Beyond Streamlining: Drag reduction in the 21st century

I don't believe Mr. McGinnis had released much information about the powerplant on this thread, so at least now we know that we are talking about a nearly 200hp diesel.  I have to credit he and his team for tackling such a large aircraft for this challenge.  We know John's opinions on the matter of size and seating because of this thread, but it's still interesting to see Synergy lined up with its competitors, none of which are shooting for the same cruise speed or passenger count and only one of which has 4 seats and similar power.  Most entries are motorgliders or variations on that theme.  From this perspective, Synergy seems the riskiest in terms of hitting the marks it has set for itself, but a six-seat aircraft actually should have scale advantages even disregarding John's approach to aerodynamics.  More seats are cheaper.

I'm not trying to hype Synergy and am interested in all of these aircraft in terms of what they can do with fewer calories per butt-mile, but Synergy seems to be one of the few "real" aircraft in this group that people would buy and fit into typical hangers.  Single-seaters are okay for homebuilders, but there's little market for such things.  The motorgliders are neat and I personally like that kind of airplane, but they also have low market appeal and will undoubtedly cost a lot per seat.  If CAFE truly intends to push for real-world aircraft, very few of those 13 entries seem to qualify, though I admittedly have not seen these aircraft and look forward to learning more about them.

I hope all the entrants show up with operational machines and that this wide spectrum of aircraft push personal aviation towards higher safety, efficiency, and full-costed affordability.  Should be an interesting competition.

In order to demonstrate 200 passenger miles per gallon, a single seat aircraft must get 200 miles per gallon. Two seats cuts that in half, and four seats brings us down to a very reasonable 50 MPG. Several four-seat aircraft with appropriate modifications are capable of achieving that target, all of them at speeds over 100 MPH, while carrying sufficient fuel for the 200 mile flight. I have been reluctant to rant publicly about this, for entirely selfish reasons.

Synergy is a five-to-seven seat (capable) aircraft by virtue of its roomy laminar flow, pressure recovery fuselage. This puts it squarely in the sweet spot for the GFC. I was told that a NASA study pegged the ideal seat count for the challenge at six.

We need only achieve either 40 or 33 miles per gallon for 200 miles, which we can do at a speed I am not about to disclose, using five or six seats, respectively. GFC scoring rewards a large spread between the economy and speed differentials in the two scoring flights, which large wingspan electric aircraft will have an extremely difficult time achieving.

We believe our comprehensive drag reduction advantage will allow a competitive spread between our fuel consumption scores, and an unmatchable scoring speed. Synergy is powered by the DeltaHawk 180HP Diesel engine , in collaboration with DeltaHawk. It was actually designed specifically for this engine, although its roots and its future lie in electric propulsion.

The rules requirement at low speed will call for us to demonstrate a maximum lift coefficient of 2.4. This is within range given our approach to the problem. Synergy has shown excellent handling at low speeds to date, but the GFC aircraft is unproven in this (and every other) regard. Our approach, both specifically and in general, has been considered promising by many in a position to know.

As of this writing, our greatest obstacle to winning the $1.65 M total prize is the chronic absence of financial support despite multiple opportunities. Fear and greed may indeed power the world today, but I find it deplorable.

Regardless of the Green Flight Challenge incentive, and/or the money to be made through subsequent business development, the risks we have to take in order to advance aviation out of its present stagnation are nothing compared to the risks taken by generations before us. As I've said before, I'm the biggest chicken in the room, and Team Synergy's GFC strategy is akin to shooting a gnat with a bazooka. If and when we can prove that, the contest itself will have been nothing more than a small catalyst in the right direction. I applaud CAFE and all the visionaries who recognize that aeronautics has been sitting on its hind end too long. It takes hard work, guts and money to shake things up, and with a small dose of all three I believe they've done so.

John;

I have tried to read this long thread and now have goine to the CAFE site.  I wish you and your team good luck and safe flight.  I hope you will post more on the craft and progress.  I assume given it will compete in July that it is well on the way to flight.

Is this being filmed for TV.  I hope so. the DARPA challenge was very interesting.

Too bad that the FAA and other letter organizations are not getting behind this and helping.

It would be great to see all of these aircraft at Oshkosh after the compete.

