A swept wing with(out) a twist


New configurations deal a new hand to old airframers. Aerospace Manufacturing reports on the revolutionary ‘box tail’ wing developments taking place at Synergy Aircraft.

Poker players can sit at the table for a lifetime without seeing a Royal Flush, but they do exist. Betting large on the potential of such a winner emerging, minus the Ace of infinite capital? Now that’s gambling at its finest.

When it comes to wing planforms and basic aeroplane architecture, we’ve seen everything already, and there’s no such thing as a hidden card waiting to complete anyone’s hand. Or is there?

Albion Bowers, Chief Scientist at NASA Armstrong Flight Research Centre, has spent much of his career pointing out that even something as basic as a swept flying wing still has secrets locked up in history. In the 1940s, Reimar Horten picked up on Ludwig Prandtl’s lesser-known improvement to his own famous elliptical spanload concept, in which a negative tip loading improved the real-world results for the wing as a system. The Horten Brothers’ flying wings quickly promised a solution to problems of induced drag that lead to adverse yaw - and thereby to the conventional aeroplane architecture.

The twin horizontal tails articulate, providing simultaneous control over roll and pitch

By re-examining their premise in great depth, Bowers saw that if, like birds, airplanes needed no vertical tail, then a major architectural shift was indeed possible in airframes. His research revealed that the Bell-Shaped Lift Distribution (BSLD) - in which the outer portion of a wing of increased span produces a typically negative lift prior to ending at a zero-lift condition - did in fact work as advertised. In its most recent iteration, a flying wing called PRANDTL-D convincingly validated the physics of Prandtl’s theory.

However, flying wings have always had architectural problems of their own, such as a reduced efficiency due to sweep and a limited range of balance. Though not commonly advocated, adding a fuselage can counter some effects of sweep at the wing root, and obviously, it creates more volume for needed payload. Yet any lengthwise fuselage merely amplifies the potential balance problem and creates centrally concentrated loads. This ensures that the structural cost of added span, nicely moderated by negative tip loading in the cruise condition, will jump immediately back into the spotlight when high angles of attack erase that benefit and substitute their unwanted, full-span leverage as the tips turn into a lift source during high-alpha manoeuvring.

Dimension comprehension

Putting a fuselage and tail on a flying wing is clearly anathema to any purist, but should it be?

John McGinnis has quietly been saying no. McGinnis, a 51-year-old aircraft designer who was unknown until his futuristic five-seat design was revealed by the Experimental Aircraft Association, thinks more in the higher dimensional spacetime of fluid flows than in the 2D world of representative design equations. Noting that when three dimensions are considered, non-planar wings show much higher span efficiencies than flying wings and that wing structures don’t have to make lift in any particular direction to potentially be used in the fight against airframe drag, McGinnis folded up as much laminar-flow wing as a sailplane would have, artfully turning most of it into structures that provide stability, control, and drag reduction at the same time.

Synergy’s unique configuration makes proven high-speed drag reduction technologies easy to implement

The result, evocatively debuted in his family aircraft project he called Synergy, creates high span efficiency by exploiting well-known attributes of generic non-planar configurations, in particular their lower induced drag at a given span loading. The compact yet roomy laminar-flow fuselage terminates in a wake-ingesting fan for quiet thrust, and the ultra-clean design adds enough ‘active’, high speed drag reduction techniques to step into new territory for performance and efficiency.

McGinnis’ novel repackaging of a wing system had a problem, however.

“Everybody thought it was a box wing,” he explains. “I kept saying: ‘No, box wings suck, it’s not a box wing! Box wing designs have been muddying the waters for nearly fifty years now, and most are just a bad idea.”

While online followers started calling the twin boom, tip-less, swept 3D configuration a ‘McGinnis wing’, the designer switched over to calling attention to the difference between box wings, which lift, and what he now calls box tail and double boxtail (DBT) configurations.

“The horizontal tails, above and behind the wingtips, actually push down, intentionally. Their ideal download increases with positive-G manoeuvring to stay relatively in proportion to the weight carried by the wing, always offsetting part of the root bending moment and the induced drag rise,” he explains, referring to the forces trying to bend a wing upward in flight and the drag penalty of increasing weight.

McGinnis intends to make the box tail configuration available via affordable licensing

The twin horizontal tails articulate, providing simultaneous control over roll and pitch. In all phases of flight, the design provides a controllable, outboard download above the wingtips and a lift distribution tailored to the same anti-tip-vortex physics as the Prandtl-Horten-Bowers BSLD, thus providing the same yaw-coordinated turn behaviour, but without a wing tip and without complexity.

“From a simplicity standpoint, what you have is a wing that doesn’t need any controls in it, and a nice, tall, stabilised winglet supporting a simple airfoil up above, that you can hinge or pivot for control,” he says. Seldom-used rudders are placed on the inboard V-tails for direct yaw control when needed.

From design to fruition

McGinnis, a senior member of the AIAA and chairman of MV Aero, a CFD services provider, started building various Synergy models in the computer and in real-life almost immediately upon filing for patent protections in 2007. He said he felt compelled by the inherent attributes of various DBT designs and his experience to explore the space fully.

“Well, for one thing, several of the early models just wouldn’t stall. They acted quite like Rutan canard designs in that respect. I needed to make them stall anyway, and that was another place where we saw big differences compared to boxwings.”

Experiments led to a 25% scale electric model that was capable of fully-controlled deep-stall descent followed by ‘instantaneous’ stall recovery, according to McGinnis and his videotaped flight testing.

poss-background“When large tails are part of the wing system, they act immediately to reattach separated flow and add effective wing area, not just to command nose down in pitch,” he states. The end of each fully-stalled descent was ‘instant level flight’, not a perilous dive to regain flight speed.

Once, while constructing a full-scale, carbon composite version of the latest model in his family garage, the need to monitor a leaky vacuum pump provided McGinnis an evening to spend writing about the many differences between box wings and his ‘DBT’ technology. It’s a surprising list, even after leaving off one of the most intriguing aspects, the ‘constructive biplanar interference’ benefit detailed on his website. After making clear that structural problems are more caused by box wing configurations than overcome by them, and railing against their stability, control, and stall recovery shortcomings, McGinnis shared that he intends to make the configuration available via affordable licensing to UAV, RPV, and manned airframe manufacturers, as well as through a start-up having rights to develop his Synergy Prime version. Any gamblers in the house?


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