Flying wing for high efficiency

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by karoliina.t.salminen, Jun 14, 2011.

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  1. Jun 14, 2011 #1

    karoliina.t.salminen

    karoliina.t.salminen

    karoliina.t.salminen

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    Hello,

    I picked up one comment by autoreply in Synergy thread about flying wing for attempting very high efficiency. So I went and wrote to him a private message about it because I have felt some interest towards this configuration for quite some time, largely for two reasons 1) looks cool and 2) simplicity, potential for fewer part count. I have searched some old threads about this topic also from this forum and elsewhere and have read them. I had sort of dismissed the configuration earlier but have came to second thoughs about the applicability of the flying wing for high efficiency concept that targets for cutting the cost/mile to similar level than airline ticket on the same distance, in other worlds, not flying just for fun, but doing practical travel (and having fun along) with private aircraft where today's GA is largely unsuitable for (I fly Diamond DA40, but even for domestic trip I have to go with airline if I want to pick the less expensive alternative, although the queues at airports are super annoying and same applies for the ridiculously small leg room, I have to wonder how 2 meter tall guys can travel in economy class, the Diamond is luxury in comparison). However, my speculation is about efficiency for practical aircraft that can be used to fly from point A to point B (medium to long distance, with minimal fuel cost and also being able to cover that distance also partly because the trip gets the more expensive the more there are stops in between as there are landing fees and navigation fees and even noise charges for every unnecessary landing between point A and B).

    However I have seen few challenges on the flying wing concept when the aim is high efficiency: instead of dragging a fuselage around, one has to drag around extra wing area simply because the shape of the plane makes the wing area rather large. The lower achievable Clmax for the tailless design does not become a problem for achieving less than 61 kts stall speed that easily because the rapid expansion of the wing area when sizing a flying wing up sufficiently so that e.g. four people can be accommodated inside the wing center section, or almost inside it without canopy or blisters for doing that but instead blending the cockpit in seamlessly (as the aim was high efficiency).

    I noted his comment about blowing and suction, but then raised a concern (in the private reply) that it would be possibly challenging system in real world use where bugs, dirt and ice can block the suction.

    Autoreply gave me permission to include his private message reply on this topic. I was requesting him to create the topic, but ended up creating it myself after all.

    Here is his answer (copy paste with permission):
    I have been reading of course all kinds of papers about active boundary layer control. In Goldschmied papers it has been discussed for fuselages and in some others it has been discussed for glider wings. However, I have been thinking some time that the thick airfoil of a flying wing (especially center section) could be a candidate for boundary layer suction.

    Obviously the plane can't fly with blowing the sucked air alone, but needs some main propulsion. Whether that would be a propeller (pusher behind the trailing edge) or several smaller pusher props on top of the trailing edge or series of ducted fans is an interesting question also. Several fans or several props could be more easy to arrange in case of electric propulsion. However, the power plant for electric power generation and battery pack would be quite heavy with current battery technology and would lead to rather poor power to weight ratio. But large drive shafts, couplings and gear reduction drives are not without weight penalty either. I have been speculating that in case of a flying wing, since the wing area is large and wing loading is low, the capability for additional weight penalty is somewhat greater (example of this is actually some of the old Northrop wings, they had rather poor power to weight ratio) than on traditional configuration with minimum wing area.

    So I have the following questions initially:
    1. The wing area for reasonably sized aircraft becomes very large on flying wing. So the wing loading will be light (much lower than necessary for the stall speed and climb performance). Does the lack of fuselage compensate for this unnecessarily large wing area?
    2. Flaps might not be doable in this configuration. On the other hand, since wing loading is low, they are not that much needed. Does dragging around the extra wing area compensate for the lack of fuselage drag?
    3. Aspect ratio tends to become lower than on fuselage + high aspect ratio wing configuration. It is of course possible to design a high aspect ratio wing, but still, the center section is rather low in aspect ratio. Question then is that which one is worse: lower aspect ratio or dragging around fuselage for no benefit. On the other hand, the flying wing can have equal span loading with lower aspect ratio than high aspect ratio conventional layout. I been thinking this a long time and I do not have a clear answer. What is your opinion on this?
    4. If the main propulsion is with propeller, which pusher propeller configuration is lesser penalty: propeller behind the thick wing or propeller behind a small high aspect ratio wing, taking in account the boundary layer control opportunities from the wake propeller vs. potential for undisturbed flow on small wing with large diameter prop. Any takes on this?
    5. If the suction is in the full span elevon hinge, where would be air then blown out? Consider that there would be one, two or several propellers at the trailing edge, where would be the exhaust or exhausts where the air would be blown out?

