My attention was called to tail-sitter aircraft, like these two:
These aircraft land on their tails but transition to normal flight during cruise.
But what I want to know is, why are the propellers so small? Disk loading is a big part of efficiency. That's one reason why helicopter blades are so big. It is more efficient to accelerate a large mass of air to a slower velocity than a small mass of air to a faster velocity.
As far as I know, the disk loading factor is valid for propellers in horizontal cruising too.
So why aren't the propellers any bigger? They have plenty of room at the nose. No worries about ground clearance. I would think for a helicopter-like takeoff, this puts a lot more strain on the engine and thus efficiency would be even more important than usual.