Virtually all airplane wings are convex on both the top and at least part of the bottom. But thus convexity on the bottom creates suction which reduces lift. I can only think of two reasons to have it: negative AoA performance (important only for aerobatics), and the need ri accommodate a spar. Suppose these two were possible to neglect. Would a concave bottom give a better L/D?

When you look at applications where the structural an AoA concerns are absent, it does seem that the answer is yes.

The blackbird is small, hence there are less structural concerns. It has a concave bottom.

Turbofan blades are also relatively small. Also, I think the forces are dominated by centrifugal force, not bending force. Once again, the bottom.is concave.

The same is true of the turbine blade.

Curiously, the propeller, which also has similar conditions, has a convex bottom.

Virtually all airplane wings are convex on both the top and at least part of the bottom. But thus convexity on the bottom creates suction which reduces lift. I can only think of two reasons to have it: negative AoA performance (important only for aerobatics), and the need to accommodate a spar. Suppose these two were possible to neglect. Would a concave bottom give a better L/D?

So, is the L/D a cruising aircraft best when bottom is concave?

When you look at applications where the structural and AoA concerns are absent, it does seem that the answer is yes.

The blackbird is small, hence there are less structural concerns. It has a concave bottom.

Turbofan blades are also relatively small. Also, I think the forces are dominated by centrifugal force, not bending force. Once again, the bottom.is concave.

The same is true of the turbine blade.

Curiously, the propeller, which also has similar conditions, has a convex bottom.