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Old Jun 24, 2015, 11:31 AM
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A clean, swept wing design.....

Hi,
Although not beeing new to aeromodelling....when it comes to flying wings, I find myself confronted to some (interesting) questions.....

My aim is to build a swept wing with nice self-stabilizing properties.
I'm thinking "stick & tissue".
I'd love an ultra-clean design that flies on its own built-in good aerodynamics, not needing any "extras"....
This finally acheived, I can go on trying to control the wing with rc....at a later stage, that is, when the aerodynamics are sorted out.

I started by making a simple paper/grass (!) "prototype".
As you cannot "twist" paper in a true way, a couple of "spoilerons" were added instead.
How this was done, you can watch HERE

Next, I cut another prototype of foam (enlarged to 250% = about 25" span), now with a 6° linear negative twist (washout). (sorry, no photo)
The result was "interesting", although somewhat encouraging.
Extremely sensitive on CG, the slightest wind gust would put the wing out of control, each time resulting in a dive, which turned into a negative loop (bunt)..!
I'm curious about the "bunt" tendencies....with the increased speed, the wing twist should have raised the nose...but not so....WHY...??

Just in order to learn something (?), I added a pair of negative wing tip spoilerons (as per the paper prototype), and the "bunt" problem thereby seemed to be solved....still a bit unstable, though...

A second foam prototype was cut (sorry, no photos), this time with 7.5° linear negative twist.
A much better result this time, although the "bunt" tendencies remained, no matter where I moved the CG..!....less sensitive than the 6° version though....

Adding "spoilerons" to this 7.5° version didn't require as much deflection as with the 6° wing, (very slight, really), and with the CG tweaked to the "right" position, this wing now flies in a beautiful manner...!
It's perfectly stable, and it rather seems to take advantage of the wind gusts, and gain height, instead of going out of control... WOW..!

Great..!....however, I hate to see those "spoilerons" poking upwards....
My wonderings are:
* Am I on the "right path"...?...would some more wing twist finally eliminate the need of control surfaces...??
* What caused these "bunt" tendencies....?..I find this strange...a "proper airplane" would raise its nose with increased speed, due to its decalage....

Any thoughts....??...


//Adding: The wing section was in both cases a "std" Clark Y 11.7% thickness throughout the wing //

.
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Old Jun 24, 2015, 01:59 PM
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Gluehand, I'm thinking while I'm writing! First, your planform isn't the problem. In fact, it's quite elegant... nice proportions. And we all know that beauty is inherently uplifting, so the problem is elsewhere.

Since your design tucks (bunts) with twist and even with forward CG, it must have something to do with the airfoil. A very thin, planar airfoil has a near-zero zone of zero lift; that is, it transitions from positive to negative lift almost instantaneously. Try making a paper airfoil that includes a leading edge radius of, let us say, 3 mm. If you roll the paper over whatever you use for the radius, and reattach the paper to the main panel some 12 mm behind the leading edge, it may work.

If it doesn't work, I'll refund my consulting fee!

ed
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Old Jun 24, 2015, 02:11 PM
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Washout along with a change in airfoil is what gave you your success. Adding the reflexed "spoileron" tabs alters the airfoil shape to one which has some reflex. In effect you used both the washout along with a change to a lower camber value airfoil at the tips to work together to achieve your stability.

So why did it seem stable but then go into a bunt when the speed went up? At the low Reynolds numbers our models fly at I suspect that the pitching moment changes with speed and angle of attack. And in fact when you look at charts of the Cm (pitching moment) of the airfoil with respect to the angle of attack it does in fact change over a range. Add to that the odd way that airflow separates and rejoins at "our" model speeds and you can end up with a lot of odd things happening. Especially when the CG and washout are on the fine edge of stability.

One option which is often used is to start with an airfoil that has a lower pitching moment value. That way it won't change much on you with changes in the flying speed. And with that your washout can be all the solution you need.

Another is to change the airfoils along the span for a swept wing style of this sort from positively cambered at the root and more forward area to symmetrical or even negatively cambered at the tip. A simple but rather extreme example of this is the Paoli swept flying wing;

http://scherrer.pagesperso-orange.fr...sh/paolie.html

The reason why we need a lot more washout than we would imagine is that on swept wings the air flow tends to roll around the swept leading edge and alter the local angle of attack along the span of the wing. So the airflow experienced at the tips appears to be coming from below which increases the local angle of attack. And this appears to be speed related which might explain the apparently stable flight which changes to a bunt when the speed comes up.

