Lifting body discussion

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Topaz

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...I heard that the SA-882 had been removed, is that true?
Yeah, it seemed to disappear a couple of years ago. It was hung in the position from your photo for a number of years, although I saw it sitting on the floor once early on. The docent I asked of it about a year ago didn't know what had happened, or where it was. I suspect it either is sitting out in a boneyard somewhere or they tossed it in as a 'free bonus' when they sold the Me-262 to the museum up north.

I haven't heard of Storck's book. Is it a history, or is it a design guide?

I ran a BabelFish translation on the French site regarding the Pelican - I'm not sure whoever was doing the writing really has a lot of experience. Some definite misconceptions in some areas.
 

Norman

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It's a history. He's an engineer so it should be more technically accurate than some of the junk I've seen but I don't expect a general treatise on how to design a flying wing. One or two good examples of how a designer solved a specific problem would be fine with me.
 

Norman

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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
I was hoping that one of the engineers would comment on this but since they haven't I'll give it a shot. Hopefully one of them will be kind enough to jump in if I get my foot lodged in my mouth.

In a deltoid planform you've got 3 factors that all tend to influence a phenomena called "induced angle of attack". In 2 dimensional flow upwash is equal to downwash except that upwash only temporarily displaces the flow whereas downwash permanently displaces it. Both are caused by the bound vortex as illustrated in the attached drawing. All wings have this flow pattern in 2-D, it's when you start doing real world things like limiting span and adding sweep & taper that things start going all wonky.

Take a wing without taper or sweep of reasonable aspect ratio. At the root the circulation will be pretty much 2 dimensional. Because of the tip vortex sucking off some of the circulation energy the lift gets weaker as you go outward toward the tip. Oddly though when you get within about 1/2 a chord length of the tip the circulation suddenly changes and the upwash angle increases. This makes the tip behave as if it is at a higher AoA but the tip vortex also delays the stall in that area, very weird. As the aspect ratio is decreased the affected tip region becomes a larger percentage of the wing. At AR below 2 the whole wing is engulfed in the tip flow.

On swept wings the air that would normally stop at the stagnation point slides outward toward the tip. This "stagnation line" is the only part of the boundary layer that actually flow's spanwise. The rest of the BL flows from leading edge to trailing edge but in a distorted S-curve. The upwash is distorted too. At the root there's a reduction in the upwash angle and a corresponding reduction in the lift slope there. However lift is not lost because of sweep, it's simply shifted toward the tip. If you put AoA gauges every foot along the span of a swept wing you would find that at any given time the local AoA of every station was slightly higher than its inboard neighbor. At low AoA the difference between adjacent stations is very small but as the airplane AoA is increased the circulation increases and with it so does the upwash. Since the upwash (and thus the induced AoA) is shifted outboard the wing acts like it has a certain amount of washin and that apparent washin increases with CL. This is why you have to have washout on swept wings. It's also why swept 'wings are usually point designs since greater CL equals greater induced AoA. The answer, of course, is vareable washout which is what elevons at the tip give.

Taper just serves to aggravate the other two effects.

The net result of these three planform features (low AR, high sweep and high taper) is that the tips will be flying at a much higher AoA than the root if you don't build in a lot of washout. Can you say "tip stall at high CL"?
 

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RonL

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If anyone has information in english, or how to obtain it, about the Pelican, i would really like to find as much as possible.
Some of my experimental ideas might work very well with this design.

Thanks
RonL
 

Norman

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2007 SAE AeroDesign winner to speak

Dan Dougherty from California State University Long Beach is scheduled to be the guest speaker at the September 15 meeting of T.W.I.T.T. in El Cajon. The CSULB team won the heavy lift competition with a flying wing this year. Just thought you designers in the So Cal area might be interested.
Ya never know, ya might learn somethn':ban:
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Dana

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A lifting body (or any very low aspect ratio wing) is horribly inefficient from an induced drag standpoint (induced drag is inversely proportional to AR), which translates to poor performance at high AOA. This translates to poor glide performance and high power requirements at lower speeds. On high speed aircraft (fighter planes and the like) the high speeds mean low AOA so it's less of an issue.

Years ago when I was attending Parks College a NASA engineer visited the campus gave a talk on the Space Shuttle (which had yet to fly). I asked him if the very high AOA on re-entry meant the wing was stalled, and he said the circulation over the leading edge strakes provided a lower effective AOA, as Norman described. It kept it controllable, but still lots of drag (desirable in this case, to slow down from orbital speed).

