Tandem wing for high efficiency? Case Proteus

I was asking earlier about the flying wing. I would like to understand throughly the case of tandem wing as well as I want to rule out not good configurations that do not meet the criteria for my target:
- In conventional aircraft you do not have the extra wetted area from the extra wing area that you need for the tandem wing aircraft because of the low Clmax of the rear wing.
- However, in conventional aircraft you might not want the main wing airfoil to have very high pitching moment as for one of the reasons high pitching moment would cause trim drag or would require a very long tail arm. On the other hand, there are examples of conventional aircraft flying with high pitching moment airfoils, one of the most common is Diamond DA40 with FX63-137. I have looked the bottom side though, and I am quite sure it is shortened FX63-137, I think it is cut earlier than the FX63-137 trailing edge would end as the Diamonds wing is not that extremely cambered, it is just heavily cambered.
- With a high pitching moment airfoil it is possible to get very high L/D. In tandem wing aircraft the forward wing is highly loaded, operates at high Cl where such airfoil gives its best L/D and pitching moment does not have moment arm to turn the nose down on the aircraft. So this kind of "super airfoils" could be utilized on the forward wing. It possibly could operate nearer to section optimum than a conventional configuration - for the forward wing case.

So I think the conclusion is that (what Burt Rutan has also said himself) that traditional canard (like Long-Ez or Cozy) should not have that configuration if the goal was ultimate efficiency and that he invented the configuration just for stall prevention.

However: Global flyer is conventional layout, but Proteus is a tandem wing. Why the Proteus would not be conventional if the conventional plane could have possibly gained a little more altitude if it was so much worse than the conventional plane? Has anybody done analysis on how a very optimized tandem wing which takes all the advantages available from aerodynamics of the forward wing compares to a conventional configuration?

I would like to know all the aspects before ruling out a configuration. Daniel Raymer warns about being too much in law with a specific configuration. However, I don't think the configurations can be ruled out with generalizations from literature "yes, it is [usually] worse" or "that's [usually] better" as it could turn out that in some corner case some other configuration than the generalization might be a better compromise.

I know one explanation for Proteus configuration, exchangeable middle part. However, how often does Burt Rutan do a thing for one reason only? I think in most cases if he designs something, it has neat multiple purposes built into one. What would be your take on this? Is the Proteus tandem just for practical reasons or is there something on that or is that one of the "lessons learned" what have made Burt himself to believe that canard type configuration should not be considered if high performance was the goal.

How high the L/D and Clmax of the forward wing should be to compensate for the low Clmax of the rear wing when compared to a highly optimized conventional layout (e.g. Globalflyer)? The forward wing on the Proteus is of substantial size and span. Common sense says, it would be getting more benefits than a canard aircraft from the aerodynamics of the forward wing, and on the other hand, larger portion of the main wing would suffer from the downwash of the forward wing. The altitude where Proteus can fly is not easy to achieve, and that speaks for that even if the common rule is the canard is worse, Proteus can not be so bad at all from aerodynamics standpoint. Also I think the Proteus was designed years after Burt Rutan had concluded that canard is not best configuration for high performance. There must be something else than practical, if it is not aerodynamics, it must be then structural. He usually seems to design synergetic benefits into his planes.

There is one thing above aerodynamics on tandem wing - a tandem wing with high aspect ratio, looks very beautiful. I think Proteus is the most beautiful aircraft Burt Rutan / Scaled Composites has designed and built.

And yes, I know the drawback of Proteus; fuel management because fuel is stored off-CG and in prototype the fuel needs to be manually managed by the pilot. I have heard it was going to be automatic in production version if that would have happened. And of course in production there is a downside for needing molds for two set of wings rather than one. But we can ignore that in this analysis.

Of course there is this one problem:
- induced drag depends on span
- with two surfaces, you are going to have less span for same aspect ratio than with one
- therefore for the same wing area, tandem might be therefore worse unless something else substitutes for the difference

 

With a large rear wing it seems that you would be replacing trim drag with form drag. A low aspect ratio rear wing such as a delta, would have a much higher stall angle because of the vortices coming off of the LE and could be smaller, but I'm not sure about the linearity of the effect and this could cause problems with longitudinal stability even though the tail plane would still be creating lift after the main plane has stalled. This is all supposing that you could move the CG back to counteract the pitching moment of the wing rather than increasing the downforce on the tail:

From Wikipedia:

Some of the above (high induced drag, loss of total lift...), would not (necessarily) be valid for a very lightly loaded tail plane. I wonder if something like a Payen with a much larger front wing could be made to take advantage of the properties of a high pitching moment main wing (unfortunately, it is ugly):




One thing that I did read about Proteus is that it can be lengthened or shortened depending on the payload requirement.

