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alfakilo
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« on: November 04, 2019, 04:50:58 PM »

Here's a question for the structural engineers in the group!!

I'm considering adding some 1/16" stringers to a very basic wing structure, the idea is to 'stiffen' the wing a bit to resist twisting.

In the attached pic, the blue stinger runs from the top of one rib to the bottom of the adjacent rib (three dimensional). The green stringer shows a stringer running from the bottom of one rib to the bottom of another (two dimensional).

Does the 'three dimensional' arrangement provide more resistance to twisting than the 'two dimensional' arrangement?

Should the stringers be placed in a 'W' (sawtooth) arrangement, or should they be placed parallel to each other?

Or should I use some other design?

I've included the model in question, a Cessna AW with a 20" span.
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atesus
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« Reply #1 on: November 04, 2019, 05:02:17 PM »

I'm not a structural engineer but I would do the second approach but with the lower braces going orthogonal to the top ones in each bay.
--Ates
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RolandD6
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« Reply #2 on: November 04, 2019, 07:12:33 PM »

Generally speaking, my first choice would be to use a 'W' pattern between the TE and the spar. The resistance to wing twisting relies upon the bending stiffness of the stiffening members in the plane of bending, in this case the vertical plane perpendicular to the bottom surface of the wing For equal weight, tall thin stiffeners are better that squat square ones. There is a limit though because very tall and thin beams are inclined to buckle laterally, that is perpendicular to the plane of bending.

The 'X' pattern you have drawn may be sufficient for this particular application and would be better if you add a short vertical piece of square balsa that joins the 'X' members in the middle where they cross each other. It is a long time since I have done any sums on thin metal structures and calculations for wooden structures are a different ball game entirely.

I suggest you make a test wing panel with a couple of adjoining rib bays that use the various bracing designs then apply a twisting couple and see which bays distort the most. Use the one that distorts the least.

Below is a typical example of my preferred wing structure. It is a wing panel for a bostonian.

The spars are 1mm, the ribs 0.6mm and the diagonals 4.5mm high by 0.5mm thick.

Paul
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DHnut
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« Reply #3 on: November 04, 2019, 08:49:35 PM »

A totally different approach is to make a D box. I have done one on a 20" Ol" Ironsides and it gives a very stiff, light wing that has not moved after covering. It is also quick to build. The main spar is 1/16", a 1/8" leading edge and shins of 1/32". Unless I am building kit scale this will be my preferred method in future.
Ricky
 
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Indoorflyer
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« Reply #4 on: November 04, 2019, 11:15:17 PM »

A "Dave Rees" style wing is reportedly quite warp-resistant.  You can look at some examples if you bring up the Dave Rees category of designers, in the HPA plan gallery. He used sliced ribs, generally two spars, and a set of diagonals between the spars... (there is a cantilever high wing Lockheed in the collection)

Since you've already completed the cantilever wing of your AW, perhaps just adding triangular gussets to reinforce all the rib to TE joints would be sufficient. Gussets (2 per rib/TE joint) would help stiffen the aft part of the wing without much of a weight penalty.
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Don McLellan
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« Reply #5 on: November 04, 2019, 11:21:31 PM »

A wing I built for my ducted fan rubber powered Mig.  Was quite stiff and reasonably light.
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lincoln
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« Reply #6 on: November 05, 2019, 06:56:52 PM »

What you need is the spar in the first picture combined with the bracing in the second picture, plus a connection between the upper and lower spars. A very thin shear web between them would work. Or, another way to do it is, in the first picture, move the blue piece inward one bay so it was over the green one, plus add a shear web. I'm pretty sure even a 1/64 shear web would do the job. It could be vertical grained and butted to the upper and lower surfaces of the lower and upper spars, respectively. You could probably even make it with paper, though I think 1/64 balsa would be lighter.

If there was some kind of thread that expanded and contracted the same as balsa, that would be even lighter, but you'd have to have twice as many of them.That is, each open space would have an X, not just one brace.

DJ Aerotech use diagonal, triangular half ribs to stiffen up their wings. A little more wood, but maybe a little easier. Here's a picture that sort of shows it:
https://cdn10.bigcommerce.com/s-87wco3a/products/93/images/296/20161116_134354__14461.1489759539.1280.1280.jpg?c=2
Keep in mind that this wing already has a shear web. Since the diagonal ribs are triangular, they don't touch the covering. They could be replaced with sticks that replaced the top and bottom of the diagonal rib, leaving a space in between.

