Lifting body discussion

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Jeremy, Jan 31, 2007.

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  1. Jan 31, 2007 #1

    Jeremy

    Jeremy

    Jeremy

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    I'm sitting at home with a nasty cold, so have been catching up on a pile of stuff I've printed out over the last year or so, meaning to study again one day. I have just re-read Barnaby Wainfan's papers on the Facetmobile, and as usual it's sparked my curiosity as to just why this design (or any other similar lifting body) hasn't been taken up.

    From the performance data it would seem to be quite a good solution to a narrow/single design point aircraft, not dissimilar in many ways to the plank type flying wing I've been working on for a while now.

    Try as I might I'm finding it hard to see just where the downside is to such a design. For an ultralight (under the new UK rules) it's got a lot going for it; light weight, structural design efficiency, good performance on modest power, loads of wing area for it's span, safe handling and relatively simple construction.

    What the heck am I missing here? There must be some really big downside that I haven't spotted, or else our skies would be littered with them. Maybe it's just conservatism that's stopped people following in his footsteps.

    I'd welcome any views you guys may have, as it might prove to be an interesting and educational debate.

    Jeremy
     
  2. Jan 31, 2007 #2

    Topaz

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    There have been a number of discussions here about the Facetmobile. Running a search on that name would probably turn up a lot of information. I don't have any first-hand knowledge about the machine, but others here have alluded to some possibly unusual flight characteristics, especially at lower speeds.
     
  3. Jan 31, 2007 #3

    etterre

    etterre

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    How about these threads:
    http://www.homebuiltairplanes.com/forums/showthread.php?t=1739
    http://www.homebuiltairplanes.com/forums/showthread.php?t=1591
    http://www.homebuiltairplanes.com/forums/showthread.php?t=2218
    http://www.homebuiltairplanes.com/forums/showthread.php?t=2241

    It looks to me like it boils down to a couple of issues:
    1. Most folks don't think they look "right", but to me that's a plus :)
    2. Problems with control authority - much like the plank that you're designing.
    3. My favorite issue: view out the window

    Point #1 is all about the current conservative nature of aviation, so no big surprise there.
    Point #2 is pretty familiar to you... but you have to admit that it boils over into a lot of things. If nothing else, it makes your allowable CG range smaller and that really bites into the utility of the airplane.
    Point #3 is important to me, but maybe not a lot of other folks. Boeing was going to do a BWB as a passenger liner, but the idea died after a few sketches since almost no-one would get a window seat and (way more importantly) it would have required changes at the airport gates.
    Maybe, but a point design really has to appeal to the designer for it to have a chance to see the light of day. The most popular homebuilts here are the RV-#. Why? Because they do most things well - the 2-place ones do mild aerobatics, they're all pretty good cross country machines, and the kits are really high quality. A BWB point design, on the other hand, isn't going to be able to do all of that and it will be harder to design. So the designer spends more time/money on a project that won't earn as much.
     
  4. Feb 1, 2007 #4

    Norman

    Norman

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    Me Too:tired:

    Because, even though you can achieve a good glide by keeping S/W low, sink rate is going to be lousy because it's proportional to B^2/W. And visibility is almost always inferior to "conventional" designs because the pilot's eyes aren't out in front of the wing where they belong

    Just my .02c
     
  5. Feb 1, 2007 #5

    Norman

    Norman

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    Re: BWB

    Back before Boeing dropped the passenger version the talk was that everybody (or at least first class) would get an LCD that they could switch between several external cameras and the in-flight movie. When it was still a McDonnell-Douglas project it was supposed to be an 800 passenger behemoth. The idea was that such high capacity would accommodate the growing flying public without increasing air traffic congestion around international airports. Also those 800 passenger BWBs would have taken up just slightly more ground space than the 550 seat A380 (much shorter length but 30' larger span) and spread the weight out over a wider area. Yes, that pushed it out of the box that current terminals can accommodate and it may be why Boeing reduced the BWB to 600 seats after the takeover. However 5 years ago they were predicting a first flight sometime after 2020. That size and schedule would have meant that they would be selling to a market segment that Airbus had already been serving for 15 years. Hmm, could that be the reason the passenger version disappeared? A cargo version is still going ahead though with military support. Current projections are for a plane that can cross the Pacific in one leg with a 200,000 pound cargo
     
  6. Feb 1, 2007 #6

    etterre

    etterre

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    Re: BWB

    That's probably a more compelling reason for the suits to yank it. :gig:
    That sound's pretty cool!:ban:
     
  7. Feb 1, 2007 #7

    Jeremy

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    Very many thanks guys, and please accept my apologies for not having the foresight to have thought of using the search function; I can only plead that this blasted cold blurred my thinking.