-Dana

When Columbus came to America, there were no taxes, no debts, and no pollution. The women did all the work while the men hunted or fished all day. Ever since then, a bunch of idiotic do-gooders have been trying to "improve" the place.
 

orion

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Although somewhat picking-at-nits, there should be a differentiation between a lifting body discussion and that of low aspect ratio wings. On the subject of the latter, the classic thought is that the short wing configuration is much less efficient, resulting not only in penalties for cruise, but more so for more critical phases of flight like take-off and climb. Most treatises of the matter zero in specifically on the aspect ratio of the wing, often forgetting that there is more to the equation, namely the squared lift coefficient in the numerator.

Essentially, what the equation states is that at times there can be a trade-off between aspect ratio and area. Dana is right in that for high speed applications the penalty is lower due to the lower lift coefficient encountered by fighter type configurations. But that same thought can be applied to lower speed envelopes. One GA example is from back a few decades - I recall a few high aspect ratio proponents who were somewhat critical (or doubting) during the development of the Dyke Delta. But contrary to their statements, the airplane proved to be a very good performer.

The more classic example of this subject was back from my basic aero courses - when you normalize all the coefficients and account for the differences in layout details, it turns out that the Avro Vulcan seems to be just a hair more efficient than the venerable B-52.

Couple that with the better structural efficiency of the low aspect ratio airplane (especially in a blended wing/body configuration), and I think that it's one particular subcategory of airplane that should not be dismissed all to lightly. No, the Facetmobile was not exactly the best example of the technology but if one takes the same thought process and develops something a bit more optimized, who knows? Could be pretty cool looking.

The extreme subcategory of all this then is the lifting body, the examples of which are "airplanes" of the sixties such as the HL-10 or the X-24B. But then those were so critical in nature I'd never recommend their application to GA.

There have been other, more conventional attempts at lifting body application, most of which looked to combine the lifting body with a set of conventional wings. But in most of those applications, the body is of such low aspect ratio (Hiperbipe for example) that it is likely that the contribution of the body is although not negligible, relatively minimal. Furthermore, in developing this configuration it is extremely critical to account for the differences between the body and the wing since an improper positioning could result in unpredictable trim changes with angle of attack.
 

Norman

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No, the Facetmobile was not exactly the best example of the technology but if one takes the same thought process and develops something a bit more optimized, who knows? Could be pretty cool looking.
If you connect the corners of the facetmobile's planview with a smooth curve it would be awfully close to an inverse Zimmerman. IIRC Mr. Wainfan acknowledged (possibly in the PAV report on his web site) that a "smoothmobile" would have been more efficient but not as interesting
 

rotax618

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G'day,
I would like to apologise to Barnaby Wainfan, first for misspelling his name, and second for any inference that the performance figures quoted for the FMX-4 were exaggerated. As I said in my post Barnaby is a talented engineer, and a legend in aeronautics.

Attached is a photo of two of my experimental 1/4 scale R/C faceted flying wings - both fly very well, and have won several flying competitions a the local model club.

Cheers
 

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lr27

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I don't tend to believe that Wainfan makes things up, but either he's making things up or the FMX 4 worked pretty well.

As far as the alleged cg sensitivity, the FMX 4 has the elevators pretty far back from the CG. Wainfan said when he went to Oshkosh he stuck a case of oil quite a ways behind him and didn't notice a change in the handling.

At cruise, it's supposed to be somewhat more efficient, in terms of power used per payload moved, than a 152.

The crashworthiness has got to be much better than a normal airplane.

etc. I'd rather have a two place Facetmobile than a 152.
 

rotax618

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G'day,
I was only relating my experience with the quarter sacle model I made of the FMX4 - the model was built from scaled up plans that appear on the Facetmobile web site - either the plans are not as accurate as they should be or I made an error in some way, because the model flew very poorly. I was stable enough but literally fell from the sky when power was reduced.
Obviously Barnaby would not have been able to fly the FMX4 to Oshkosh if it handled like the model I built.
My subsequent faceted aircraft models had about the same wing loading but they had a much more acceptable glide and really outperformed more conventional models I have built.
I believe that a faceted aircraft similar to the FMX4 but with an improved L/D would make a very practical light (2-3 seat) aircraft with many advantages over the conventional layout.

Cheers Tom C
 

lr27

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I think Wainfan said there were a couple of little tricks in the FMX-4 that would make it difficult to copy if you didn't know what they were. I don't know if he was just trying to discourage copying or if there really was something.