Wow what a great thread!  It just sucked up a 1/2 of my day reading the last 15 pages!  You're firing the imagination here.  It would be great to see another leap forward in successful aircraft design.  You would probably also reprice the turbine market, which would be nice to see..

I'm not a homebuilder but you've got me waiting to clean out the garage and build this aircraft.  Thanks for your time and insights here. I wish you success and hope to see your plane at Oshkosh this year.

 For some time now I have hoped to discuss the role of wind tunnels in advancing (and retarding) our aeronautical understanding. Due to multiple conflicting demands on my time this won't get nearly the treatment it deserves, but the topic remains important to acknowledge anyway as it sheds insight on one of the last ingredients in our recipe for next generation aircraft designs.

Wind tunnels have served us well, and there is no question of their value to history. In modern times they remain an effective and economical way to validate designs of both a casual nature, and especially those that are too complicated to submit to numerical analysis alone. Yet somehow, the concept of wind tunnel testing has been elevated to a stature that it does not properly occupy, and the result of this perception among the public and scientists alike has led to inaccurate data, missed opportunity, and apathy toward promising technologies.

In the long version of this story we could provide specific examples, but in this case the influence of our tunnel-centric perspective is nearly universal with respect to its effect on the two things we most care about in this thread: laminar flow and induced drag.

Since our premise is that the progress we need lies on a vector driven on the one hand by high speed drag reduction and on the other by low speed induced drag reduction, our sensitivity to the possibility of a combined 'double penalty' coming from our most trusted test apparatus is very high. If wind tunnel studies have cast uncertainty on both the achievability of laminar flow and the magnitude of the induced drag penalty incorrectly, this error would have very little effect on the vast majority of practical aircraft designs. Most designs do not go anywhere near the fringes of our understanding of matters.

But what about those that did? Wind tunnels are clearly not their friend.

Undoubtedly many are more intimately familiar with this problem than I am. I recall hearing, every once in a while, true horror stories so scary (by the magnitude of the dollars and the jobs at stake) that I feared even to memorize their details. When tunnel testing and flight testing tell drastically different stories, heads will roll.

Our interest should be in understanding why and how this could occur, from a physical perspective.

It's dirt simple, really. The atmosphere we fly in and the tunnel environment are not the same thing. Not even remotely.

Oh sure, they're very related, and we know a lot of fancy tricks with fancy names designed to give us the proper fudge factors for any given objection, such as the influence of tunnel walls, Mach number, and microturbulence. The problem is that we forget that those bandaids are holding our foundation together, and we build as if the blueprints couldn't possibly be wrong. After enough excellent correlation we might believe that our corrections accurately model the real world.

Maybe they do. Maybe they don't.

Since Bruce Holmes et al did an excellent job of exposing how conservative tunnel predictions could be toward the attainment and maintenance of laminar flow in full scale flight testing, I'd like to focus full attention on the other problem, perhaps the larger one: the inaccuracy of wind tunnels with respect to uncorrected prediction of induced drag (which forces our reliance upon our self-generated corrections).

For this it's best to remember what induced drag is. It is NOT the 'drag that is induced'. For purposes of bookkeeping, induced drag is mostly the penalty of having a finite wingspan. It can be and often is lumped together with other viscosity-related drag terms, but the way I avoid discussing 'vorticity vectors' and other math-related visualizations is to remind the reader that heavier-than-air machines usually use wings to throw air downward by passing through it at a shallow angle with a curved camberline. Induced drag is (properly) the difference between the drag we would measure while making no lift and the drag we make while generating lift.

(I have to restrain myself at this point from going off on a wild tangent about the terrible injustices this simple view incubates, such as the awful zero-lift drag coefficient concept. Students apply this teaching by looking at their drag polars, using the drag at CL=0, which is often very high... higher even than when making cruise lift!)

As it became clear that induced drag prediction by correlating tunnel drag and with experiment (through calculation) looked more like guesswork than science, a whole lot of shaky ideas were braced up enough to become the scaffolding we depend upon in aeronautical calculation today. Oswald efficiency, which uses the term e, became incorrectly synonymous with span efficiency, also e, both of which rely upon the untruth that elliptical planar loading is the only ideal. George Greene established that viscosity must be considered (leading to a parabolic ideal), and many have learned that any non-planar wing design creates a new loading ideal that does not deserve to be penalized, mathemathically, just because it departs from an assumption.