    These are just some thoughts and questions on the flying wing and high efficiency. I would be interested to hear your opinions on the flying wing configuration.

    Best Regards,
    Karoliina Salminen
     
  2. Jun 14, 2011 #2

    Topaz

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    In terms of wetted area, it can, but the details will be in the individual configuration you choose. Also realize that for a given total wetted area, more of that area is taken up by low-drag flying surfaces on a flying wing than on a traditional wing-fuselage combination. So even if the wetted area is larger, you still may see a net reduction in parasite drag. Flaps are certainly possible in a flying wing, but they have very tight constraints on where and how they are applied to the design. Even plain flaps can be used, if their deflection results in zero pitching moment change.

    Optimize your aspect ratio to the mission specifications, and then figure out how to put people in it. With a sportplane-class flying wing, you're almost certainly going to end up with some sort of fuselage 'pod' or 'bulge' in the center section unless you go the Blended Wing/Body route, which is harder to analyze than even 'plain' flying wing. As you note, wing thickness for reasonable chords is usually insufficient for comfortable seating. A somewhat enlarged pod of the type on the XB-49 might do the trick, and you need a mounting bulkhead for the motor anyway. With four seats (and baggage, one assumes), the real trick is going to be CG control. Flying wings tend to have very small CG ranges, and you need to be able to accomodate all the variations in useful load without busting those limits. Span-wise distribution of the load can help, but worsens your "thickness" problem and gets you into lateral CG issues and the need for fairly powerful aileron trim. Off the top of my head, a single pilot forward and the passengers lined up abreast behind him seems the most-likely route, but you'll need to do some study to figure out the best compromise.

    I'll have to defer to others with regards to the active boundary layer control stuff, but like I said, you're unlikely to be able to house the occupants in a reasonable wing root section anyway, without some sort of pod or bulge. So I'm not sure this question is applicable, as-stated.

    Just my thoughts.

    Also, keep in mind that you'll want to pay attention to induced drag effects on climb power required. The lift distribution at high CL is going to be far from optimal (because of deflected pitch control surfaces), so the induced drag will be relatively high for a given span loading. The low wing loading will help quite a bit, but all of this needs to be taken into account when you do your sizing and determine the power required.

    A relatively high-power, high-speed cruising sport airplane seems a natural application for a flying wing/tailless airplane to me. It'll be interesting to see this develop for you!
     
    Last edited: Jun 14, 2011
  3. Jun 14, 2011 #3

    Voyeurger

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    Karoliina,
    I just want to say, I've been flying 500 miles (each way) via commercial airlines twice a week, almost every week, for over twenty years because, as you say, GA cannot even come close cost-wise. So, I wish you all possible luck and speed in getting this puzzle solved. How do we put up with the small economy size seating? (I am over two meters tall). We use every possible trick to get first boarding positions and sit in bulkhead or emergency exit seats. The security checkpoints and airport queues are miseries I would do anything to escape though.
    Regards,
    Gary
     
  4. Jun 14, 2011 #4

    Jay Kempf

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    How can you get higher efficiency with short coupling and virtual or actual twist that adds up to more trim drag than a conventional configuration or canard for that matter? There is a reason why a glider is shaped like a glider. BWB with some sort of tail on a stick seems like a proper way to distribute structure and get more of the wetted area lifting or fuselageless design I guess you could call it.

    What a tailless design is good at is parts count and ease of assembly which is why simplistic designs like ultralights and hanggliders get a benefit from the config at the sacrifice in efficiency or performance no matter how it is defined. Or am I missing something?

    I guess another benefit would be the ability to just load the wing with components along it's span to reduce bending and to then reduce structural needs...
     
  5. Jun 14, 2011 #5

    karoliina.t.salminen

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    Indeed interesting puzzle to solve. It has taken for some time (and one time to fly North Atlantic with the DA40) to find out (for sure) that this is what I am interested in and why I think it is what is important. The increasing prohibitive operating cost of GA is in my opinion a major contributor for its decline and it has led to situation where planes are not like cars, but instead more like sports equipment.

    By the way, what comes to seating arrangement, prior experience says that side by side seating is must on long trips, ability to reach all instruments from both seats is mandatory requirement. However, I think the back seats don't need to be so back. If thickness problem is ignored, back seats could be on the both outer sides of the pilots, a bit behind (like on Stagger-Ez) and the space between the back seats could be for baggage and a place where the hatch would be located (there would not be multiple doors, but one hatch for several reasons, one of them being that a hatch is much simpler, lighter and easier to implement).

