Video: Synergy Aircraft Presentation, Airventure 2011

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autoreply

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Every time it comes back to "constructive biplane interference" we see a variation of the same picture, with negative net lift. Anybody who has seen other pictures or explanation of the "constructive biplane interference"?
 

Synergy

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oh and sorry about the neg net lift image. I have others somewhere but for now please ignore the specific condition, which was just to make the point.
 

Aircar

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I actually posted a link to that Max Munk paper on the Oshkosh 365 thread "beyond streamlining" etc started by John (and now archived in the EAA forums )-- likewise the Greene paper on minimizing induced drag " viscous induced drag ..entropy etc " -- as expected the Greene paper merely arrives at the same conclusion as several earlier papers where the structural interactions are taken into account and span is not a constraint --there is no 'new' concept of induced drag involved .

Clarence Cone published a paper on the minimum induced drag wing in the 1960s (re published in Soaring magazine and available on the backissues link ) --simply speaking it pays to extend the span of a wing of given area or fixed weight and deviate from the 'ideal' elliptical planform shape by swelling the root end chord and reducing the tip chord . the net effect is to deepen the root spar and so save spar mass or allow the span to be increased in order to increase the streamtube mass flow and hence reduce the deflection of the airmass and lower induced drag.

I posted pieces on the SoarIdaho website (dealing with Schreder sailplane designs) some years ago on this well known concept --I described the wing planform as of "Eiffel tower' shape as an easily visualized analogy -- the wing outline can be concave rather than convex as for an ellipse but the straighttapered or compound tapered wing comes close enough .
Most jetliners have such a planform --more apparent if you unsweep them --and gain a great deal of strength and weight saving ,fuel volume and landing gear stowage etc as well . I disagreed with Autoreply that spar mass is almost neglectable in calculating wing weight and that skin weight is dominant (on another thread on HBA) --and this planform concept is the physical embodiment of the primacy of bending material mass . (there are a couple of other reasons for adopting the "Eiffel tower' shape in swept wings --eg the Horten flying wings -- the 'lift deficit' in the centre section being one --an effect of the isobars sweep and spanwise flow , also possible thinning and decamber of the root intersection ) .

The Munk theorem is way too mathematical for my being able to follow the development of the equations nowadays (and likely way back when,,) --however the basic fallacy about increasing mass flow by matching pressure fields --the 'venturi effect' -- is that induced drag is created by any deflection of an airstream --whether up or down and the effect on the leading wing is to cause it to carry not only the weight of the aircraft plus the trim down load ( the Cl squared term applies) and then the rear surface again deflects the airstream with another induced drag term -- only in the case of extreme deflection of the main wing flow field could there be any forward vector on the downforce of the rear wing --probably only near stall in practice where the drag overall has become non linear and uneconomic to fly anyway (minimum power speed is below max L/D but only of interest for maximizing duration )

It is still the case that you cannot beat simple span to minimize power to fly in the Coefficient sense --but there is still the structural weight penalty for huge cantilever spans which is why the GFC entries turned to distributing masses along the wing a la Voyager etc (and would have just as atrocious handling ) --the Boeing SUGAR study came to the concludion that an externally braced wing of higher span would still be better than a cantilever --or a biplane at the other extreme --what the Synergy equates to though is a cantilever biplane such that it suffers the drag cost of short physical span but does not gain the structural benefit of connected biplanes or strutted wings or the "eiffel tower" shape --no doubt using Grafil ultra high strength carbon minimizes the actual penalty but it is hard to credit any comparative saving in induced drag over the alternatives --some figures would be nice .
 

Synergy

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Ross,

I don't suppose you're hopping over to Flair in NZ next week, but if you did it would be awesome to meet you. I have to be brief as I'm not quite ready to leave yet.

Sometimes the hardest people to reach are the ones most knowledgeable. There is no doubt we are on the same page about things when you get to fully explain yourself. I still entertain hope that in face-to-face communication you'll find I'm saying nothing but things you know are true.

Poorly, perhaps.

Greene makes two key points that have extensive experimental corroboration. One is that induced drag is Reynolds number dependent, and the other is that it is sensitive to streamwise and vertical positioning functions. These are important and quite novel assertions, and I find them essential to a physical understanding of matters.

Your spanload ideas are good and I agree with your position.

As to Biplane theory, Munk doesn't really blow the horn he finds most interesting, the case of the staggered biplane of counter-circulation (positive wing lift, negative upper wing lift, therefore positive decalage), but there is no question either here or in subsequent investigations that 'matching pressure fields' as you put it is key to efficient flow amplification (in the manner of a venturi or any number of multi-element designs.) This use of sequential pressure maps is one of the bedrock principles of efficient fluid dynamics. (Although I'd note that THAT statement has been an oxymoron... in many industries... for a long time.)