    I've read through most of those threads and found them very interesting indeed. What seems clear is that there's no really good technical reason as to why an aircraft of this type couldn't be successful as a single seat ultralight type (using the UK definition). My best guess is that us amateur designers are a conservative breed on the whole, so fight shy of things that look a bit radical.

    It's a shame that Barnaby seems to have other commitments that have put the Facetmobile on hold for a while, but I can sympathise with him, I have strict limit on the number of "play projects" I dare get involved in at any one time!

    If my head clears a bit tomorrow I might just see how a smaller Facetmobile scaled to about half the empty weight would pan out. My guess is that getting the weight down should be reasonably easy, especially if one of the well-proven paramotor engines were to be used, together with maybe a composite frame or semi-monocoque. I've already built and proven a carbon fibre semi-elliptic spring landing gear that is both very light and very strong, so I'm sure that the potential is there to get down to below the UK deregulated aircraft weight limit of 115kg.

    I'd better get back to the flying plank project for now though, before I risk getting further distracted!

    Jeremy
     
  8. Feb 1, 2007 #8

    Jeremy

    Jeremy

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    Just thought I'd finish this off by adding some data obtained from running the Facetmobile centre section "aerofoil" (less the engine cowl, just the five point profile) through Javafoil, to get some very rough idea as to what the polar etc looked like.

    The results were very surprising indeed. Believe it or not max Cl is around 1.44 at an AoA of 35 degrees(!), Cd is near constant at 0.06 for values of Cl from zero to about 0.73 and Cm0.25 varies from +0.04 at 0 deg AoA to about -0.25 at 35 degrees AoA.

    The high Cl at such a such a high AoA is what surprised me, it may partially explain why the Facetmobile was able to fly well at slow speed and high AoA. Cl falls off slowly beyond 35 degrees, which may help explain the benign stall.

    I'm happy to be corrected if the above conclusions look to be wide of the mark.

    Jeremy
     
    Last edited: Feb 1, 2007
  9. Mar 15, 2007 #9

    Norman

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    bump

    Some items that have shown up on several flying wing mailing lists recently.

    First some YouTube clips:

    This one shows


    This one is labeled H-IX but the





    from Boeing News Now
     
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  10. Mar 15, 2007 #10

    Topaz

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    Re: bump

    Very cool! Thanks, Norm. I'd never seen any of that footage before.

    Almost all the inflight is, indeed, the H-III, but the color 'restoration' footage is either an H-IV or H-VI (there's a brief view of a paper tag on the airframe that's labeled the latter). And there's some later flight footage of either the 229 or the H-III with the mockup engine nacelles for that project.

    Did you note the segment of the H-III with the small canard immediately in front of the center-section nose? I've never heard of the Hortens experimenting with that. I wonder what it was for?
     
  11. Mar 15, 2007 #11

    Norman

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    Re: bump

    I don't recall a high AR wing but I've slept sinse the last time I saw that clip.


    There was never an H-IX (later Go-229) or H-III with dummy engines. One of the H-IIs (smaller version of the H-III) was fitted with dummy engines and a nose.

    The forewing is so close coupled it is more like a slotted center section than a canard. It was an attempt to compensate for the "Mitteleffekt". It didn't work, probably because the theory was wrong.

    can't view youtube at home. I'll go to the lab later and watch more carfuly.
     
  12. Mar 15, 2007 #12

    Topaz

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    Re: bump

    The view is only of the center-section, being turned over by a couple of guys. The 'blended' canopy is characteristic over the H-III, which had more of a bubble canopy. Plus there's that hand-written paper tag.

    No doubt you're right - I remember seeing an in-flight photo, but I may be wrong about the actual glider.

    Interesting! I hadn't heard about that work.
     
  13. Mar 16, 2007 #13

    Norman

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    Okay, I looked at the clip again. This time in full screen mode. The shots of the glider appear to all be H-IIIs (there were several different versions built) but there may have also been a few seconds of an H-II. The plane with jet exhausts is the H-IX V2 ("V" for "versuch" which means "experimental" (it's like a whole other language)) apparently with the "tail" painted white. The color restoration footage is apparently from when the Smithsonian sent their collection of Horten gliders to Germany so the people with the blueprints could restore them properly. The H-VI is now on display in the Udvar-Hazy center at Dulles[SIZE=-1] [/SIZE]International Airport. In exchange they got to keep the H-II.