    Obviously on this seating arrangement the back seats would need to be more reclining than the front seats to optimize the center section thickness lower. But I think that would be ok because the back seats would be most of the time just baggage. I think that in most cases there are two people + baggage in 4 seat aircraft. In cars most of the time there is 1-2 people in car that can accommodate 5. The airline ticket price should be divided by these two seats only because most of the time on practical travel, you can't get the plane full and even if you would be able to load it full, you would not be able to split the cost between all the occupied seats. For example my previous work trip there were 2 persons from the same city traveling to the another office in another city. Most of the time (at least with my experience) this number is not large, especially on domestic trips. So additional back seats are rather for friends and family and on most of the trips (e.g. on business travel) empty.

    This example business trip (that could have worked with private plane if the operating cost was lower), was the following:
    - less than 2000 km total distance traveled
    - two persons
    - airfare about 300 eur per person
    To compete, a plane would need to travel with two persons on board 2000 kilometers with operating cost of 600 euros maximum. The Diamond would have taken counting gasoline price at least double that. Therefore the concept would need to arrive at over 2 times more efficient compared to DA40 (if AVGAS would be assumed, AVGAS price is 3 eur per liter nowadays, rather superexpensive). On this trip it could be achievable without rewriting the laws of physics. However on longer trips, the airline ticket price does not increase linearly. Based on experience, crossing North Atlantic on airline on discounted price can be as low as 400 eur one direction (800 round trip). Based on personal experience, flying the same route with DA40, can easily exceed 10000 (large amount of the cost comes from staying nights in places like Kuujjuaq, where motel grade hotel room costed 300 per night). With skipping few places to land and having more efficient power plant (e.g. diesel cycle) would bring the figure closer to reality. However, some further breakthroughs are needed to cover the fuel price alone with the airfare price. Further than that may be reinventing physics.
     
    Last edited: Jun 14, 2011
  6. Jun 14, 2011 #6

    WonderousMountain

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    I'm not entirely sure I understand the mission parameters. Near the price of large airlines for very much smaller seating capacity airplanes? Or use considerably less fuel for the same flight? I can tackle efficiency marginally better than airline competition.

    As for drag it's, it's part wing, part fuselage, part getting those two together in a safe and sane manner. Structurally speaking, both span & aspect ratio will increase wing weight; however air loads, landing loads and pretty much everything else effects structural requirements so don't expect a perfect relation to drop out of the sky. The information I have seen on wing-body interaction is ancient, so I can't really help you there.

    Something rather important to remember is that efficient flight with less than a few dozen people is likely to be done @ considerably slower speeds. Passengers may be willing to take longer flights if the wait to fly is less. For very long flights convincing people to sit in a plane for twice as long would be a hard sell.

    If you're still excited about the prospect as I expect you are, there are basically two approaches, get rid of the tail, and do the flying wing. Which if you can get a low trim drag @ cruise, the efficiency will be quite good compared to conventional aircraft with even very modest AR. Or imitate the lead of gliders, and reduce fuselage drag as much as possible and use a high AR wing for very low cruise drag. The problem with this second approach, is while it gives very good airplane efficiency, it does little for the fuel use/passenger which is the standard efficiency measure.

    Span loading is a great concept, but landing it could be strange. You'd need many wheels across the span to support on landing, and it's not necissarily easy to convince people to sit in a wing that will shoot them up and down when turning.
    Of course if you're doing an all wing AC you want whatever the lowest drag of a useful wing is, Seems possible to get a L/d of 50 or more with a flying wing of moderate scale, but meeting all you're other objectives are going to be a bit of a challenge.

    Just starting,

    Wonderous Mountain
     
  7. Jun 14, 2011 #7

    henryk

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  8. Jun 14, 2011 #8

    karoliina.t.salminen

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    The speed consideration:

    WonderousMountain: Yes; speed is not the most important factor when the target is efficiency. However, I have been thinking these hard and there is a relation between maximum range, efficiency and speed. Faster plane often means larger range because it takes less time to cover the distance, and shorter time to burn the fuel. On the other hand, the larger speed usually means larger fuel burn. And it becomes art when the aim is to go fast with little.