Non-Planar wings can push the air outward, not just downward. In doing this they can act against induced drag as well as a longer wing. Their key ability is to delay and moderate the intensity of wake vortex. This is the key job description of any wing intended to reduce induced drag.

The full force of the induced drag penalty can only be felt when enough airmass has been stirred up in our wake to allow a tornado to form on our wingtip. Most of us know that it actually takes a lot to form a tornado. They depend on surrounding air moving just right, and dissipate quickly when it doesn't.

By limiting and ingesting the surrounding airmass, wind tunnels destroy wake vortex before it grows to full strength. That is why NASA began researching wake vortex using moving models (stationary fluid mass) in tanks and 'tunnels' thousands of feet long. Under these conditions, the miniscule influence of non-moving airmass, connected ever so delicately to the disturbed airmass (by viscous forces), could nonetheless play its role in amplifying the velocity of wingtip vortices.

The reason I wanted to clarify these points is that we're about to unveil a number of new ways to mitigate induced drag without as much penalty as in prior designs. The first thing the average armchair aerodynamicist wants to do is put it in a wind tunnel. This is the equivalent of pouring salt all over the chef's meal just prior to making your culinary review.

To the designers of every laminar flow, low induced drag airplane whose dataset was poisoned by such well-meaning but ill-informed thinking, I feel your pain. Even if Synergy fails to please us to the extent hoped, it will at least have shown that ideas deserve to be taken farther than they typically are before being subjected to an onslaught of lukewarm numbers generated under faulty premise.

Many thanks to the supportive readers above. Today's rant:

Oswald efficiency, Biplane Interference

If my posts sound a bit cranky lately, don't be mislead. Relief is happening all around us! My point is that as an industry, until now We've been driving our 'interstate highway' of the status quo every day, right past the most valuable real estate in aviation. Some have suspected that through the bushes may lie a paradise that's not even on the map, but whenever a group or two stop to investigate, the combined arguments of wind tunnel data and preliminary calculation have consistently left the explorers alone and without financing.

The argument is usually over the structural cost of low induced drag (whose real value is not even fully appreciated until it is placed on an unfamiliar pedestal). Yet in cases where structural issues are not the problem, there are still two dealbreakers lurking in our equations to spoil the adventure.

Bruce Carmichael has frequently led the way in exposing that Oswald efficiency (Oswald's bugger factor, as he calls it) isn't capturing the real physics required to help us nail down our induced drag before we get to flight test. For many designs, it's off by a mile, despite several noble attempts at improvement. It takes a lot of time and money to get to the point where a design can be submitted to more advanced methods of calculation, such as Large Eddy Simulation, so most individuals and companies evaluate designs at the conceptual and preliminary levels by using software programmed to apply textbook methods, such as those by Glauert and Weissinger below. For a straight wing aircraft, e is given as

e = 1.78(1 - 0.045A^0.68) - .64

and for swept wing aircraft of 30+ degrees,

e = 4.61(1 - 0.045A^0.68)(cos LE angle)^0.15 - 3.1

In other words, we have no idea what's going on physically.

Other methods teach us a slightly better induced drag factor 'k', but do so as if it were a constant. Not in the real world, it isn't. Worse, with k we muddle local 2-D and large scale 3-D concepts together while grappling with the (inviscid) ability of an airfoil to have zero 'drag due to lift' mathematically, instead of targeting the systematic creation of that actual phenomenon.

Arbitrary coefficients and exponents have no place in evaluating the 3-D aircraft concepts of the future. For this, what we need is a better framework for applying the equations of fluid motion. We'll need to STOP teaching the components of lift, drag, and thrust, and start teaching the physics of unconstrained fluid reaction. Let the computers do the simulation, but program them with nature.

For example, non-planar wing designs frequently use multiple airfoil surfaces in close proximity to one another. Whenever this happens, we need to consider their interactive effects. Like more sophisticated induced drag calculations, this takes place in software. However, Biplane Interference is treated in most software as a one-way effect: always negative. That is not the case. Even though for boxwings and such the influence is negative, there is such a thing as constructive biplane interference, and in my view it should be created and exploited. This is the operational equivalent of creating a venturi.