For best results, all the intersections should be "triangulated", such that each is constrained by tensile and compressive forces along the bracing, spar, or trailing edge. As it is, you're going to be bending some ribs when you twist the wing and it will still be somewhat flexible. In effect, you are building a three sided box along the wing.

I have to go vote now, but if I'm feeling ambitious when I get back. I'll draw something up.
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OZPAF
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« Reply #7 on: November 05, 2019, 07:28:10 PM »

If the aim is to provide better torsional stiffness - ie in twisting, then the front section of the wing ie between the LE and spar, is where the extra structure should be placed. A 3 dimensional and triangulated strut arrangement would work well there even better with light shear webs between the struts as Lincoln has mentioned.

However the problem is that to be effective - the joints should be good and it would be a pain to do in practice. On a strength to weight basis a simple light Dbox as mentioned by Dhnut would be very effective - perhaps more than required and thus heavy - unless it was something like a second layer of tissue. However it would for best effect need the spar to be split between top and bottom with a shear web between - ie an I beam.

The other variations mentioned have a lot of merit and are easier and perhaps lighter but would not be as strong and would of course be new wings.

For your wing why not add diagonal sub ribs between the spar and the LE. These could be light balsa - 0.8 or so and even lightened with holes. Place oversized blanks between the ribs and trammel sand them to match the existing ribs. This would be close to a triangulated 3d array of struts in this area and much easier to do - although a bit heavier.

However I must ask - do you really need the extra stiffness above what the finished covered wing will provide. The uncovered wing has a fraction of the final covered stiffness.

Nb I have just noticed that Lincoln has already covered this last half rib idea in the reference to Dj Aerotech..

John
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alfakilo
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« Reply #8 on: November 06, 2019, 09:22:22 AM »

Wow!! Many thanks to all for some excellent instruction. I added a 1/16" spar to the bottom of the wing and gussets to the TE rib joints. I'll leave it there and see how much the tissue improves things. I'll pre-shrink Esaki and once applied, will use alcohol if further shrinking is needed.

I'll keep the shear web idea in mind for future builds as well as the diagonal rib concept. Not too sure that the D box is something I can accomplish!

Thanks again for everyone's help! Here's the wing as for now:
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Big G
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« Reply #9 on: November 06, 2019, 10:02:40 AM »

Quote
If the aim is to provide better torsional stiffness - ie in twisting, then the front section of the wing ie between the LE and spar, is where the extra structure should be placed. A 3 dimensional and triangulated strut arrangement would work well there even better with light shear webs between the struts as Lincoln has mentioned
.

Absolutely correct. I used to stiffen my old 1/2A FF wings via this method, much better results than adding struts behind the spar. Another poster suggested a D-box construction, and if the design allows it, this is the way to go - just look at an F1B wing.

G
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gman
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« Reply #10 on: November 06, 2019, 10:39:30 AM »

If you can stand the shrinkage then double covering with tissue in front of the (webbed) pair of spars is remarkably effective. A "tissue D box" which takes no building. Also works very well over Mylar.
Gavin
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alfakilo
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« Reply #11 on: November 06, 2019, 11:27:37 AM »

If you can stand the shrinkage then double covering with tissue in front of the (webbed) pair of spars is remarkably effective. A "tissue D box" which takes no building. Also works very well over Mylar.
Gavin

Question on that. I'm comfortable applying tissue on top of tissue when applying markings. Should I do the same here? Or should I apply the second layer as if it were the first, by running a glue stick along the LE, ribs, and spar?
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strat-o
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« Reply #12 on: November 06, 2019, 01:14:13 PM »

Shear webs!  (if needed, always if needed)

I think that what everyone has said about shear webs is spot on.  The one exception I have is what Lincoln was saying about using thread.  I think that shear webs, while definitely are needed for tension, they are also needed for compression.  Compression is what threads would not provide.  So that would be my recommendation against employing it.

The following comment applies to wing structure, not specifically for warp prevention:  Wings are usually built stronger towards the center and lighter towards the tip.  If you think about it you can see why: The center section needs to support ~100% of the airplane's weight.  Mid span, however, the wing at this point only needs to be strong enough to support 50% of the airplane's weight.  And at the wingtip, 0%.  From this you can plan the materials and structure you need to use according to exactly how much strength is needed at each location.  (Just don't cartwheel it!  Tongue)

But this is why sometimes you might see sheer webs in the center two bays only, but then none!  Why? because the center needs the strength and the outboard part of the wing doesn't.