    This speed speculation is rather off-topic, but it gives some background for reasoning:
    For a reference about what kind of speed is desirable, I think the DA40 is slightly too slow. Example: We took off from Palo Alto early in the morning (something like 7am) and could not make it the same day to Oshkosh. Because of the landing time restrictions at Oshkosh, we lost about one day of the air show by needing to land one airport short of KOSH and spend the night there. Same thing: last summer we were flying from Honningsvag to Helsinki. We had to circle around a thunderstorm and in that detour we lost some time. As a consequence we had to land one airport short of EFHF (and spend night there) because of the noise regulations preventing flight at EFHF after 10.00pm. Or that when we departed from Wabush in CAVOK weather, and landed on Kuujjuaq (still CAVOK), but by spending night there, we lost two days waiting for weather there because it was already too late to continue to Iqaluit since refueling from barrels was slow and day was turning to late evening. Or when we departed Iqaluit for Kangerlussuaq, to double check our refueling stop (Cape Dyer vs. Pangnirtung) at tower, caused being late from closing of Kangerlussuaq and that was 1000 dollar bill if I remember the sum right. We did not miss the closing time so much, but just slightly, but still their rules say: you are late, you pay. There are many examples which have been just about almost ok but slightly on the slow side. The Diamond cruises about 127-130 kts TAS at 55% power.

    If we consider a short domestic business trip in Finland. For example EFHK-EFOU 521 km. Airline spends 1 hour to cover the distance. If a concept aircraft would economy cruise 145 kts TAS, it would cross the distance in 2 hours (taking in account takeoff and landing). To fly with the airline, one has to arrive at the airport 2 hours before departure, wait in queues and go to checks. Required time in advance is 1 hour, but one can't make it to the plane from the queues in 1 hour, I have tried and learned the hard way. Total travel time thus becomes 3 hours despite the flight time is short. Pre-flight for the plane takes maybe 30 minutes including refueling. The distance would be covered in 2.5 hours. With the much slower plane one would arrive sooner at the destination than with the airline. Even 145 kts cruise speed would be good enough to cover short distance travel, e.g. regular trips to another city. Of course this should come without the prohibitive cost of fuel. First of all the burned fuel should be less expensive (AVGAS price is getting sky high before becoming completely unavailable at any price) and secondly the fuel burn should be more modest than in current generation of GA.

    What it comes for speed, altitude would be helpful. Maintaining 100 kts IAS would give pretty nice TAS reading at 32000 ft. In fact this would be already good enough in the case it would be achieved with very little power.

    However, back to the technical topic from reasoning and background:
    Any other opinions on flying wing and active boundary layer control?

    Any opinions (pros and cons) for C-wing tip on flying wing (making it not pure flying wing)? Consider this: Control would be from elevons, but in a small C wing tip there could be small pitch trims. For example on takeoff it would be possible to decrease the need of the upward reflex on the elevons when there would be some downforce on the C-wing tip. This could decrease the induced drag on takeoff/climb. Any counter arguments, why it would not work?
     
    Last edited: Jun 14, 2011
  9. Jun 15, 2011 #9

    karoliina.t.salminen

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    Fitting people into airfoil:

    Example:
    KS702centersection.jpg
     
  10. Jun 15, 2011 #10

    Denis

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    I think one needs not to divert from the normal plane layout for further increase in transport efficiency beyond the DA-40. Moreover, the BWB concept was considered very seriously as a polssible base for future ultra-efficient airliners for a long time. Now the multidisciplinary studies reveal thatmost likely the winner will be a conventional high-wing monoplane with struts. This classics taken to extremes shows a promise to get a cruise L/D close to 30:1 or even more than 40:1 with a truss-braced wing. Indeed this applies to a pressurized airliner cruising at its best L/D speed at high altitude.
    A simple GA airplane has more limitatioins as the corresponding airspeed at realistic operating altitudes may be too low. Indeed, I look optimistically at a prospect of about twofold increase in fuel mileage for a light plane with payload and dimensions of DA-40. Such basic four-place plane may need just Continental O-200 or IO-240 or Lycoming O-235 instead of IO-360 for the same job, wioth corresponding reduction in fuel burn with the same or slightly larger wing span than that of DA-40.
     
    Last edited: Jun 15, 2011
  11. Jun 15, 2011 #11

    henryk

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  12. Jun 15, 2011 #12

    Norman

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    Was:confused: I think it's a bit early to put it in the past tense. The X-48b finished its tests a few months ago and the X-48c wind tunnel model is ready to go.