If you made your model from the 3 view, I would guess that some things are missing. I know there's a commercially available scale model kit for the FMX-4 that's supposed to fly pretty well.

I would think that if you tried to slow down just a tad too much the FMX-4 would start to glide like a lightweight brick. I remember some stick time on a weird RC model that had an aspect ratio just over 1. It felt pretty normal until you tried a tight turn, and then it needed LOTS of power.
 

rotax618

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G'day, I posted some video clips of one of our faceted lifting body aircraft on Youtube, the link is attached.

http://www.youtube.com/watch?v=_Cd0lFPyP4I

As you can see from the video the craft has a very good L/D for such a low A/R - it appears to be stallproof and will fly quite fast and very slow without any high lift devices other than its shape - the wing loading of this model is about 3/4 of a conventional model of this size (brought about by the larger wing area).

Cheers Tom C
 

Jman

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WOW...Now a manned version of that system would be a real hoot! (going to ebay to look for a used space suit)
 

Bart

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For those who've been building flying models of the Facetmobile (and for those who haven't), have you considered construction of the much simpler MUGI?

Mugi is a delta flying wing design from Britain, and as simple as pie: Just fold over a rectangle of corrugated plastic sheet into a triangle, and add vertical fin(s). Superb flying and acrobatic characteristics, very cheap to build, very strong, and very damage-resistant.

At first glance of the Mugi video clips, I could not help but think this design would be simple to scale up for a manned version.

The double-wall extruded plastic sheet that it's made of comes in various thicknesses, and is commonly used for making advertising signs, etc.. Very tough stuff, and cheap.

Google for Mugi and see what you think.

Or:

Or:

There are a bunch of other Mugi videos on YouTube, etc..
 
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Topaz

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...Superb flying and acrobatic characteristics, ...
Well, that certainly is a neat RC airplane, but scaling it up would be problematic. As is the case with most RC airplanes, very low wing loading hides a multitude of sins that would jump to the fore in a full-scale aircraft.

I'm not saying it couldn't be done, but like any flying-wing, there's a LOT of work involved to make it an airplane that would fly as well as (or better than, in some respect) a more conventional aircraft. We had a nice discussion of flying wings (a personal interest of mine) over in the "Affecting Thrust/Propeller" thread some time back. Covers a lot of the issues for flying wings, and the Mugi is a highly swept, highly tapered flying wing.

There's been a lot of discussion here and elsewhere about how a Facetmobile/Mugi-style airframe lends itself to relatively cheap, easy sandwich construction. I think the actual 'plastic corrugated' material used in the model would be far too heavy for a real aircraft, but the structural concept is there.
 

Bart

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Well, that certainly is a neat RC airplane, but scaling it up would be problematic. As is the case with most RC airplanes, very low wing loading hides a multitude of sins that would jump to the fore in a full-scale aircraft.

I'm not saying it couldn't be done, but like any flying-wing, there's a LOT of work involved to make it an airplane that would fly as well as (or better than, in some respect) a more conventional aircraft. We had a nice discussion of flying wings (a personal interest of mine) over in the "Affecting Thrust/Propeller" thread some time back. Covers a lot of the issues for flying wings, and the Mugi is a highly swept, highly tapered flying wing.

There's been a lot of discussion here and elsewhere about how a Facetmobile/Mugi-style airframe lends itself to relatively cheap, easy sandwich construction. I think the actual 'plastic corrugated' material used in the model would be far too heavy for a real aircraft, but the structural concept is there.
Agreed, in essence. That said, scale models (including that of a Facetmobile) have proven to be reasonable predictors of performance for full scale versions. If I were looking at Mugi as a planform for a full scale manned aircraft, I'd use aluminum, fiberglass, or perhaps polycarbonate skin. I'd also be truer to some known reflex airfoil than a simple Mugi-like fold would impart, and maybe throw in inflatable stiffeners for good measure. This could wind up being a very simple plane to build, yet with a very generous performance envelope. In other words, a Mugi with mods.

It would also be interesting to see a scale flyoff between Facetmobile and Mugi in, say, 48" span.

Speaking of Facetmobile, has anybody done a model of a Smoothmobile? Wainfan wrote some years ago that a Smoothmobile would have ~20% less drag, as I recall, and that the facets were a function of design simplicity and limited funds, rather than any desire to have facets, per se.
 
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