I am not disputing max drag will be at 90 degrees
Just that prior to that (45-50 degrees) the lift will still be significant so much so that the drag compnent in the vertical plane with the addition of that lift will be greater than just pure drag component flat.
This will give a lower rate of decent than a pure flat plate effect.
NASA has done tons of work on turbine blades in the last 10 years from a quick search.
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I needed to work out the max moment for the base support for a FEA contract job years ago which was when I found out that taking the max force at flat wasn't a good idea.