Where we probably are at odds is that I do not consider multi-element 2-D drag coefficients or any of this 2-D drag discussion to be a proper part of the induced drag bookkeeping, which is most certainly a large scale 3-D issue. I would never describe biplane interference as having anything to do with induced drag, for example.

When I speak of induced drag, it is of the drag that is a consequence of finite wingspan. Regardless of monoplane or biplane configuration. If span were infinite, the drag-affecting behaviors described by Munk would still hold, both for normal, destructive biplane interference, and also for favorable, constructive interaction. Yet in this case 'induced drag' would be zero.

Now that I've said it, we see the issue as it really is: Induced drag is a term used in bookkeeping the 'components' of our drag in flight. Where to draw the lines has been and continues to be the subject of much conjecture and debate, most of it based on the chosen mathematics in use.

I won't be able to respond, unfortunately, so now is as good a time as any to tie me to a stake!
 

Synergy

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As to no benefit from Synergy's use of the double box tail, that is false. For its span it offers higher efficiency with other benefits. The physical mechanism for its low wake disturbance is well known and can be studied in the literature. Ilan Kroo is a good place to start.
 

Aircar

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Prepare to be flogged at the stake ... no I wouldn't do that . The phenomenon of induced drag still HAS to occurr even for an infinite span when lift is created --the rearward tilt of the lift vector (roughly splitting the angle between the upwash in front of the wing and the downwash behind has to be tilted rearwards relative to the direction of gravity (the weight vector ) --this is a result of the creation of an upwards force by Newtonian physics and the creation of a downwash velocity means that --without any profile drag-- the freestream has lost a small amount of it's original momentum in the flight direction --the rearwards tilt is equal to sin 'theta' times the flow velocity at infinity -- in the case of tandem or vertically isolated paired wings the flow turning at each wing should be something like half the amount if only done by one wing --the sine of half the turning angle times half the mass ( if each wing lifts equally for a first cut ) at each wing should halve the induced drag as compared to one wing of the same span - (in practice Cdi comparisons do follow a sinusoidal pattern from one to a minimum of .6 for the least (equal span area and loading and separation --in effect two streamtubes) --but connecting those two wings tip to tip (doubling the span) will reduce induced drag to 25% of the original --for an 'infinite' span, in the sense of no wing tip vortexes, it will still exist as a result of the lift vector being tilted back ( or you can deduct the same component from the vertical vector of the lift to keep it equal -- in practice you would need to add power to maintain level flight or sink slowly and in either case the small difference in power as compared to the non lifting infinite span wing is induced power and can be related to induced drag -- I guess that somehow the actual wing must 'feel' a streamwise force in order to experience any sort of drag and in that respect it can be said to be 'viscous' --integration of pressures etc on a 'perfect' airfoil without profile drag should cancel out (D' lambert's paradox and approaching the Goldschmiedt ideal ) in which case the induced drag still has to be felt by the wing -- here is where I find it difficult to credit that placing a large underpressure close to and behind the front wing can reduce drag --it should reduce the 'base' pressure and tend to promote separation . The pressure deficit /thrust deficit for a downstream pusher propeller acts in a similar way .(ie tends to suck back the body in front of the prop )

I recall some text or other describing the effect of a passing aircraft as like floating a boat in the bath --the fluid reacts the weight of the thing immersed in it but ultimately the weight must be reacted by mother Earth (more weight on the bath legs or a pressure wave that extends to the surface of the Earth and eventually reacts to it --a very small one as the wave expands to take in more air with ever less velocity but still there -- sonic bangs are a real example . In any case the balance of momentum forces still has to be accounted for and induced drag is unavoidable and proportional to the span squared with only small variations ( and wing interactions can not reduce the theoretical minimum overall for paired wings if the correct streamtube is taken into account --the Nenadovich biplane is the opposite of your thesis for example with the highly lifting rear wing creating enough upwash for the front wing's lift vector to tilt forward of vertical as apparent negative drag but the cost of the total lift is still not less than the ideal tandem airfoils --it is just a shorter overall arrangement and much like the Prandtl tandem lifting system --all lifting surfaces again . It would be convincing to set up a physical lifting model test and measure the results with all possible arrangements of mutually interacting wings --not in a wind tunnel with constrained flow as you state but in free air --that would change my mind but so far I am unconvinced from the argument.

better post before it goes,
 
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Aircar

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John, if you aren't already incommunicado, ''what is "Flair in New Zealand? " I vaguely recall a venture capital firm from NZ on the EAA Oshkosh tapes --maybe related ? You might want to look up the firm there making a swashplate or radial piston type of aircraft engine --something like Duet or Dual from memory (my email archive has been wrecked by an 'expert' installing another program for me and still trying to retrieve it so I can't look up my history ) --the Dyna Cam engine was another of this type (based on the Michell engine developed in Australia in the 1930s... ) it would fit your cowling and configuration .

regards Ross.

PS can't afford to go to NZ but look forward to meeting sometime.
 

Aircar

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Just found FLAIR --and note it is Duke engines (not too far out ) --seems interesting --who says Kiwis can't fly ?