 

I read once where Burt stated the configuration was due to two things: exchangeable middle payload area that would cause varying lengths, and wingspan was limited by the size of the current hanger....

Even the big boys are sometimes constrained by other practicalities...


~Bill

 

In fact the tandem wing can have as little as 0.6 of the induced drag of a monoplane of the same span and area ("equivalent') --that is well known and I would have thought Raymer would know this from Prandtl --the Proteus is very similar to a couple of designs done for Vought by Julian Wolkovitch for carrier ASW use . (and a Goodyear advert from the 1940s showing such an 'aircraft of the future' in Aviation mag --I gave SCALED a copy of that ad and a 1947 British Aeromodeller magazine having a model of the exact Pond Racer configuration (and also an Airspeed project that was not built but published in the company history book )--on a visit to Mojave in 1990.

What exactly are your 'criteria' ? --from the sizing and other determinations a feasible or "optimum" configuration can be developed if the 'imagination' is there as well as the understanding of interactions and constraints,
(for example, apart from your extensive 'analysis of the Proteus there is also the case that flying at great altitude the dynamic damping of an aircraft gets less and a tandem wing benefits from the extra damping given by the plunging of both wings around the (mid fuselage) centre of gravity -- splitting a wing into two also lowers the ReNo as does high altitude so that laminar flow might be sustained ( need about 1 million )

 

A quick "PS" --you can look at the figures for the Quickie and Dragonfly tandem wingers to see that range and efficiency are not less than for a monoplane (the Q2 held the 'most efficient two seat aircraft' at one time and only beaten by Klaus Savier's ultra cleaned up Vari Eze I seem to recall --the tractor Q2 and Pusher VariEze coming out nearly equal but side by side in the Q2 ....

 

Two "fundamental" advantages I can think of, except from the above. Wider cg range, which is important for slender wings (low chord, high AR, for cruising or holding efficiency) and possibly lower production cost because you have only 1 set of molds for the wings out of which you can make 2 wings, instead of separate molds for the tail. Afaik, that's not the case for Proteus.

There's a third, supportive argument, but that's less obvious. We all know that if we have a wing with a certain area, taper and so on, it has a certain weight. If we then raise the aspect ratio, we all know the wing gets heavier don't we? Spar loads increase, while the skin area remains the same.

This however is NOT true for solid foam wings. There, a higher aspect ratio will frequently yield a lower total wing weight. While the spar loads go up, the weight of the foam goes down and more than offsets this. Just think of it, if we increase the aspect ratio by a factor of 4, the span has doubled and the chord halved. Foam volume is chord*(chord*profile thickness)*span. If we keep the aspect ratio (span/chord) constant, we see that foam volume/weight is proportional to the chord or to the square root of the aspect ratio. Here too, two separate wings will have a much lower foam weight, albeit their spar weight is considerably higher.

From an aerodynamic point of view, tandems are usually sub-optimal (especially since flaps are hard to implement)

 

But that's apples to oranges. The biplane spars will be much higher loaded, since it's profiles are only half as thick, the loads in the spar have just gotten up by a factor of 4...

If you want to do apples to apples you have to compare with equal aspect ratio and same total area. This will give you results that are worse then a monoplane.

 

I am quite confused about this statement.
I have been led to understand that the lower the Reynolds number gets, the less favorable the laminar flow is (and more avoidable it is). When it gets really low, then there will be problems with separation bubble etc. Also when Re goes low the profile drag increases and lower gets the obtainable section L/D. Consider this: the NLF414F airfoil in the Vmax probe was designed for Re 500000-10000000 high speed flight. I am sure it would have worked at that Re, however, at the Re the plane was supposed to land, the boundary layer might have been already unstable.

Here is example simulation for a "laminar flow" airfoil 5 million Re vs. 1 million Re in section L/D. The 5 million gives excellent L/D, however, the result can be actually worse at 1 million than the result would be with a turbulent flow airfoil.

Can you elaborate a bit what you meant with needing Re 1 million.

