The Double Boxtail Configuration:
Catalyst for Efficiency

A: Synergy's patent-pending double boxtail configuration, 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 mentioned elsewhere, such as laminar flow, wake propulsion, and boundary layer control. Taken together, aerodynamic drag is reduced to the Gabrielli-von Karman limit, which is a benchmark anywhere from two to fifteen times as efficient as typical powered aircraft, depending on speed. No manned aircraft intended for volume production has ever come remotely close to such fuel-efficient high performance in the General Aviation speed range.

1. The 'non-planar' Synergy configuration lowers induced drag (the drag due to lift) to the theoretical limit for a given wingspan. Its 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 flight at higher speeds than a long glider-style wing. Decreasing the induced drag of an airplane provides benefits in climb, at lower speeds, and at higher altitudes.
2. The Synergy configuration allows a bigger, thicker wing in comparison to typical high performance design teachings. Providing increased fuel storage and slower landings, these features make the wing stronger and stiffer for a given weight. Wingtip twist, associated with swept wings, is countered in the system by the downforce from the tails.
3. The Synergy configuration moves the engine close to the center of gravity and support structure. When flown solo and light, 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.)
4. The Synergy configuration eliminates complexity. Simple wings are freed of control surfaces and the many parts they require. Only two moving surfaces are needed to provide a majority of flight control, and these 'high aspect ratio elevons' are supported at both ends. Thanks to its natural turn coordination, Synergy's twin, V-tail-mounted rudders are rarely required in flight.
5. The Synergy configuration creates a novel method for the prevention of stall, and exhibits pre-stall response behavior similar to the canard configuration.
6. The volume of air that is progressively displaced by Synergy in flight changes smoothly and properly in a way that matches the identified optimums for a simplified 'body of revolution' in its speed range. This 'subsonic area ruling' creates a minimum 'pressure field disturbance' condition in all phases of flight... but it takes an extraordinary aircraft to benefit from, or preserve, refinements in this category. Normally they are lost before they can be seen, let alone studied.
7. The Synergy configuration exhibits superior handling at all speeds, including ideal turn coordination.
8. Several versions of Synergy show promising control behaviors during intentional 'deep stall descent' at relatively low speeds.
9. The Synergy configuration creates a smoother ride in turbulence, and a noticeably more stable platform overall.
10. Synergy's double boxtail configuration also creates constructive, beneficial biplane interaction, rather than destructive biplane interference, allowing wing and tail to cooperate together for lower drag.
11. The placement of the large tails provides instant positive lift and wing lift recovery, along with the usual nose-down control input, when recovering from any high angle-of-attack maneuvering.

A: Interference drag occurs when the interaction between objects moving through a three dimensional volume creates unfavorable pressure and velocity distributions, resulting in turbulence. The amount of turbulence that can be created at intersections between wing and fuselage, and wing and winglet, for example, can be surprisingly high. Knowledgeable reviewers of the Synergy conceptual design (shown in early work without a wing fillet) are therefore quick to point this out.

However, shaping these elements is a critical design task. Rather than blindly implement the usual wisdom, which oversimplifies, we strive to work the problem parametrically in 3-D. This approach eventually yields a superior result, without compromising wing placement, or detracting from the propulsive potential of the fuselage in pressure thrust. Due to a nearly ideal volumetric displacement and laminar flow, we also have a more tolerant condition than meets the eye.

Early on, wing-fuselage interference was intended to be captured for cooling. Later optimizations allowed a cooling thrust design, so a preliminary fillet was designed around high-recovery pressure thrust attributes. This feature will be highly refined for the final product.

With continuity of higher pressures on one side of the airfoils and continuity of low pressures on the other, Synergy doesn't create the kind of conditions that cause 'interference drag' (which is really just a catch-all term for unanticipated turbulence.) The intersection of our airfoils is also optimized to provide minimal shed vortices, which is likewise a symptom of discontinuity. Our approach goes beyond a blended winglet design in favor of a temporally optimized volumetric displacement, a true 4-D solution.

A: Although it reaches unprecedented speed on only 200 HP, let's just say that Synergy is faster than we need to assert at this time. For safety reasons, our service ceiling is 25,000 ft, but in simulation we've flown far higher. Our low induced drag advantage especially shines at high altitude.

A: Yes! We keep forgetting to say so, as many are already familiar with the seating arrangement advantages of Synergy.

There are actually THREE places where we can fly the airplane.There is side by side pilot-copilot seating, with a choice of control configuration, right behind the front seat. "Solo" can only be flown from up front, but "dual" can be flown side-by-side OR tandem.

You can also have one instructor with two students, or a VIP passenger up front. (It's hard to tell the first side-by-side seats are pilot locations because the back instrument pylon isn't visible yet and the controls attach to the wing center section, still being built.)

