Technical Information

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The Need for Active Measures Against Atmospheric Viscosity

Personal air vehicles that fly in the 150-400 MPH range have almost never actually achieved efficiency. In fact, they’ve been so universally bad for so long, most people never knew there was any issue. Adding enough horsepower and speed eventually blasted us through these limitations, and by the jet age we were flying high enough and fast enough in very large airplanes to overcome the inherent stickiness of the air. This created economy, but at the cost of huge airports and  limited destinationsand it never really solved the original problem: efficient, personal group flight at midrange speeds.

A hidden problem leading to a 6x deficit in comparative efficiency turns out to be the real reason why small planes lost to big planes, and why they never got comfy, quiet, or affordable.

Our great-grandparents knew that a mile of highway can only take you a mile, but a mile of runway can take you ANYWHERE. So they built 50 times as many airports as we presently use commercially, and waited for the futuristic family airplanes that never arrived. Synergy Aircraft is the only company to fully understand why that occurred, and to commit 100% of our resources to using the required technologies correctly.

GvK Chart

History Repeats Itself

To assess the fundamental efficiency of a vehicle, it makes sense to compare the ‘total energy consumed’ with the momentum obtained (speed times weight). Clear back in 1950, Gabrielli and von Karman published “What Price Speed?”, a sweeping look at this topic for various forms of transportation.

Unfortunately, what it revealed is still true: the economics of flight are only reasonable when we fly big, heavy airplanes very, very fast… or when we fly very slowly with lots of wingspan. Airplanes in the middle miss the achievable efficiency target… by a factor of six!

To ask why until real answers emerge is to discover one of the largest opportunities in the modern world.

Two reasons especially stand out. One, airframe drag under power has always been assumed to be essentially the same or higher than the drag when towed or gliding. That isn’t true, nor should it be. A source of power can be used to cut drag by huge amounts, especially in the largely unexplored domain in question, if used to promote free circulation.

This can be illustrated, first by understanding there actually is a ‘static condition’ implicitly assumed in the math of lift and drag. It essentially says that if one drags a streamlined hull down a canal with a team of horses, the ‘horsepower’ it takes at a certain speed can be measured in the rope tension.

However, long ago it was learned that if one puts a motor of roughly half that horsepower on board, with proper propulsive design it can essentially achieve the same speed. Boat and aircraft propulsion is acting, not upon a fixed, immovable earth, but upon the fluid medium directly, which can be naturally circulated in a way that recovers much of the initial resistance. This is one reason why there are no tractor-prop submarines.

Second, the basic equations of drag and lift used for prediction embed this fixed, static assumption, rather than a dynamic circulation, which can only be directly computed upon a completed design.

It turns out that this assumption in the simplified math doesn’t approximate the physics of airflow in the unconstrained atmosphere equally in every domain, which can be seen by the changing coefficients required to make it work out at different speeds. Yet virtually every tool available early in the design process, or to a casual reviewer, relies entirely upon these representative equations.

The track record of GA inefficiency makes it obvious that something deeply hidden must be wrong in standard aeronautical practice. The above gives many clues about what it is.

In fact, a different aspect of this same topic once held center stage in the debate between ‘mathematically certain’ Cambridge physicists and technically competent German engineers. It took decades for Britain to catch on and accede to the obvious experimental results of Germany’s mastery of the circulation theory of lift. Now it is time for the circulation theory of drag reduction to be similarly advanced.

For the practical needs of aircraft designers, at the beginning of the 20th century Ludwig Prandtl and his student Blasius resolved d’ Alembert’s lossless fluid circulation paradox, a mystery that had been central to the puzzle of drag for close to two centuries, by postulating a physical basis for a mathematically required assumption. Their work gave us the ironically named ‘dynamic pressure’ concept; the ‘1/2 density times velocity squared’ portion of the basic lift and drag equation.

The irony of this postulate is that the required condition to allow our cavalier acceptance of this version of the standard ‘kinetic energy equation’ …is that there is…there MUST be…a non-dynamic, ‘static condition’ in the system somewhere, wherein air molecule movement is simply not allowed.

Since for a gas that’s not even remotely true, our use of the dynamic pressure equation depends upon a falsehood. This equation is used early, and pervades throughout aeronautics. It has preemptively cut off half of the experimentally demonstrated potential for drag reduction by labeling such results as anomalies and fictions.

What can be said now, as the true physics of circulation are finally reapplied to the topic of drag, is that the static condition assumption is only approximately useful for the flow regimes where either viscosity or inertia have the upper hand. In those ‘fast’ or ‘slow’ domains, this non-physical assumption holds reasonably enough, and consequently in those domains we’ve done quite well.

In the flight regime in question, though, actual boundary layer behaviors differ, sometimes dramatically, from the assumptions of this century-old mathematical model.

Even today this fact has remained largely invisible, although experimenters found it strongly implied for at least half that time. The modern version of the debate considers the merit of a host of turbulence models for CFD codes, each having improved accuracy in a particular domain, yet each conflicting and artificial.