The images below have been coaxed to illustrate how biplane interference, blindly applied by equations in the usual destructive manner, can affect the evaluation of a pair of constructively interfering airfoils. In the first, a trailing wing that should be making negative lift is evaluated as if it were lifting in a positive direction, like in a staggered biplane, to graphically illustrate what biplane interference seeks to consider.


In the second image, the actual constructive interference between the wing and the trailing airfoil is shown properly accelerating the flow over both airfoils, the green areas showing how their interaction combines to affect a larger volume than either influences on their own. The difference, obviously, is huge, but such opposite treatments can be hidden deep inside spreadsheets, simulators, and MDO software... and they won't volunteer to tell you when they get it wrong.

Many readers have written to take a guess at what Synergy must look like, but so far no one quite figured it out. (As far as I know... according to EAA, Burt Rutan is building one more airplane, so maybe now is a good time to lay down my cards!)

But seriously, as I post this, Dr. Larry Ford of the CAFE Foundation is introducing the aircraft of the 2011 Green Flight Challenge, including Synergy, at the CAFE Electric Aircraft Symposium V in Santa Rosa, CA. Later in the afternoon, John Paul Noyes and I will be presenting on the topic "Stability and Control Through Induced Drag Reduction," which will be introduced by a showing of the video that was shot last fall in documentation of Team Synergy's GFC effort.

This is that video.  
 

The energy of wake vortex is put there by our flight, not by anything we design or put on an airplane, and it is 100% lost to us unless we can make use of it for something. As mentioned, Burt did so marvelously with the first-ever application of Whitcomb's non-planar winglet technology. Being vertical stabilizers located behind the CG, Rutan winglets create yaw stability while reducing the energy of wake vortex.

Let's take it a step farther. 
Since the swirl of wake vortex acts in a 360 degree circle, we can put airfoils in it as we like, and depending on where they are located and how they are oriented, we can redirect the molecules previously whipped off into a frenzied spin by using them to create stabilizing forces- while leaving them slightly less energized than they were when we slammed into them. (!)

Since vortex happens- regardless- the 'cost' of doing this at the wing tip is mostly 'paid for'; whereas on most aircraft we pay double: first we throw air at the ground, paying an induced drag penalty, then we catch some of it (right in the middle of the downwash, where we tried to move it the fastest) and pay another induced drag penalty at the tail while we reduce the lift we just made. Even when we minimize the downforce of a tail at the expense of stability, inevitably it comes right back into focus because our most efficient airfoils have a strong negative pitching moment.

I have come to the inescapable conclusion that the right place for a tail, aerodynamically, is having it connected to a wingtip by a tall, shared vertical winglet. In this location a deliberately strong negative lift from the tail can achieve its stabilizing action by reducing the strength of wake vortex, while constructively interacting (biplane interference) with the other flight surfaces from the proper safe distance. Its downward force reduces upward wing bending moment, and in a swept wing design, counters wing twist. It adds beneficial wing loading in flight without the mass or low speed penalty, 'doubling down' on its already-smoother ride thanks to increased decalage.

Synergy achieves remarkable stability with responsive control (in all three axes) primarily by means of this induced drag reduction. In fact, the scale prototype shown in video has only two moving flight surfaces.

 

 

You are absolutely in the ball park ...I can intuitively  discern regarding your statement / comments  are accurate - 'Since vortex happens" and  that you were inspired from watching Ann Margaret dance in Viva Las Vegas...  It has been a blast watching this get to this point. Looks like it will be applicable in a big way commercially... It is going to be fun  finding out how you set up the control  actuating systems and the inovations you developed ...   Just remember , I was the first to have a Moose Antler for a side stick handle....  

Your secrets are safe with me.

With great joy my brain has been twisted to seek more, and not blindly follow the heard of sheep.

You have schooled us, ( like fish ) Low drag in fluid....

 

John, congratulations to the team for where you're at and where you're headed.  God speed as you guys work to tie it alll together for July flying!