Either way the option is to build in the washout either with twist or from the use of different airfoils from root to tip. Or a combination of the two things.
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Old Jun 24, 2015, 02:27 PM
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Is that strike anywhere match?

I'll give you a kr1 if you can get that match to strike (and light) upon landing. LOVE the match fuselage!

Baffled by your odd results with wing twist on the foam wings. So the wing tips have 6 degrees less angle of attack than the root. That is a lot of twist, that should allow for a very forward CG that should provide stable dampened pitch behavior. It should recover well from gusts. More twist would certainly allow for no elevators at the wing tips with this airfoil.

A low pitching moment airfoil at the root (PW51) with a symmetrical airfoil at the washed out tip may provide good stability with less wing twist and thus less overall drag.

I too am wondering............................could a paper version have a double surface for the front portion of the wing and thus provide a torsion tube to set the washout?
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Old Jun 24, 2015, 05:27 PM
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Back when I was a high school student (YES! THEY DID HAVE SCHOOLS WHEN I WAS YOUNG! ! ! ! ) I did this little folding trick one day in class instead of listening to whatever they were parroting at the front of the room.

I took the idea home and made a small all paper glider using the cheezy school notebook paper. If used along the 11 inch direction for a 22 inch overall wing span it was too weak. But the other way to give a 16 inch span the resulting model was very flyable and flew as well as a flat wing sheet balsa model.

With better heavy weight paper there's no doubt that this could work to a 22 to 25 inch span size just fine. Or perhaps with business card stock for the center panel and dihedraled tips from good printer/copier paper that we could produce a 27 inch span model wing.

Of course the airfoil shown isn't much good for a flying wing. ...... or maybe it could be? If we start with the shape shown at the root then taper to the tip by cutting the trailing edge off to the seam that would induce a fair washout angle. On top of that if the glued seam was then creased upwards then we'd have some camber reduction or even some reflex.
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Old Jun 25, 2015, 02:00 AM
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Just another thought that hasn't been raised: With a paper airfoil (not very stiff) I imagine all kinds of warping could be happening - on the wing itself and for the spoilerons. And the extent of warping may be varying wildly depending on the strength of wind gust.
As others have mentioned, a clark-y airfoil has a really strong pitching moment and is much less suitable for a flying wing than an airfoil with a less aggressive pitching moment.
Finally, scaling up will vastly improve handling characteristics because of very "sensitive" behavior of low Reynolds number (also mentioned by others).
A 20" span will be much more skittish than the same exact plane scaled up to 60" span.
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Old Jun 25, 2015, 08:58 AM
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What I described, and what Knoll and BMatthews drew, is a paper version of the Jedelsky airfoil that is so easy to build in balsa. I used to build free flight conventional gliders that way many years ago.

If you combined the paper Jedelsky with a fold that produces the Paoli reflex toward the tip, you might have a winner. The only problem I can see with the Jedelsky is that it has a large pitching moment, so you'd need plenty of washout. Might be inefficient.

There is a paper wing that looks almost exactly like yours and seems to have a great glide. It's called the Omni Wing, on Youtube:
Building the OmniWing Paper Airplane (9 min 51 sec)
.
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Old Jun 25, 2015, 10:59 AM
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Had to try it......finless

I tried a sharp LE, 1/8" spar, with a double surface to the TE. Inasmuch as I am largely unfamiliar with type of model, there were many construction problems, but ended up with a rough simple wing twist. First a CG was set to get close to a glide, then that set, the wing twist was adjusted to improve the glide. Instead, just setting the wing twist then fine tuning the CG to improve glide would probably be a better method. Fine tuning the CG would be a little tricky at this very light weight. I'm sure that there is a method.

Although it was hard to adjust wing twist, I think that there is a good glide in there some where, without elevators.

paper airplane (0 min 29 sec)
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Old Jun 26, 2015, 09:48 AM
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Try 2

although I am not having much luck with the paper construction method, I do have a wing with minor twist, no elevators, an OK glide and no bunt.

paper airplane 2 (0 min 7 sec)
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Old Jul 03, 2015, 01:58 PM
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Sorry for this delayed reply....
For a good week I've been tormented - day and night- by ache in my right shoulder and hand....which yesterday was diagnosed as Podagra, or Gout. Hopefully the prescripted cure of cortisone will sort it out.
Yeah, aeromodellers get older....