    [video=youtube;NQfByW5BsCU]http://www.youtube.com/watch?v=NQfByW5BsCU&feature=related[/video]
    [video=youtube;GHz2B_4g2tM]http://www.youtube.com/watch?v=GHz2B_4g2tM&feature=related[/video]
    [video=youtube;kJDaVoRi7ro]http://www.youtube.com/watch?v=kJDaVoRi7ro[/video]
     
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  13. Jun 15, 2011 #13

    Denis

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    Detego likes this.
  14. Jun 15, 2011 #14

    karoliina.t.salminen

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    Interesting philosophical question/argument:
    What is the difference between flying wing with reflex airfoil and tail that is behind a boom?
    Each actually function as a tail in fact, don't they?
     
  15. Jun 15, 2011 #15

    caribeanbound

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    Just in case you havn't read it or even looked at the photos, find a copy of "NUFLUGEL" by Reimer Horten and Peter Selinger. That is the most intresting book that I have ever seen on flying wing aircraft. The Hortens built more test models of aircraft than any one else. They had an idea and simply built a flying model to check it out. Their flying wing sailplanes were a thing of beauty for sure. The specs are in there as well. I fully believe that if the war had not ended we would have seen the flying wing "Americas" bombers from the wrong side. No sense reinventing the basics. Take what they learned and add modern aerodynamics.
     
  16. Jun 15, 2011 #16

    Denis

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    The conventional tail is placed at a large arm behind the wing. This is a serious advantage, it can balance almost any diving moment of the wing. Further, the conventional configuration can be simultaneously optimized for minimal drag both at cruise and climb conditions, and within a wide CG travel.
    Reflexed airfoils have limited pitc trim capabilities and seriously compromised drag polar, their profile drag typ[ically rises with increase in Cl. A flying wing stabilized only by such airfoil will hale limited airspeed and CG travel range. Therefore the better way to creat a fluying wing is to use the washout and twist. These airframes indeed have lower span efficiency and limited pitch trim authority with flaps. Maybe the best compromise between these issues is obtained in the SWIFT glider, where a root flap on a back swept wing is used for both the CL increase and pitch trim. Indeed, this glider is well behind the capabilities of a conventional layout and its tailless design was dictated by special requirements.

    As for the possible use of a flying wing configuration ina 4 place GA plane, let us look at the numbers. Such ultimately eficient plane may need quite small wing area, les than 10m^2/ but at the same time a large span, about 12m. Such wing will be very thin. A fuselage with a comfortable four place cabin also can be quite compact and its wetted arera will be not much larger than thatof a two-seater plane. We can mate this wing to this fuselage wit unmeasurable interfernce drag andwe'll get very low Cd0 and very high e, sometimes even greater than unity. Definitely it will be difficult to beat this solution.
     
  17. Jun 15, 2011 #17

    autoreply

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    Sorry about that. Until now I haven't had the time to give an appropriate reply to your concerns.
    My thoughts exactly. Usually the "traditional" solution is a double boom and tail (ala the Adam 500), but a single boom (not symetrical), to keep the structures and design a lot simpler.

    But let's start at the beginning:
    Image03.jpg

    BWB's, even for 4-6 passengers need an elongated "pod" thing in the middle, simply because the human dimensions (height) and required wing area don't match, even if you accept twice the wing area that you'd normally need. Visibility, entrance and landing gear design (laminar flow over a door is hard to achieve) are all challenges.

    My reasoning for the BWB is the possibility for extensive laminar flow. That would be an ideal platform for a radical drag reduction. This started with my comments on radical designs that could run on a fraction of what we see nowadays. The nice thing too about a BWB is that - even if your suction system fails - you still have a normal, stable flying aircaft.

    Ok, enough about that.

    For a BWB you can have 2 ways to control the pitch and have stability. An airfoil with a variable Cm (classic tailless aircraft), or remote pitch/stability controls. The Atlantica has those (strong sweep and thus "elevators" at the tips), making it possible to use flaps. A remote tail would do the same. An interesting option might be a strong forward sweep. This way you can use the rear of the pod as elevator, and you don't have to worry about c of g either.
    Visibility is a major problem. Even with a pod, your sideways visibility is limited. The large wing area can be a problem. Your dCl/dAlpha is the same as for aircraft with a higher wingloading. Thus, you will be a lot more sensitive to turbulence and thermals.


    Pressurization is a nightmare. Pressurize a coke bottle and then a flat shampoo bottle. Double-bubble is possible, but structurally very, very complex to analyze for such a shape. The theory is solid. Go higher up and fly faster for the same cost/fuel per mile. That's where the problems start. Above FL180 is hard, above FL250 is in many countries (close to) impossible. FL180 with oxygen is no problem at all, 250 could still work with oxygen. That gives you a maximum of less than 50% more cruise speed. Significant, but not enough to justify excessive drag.