If we consider a conventional and tandem in imaginary case:
- span would be limited to some specific limit, such as 15 meters.
- with conventional, the Re would be 2 million
- with tandem, the Re would be 1 million and the AR of each wing would be 2 times higher
=> how the 1 million would be better than the 2 million if the 2 million would give better profile L/D and the wing L/D is all about the wing span, which in this case would be 15 meters in both cases. I was originally misguided by some books claiming that L/D has to do with aspect ratio, but it is not true, it is all about the wing span (how many meters of span rather than how long chord).
-> would the 2 x 15 m wings equate to better plane L/D than the 1 wing with wingspan of 15 meters?

In the attachment; the green line is Re 5 million. The blue line is Re 1 million. The polar represents section L/D vs angle of attack.

 

Autoreply, can you enlighten why wider cg range is important for slender wings?

 

karoliina,

Yes, a higher Re can lead to lower drag, but that's a fairly complex topic. Gliders typically have up to 5% better performance at the higher wingloadings (30 vs 55 kg/m2), but those are pretty specialized applications and I guess that the gain is much smaller on a typical powered aircraft.

In general, the cg range is proportional to the chord. That's not a rule, but more a common result, if you raise the aspect ratio, your required horizontal tail (volume coefficient) goes down, so you make a smaller stab, resulting in a smaller allowable range for the c of g. Look at the gliders for some extreme results.

 

The induced drag of a tandem wing airplane with 15m span is almost identical to a monoplane with 15m span. Extreme vertical separation can reduce the induced drag of the tandem. A stable tandem wing airplane will have the canard operating at about 3 times the Cl of the main wing. This will call for very different airfoils. The canard limits the maximum lift, fortunately we can use a high lift airfoil with a full span slotted flap effectively for both maximum lift and pitch control. Ailerons are needed on only the aft wing. The lower reynolds number is troublesome to achieve high section L/D. The tandem wing airplane will have higher spar weight as the height of the spar may be half that of a conventional configuration. This can 4 times the spar weight. The wings are not as convenient for fuel as neither is near the center of gravity. Fuel management is required.

 

Virtually every airframe development program I've been involved with goes through a configurational study where various trade-offs are presented and analyzed for a number of variables, each of which is prescribed for the mission at hand. In each case, these studies examined pretty much all the primary configurational variants, as well as several offshoots of each, in order to find a near optimum compendium of design features to meet the specific operational goals as specified by the customer and/or program. It's an interesting side note that in none of the cases that I've seen was a canard or tandem wing configuration selected.

But to the question at hand, maybe I can answer it this way: As many have stipulated, there are some advantages to the tandem wing layout however in virtually every case those require a near point design where any other consideration becomes inconsequential. So far the work I've seen (by my teams and by other organizations) suggests that there are really only two mission variants where the optimal features mentioned above can be advantageous to the goals. The Proteus is a great example of one - this is essentially a point design meant to do one thing well and one thing only: Fly straight and level with near optimal efficiency. No, it's not the only option but the tandem wing layout fulfills this relatively well. The level flight requirement is the over-riding goal but to do this the airframe will by necessity have a limited CG range, will have limited maneuverability, and will not be able to operate out of more than a few selected airports where the runways are relatively long and the approaches are clear, allowing for a very flat and level glide path. And a second benefit of the layout is a clear fuselage between the wings that is unencumbered with wing systems and associated structural assemblies - again a great feature of the Proteus layout.

The second example where the tandem or canard shows optimal characteristics is in a typical fighter mission, especially where high agility is part of the flight requirement. The latter is interesting in that many assume that with modern standoff weapons the aerial dogfight scenario would be outdated. But the typical mission analysis still continues to show that maneuverability and efficient energy management are keys to superiority and survival. For this the canard shows a slight edge over more conventional layouts and we do see that not only in the choice of airplanes in the Swedish Air Force but recently also in developments being done in France, Russia and China.

But outside of these two examples I really can't see an optimal tandem configuration - yes, airplanes like the Ezes are interesting, and for their day, very unique but in reality, it is particularly that uniqueness that has gained them their notoriety, not any particular advantage or superiority over more conventional arrangements. In most cases just about the opposite is the norm.

