Controlled Descender HELPPPPP

[al_garnett]

I have been tasked to design a controlled descender to be used on a building site to lower tools/rubble to the ground.

I'm trying to design a product like a fan descender like that seen at powerfan

Could anyone help me with the theory side of its operation. Obviously it uses the fans air resisstance, but i need to complete calcs to prove my design would work. If anyone could point me in the right direction or to a suitable source of info it's be greatly appreciated.

Thanks
Alex