    Going to 20 m2 (200 sqft) for a BWB is just fine, that's still roughly the same drag as a Lancair IV or so.

    One last note; the "tail on a stick" can be pretty simple. A tapered tube, connected to a rib with 4 identical tail surfaces ("mass" production) can cut down cost and time a lot.
     
  18. Jun 15, 2011 #18

    Jay Kempf

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    This is what I have been looking at. I like the inverted independant full flying V tail but I have been looking at multiple surface high aspect ratio designs. 3 or 4 surfaces on a stick at differing angles and with the stick dead center of the span of the tail surface are very efficient and simple structures. I am partial to pusher designs so I have been sketching high aspect ratio glider type wings off of a central lifting pod that blends out to the wings that attach like glider wings. You can put on different length wings and tail booms for that modular approach. Properly shaping the center section with adequate depth is quite easy. Pressurizing is another story altogether.
     
  19. Jun 15, 2011 #19

    Norman

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    "Nurflugel" has been out of print for over a decade. I was lucky to get a copy from the translator. A new book came out last month titled "Only the wing". I haven't ordered mine yet but it's on my wish list
     
    Last edited: Jun 15, 2011
  20. Jun 15, 2011 #20

    karoliina.t.salminen

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    1. For structural consideration, how about embedding a pressurized cylinder inside the blended shape?

    Pros:
    - Pressurization no longer a nightmare.
    Cons:
    - Weight penalty for double structure and waste of internal space, sort of diminishing the internal space advantage of the blended wing body.

    2. Tail on stick issue: So you mean that instead of elevons, there would be several tails on stick behind the blended wing body? I would like to ask that why do you think several tails on stick would be better than one or two?

    3. What do you think about using C-wing tip as elevator trim or elevator? When there is sweep on the wing and there is some sweep on the winglet too, the top part of the C is conveniently farther behind.

    4. What do you consider the practical AR for flying wing? Horten/Northrop type AR, or perhaps higher? I was thinking about Horten/Northrop like center section and then wing would continue as more narrow from that in such way that the AR would be quite high.

    5. Forward swept blended wing body would look quite odd I would say. Like B2 flying backwards. When it comes to looks, I think the back sweep is more cool in case of flying wing, at least the forward sweep would kill the UFO-coolness-factor of the looks. Somehow this B2 style just tends to appeal to my eye despite the forward swept would be even more novel and less conventional where I in general tend to prefer unconventional, and I am quite sure that I am not alone with this. Maybe if you can draw a picture that appeals to me, it could convince me (the looks part).

    I think that look of an aircraft is part of the product. Airplanes tend to have very practical and functional look, but I think people in general really would like something that looks stylish. As such, current GA selection are rather poor products as a whole, even if they were technically quite good. Like I love my unibody Mac laptop partly because of how it looks, could not compare it with a plastic Dell on the same day even if the Dell was cheaper and matched the CPU and GPU speed of the Mac. Conventional and functional looking planes (compare to Willy's Jeep) tend to appeal for old time pilots, but non-pilots that could be prospective pilots seem to look as a first thing how it looks like and the conventional (C172-like) does not have any merits anymore. I have seen this when talking to some non-pilot workmates. For example one was lusting for Smartfish (before they spoiled the looks by designing the huge duct below the wing - the original looked like the HyFish or SpaceFish (check from their page)). I told him that ordinary Diamond or Cirrus already is quite fun to fly and there is enough challenge for a beginner even on the simplest trainers out there, but apparently he doesn't care and just wants a Smartfish. I think this a bit describes what prospective customers want and what the GA selection do not offer: product that also appeals with the looks for also non-pilots which get inspired enough to invest the huge amount of time and money for a license. So I have a feeling that backward sweep is a good idea for product appeal in case of flying wing. Pure flying wing in my opinion, might have the greatest product looks appeal of the configurations. It would have something same when properly finished than Bugatti Veyron has for cars. Not a race car, but totally awesome thing that blows everyone away. I would personally love the multi-fuselage look of Burt Rutan, but I think blended wing body flying wing appeals to more people than those.

    I have made some airfoils for blended wing body/flying wing which have reflex in the trailing edge which according to simulations have high L/D across the useful Cl range. Theoretically the reflex is not necessarily a that bad thing that would make the airfoil inferior other than about the Clmax. I have to do some wing simulations with XFLR5 someday.
     

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