 

I worked on Proteus (after it was flying, not the design phase) and this was definitely a point design aircraft with a very specific payload. It was designed around carrying a belly mounted telcom antenna as a low cost satellite replacement to provide broadband service over urban areas. It needed high altitude, lowish speed, long loiter capability, combined with the need to not blanket the line of sight of the antenna hanging below. The tandem arrangement was not to the best of my knowledge aero/efficiency driven. The strange dihedral/anhedral angles in the main wing were to clear LOS for the antenna. In fact, the outboard wings are glass, not carbon, to keep from interfering with the signal. The aircraft only became a high altitude testbed when the telcom company tanked and the project evaporated and Scaled needed to find other uses for it. It's a remarkable plane and it says a lot about the original design team that it now has over 3500 hours on an airframe that was intended for a 100 hour lifespan!

Patrick

 

Just FYI, the term point design does not refer to payload - it refers to mission profile. And per your description of the program and the aircraft's goals (thank you for that insight BTW), it meets that criteria.

 

I missed some of the discussion but the question was about aerodynamic advantages for tandem wings (or actually multi wings -- in any event "two thin versus one thick" ) Keep in mind that the only USEFUL part of wing structure is the shear web --it transmits the lift loads to the payload -all else is neccesary evil in terms of extra weight (sparcaps are there to push the wing back down and torsion loads require a shell or separated spars ) --the best form of tandem wing is the joined wing because you no longer suffer the penalties that arise from splitting the wing . Just take a lumber beam and see how strong it is :--then cut it into piddling little sticks and build yourself a Warren truss of the same weight and then see how much load it will carry --you can make axial loads (spar cap forces -by far the largest ) actually pay for themselves and carry most of the payload weight and trick the wing into thinking it is six to eight times as stiff and strong ...anyway another subject.

the thinner wing will be able to maintain laminar flow, with the same surface quality, over a greater proportion of it's area and the sort of optimum ReNo for natural laminar flows seems to be around one million --I can't quote the reference offhand but you can see a non linearity in the laminar versus turbulent flat plate drag curves -- flow will break down after a definite physical distance which is influenced by the re no. but somewhat independent as well -- 'starting again' with a new leading edge gets more laminar skin as a crude analogy (with real life bug guts etc involved it is even more important )

The traditional means of specifying centre of gravity in terms of wing MAC is actually arbitrary but convenient (it is of course very often well outside the wing itself in many cases of unconventional layouts ) - Hurel Dubois noted this with his ultra high aspect ratio designs of the 1950s --the HD10 for example had a wing chord of about 13inches from memory --and an aspect ratio of something like 25 (forgotten the exact figures) and it is the tail volume that is important and with a very small chord wing this is easily made very large as is the trimmable and stable CG range . The remark about damping in pitch on the proteous or similar configs might not be that significant in terms of a design driver otherwise Rutan might have put a trim surface on top of the remote fins (as some Quickies and the tandem Amsoil racer did -- or his later Global Flyer etc --BTW the 'unconnected tails' thread crosses over with tandem wings on some of Rutans other designs (eg ATTT ) or SCALED experimentals and the wing tip control surface and 'tail boom' at wing tip design was also tried by them for the Proteus role I think --see "stargazer" website for a few variations and their "tandem' tip tails ' design.

 

Actual results (sailplanes) puts that at around 2 million, with a considerable gain (a few percent), starting from a million.

 

I seriously doubt that even 2 million would be any good Re when a power plane is in question. As a reference: Globalflyer; Re most likely much greater than 1-2 million, and 75% laminar flow on both surfaces, resulting 177 in section L/D according to John Roncz presentation. Same section with 1-2 million would barely reach section L/D 100 - rough estimate based previous experience on simulated results.

Actually I would claim [based on simulation results, and claims of Bruce Carmichael] that optimum Re for laminar flow sections starts from 4 million and extends a bit over 10 million. The target also is to get the high L/D for a low Cl, rather than high Cl because the power planes do not cruise most efficiently at ldmax, but a bit higher than that, I think some rule of thump was 1.35 x ldmax speed. So to get high efficiency on that regime, would mean to design airfoil that exhibits high L/D at low alfa and low Cl. For example Lancair IV or Stallion cruises even below 0.2, however, getting high L/D below 0.2 Cl is not very easy. The area of Cl in cruise for highly efficient plane thus would be around 0.2-0.8. There the 2 million Re two skinny wings would be less favorable than one wing that would reach at least 5 million in cruise (in case the span is limited by practicality, 15 m wing could be manouverable at airfields, but much higher than that, parking and taxiing would become a problem without folding the wings first). Of course at very high altitude the difference in TAS can somewhat substitute for the additional penalty of the low Re, which two skinny wings would produce.

