Truss-braced wings could bring new look to runways worldwide

Wing tanks keep the fuel from occupying the fuselage.

While in theory you could store things elsewhere, practically the logistics of loading and unloading a craft require containers that fit into the airplane. Which means those containers that fit into a cylindrical section.

A small amount of space used by the stewards for supplies or storing carryons might work in limited amounts, say for first class.

I think you are reading that backwards; the trusses allow lower costs, so can be more profitable with the same payload.

Biplanes have supports between the wings that add drag. Also, biplanes typically don't have swept wings; here, the truss sweeps forward while the wing sweeps backward.

Mentour Now! video on Boeing and transonic truss braced wings: https://www.youtube.com/watch?v=zkrdIw8LWrM

The reason why you use a biplane design is to get more lift per unit span, so you can keep wingspan within some arbitrary constraint.

As with all engineering, there is a tradeoff involved, and with biplanes you don't get double the lift because of the boundary layers of the wings interacting.

You also pay a drag penalty as you have twice as much skin area exposed to airflow, and it complicates the design because in the transonic région you can get interactions between shockwaves causing flutter and other not so fun effects.

Biplanes have twice the number of wingtip vortices, so lots more drag. I suspect the reason biplanes existed in the first place was due to materials constraints. We didn't know how to make the wings strong and light enough to be very long, so kept them short and added a second set. This caused both more lift and drag. The drag was compensated for by running the engine harder (or using more powerful ones).

We now have sufficient materials knowledge that we don't have a problem making wings long enough to lift the plane. We're instead working on reducing the drag, so we can make engines work less hard.

At a guess (I know nothing about the field):

- because the fuselage itself isn't important for testing - older chunkier design is more amenable to bolting radical new bits on - 737s still have market value

Low wings work better for commercial airliners because the structure can go through the cargo hold instead of the passenger compartment. There are some other smaller benefits like easier refueling and ability to retract the main landing gear into the wings on many aircraft.

The structure between the wings is one of strongest parts are the aircraft. With low-mounted wings, the fuselage sits on top of that structure and only needs to support its own structure. The wings are pushing the plane up.

High-mounted wings, on the other hand, pull the plane up. This requires the fuselage to be stronger. Otherwise the wings would rip off the top of it.

High-mounted wings require the structure holding the wing to be suspended higher in the fuselage, requiring it to be stronger. Their main advantage is to avoid obstacles in the ground, which is desirable when landing in less developed airfields, but it's not a problem airliners have.