Ried

For the past 15 years I have gotten joy watching inspiring aviation projects.  I am envious of the successes and take stock in the failures.  There are some that hype and promote (and usually fail) and others that don't.  They just quietly "do" (and often succeed).  Your project seems to have the right amount of "do".  

I have been following this thread for a long time and while I understand only a little of the technical aspects of your project, I am inspired in the endeavor.  Your project embodies the american ideal that somewhere in a garage lies innovation.

Thank you for this thread.  Your plane is beautiful.  Wishing you vigorous success in pursuing this innovative concept.  Your passion is contagious and hoping the dream will come true.  I look forward to Airventure 201(?) and seeing this innovation take flight.

Daniel Findling

Thank you all. I've always planned on an AirVenture 201(?) debut, and like every EAAer, expect to be completed on Wednesday (unspecified.) Thanks, Daniel for putting a smile on my face and keeping it real.

It is certainly still within reach for us to fly on the aggressive schedule required, but not by any means certain as we have been without funding since September. (!) In response we changed from kit prototyping mode to rapid prototyping mode, but having to work sixty hours a week to support twenty hours of build time brings progress back to the homebuilder's usual schedule.

Since our unveiling the volunteers are plentiful, but there are many costs to meet now. Please watch for www.synergyaircraft.com to go live 5/6/11 where we'll have souvenirs available for those who'd like to help.

Wow,

 

Thank you internet.  I had to smile at the video when John and crew were asked "Where are you guys FROM?"  Here I've been working for years in parallel and didn't know it.  Such is the divergence in transportation technology - I landed in automotive engineering sixteen years ago and infrequently find time to dip my finger back in on GA, and yet here exactly the same kind of game changing discussions are happening.  Although I have to say my speciality is thermodynamics, so you might say we are really moving the needle on the supply side of efficiency instead of on the demand side.  Knocking the demand down dramatically is the icing on the cake.

 

Anyhow, it took me a day to catch up on the whole history of this topic - and I'm glad the Synergy looks like it does.   Just can't wait to hear the actual performance.

 

I have a question though - why not spanloaders, especially like Goldschmeid's application of the GLASII in AIAA 90-3198 "Thick-Wing Spanloader All-Freighter Design Concept for Tomorrow's Air Cargo"?  Seems to me this paper study should ace the AEI metric.

 

Good Luck John.  Maybe I'll see you in July.

 

John

Finally out in the open!! 

I just want to take a second to publicly thank Eric Peterson, Justin Daugherty, Howard Handelman, Larry Rhoads, and Jack Moon for supporting our project heart and soul. Take a bow, gentlemen, you've earned it. 

John, 

I just realized I answered your question on the wrong forum. Maybe I need to invent the cure for induced senility...

Here's what I said.

A spanloader or other flying planar wing design makes sense to our 2-D way of thinking, but the challenge of creating and responding to real-world airflows efficiently has forced 3-D (and 4-D) thinking upon us. Now that we have the computational tools to handle it, the clear advantage of 3-D non-planar designs (even over idealized spanloaders) has become fairly clear even though we are barely beginning to apply the teachings. I am not advocating for tube fuselages over all-lifting bodies, but when we integrate propulsion with fuselage drag elimination (only under power, of course) there can be an equivalence or advantage reached aerodynamically with less complication than a Griffith-style thick airfoil. 

Thanks for writing.

John, I just got back to this forum after a long absence and finding your design disclosed on the CAFE site (and discussed on the homebuilt airplanes forum )  Firstly it is good to see the actual hardware and see that it is also technically feasible and in fact quite beautiful so congratulations on both scores .    There are a few 'buts' though which come to mind .

Your voluminous descriptions and observations are hard to summarize or respond to but I will try to add some hopefully useful comments .