In the event of an emergency, we intend for all passengers to have access to the new VP-400 system from Vertical Power, which is a new technology to safely and automatically put the airplane on the safest available runway simply by "pushing the Runway Seeker button." Even if that's not looking too good for some reason, a pull of the "big red handle" will deploy the ballistic airframe parachute.

Many other safety features are designed right into this aircraft from the conceptual level onward, including outstanding low speed handling.

A: Actually, Synergy with one or two seats filled will easily outperform most two seat aircraft on the same or less horsepower. It's also smaller than a 172 in terms of hangar footprint (L,W,H) even though it can seat six without anyone touching anyone else.

Our Deltahawk diesel engine has already shown a 4 GPH cruise in the Velocity aircraft at 146 kts. We can do better than that.

Build it as a one seater with a pickup bed in back, if you want.

A: Synergy has an empty weight of 1650 lbs, 200HP, and 156 sq ft of wing area on a 32 foot wingspan. Casual use of these numbers would be misleading, however, because of our high span efficiency, laminar flow fuselage, and powered lift / powered drag reduction system.

Synergy will offer high performance at both ends of the speed spectrum, but specifications won't be used for marketing purposes until they have been demonstrated in full scale atmospheric flight testing. More details are found on the technical info page, and with formal non-disclosure agreements we can share more detailed information with prospective partners and collaborators.

A video explaining the basic principles at work in the design is found here: http://youtu.be/UdUNcByJ5eM

A: This issue is driven by things that happen in the wake of a presumably successful flight test regimen, followed by production development and production flight test.

It is rare to have the opportunity to influence such things fundamentally and at the system level. So, given that we know how much it costs to build similar aircraft using less efficient methods, we can confidently predict that it will be possible for Synergy to substantially lower the cost of market entry. Early adopters will find the price highly attractive and competitive, but costs will drop when production hits its stride. Our goal is to offer unmatchable value.

A: Yes, although a better choice would be a small multiblade turboprop, in terms of efficiency and in keeping with the quiet advantages of the concept. Other engines are also possible, but the configuration is not highly suitable for use with air-cooled engines. This intentional bias may help advance the use of liquid cooling in aircraft. Originally, John intended it to be electric powered, which is how every detail came to be scrutinized and the aerodynamic breakthrough occurred. High torque electric motors are the preferred eventual powerplant for Synergy designs. With regard to jet propulsion, centrifugal turbines become a possibility, avoiding a forward intake. Hopefully the jet engine folks will study this work as a new opportunity for unducted suction propulsion (instead of pushing the rope). Reaction thrust jets are obsolete.

A: Not yet. Publication of technical papers is planned for after flight test demonstration. However, at least nine respected aeronautical authorities received an early look at Synergy under formal confidentiality agreement. Their opinions, while not purchased for publication, were positive.

Reactions varied initially, as most thought we had 'another box wing.' Upon walking them through the design and our technical library, all but one agreed that the premise and its execution have significant merit. No one raised any issues not already considered, and the consensus was that the work will succeed and will speak for itself.

Since our unveiling, the acclaim has been universal, even though little can be evaluated. Extreme interest has been expressed for getting the full scale prototype flying! Most engineers can't add much to the body of research we have produced to date without it.

A: Synergy clearly promises the largest practical fuel economy breakthrough in history. However, before the full scale vehicle is completed, validating solid answers to the question is as expensive as simply testing the real thing.

As reviewer George C. Greene put it (FAA Chief Research Scientist; NASA Langley, retired), "the thing that makes it so hard for me, and probably for others, is the synergy. You are doing so many things (well) at the same time that you have to look at all of them (together). And when you talk about synergy, as you know, they don't add in a linear way as most classical aero stuff assumes... I don't think I could put the pieces together the way you did - that is true insight."

Eventually our debut will allow universities and industry to study the project openly, comparing actual data to predicted performance. Until then, the bottom line is that Synergy correctly and intelligently combines four proven technologies, each having a huge impact on power requirements and fuel consumption. It does so usefully; with strength, safety, and manufacturing economy, and without complexity; while eliminating trim drag and cooling drag.

We feel it's worth the minimal remaining build cost to find out how high the bar has been raised, and in doing so, we create maximum value. Time to build it!

There is no reason to think our results will not match those obtained in previous work, and, thanks to modern tools, much reason to expect we'll exceed them. Still, until it's publicly demonstrated, it is best to make slightly more conservative statements. While claiming 'extraordinary' drag reduction, we merely match Bruce Carmichael's figure for 100% laminar flow, ignoring pressure thrust and propulsive synergies altogether.

A: At cruise Reynolds numbers, without active BLC or propulsive influence, X-Foil and other 2-D analysis codes yield fuselage C_d = .0026, wing C_d = .0026-.0036, and stabilizer C_d = .0045. Since these values reflect natural laminar flow beyond 62% of chord, expected C_d for the 100% laminar flow condition is actually less than the conservative .0020 polar minimum used to calculate our suction BLC condition. Confirmed values of .0008 to .0014 have been demonstrated experimentally by Pfenninger and others.