It should therefore not be surprising that the natural application of circulation to drag reduction, using power, has been a sometimes controversial and always very poorly-understood topic. For example, the result shown here is ‘news’ known to some although applied successfully for forty years.

Today, the basic architecture of most aircraft reflects the pervasive influence of these underlying 2-D mathematical assumptions from a century ago, rather than the proper 4-D, dynamic physics. In fairness, it was until recently impossible to analyze such behaviors accurately, even though many designers have displayed a gift for its governing principles.

Nevertheless, the conventional aircraft architecture is fundamentally at odds with circulation methods of drag reduction, and therefore existing aircraft hardly benefit from concepts that require the body length and fineness ratio to be appropriate for the Reynolds number involved. Synergy uniquely resolves these issues in a transformative manner.

Synergy GvK

How It comes together in an elegant design


Car sized airplanes cannot succeed economically until they fly faster using elegant physics, the correct fluid mechanics for a domain in which (high speed) inertial effects and (low speed) viscous effects of the atmosphere equally impede flight. Brute force application of power, with its many downstream consequences to systems and structure, is no longer sensible for this size of craft.

Synergy takes a direct path toward fundamental efficiency by putting the priorities of a sailplane in a form that can carry a lot of weight and fly very fast, yet quietly operate from fifty times as many local airfields as the scheduled airlines can reach.

This has never been an easy or obvious thing to achieve, but John McGinnis’ landmark patents opened the door to gaining stability, control, and stall resistance in a span-efficient form that works for high speed flight. That allowed the adoption of proven measures targeting the root causes of drag in each of the three scales of atmospheric response: dissipative, inertial, and energetic. Thanks to a decades-long, proprietary headstart in the applied technologies, Synergy gently and respectfully deals with inertial and viscous effects at every scale, from the Mach farfield to the boundary layer.

Solving this problem with a safe aircraft means that any random place is a quick drive, by air, from anywhere in a region, and people can get busy making lots of affordable, smart airplanes that won’t demand as much (or anything) from their operators.

Passive Drag Reduction

Synergy begins by enabling conventional drag reduction techniques at low cost. Every part of the aircraft works together to maximize Natural Laminar Flow… while minimizing interference drag, induced drag, and turbulence. Its 3-D wing/tail system reaches the highest possible ‘span efficiency’ and is the perfect aerodynamic catalyst for economy at high speed. The large tail configuration creates exceptional stability and control by means of induced drag reduction, plus great low speed handling and stall resistance.

Double Boxtail

Synergy’s patented ‘Double Boxtail’ arrangement (US 8657226; 9545993) allows a simpler, stronger wing of the minimum induced drag for a given wing span loading. The two horizontal tails intentionally push down, moving air upward at the wingtip in opposition to wake vortex. (Their resulting “constructive” bi-plane interaction with the wing also reverses the usual biplane interference penalty, increasing laminar flow and span efficiency.)

Negative loading of large tails, at just the right amounts to optimize these counterintuitive drag reduction benefits, also provides a smoother ride, more stable flight, stall resistance, and better handling, especially at low speeds when it is critical to safety.

Neither wing nor tail have any control surface hinge lines or spanwise breaks in their high performance airfoils, totally sealing the pressure side from the suction side, reducing drag and drastically simplifying their construction while reducing weight and complexity. Simple parts are inexpensive parts, with less to go wrong.

Perhaps more exciting, the configuration opens a door to efficient high speed flight for the first time. Synergy’s Double Boxtail configuration becomes a true breakthrough by allowing proven drag reduction methods to be applied directly and without complexity.

Natural Laminar Flow

‘Laminar flow’ means no turbulence in a fluid flow. Through proper 3-D shaping, each part of the airplane is mathematically optimized for significantly less drag at its design speed. For example, body length is reduced to known optimums for its fuselage width at high speeds. Synergy uses custom ‘natural laminar flow’ airfoils which have a very flat pressure and velocity distribution, easily maintaining laminar flow for 60% – 80% of the ‘chord length’.

Subsonic Area Ruling

The volume of air that is progressively displaced by Synergy in flight changes smoothly and properly in a way that matches the identified optimum for a simplified ‘body of revolution’ in its speed range. This optimized volumetric displacement (according to subsonic, not supersonic ‘area ruling‘ principles) minimizes sudden disturbance to the pressure fields developed around the aircraft, in all phases of flight.


Synergy cfd example

Active Drag Reduction

In addition to its comprehensive use of ‘conventional’ aerodynamic advantages rarely seen, Synergy uses experimentally proven techniques for active drag reduction. A small amount of energy is used to control the behavior of air close to the skin, resulting in a tremendous reduction in overall power requirements. On this aircraft, boundary layer control minimizes drag by creating ‘pressure thrust’ and up to 100% laminar flow.

To explain what was happening as experiments began violating convention, in the 1980’s NASA began describing this equation-changing field of “powered” drag reduction by the term “open thermodynamics,” because we are bringing energy into an otherwise closed system. It’s not really about how we use heat, although that is still relevant.