"Biplane interference" -- it has long been realized that biplanes can be favourable for reducing induced drag -there is nothing mysterious about that (it has to do with the streamtube being influenced by the wing not being a circle equal to span as Prandtl determined but a 'racetrack' shape having a parallel section inserted equal to the wing gap and therefore dealing with a bigger volume and hence mass flow of air so needing to deflect it less to get the same momentum reaction that we call lift . Newtonian physics is dafe still .    Nenandovich in France published a series of papers showing such effects and various other SAE and AIAA papers over the years have followed suit  --one man powered aircraft intended to use the concept and was in fact a joined wing well before Wolkovitch .   The tip feathers of a soaring bird are an example of non planar multiplane and the Swiss tip grid is another application --Francis Rogallo did work for NACA on multi element airfoil arrays along with others over the years  (nobody called homely old biplanes 'non planar wings' back then )

Your computer flow simulations a few posts back look somehow wrong --the upper one shows negative lift and strong upwash from the rear airfoil with little overall lift while the lower one at least shows some overall flow deflection and therefore lift -- the descriptions are opposite to this as far as I can tell (?)    Wolkovitch designed a staggered tandem wing aircraft for Vought that had in effect a huge T tail and calculated induced drag only 0.6 of the Prandtl value --this was the predeccesor to his joined wing work when he realized how relatively unsupported the rear wing was and joining the two tips by sweeping them together solved the structural issue  --when I met him in 1990 and had that paper with me he gave a wry smile and asmitted that this was how he came upon the concept (not the 'married man's hang glider' as he quoted --this was I think to 'freshen' the idea in terms of patent life ) 

In one post you compared the efficiency of a 'towed' vehicle ( a barge on a tow way pulled by a horse) as against a vehicle 'pushing' on a fluid -- indeed the reality is that not only do you scrub the air with the wetted surface of the aircraft as you would also if pulling it via a cable from the ground like a glider being winch launched but you also scrub ,at much higher velocity, the wetted area of the propeller in the act of thowing air rearwards (and the power absorbed being proportional to the cube of  local airspeed means that tiny spinning surface absorbs the whole of the power of the engine with the lift reaction created then indirectly propelling the rest of the aircraft .  This is undoubtedly inefficient and results in the wake of the aircraft being composed of one highspeed blast going going rearwards accompanied by another lot of air going forwards which is the wake of the aircraft itself (as seen by a stationary observer in a balloon basket as you pass )

the "ideal' aircraft would leave no wake in effect like being pulled by a cable from in front but adding enough power to cancel out the drag which was leaving energy in the wake --seen as forward moving air to the observer.   Goldschmied attempted to get to that condition for bodies (airships and submerged torpedoes,submarines and then fuselages)

Bouyant bodies create no induced drag by needing no lift but flight requires lift and still faces the very large ( 66%) cost of induced power to stay airborne at least power point and 50% at best L/D -- a huge penalty as compared to a rolling or floating vehicle .  To reduce that cost significantly will require some form of Goldschmied like wake energizing for the wing itself --in fact a propelling wing rather than one that is dragged along by a discrete propulsion system elsewhere .

This too has been known for decades -- every flying creature uses in fact a self propelling wing --we call it flapping .

The nature of flapping flight is that the wing can generate thrust at a maximum at the wing tips --where the local drag is the highest due to tip vortex and along the span the plunging velocity drops so that there is a net drag on the inboard part of the wing -- a combined plunge anf flap could match the local drag and wing thrust to give the ideal of a constant wake velocity along the span much like the ideal downwash distribution .  Migrating birds exploit tip vortex energy recovery by flying in formation and 'unwinding' each other's tip flow ( they in effect 'surf' the upwash that  occurrs outside the span --the start of the tip vortex roll up )

I suspect you are hoping to realize a similar sort of gain with the Synergy wing -- it is somewhat related to the spiroid winglet, the "C" wing or the tip tail configuration as well as the Whitcomb winglet in geometric terms and effect  -- I would question how much value this will be in cruise UNLESS you can operate at a very high Cl which implies some sort of very high lift system or ideally variable geometry -- since induced drag is so low at higher speeds (without climbing into the stratosphere like commercial jets ) the potential gains are only small --gliders can get a net benefit by 'cheating' the span rule for 15Metre class and by spending half their time at very high Cls when  thermalling --for the usual lightplane the performance benefits are minimal if any .    (handling might improve and climb rate )

I don't agree that the Synergy is 'revolutionary' or breaks new ground conceptually but it is a fresh look and welcome in terms of experimentation --it is NOT novel in configuration though ( several papers on the joined wing sensorcraft depict a 'gull rear winged' joined wing with the same curious gap in the rear wing with the rear wing returning to the body via a pair of fins --jet engines are nestled in the 'armpit' region -- Several tip fin joined plus twin finned but long span configurations also exist -- Wolkovitch's  design with twin turboprops at the fin(s) and rear wing intersection is an example and I think Rutan might have sketched something similar .

