My first thought when looking at this were the "flaptors" in Studio Ghibli's movie Castle in the Sky - machines that have almost exactly these types of wings. So you could check some frames from that movie for reference. As I recall, the wings there become almost invisible and are mostly drawn as a series of blurry, white lines to indicate how they are vibrating.
I had a similar struggle a while back when I tried to draw the character Tails from Sonic the Hedgehog, with his spinning tails. Looking back, I think it was the right approach to just draw a blurry shape, where you can't see the wings (or tails in this case) at all - but I think I should have reduced the opacity a lot more. Now it feels more like a solid mass than something rapidly moving.
Hope that helps!