A: Defined on the basis of span efficiency and aspect ratio, K = .0307. K reaches similar values using either e = 1.46 or AR_eq = (b_s + .45b_h + .45b_e )/ C_avg with e set to unity.

A: Yes. None of our performance claims depend upon having Oswald efficiency greater than one. Flight simulation has typically employed a highly conservative Oswald e = 0.985. Our actual true span efficency is, like many real aircraft with optimal loading and nonplanar wings , much higher than 1. Kroo, in reference 5, summarizes wake-based studies confirming values of e reaching all the way to Prandtl's theoretical limit of 1.47 for a wide range of nonplanar forms. Though often confused with span efficiency, and (incorrectly) used interchangeably, Oswald efficiency is based upon the teaching that elliptical loading is the only ideal, whereas for nonplanar configurations this assumption is false.

A: No. The presumption of loss due to swirl is a man-made concept. The objective of Synergy propulsion is minimum entropy mass flow. An unimpeded, lightly swirling propwash displaces a highly laminar column of airflow efficiently downstream of the aircraft. Moreover, Synergy creates a majority of its thrust through suction.

A: Definitely. In both Single Stage To Orbit and aerial launch to space, aerodynamic drag is a major design factor. To focus briefly on the comparative relevance of Synergy toward the WhiteKnight/SpaceShip One model, it can be seen that a successful mothership vehicle requires attention to low induced drag, high strength, light weight, and high payload capacity. Uncomplicated twin fuselage design is also helpful.

For the reentry vehicle, favorable wave drag and transonic behaviors; short, strong wings; and control under high-alpha deep stall conditions are required.

In all of these respects, Synergy introduces new and useful solutions. High span efficiency and greatly reduced fuel requirements allow designers to imagine much higher achievables than allowed by prior technology.

A: By analyzing the pressure and velocity distributions required to maintain an attached boundary layer. Pioneering work by August Raspet in the 1960s showed that 100% laminar flow is surprisingly easy to achieve at general aviation speeds, using power. Suction applied to perforated wing skins at a rate of 0.0137 horsepower per square foot of wing area provided laminar flow on turbulent airfoils, and very high maximum lift coefficients, on full scale aircraft.

Our 'natural laminar flow' airfoils have a very flat pressure and velocity distribution, easily maintaining laminar flow up to 60% of the wing 'chord length'. Suction is applied beyond this point, which not only stabilizes the boundary layer for 100% laminar flow, but creates pressure thrust for extremely low drag. (We use a number of commercial grade airfoil analysis codes to compare similar airfoils having flight test data.)

In addition to issues of safety (see FAQ), there are several reasons why boundary layer control failed commercially. First, some aspects, such as contamination, water entry, maintenance, and so forth, require serious effort to address. Second, aero research at the time was all about high speed and supersonic flight, where it was hard to achieve and not helpful.

Third, BLC is seldom seen as the easy recipe that it is. Much emphasis is given to using specific 'proven' details of hole size, pattern, placement, and so on, without insightful consideration of the dirt-simple mechanism (pressure gradient) that we're using. Fourth, airplanes that could use it really weren't changing, and it didn't adapt well to old designs. Finally, a push toward blowing, rather than suction, goofed up the ability for designers to know what they're doing in physical terms. Thus the attitude: active laminar flow is complicated and expensive. Probably not worth it!!!

A: Yes. A small series of tests has been run using Large Eddy Simulation for the v.31 aircraft in the non-powered condition. The results are very exciting, and in particular have confirmed our success in refining the flow in areas that are more difficult to validate using volumetric and 2-D methods.

Since our software was provided on a trial basis with various legal restrictions it may be a while before we can share any results publicly, and we remain of the opinion that full scale flight test is essential to proving our premise. In the surprisingly candid opinion of many who use it daily, CFD makes pretty pictures for the marketing department. For the most part we have to agree with that view, and it is important to understand that CFD is far less important than CFU (comprehensive fluids understanding!)

Computational Fluid Dynamics is an attempt to solve simplified forms of the equations of fluid dynamics, usually the Euler, Navier-Stokes and Barnette equations. While solutions to these equations typically DO NOT exist, and their approximations remain difficult, CFD software has been refined to the point where a reasonable degree of accuracy can be obtained for well-known geometries and flow conditions.

However, the liabilities hidden in today's level of analysis are many, and defensible results require much care and insight. In any 'open thermodynamic' configuration, for example, the results obtained under power are expected to be completely different than the results obtained when passively dragging the body through the air. We are particularly enthusiastic in that regard, however, as Synergy appears to show very little drag outside of areas we've designed to benefit from the use of power.

A: No, they're consistent and flow from classic methods including 2-D flow solvers, spreadsheets, proprietary analysis, and panel codes. We feel that a completely different mathematical basis is required for accurate, low cost, and potentially real-time CFD solutions, and it is a focus of much research. Stay tuned!