The big gap in the rear wing, or tail, cannot fail to increase induced drag regardless of the direction of the lift vector --the flow is so far from the wing tip that the flow will be essentialy "2D" and the danger is that very high local changes in Cl will threaten separation of flow from yaw inputs --leading to flow breakdown from the fins spreading to the wing and hence giving rolling and pitching moments .  I don't like the look of it in that respect --at least a wing fence might be needed .

The Ligeti Stratos crashed (twice) due to interactions from the tip fins and the pitch surfaces --once in a flat inverted spin and the next in a crosswind take off  -- destabilizing (or rather being super stable but uncontrollable) when inverted is a consequence of the highly loaded trim surfaces on canards and close coupled joined wings,tandems, staggered biplanes (flying flea included) and the geometric shift of CG and lift centre --make sure you have a tailchute ( in your case from one or both booms I guess ) and do some captive 'free flight' model tests at unusual attitudes on a car mounted rig or the like (that will get around your distaste for wind tunnels as well !)

 I'll wind this up before the link expires Again..)  Might post again after some more thought.

Good luck with further test flying.

Ross Nolan.

Just located the previously mentioned tech paper with the similar configuration but this was not the original source --the ref is AIAA -2003-0605  page 5  title of paper is "Unconventional high aspect ratio joined wing aircraft with aft  and forward swept wings"  -- clearly the span is higher here and the rear wing is directly connected to the body  via the jet nacelles but otherwise is the same . (the prop version has discreet rear fins )

The Ligeti Stratos was 'inspired' by the Lippisch design done for Heinkel of the same configuration but having a nose engine and rear observer --I took an old Aerokurier article on this design to an ultralight aircraft group meeting at which Charles was attending and the previous meeting had featured the infamous incident at Oshkosh with the Easy Riser tailess biplane tumbling out of control --the Heinkel/Lippisch config would have cured that tendency by separating the two wings (unsweeping the top biplane wing ) Thence arose the Stratos.

The Easy Riser itself was essentially a copy of the 1907 Dunne tailess biplane so it is another case of history repeating itself (even further back the stable Zanonia seed inspired the Weiss tailless design which was the background for the structurally better Dunne design ) 

To recover the losses involved in the drag of a spinning propeller contrarotating co axial propellers are superior to a single prop --essentially exploiting the same momentum recovery thing as the winglets --to do really significant induced power recovery counter rotating tip propellers are much better again --the Zimmerman XF5U was the ultimate example of that and nowadays the V22 Osprey but the amount of power is a huge overkill compared to what is needed.

("Synergetic" interactions involving induced drag should exploit the rotational velocity of the tip vortex directly to maximize the power reduction --I suppose more power should go to the tip props at low speed and shift to the body wake prop at high speed as the relative drag varies .

I would test the result of 'bridging' the rear wing/tail just to see how it really behaves even if your analysis does not predict any benefit --the present structure is somewhat flutter prone by being almost disconnected between the two wings (somewhat like a two piece all flying tail which type has also suffered flutter ) and the rear surface is mass well aft of the wing elastic centre .

Cheers,  Ross Nolan.

PS  Novel configurations are technically interesting in themselves but a few knots more or less is often of little importance -to be really "revolutionary" we need to get the other 999 out of  a thousand vehicle buyers to buy,and use, the FLYING vehicle( as compared to AIRcraft only ) -- in my opinion this requires 'roadable aircraft' that MUST be of novel design as well in order to satisfy the demands involved . Over a thousand times as many cars are sold for each aircraft.

