But they never realised bumblebees have it both ways, their wings work both coming and going. One way only in a circular movement like a bird wouldn't give them enough specific impulse, came the days of high speed photography and the light dawned, Bumbles use a figure 8 stroke with canting wings and obtain lift on both be backwards and forwards stroke.
It's a bit like the case of the Kangaroo not being able to eat enough in a day to fuel moving their weight around in big jumps. The early Natural Philosophers left out the effect of the main hamstring tendon. Lift a bag of spuds of the floor and put it down, repeat... Lots of energy. Put a spring on the bottom and lift it and bounce it. First lift, more energy, following bounces require less. Kangaroos cheat physics by building in their own springs, hence the problem with the old figures of where the Kangaroo got it's mysterious ability to obtain energy from nowhere from.