is uncovered.
In Christ risen,