Do box-wings suffer from induced drag the same way as normal wings?
The box wing is only better when you compare wings with identical span. The two wings of a box wing work in different Treffz planes, so the downwash is spread out vertically. The difference in induced drag to a single wing is not big, just a few percent. The friction drag is higher (see below), as is the structural mass, so the box wing needs to create more lift. This makes the induced drag of a box wing effectively higher than that of a single wing.

What is induced drag, anyway? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! Wingtips are not involved and are not causing induced drag. Lift creation is.

If you fly fast, there is a lot of air mass streaming past the wing per unit of time, so you need to deflect the air only slightly. Your induced drag is small. Sames goes for a large span: There is more air which can be deflected, so the induced drag is small.

A box wing needs two slim wings per side, which will have a smaller chord than a single wing of the same surface area. So their Reynolds number is smaller, and their friction drag is higher. Also, the wing spar is less thick and will need to be heavier to carry the same lift!

If you drop the restriction of keeping span identical, the optimum single wing can afford to have more span (due to its better structural efficiency), and away goes the advantage of the box wing. And once you look at the full picture and add structural mass, the box wing never had this advantage in the first place.

Yes, but what about the Synergy?

The Synergy is a clever design with some advantages, but it cannot cheat physics. These are the advantanges:


The pusher prop keeps the airframe free of wake turbulence, so more area can be kept in laminar flow.
The pusher prop sucks the air from the rear fuselage, effectively avoiding separation.
The two stubby tailbooms and fins give great protection for the propeller area on the ground.
The compact layout keeps the stabilizing effect of the propeller small, so maneuverability does not suffer much.
The use of composites and glider airframe technologies reduce friction drag.
The diesel engine consumes cheaper jet fuel and is more fuel-efficient than a gasoline engine.


Note that I did not mention the box wing design?

Here are the disadvantages:


Wing sweep in a propeller aircraft looks cool, but increases drag, because the wing must be bigger to create the same lift.
In total, this configuration has four vertical tails, each of them with its own interference drag and a short chord which, again, increases drag over a comparable single vertical tail.
The stretched-out horizontal tail is also less effective than a smaller single surface with more chord and more distance from the center of gravity.
The compact layout provides little pitch or yaw damping. I wonder what the ride qualities in gusty weather are.


I would expect that a more conventional layout along the lines of the fs-28 would be even more efficient.



Akaflieg Stuttgart fs-28 in flight (picture source)