What is the primary breakthrough here?

There is a LOT going on in this counterintuitive design, but the obvious thing is also what makes it simple:

Synergy’s patented double boxtail configuration (DBT), which is responsible for the following unique improvements, serves as a catalyst for the simultaneous, low cost adoption of several proven but underutilized drag-reducing technologies, such as laminar flow, wake propulsion, pressure thrust, and boundary layer control. Taken together, these technologies improve aerodynamic performance to the Gabrielli-von Karman limit, which is a benchmark anywhere from two to fifteen times as efficient as typical powered aircraft, depending on their speed. No manned airplane has ever come remotely close to such fuel-efficient high performance in this speed and weight category. Here are some of the benefits of the unique new DBT technology, as it applies to this aircraft in particular.

  1. The  Synergy configuration lowers induced drag (the drag due to lift) to the theoretical limit for a given wingspan. Its ‘non-planar’ span efficiency is 1.46 times that of an optimally loaded planar wing of the same span, allowing a strong, compact form more capable of higher speeds than a long, glider-style wing. Decreasing the induced drag of an airplane provides benefits in climb, at lower speeds, when racing or maneuvering, at higher weights, and at higher altitudes.
  2. The Synergy configuration eliminates complexity. Its simple, seamless wings need no control surfaces or the many parts that would impose; for example, to achieve the major benefits of a sealed gap. Instead, only two moving surfaces are needed to provide a majority of flight control, and these two ‘high aspect ratio elevons‘ are equally simple, one-piece airfoils; merely supported at both ends so they can be rotated for control. (Thanks to its natural turn coordination, Synergy’s twin, V-tail-mounted rudders are rarely required in flight, but yes, we have (two) rudders on the full size aircraft!) Reduced complexity translates directly into reduced weight and reduced cost.
    The scale model doesn't even have rudders. Two simple surfaces provide fully coordinated turn control.

    The 25% scale model doesn’t even have rudders. Two simple tail surfaces provide the outstanding, naturally coordinated turn control seen in our flight test videos.

  3. The weight distribution of the structure is inherently ideal for keeping the structural costs down and the balance right. When flown solo and light, from the front seat, the balance provides nimble handling. As the aircraft gains more people or payload, it remains in proper balance and increases in stability. At maximum weights all remaining payload is carried right on the CG itself, and the aircraft achieves its most stable configuration. (Many airplanes suffer from the exact opposite condition as they get heavier: less stable, less safe.)
  4. Laminar flow and high span efficiency allows a bigger, stronger wing in comparison to typical high-performance designs. Providing increased fuel storage and slower landings, the wings can be stronger and stiffer for a given span loading. Wingtip twist, associated with swept wings, is balanced out of the system under G-loading by an always-proportionate, opposing downforce from the tails. Note, however, that Synergy doesn’t really have any ‘wingtips’. (How many do you see on other aircraft, counting every lift-producing surface?  Each one is a drag source.)
  5. The DBT configuration creates a (patented) novel method for the prevention of stall, and many DBT aircraft exhibit pre-stall behavior similar to the canard configuration. Synergy itself uses a conventional method, in that its controls provide full authority without creating excessive wing angle of attack. In testing, with controls set to allow excessive authority, recovery from intentional stalls was instantaneous and without altitude loss,  due to the large elevons becoming additional flap-like wing area (and, wing airflow control devices !) when commanded to lower the nose. Several versions of Synergy also show promising control behaviors during intentional ‘deep stall descent’ at relatively low speeds. Total control of flightpath and attitude during fully stalled flight, with instantaneous normal flight recovery on command, has been demonstrated in a range of conditions. Certification of such potential requires that every possible combination of conditions be tested. We will not be configuring to allow intentional stalls or deep stall control potential until it has been exhaustively refined in full scale flight test.
  6. The volume of air that is progressively displaced by Synergy in flight changes smoothly along its length in a way that properly matches an optimum ‘body of revolution’ shape in its speed range. This objective, called ‘subsonic area ruling‘, is quite different in character than for supersonic aircraft. Its shape is different and designed to promote stable near-field pressure gradients in all phases of flight, drastically reducing the true source of catch-all “interference drag” and turbulence. It takes an extraordinary aircraft to benefit from, or preserve, refinements in this category. Normally the esoteric benefits of pressure field tailoring at subsonic airspeeds are lost before they can be seen, let alone studied, because of high turbulence in the fuselage region. Thus, conventional wisdom ignores subsonic volumetric tailoring entirely. (Nature, however, doesn’t, and this is part of the reason we can often accurately associate efficiency with beauty.)
  7. The Synergy DBT configuration exhibits superior handling at all speeds, including ideal turn coordination. Most aircraft require a vertical tail and/or rudder input to counter adverse yaw. Synergy carves turns like a finely tuned motorcycle.
  8. The DBT configuration creates a smoother ride in turbulence, and a noticeably more stable platform overall, thanks to moderate wing sweep and effective ‘decalage.’ Synergy’s remarkable stability provides the opposite of a ‘short-coupled’ aircraft. Like a strong man with his hands and feet wedged into the corners of a doorway, Synergy intentionally leverages against the atmosphere with every flight surface, despite having a wing planform that doesn’t, technically, even require tails. (!)
  9. Synergy’s double boxtail configuration creates constructive, beneficial wing-tail interaction, rather than (destructive) “biplane interference”, allowing wing and tail to cooperate together for lower drag. In addition, all airfoil surfaces continue the low pressure or high pressure assignment of the surface  adjacent to it, virtually eliminating the famous interference drag problem common to box wing designs.
  10.  Pilots and passengers  can see in every direction, and the jet-like nose allows all kinds of doors and nose openings to be used for passengers, cargo, and medical access purposes.

Various DBT models have been making the above obvious for some time. Wanna see it fly?

Presently, Synergy Prime, Synergy derivatives, and other DBT aircraft designs are being developed and studied by governments, industry, hobbyists, and academia alike. A large, fast aircraft is required to achieve the size/speed regime where our equally valuable high speed drag reduction technologies make on-demand regional transportation possible, but it’s truly exciting that these many benefits typically result in beautiful, well-mannered aircraft that are a joy to fly.