The assumption of “closed” thermodynamic calculations (the ones always used to figure up a new aircraft) is that the drag when gliding unpowered is always equal to the drag at the same speed even when under power, which isn’t necessarily true when one is using that power to reduce drag.  Synergy uses power first to dramatically reduce drag, then to make thrust.

Active drag reduction takes general aviation far beyond mere streamlining to explore new technologies for fast, efficient, comfortable flight in the 21st century.

Pressure Thrust

A small amount of engine power can be used to reduce the boundary layer thickness of the aft fuselage. This “powered pressure recovery” can be used to create a larger area of high pressure in the back of a body than in front of it, creating a slight forward push called pressure thrust.

However, attempts to use the phenomenon for propulsion are misguided and inefficient. Done properly, pressure thrust is actually just major drag reduction for the price of a little suction.

Zero or even negative drag has been easily reached in experiment, but it quickly becomes inefficient to go beyond the goal of drag cancellation.

Boundary Layer Control

On Synergy, power can be used to create less drag on the airframe by sucking turbulent air off the wings. This not only stabilizes the boundary layer for 100% laminar flow, but with correct geometry, creates pressure thrust for extremely low drag.

Although the physics are slightly different, boundary layer control is like the difference between playing air hockey with the table turned off… or playing with it turned on.

Done well, this is neither complicated nor a safety issue. Synergy provides the ideal form for utilizing several proprietary BLC technologies developed and refined for laminar bodies by John McGinnis since the late 1970s.

Wake Immersed Propulsion

Hydrodynamicists have likewise known for decades that wake-immersed propulsion greatly reduces drag: because of the high drag environment of water, it was obvious. Synergy uses a specially designed quiet wake impeller, (not shown) which reduces turbulence ahead of the engine. Cooling drag is also minimized by pressure recovery flow design and integral exhaust scavenging.

Having the prop behind the body is stabilizing and greatly preferable over having it in front, as long as the inflow is not disturbed by turbulence as in the vast majority of prior designs. As in most areas of aeronautics, opinionated myths persist in this area due to the perception that prior work was adequate to establish a superiority of the tractor configuration. In fact, the primary reason for tractor props becoming dominant was the influence of the air-cooled engine in conjunction with very poor aerodynamics for propulsion, including among most good-looking pushers.


For more information on the historical development of advanced drag reduction technologies, or to read NASA studies on their impact when applied, visit the CAFE Foundation technical library here.

Synergy Basic Information (V31)

Synergy-GFC-intro_4_598x598Wingspan: 32.4 ft*
Overall length: 21.4 ft
Overall height, gear down: 10.8 ft
Fuselage width: 5.0 ft
Seating, 5-7 passengers: 1-2-2, 1-2-3, 2-2-2, 2-2-3
Empty weight: <1650 lbs
Gross weight: >3100 lbs
Wing area: 156 sq. ft.
Engine: DeltaHawk diesel, 200HP, 4 cylinder, turbocharged, fuel-injected, piston-ported two-stroke
Prop: Kiwi Custom Propellers, proprietary wake impeller.
Controls: pushrod actuation (from 3 seats); rudders (2) on V-Tails; Elevons; split flap; wing suction.
Landing gear: tricycle, retractable, trailing link mains.
Designed for pressurization.
Emergency auto-landing system with autopilot.
Whole airframe Ballistic Parachute.

     * Synergy has a high ‘span efficiency’ “e”, near the theoretical maximum of 1.47. For more information, see video overview and FAQs.

Performance specifications are not public at this time. When offered, they will be based upon actual flight test results. However, the following general guidance has been previously published.

Minimum level flight speed: <55 KIAS
Range: >1500 nautical miles w/ std reserve
Economy cruise: >40 MPG @ >200MPH

Note: Synergy Landing gear not shown in comparison view. It sits higher and the main gear are much further back.

On Poster V1Demand Aviation utterly changes everything: In Synergy, the same hour wasted on your Silicon Valley traffic jam would allow your commute to originate from South Lake Tahoe… or Santa Barbara, Santa Rosa, Reno, Monterrey, San Luis Obispo, Carson City…or anywhere in the entire central valley from Bakersfield to Sacramento to Redding. Draw the size of circle you want, around the place you need to be, and it will show you where you could live and how you could travel someday.

There is no need to worry about mass adoption issues, ever… the pace of change will be far too slow due to the unmeetable need for enough aircraft. History shows that society can get behind good ideas far more quickly than they arrive, but it will take time to transition from making ten thousand automobiles per hour. As that rate is reached annually, safe, quiet future aircraft will eventually, slowly soak right into the very fabric of our communities while upgrading and repurposing areas of blight.

Aviators know that traffic jams are a quaint 2-D concept that networked neighborhood smartplanes ™ will never replicate. Remember, sprawling commercial airports (and their busy air traffic control) are a legacy of packing 747s onto the same ribbon of concrete every few minutes, not a requirement of safe operations for future little airplanes that talk to each other, spread over a million times more area.

With each level of adoption comes the increasing reality of affordable, nextgen personal aviation… it’s a thousand times better idea than Hyperloop.