It's always interesting when practicality gets mixed up with "most efficient", re; the square tube he used instead of oval or round, and why. My plane (and several other kitplanes) has round spars, I'm sure partly due to the off the shelf availabilty of them, rather then some optimized built up and labor extensive spar. But recently RANS, my plane's kit company, has came up with a purpose designed extrusion that serves as a tough leading edge (when you bash into the hangar wall/hangar rash) AND the structural main spar, it's never been done quite this way before, and it has a lot of us all abuzz. They will be offering this new wing as a retro fit for my model plane, the S-7S, besides it being the standard for the brand new S-21.https://www.rans.com/s-21-outbound-progress
Livefor: that's some heavy stuff, IMPROVING the efficiency, with the exact proper type of stacked props, very cool. My understanding, in the traditional airplane world, is stacked props is a last resort, to get more total thrust in spite of any efficiency losses, usually because they have run out of room for a bigger single propellor. My plane for example, has 3" taller gear, which allows me to run a bigger prop, without trashing it on the ground. Some WW2 planes had counter rotating stacked props, because they were making so much frigging HP they couldn't use it all up in one prop, even after making the gear as tall as they dared. This drone stuff is all new, mostly, territory and I guess a lot of the old rules and pre conceptions don't apply. My own experience has shown that all other things being equeal, a tractor prop, working in undisturbed airflow, is better then a pusher, working in the disturbed air of the fuselage.