New Zealand had more enlightened aviation regulation than Australia --in 1975 I dropped in on Hamilton aerospace on the way back to Australia and at that time they were looking to produce BD5 kits and certify the aircraft in NZ since there was no cost to an applicant for type approval then --very entrepreneurial people.
 

Synergy

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I was surprised to learn that what you describe (drag due to lift, ordinary Cd) is frequently called induced drag these days. That was not the case in the time of the aerodynamic greats. They used the ordinary lift/drag relationship (Cd) separately from the penalty of finite wingspan (Cdi), and their math treats it that way.

Somewhere along the line we got started teaching the idea that profile drag at zero lift is the first part of the drag, and everything "due to lift" is the second part. This is one way of looking at it, I guess, but a very bad way to teach the subject in my opinion.

So, now when we try to discuss "induced drag" with people, some are thinking about the drag that increases with alpha... a consequence of viscosity and turbulence. This is the type of drag that responds well to laminar flow shaping and boundary layer control.

It has nothing to do with (and no fundamental effect on) the large scale vorticity that happens because of our finite wing spans. Wake vortex is the symptom of the energy classically described and quantified as 'induced drag'.

Of course, my real position is that total drag... its causes and its symptoms... is the only thing that matters. Bookkeeping is for accountants. The concept to master is viscosity, at all scales.

re: venturi flow: I have performed experiments like the ones you describe; that is part of how I got schooled on this weird phenomenon. I highly encourage you to check into it.
 

autoreply

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I posted pieces on the SoarIdaho website (dealing with Schreder sailplane designs) some years ago on this well known concept --I described the wing planform as of "Eiffel tower' shape as an easily visualized analogy -- the wing outline can be concave rather than convex as for an ellipse but the straighttapered or compound tapered wing comes close enough .
Most jetliners have such a planform --more apparent if you unsweep them --and gain a great deal of strength and weight saving ,fuel volume and landing gear stowage etc as well . I disagreed with Autoreply that spar mass is almost neglectable in calculating wing weight and that skin weight is dominant (on another thread on HBA) --and this planform concept is the physical embodiment of the primacy of bending material mass . (there are a couple of other reasons for adopting the "Eiffel tower' shape in swept wings --eg the Horten flying wings -- the 'lift deficit' in the centre section being one --an effect of the isobars sweep and spanwise flow , also possible thinning and decamber of the root intersection ).
That comment was solely aimed at the typical composite wings we see in GA and sailplanes where things like skin stability (can) drive the weight, much more, or even surpassing the influence of the spar weight as seen by monocoque sparless wings like that of the Diana II (the lightest wing in it's class). Of course, in an airliner, the wing is basically nothing more than a heavy beam, with lots of things (flaps, slats, engines) connected and my comments aren't valid there.
 

Aircar

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In the case of a "sparless" wing you are right in that it is difficult to isolate the material that is there for torsion alone as compared to the bending (and torsion induced by bending on swept wings --actually all airliners use a shell structure without discrete spar caps as I am sure you know . Separate spar caps are the exception for the majority of aircraft above the light class or gliders --where rigging imposes a constraint on the multi bolted strap or flanges usual in other monococque type wings --even the Diana gathers the bending material together but in their case they have the tongues on the fuselage and the socket in the wing to reduce the concentration weight penalty -- aircraft like the DC3 set the paradigm of stressed skins with hundreds of bolts avoiding massive root fittings and the relatively low shear loads (small payload)in gliders compared to the huge bending definitely biases the optimum wing planform to the non elliptical shape ( with ReNo constraints favouring the reverse shape of a parallel centre section and tapered tips --Wortmann described this as the lowest induced drag planform per se even though it carries the highest spar weight penalty . My 1971 telescopic wing, prone pilot, tailless sailplane (the 'ultimate' performance thing I could conceive of then and when I was focussed on competition gliding as the 'be all'...) --had parallel OUTEr wings that retracted INSIDE the inner wing which resulted in a close approximation to the 'Eiffel tower" or Greene/Cone/Kuchemann 'ideal' minimum induced drag shape although I had not heard of any of them at that time . The constant chord outer wing created no gap during extension or retraction and had no very low ReNo at the tip end --the inner wing structure was based on the "sparless" HP 10 which has a hollow interior and allowed to internally nest the outer panel . The Schreder HP15 also had no spar cap of any kind -just rolled to contour 7075 skins and a full honeycomb core --33;1 aspect ratio at 15 metres . Both amazing structural innivotions and requiring novel wing root joining --the Finnish vasama had a broad box spar without a discrete cap and the Std Austria again used a non 'solid' spar --several other earlier gliders like the Foka,Zefir etc and right back to the DH Mosquito had "sparless wings" but in every case fattening the wing to match the developed bending moment saves weight and gives rise to the non elliptical wing for minimum induced drag cost if you want to describe it that way.
 
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