Anyway; I think my conclusion is that the tandem is ruled out of the top configuration alternatives.
And I think another conclusion is that on conventional high aspect ratio configuration, the tail volume coefficient is not limited by stability (which would result smallest drag) but practicality in CG travel allowance - in other words, high aspect ratio plane will require larger tail or longer tail boom than the minimum necessary. Drag count sacrificed for practicality.

 

Simulations say nothing, unless you can verify them with real-world tests, preferably not only in a windtunnel, but also in the atmosphere with it's microturbulence.
The foils of modern gliders routinely do far better than 1:200 L/D and 95 and 75% laminar flow.

To the best of my knowledge, there are no airfoils that come even close to the performance, shown in gliders. But that's the problem here. You can't simplify a pretty complex topic like laminar flow in some optimal numbers and to the best of my knowledge, there are no simple optima, only regions (Reynolds ranges) where we know the performance improves, like in ballasted gliders, but as far as I know, that's certainly no "fixed rule", or fundamental behavior. Of course we have rules of thumb for flat plates and we can adapt them to arbitrary shapes/airfoils (pressure distribution), but this by no means gives clear or even accurate results.

 

Can you show me a glider airfoil which is better than Diana 2 airfoil? Because Diana 2 airfoil KL-002-128-F/17, according to the published paper, was about the state of the art on gliders, with about 40 drag counts at Re 1.1 million with low pitching moment as a bonus. Am I now somehow mistaken with my interpretation of the state of the art in gliders?
http://www.dianasailplanes.com/Tech_Soar_KK.pdf
To my understanding the older generation of gliders such as Nimbus 3, ASW-28 etc. use Wortmann airfoils which were developed in the 60s, 70s and 80s before computerized airfoil optimization, which are not really state of the art anymore, but provide quite nice results still today. For example the Wortmann FX63-137mod that is used on Diamond DA40 has according to simulations over 200 section L/D and tunnel data from Bruce Carmichael's publication also concludes the same thing, but it has very high pitching moment which is not very favorable for trim drag and for other reasons on a conventional layout (but would be ok on a canard aircraft's forward wing because of the moment pivot point but is not so excellent on the mentioned aircraft if achieving ultimate performance was the goal).

Would you mind sending me a airfoil dat file(s) for a modern glider airfoil which would have 95% laminar flow on one surface and 75% on another for investigation - which are better than the Diana 2 airfoil? My email address is karoliina dot t dot salminen at gmail dot com. Also if you have airfoil dat file at hand which will do this: have NLF414F drag count at Cl 0.3 at 1 million whereas the NLF414F obtains the same at 10 million. If your statement is accurate, this should be possible. I will be proven mistaken if you can prove me mistaken (and your new theory replaces my previous one), and such super airfoils do exist in actual gliders, but before that I will have hard time to believe the statement's complete validity. And also generally, the numbers I am seeing on simulations in L/D in relation to the reynolds number (that observation is not a software fault, but represents reality as well), are quite well matching the wind tunnel data for given airfoil (such as the KL-airfoil that the Diana 2 uses).

 

Beware of the Wortmann's, several of the sixties series, especially the thinner ones are extremely sensitive to contamination. As for the best current airfoils, the ASH30/31 and Arcus have the last generation "on board" The Diana II is a very impressive sailplane, for multiple issues. It's aerodynamics were comparable to "the Germans" when it came out 10 years ago, but structurally it's much better, which made it superior to existing designs. Since then we've seen a new generation that's aerodynamically superior to the Diana, while she is still impressive from the structural point of view. (70 kg lower empty weight, higher AR)

No, I won't. First of all, I don't have them, they're commercial and given that airfoils are the single best USP of sailplanes, you will never be able to lay your hand on accurate digital models, without signing a NDA or using very old airfoils. Boeing will also never give you the airfoils of their aircraft, or a cad-file of parts of their design for the same reason.

Then I'm curious which software you use, since the most experienced people working in this field, using the most advanced software (proprietary) are frequently off by quite a margin if they're working on designing new extensive laminar airfoils.

Aerodynamics might not be an art, but for engineering it comes pretty close to it.