One of John's competitors in the CAFE competition is the twin fuselage "G4" pipistrel Taurus -- itself not a practical design and would have been previously invalidated as a "monstrosity" (their term) just designed to win the prize but offering nothing in the spirit of the contest for later practical use .   The recently disclosed Brown CarPlane is a roadable aircraft (flying car) also using the twin body concept and forerunner of a host of truly innovative roadable aircraft spanning all the fields of aviation and configurations .   The original aim of the CAFE /NASA contest was to stimulate flying car design (see Dennis Bushnell "Converticar" paper )

Wow, Ross, you're not the only highly qualified person to completely miss the point, but you might want to get up to speed before putting too much more egg on your face. :-)

Please take no insult here... I won't name names, but there are many among the upper echelon who would have backed you up on this until very recently. In fact, my research reveals that virtually every investigator over seven decades reached the same logical conclusions shown by your perspective. For example, the question posed in our CAFE video trips everyone with a Master's degree or higher: 80% drag reduction? One scientist sat on a panel where John Roncz stated "laminar flow can easily show one fifth the drag of turbulent flow", only to find the same fact objectionable a few hours later. (You have to consider our drag at the speed we fly, versus the drag at the same speed in the baseline aircraft, which can't even get there. It's really not a fair fight.)

Yes, there is nothing new under the sun, and many designs bear various similarities to this one. The two points you missed are practically the entire story, however. You can argue them for all eternity if you want, but it will not change the facts. (I'm sure if you had the same evidence in front of you that I do, you would have experienced what I did when this monumental conclusion whacked me over the head: permanent brain damage. I freely admit it: crazy I am!)

But it's nice to be right in this instance.

Point number one is that Synergy is not a Box Wing, It is a Double Box Tail. Nothing, repeat, nothing remotely similar about them from a fluids perspective. Box wings are shown to be mediocre at best, dangerous at worst, and even some very well executed embodiments continually fail to overcome their inherent liabilities, which I have written about.

They do make a certain sense: if we have "wings", then make them "lift"! What kind of idiot would deliberately push DOWN, hard, on their airplane in flight?


Point number two is that fluids are frictionless, endlessly whirling and energetic like the universe, aside from their viscosity alone. It took more than a hundred years between when d' Alembert proved it and Prandtl explained the cost for inefficient aircraft, but once he had, we said, "OK, forget it. We need something easier to deal with" and so we locked the frictionless circular reaction to take a measurement of its energy in a given direction, negating its natural reversal moments later. This is the root of the concept of dynamic pressure, and it is directly analagous and equal in impact to 'action against the canal bank', a profound illustration (not mine) that you seem to disregard. It's also an illegal mathematical operation to the true physicist.

By the way, none of this is my newcomer's argument. It pervades the discussion in the literature for more than a generation, if we take the blinders off.

Our math, using dynamic pressure, is 100% valid for a closed thermodynamic system. It is an experimentally proven fact that it is 100% invalid for an open thermodynamic system. Elegant use of power to reduce the consequence of viscosity restores almost ALL of the "paradox" that has held us back in our 'proven inept failure' to conquer efficient flight in the lower speed ranges.

There is a lot of discussion here on the details of this already, and elsewhere, and I can't take time to go back over it now. It's also clear that you don't read me well, so the only alternative source I can offer for similar information is scattered through the technical literature; often found in papers you don't have, as they're unpublished (not mine).

Once you realize that Synergy acts against wake vortex in an entirely different manner than a box wing, and while doing so reverses the penalty of a biplane to create the bonus of a venturi, you'll want to cross examine our efforts to mitigate interference drag . I think, thanks to subsonic area ruling and flow control, we've done a stellar job, but even if we did not, the results of this work are abundantly clear in principle if not yet in proven full scale practice.

Biplane interference between airfoils has nothing to do with their induced drag behaviors; it's a different issue physically which only happens to impose a large scale effect that is related. The images I have created are for teaching, and teach they do when you let them. "Always negative" 'Biplane Interference' terms in equations impose the incorrect evaluation of some otherwise obvious stuff, just as shown, but it's not so obvious in a spreadsheet, especially if you have never seen what constructive interference looks like when it happens in the real world.

Ross, Synergy is disruptive and it takes time to wrap your brain around it. That is why I started talking about this stuff eighteen months ago. Man, what would you have said then? By the way, your compatriot Duncan in Brisbane ran me up the flagpole once, too. He's a changed man. 

I think you've missed more than a few things about this design as it relates to practical matters and its demonstration of non-incremental improvements, but I'm by no means done applying the principles it harnesses. Solutions you think we need are on the drawing board already.

see it fly