    BTW This may be of interest considering that this thread was started by somebody who is interested in low AR planks. I'm just about finished with the index of the last 5 years of the TWITT newsletter. Yesterday I processed the November 2001 issue. It's mostly devoted to the Debreyer Pelican. While looking for pictures on the web I discovered that the contents of that issue is on their site. As you've probably noticed I'm not excited by planks but I'll admit that the Pelican is a nice looking plane. Although I think Mr. Debreyer may have selected his airfoil on the grounds of nostalgia rather than technical merit. It's an Abrial section, one of the very first reflexed airfoils:dis: (even older than the Munk sections)
     
  14. Mar 17, 2007 #14

    Topaz

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    Nice. There's another H-IV (I believe, either that or an H-VI) at Planes of Fame in Chino. It's hoisted up against the ceiling, so you get a nice view of the planform, but not of the cockpit area.

    Regarding the Pelican and that website, have you heard anything further about the 'Vampyr' version they mentioned in '06? I've never heard anything further after that posting.
     
  15. Mar 17, 2007 #15

    Norman

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    Yeah I've seen the Horten 4 hanging from the rafters at Chino. I heard that the SA-882 had been removed, is that true?

    I love having the world's biggest library on my desk and a friendly librarian of infinite patience named Google to help me search the card catalog:ban:

    The first thing I looked at was Koen's site because he's the only person I know who's actually talked to J. C. Debreyer. Unfortunately it doesn't look like he's added anything on the Pelican in a while but there's plenty of other flying wing stuff.


    This French article looks pretty recent, at least the only part I could read said "2007". It's very heavy on the graphics. Several pictures I haven't seen and a few graphs.


    I ran across the eBay page for Rudolf Storck's long awaited book. It's in German but I'm hopeing an English version will be out within 3 years
     

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  16. Mar 17, 2007 #16

    rotax618

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    This thread, like many others seems to steer clear of discussing the original question concerning the merits or otherwise of a faceted "lifting body" similar to the Facetmobile concept.
    I have built a couple of 1/4 scale RC models, one of Barnaby Winfram's original FMX4 and one of an aircraft similar to the concept aircraft described in the NASA LARC NAG-1-03054 (can be downloaded from Winfram's site) we called the Devil Ray.
    I found that the FMX4's handling characteristics were pretty poor - the L/D was very low and the power-off sink rate unacceptable, on the other hand the Devil Ray (with the same wing loading and power) proved to be stable, controllable, unstallable and its power-off glide was excellent. We also found that the aircraft had a wide C/G range and a very low minimum speed.
    As far as the visability arguement is concerned, this can be addressed by incorporating clear panels in the cockpit floor.

    Cheers Tom C
     
  17. Mar 18, 2007 #17

    plncraze

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    What are the differences in the FMX model and the one in the NASA report? Is there a large difference in area or span or power?
     
  18. Mar 18, 2007 #18

    rotax618

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    There is no plan given for the aircraft in the NASA report, but I built the model using the sketches. If you look at longitudinal sections taken through the FMX4 you will see a very large washout, for the outer sections to have any useful angle of attack, the aircraft has to be flying at a very high nose-up attitude. I havn't figured out how to insert photos or drawings into this post or I could show you what I discovered, I am surprised that Barnaby Winfam's aircraft flew as well as was reported, the model we built didn't fly well at all, when the power was reduced the sink rate suggested an L/D of about 3:1, while there was sufficient power the model FMX4 was stable and controllable and as Barnaby reported, would not fly inverted but would immediately roll upright.
    Maybe that is why the FMX4 is still languishing in the back of a hangar in pieces.
    We were going to build an aircraft similar to the FMX4, but after analyzing the performance of the model we abandoned the idea. The new model based on the ideas in the NASA report has given us back oue enthusiasm to go further with the idea.

    Cheers Tom C
     
  19. Mar 18, 2007 #19

    Norman

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    If you use the "Quick Reply" box at the bottom of this page then you don't get many formatting options and you can't attach pictures. To post a picture use the "Post Reply" button at the bottom left corner of the last message. Then scroll down past the "submit Reply" button to the "Additional Options" dialog box. There's a "Manage Attachments" button in there. That will open yet another dialog box where you can pick the files you want to attach. Two things to remember at this stage.

    1: after you select you files make sure to hit the "Upload" button because it doesn't do it automatically
    and
    2: check the files your want to share to make sure they're less than 500 kb. That's the limit for ordinary users and it's plenty for viewing most things in a message box.
     
  20. Mar 19, 2007 #20

    rotax618

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    As you can see from the attached PDF of longitudinal sections through the FMX4 that the effective washout is around 6deg, not what I would call an effecient shape.
    Has anyone else noticed this? Obviously Barnaby is a talented engineer, but I just cannot reconcile his thinking.

    Cheers Tom C
     

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