Thanks for your responses..!...I enjoyed your film clips and photos.
Bruce's good advices have made me move towards a solution involving wash-out like before, combined with a different tip section.
Clark Y throughout the wing panel, like I used with "prototype 2 & 3", is probably the least efficient choice, requiring a bizarre washout angle....

Helped by the excellent software Profili, I've been toying with combinations of sections, such as:

First pic below: Clark Y gradually sneaking into a full symmetrical J5012/12%.
Second pic: Clark Y likewise gradually sneaking into an EH1.5-9.0
Any thoughs on these suggestions....?

I'm reluctant to choose extreme "duck-tail" sections or similar, as I wish an as clean design as possible...self-stable, without the need of contol surfaces....


Third photo shows my "prototype No 3", i.e. Clark Y throuhout, 7.5° washout, 25" span.
It was "almost" meeting my goal (great flyer), although the slightly deflected spoilerons were neccesary to finally get there.....
Note this is a very rough made prototype, put together just for test purposes.
(Those fins may not be neccesary....or at least not that large, anyway...)
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Old Jul 04, 2015, 01:26 PM
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I think you might be missing a critical point: You're "shooting yourself in the foot" by using clark-y on a flying wing (if efficiency is important to you). You mention that your most succesful iteration thus far is clark-y with 7.5 degrees washout AND some reflex in spoilerons - that's a LOT of washout and is not efficient - but you're forced to do so because of the strong pitching moment of the clark-y.
An MH45 or the like (or even a symmetrical airfoil) would require a lot less washout and be more efficient.
Finally, washout is not the only approach. Drooping leading edge cuffs (for outboard third of the wing) like the ones in the image below are easy to work with and give effectively nearly identical results (I and others discuss this at length in other posts).
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Old Jul 04, 2015, 04:47 PM
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The droop doesn't even have to be applied to the whole outboard section. A small fin where the droop would start will do the same job but it would be fragile so a 1/2 inch wide droop or even a little triangular "tooth" might be a good shape.
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Old Jul 05, 2015, 05:04 AM
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Quote:
Originally Posted by nmasters View Post
The droop doesn't even have to be applied to the whole outboard section. A small fin where the droop would start will do the same job but it would be fragile so a 1/2 inch wide droop or even a little triangular "tooth" might be a good shape.
Hi nmasters,

So what you're suggesting would prevent much of the spanwise flow, but would it also address lowering the AOA for the outboard wing?
Many examples on full scale planes have the cuff going out to the tip (like the one below). Is it overkill for this case?
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Old Jul 05, 2015, 12:47 PM
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Quote:
Originally Posted by nuteman View Post
You're "shooting yourself in the foot" by using clark-y on a flying wing
Yes....I'm leaning that way too....
It would take an unrealistic rate of washout, wrestling with the Clark Y pitching moment...
Question is though....do I really have to change airfoil of the entire semi-span...?
Wouldn't a symmetrical or slightly "reflexed" tip airfoil do the required job (gradually changing root to tip), combined with a sensible washout...?
In any case, your suggested MH45 looks to me like a sensible choice for further experiments. It would surely provide a better "bite" at the tips....

Also: Browsing some old magazines, I found a couple of FF "wings" (Pic 2 + the PDF)
The Sea Gull is nice, but is unfortunately a "flat" design, requiring "spoilerons", which I try to avoid here....

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Old Jul 05, 2015, 01:59 PM
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Quote:
Originally Posted by nuteman View Post
So what you're suggesting would prevent much of the spanwise flow, but would it also address lowering the AOA for the outboard wing?
Many examples on full scale planes have the cuff going out to the tip (like the one below). Is it overkill for this case?
A properly designed cuff only adds 3% to the chord and moves the camber forward a bit. The AoA change is only a fraction of a degree. When NASA was testing it they faired in the dog-tooth at the inboard end to see what effect the chord extension had without the vortex and found that it had almost no effect that way so the vortex that the sharp step of the dog-tooth creates is the dominant characteristic. If you want to do a cuff go ahead, they work great and are more damage resistant than a smaller feature, but the magic is happening at the step in the leading edge so just make sure that it is fairly sharp. I've compiled a short list of links on this topic here. There is a drawing of a model with vortilons here

--------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^) --Norm
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