Marlin
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gman
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« Reply #13 on: November 06, 2019, 01:48:10 PM »

Question on that. I'm comfortable applying tissue on top of tissue when applying markings. Should I do the same here? Or should I apply the second layer as if it were the first, by running a glue stick along the LE, ribs, and spar?
(I definitely haven't got the hang of this "quote" thing.

Hi Alfakilo, I can only speak as a FF duration modeller which is all I've ever been. I would normally apply a second dampened layer of tissue (maybe with the grain at 90°, if I remember to) and stuck down with thinners over the doped first layer of tissue. That might be a harsh regime on a lightweight scale model...I've done this treatment on F1Gs, tissue/tissue and tissue/Mylar and P30s tissue/Mylar so smallish models, maybe not small enough to be relevant. I've also done 35g Kevlar at +/-45° over tissue, really good on F1A and F1H and definitely not relevant here!!!
Gavin
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lincoln
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« Reply #14 on: November 06, 2019, 05:59:09 PM »

Strat-0:
Threads would work fine if you used twice as many, i.e. one thread across from two diagonally opposite intersections, and one for the other two, in an X. There are many structures like this.

Here's an example of such a structure, though you  have to zoom in and look carefully to make out the arrangement of the bracing wires.
https://airandspace.si.edu/sites/default/files/images/collection-objects/record-images/NASM-NASM2013-00640.jpg

OZPAF:
If you're trying to prevent flutter, then the extra weight and torsional stiffness should go in front of the spar. If this is a rubber powered model, it will be much too slow to flutter, although of course moving any weight forward helps with the C.G. of the entire aircraft.
-----------
I think the main advantage of all this bracing would be to prevent warping of the structure caused by tissue expansion or contraction. Adding another layer of tissue might make things worse. I suppose mylar under the tissue might help, if it isn't subject to changes during storage itself.
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lincoln
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« Reply #15 on: November 06, 2019, 06:12:54 PM »

P.S. You've probably seen bridges that are braced with metal too thin to resist compression. It's hard to find an example of just what I remember, but it's easy to find bridges that are braced with cables or rods.
https://w0cosv3kke2wxd231fhcn6j9-wpengine.netdna-ssl.com/wp-content/uploads/2019/08/bridge-pic.jpg
Come to think of it, the load on the bridge is always in the same direction, so you might only need wires going one way, except maybe for wind loads or loads from a flood. However, you can see crossed rods or cables across the top of the bridge in the example.
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RolandD6
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« Reply #16 on: November 06, 2019, 07:01:20 PM »

...
I think the main advantage of all this bracing would be to prevent warping of the structure caused by tissue expansion or contraction...

My reason for using torsion resistant structures is a lack of faith in my covering and finishing method providing much or any torsional stiffness. Taut doped tissue can be provide a lot of strength, acrylic ink sealed tissue less so. Acrylic finished Tengujo tissue will go baggy in humid conditions even though it does not shrink much past its initial dry state. I need more experience in this area to determine the ideal way of handling the stuff. For health reasons I cannot use solvent thinned products like dope and need to be very careful with sprayed stuff and dust.

Paul
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lincoln
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« Reply #17 on: November 06, 2019, 11:33:59 PM »

I take it that applying dope outdoors with a brush isn't good enough? There are cheap ways of doing a remote air supply using a long corrugated hose and a 40mm  computer cooling fan module or two. But, for all I know, you might be more sensitive than that. If so, the btacing being discussed here ought to do the job.
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RolandD6
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« Reply #18 on: November 07, 2019, 03:37:53 AM »

I take it that applying dope outdoors with a brush isn't good enough? ...

Nope.

It would need to be a breezy day and there are already enough things that limit my modelling time.

My work shop is equipped with multiple exhaust fans that theoretically can turn over all of the air in about 15 minutes with the external door open. Looks good on paper but in practice it is not enough. The weather most conducive for working outside is spring and early summer but at that time of the year I am fighting off hay fever and asthma.

Years ago I spent a lot of time in that work shop casting polyurethane plastic, sufficiently so that I have become sensitised to it and many solvents. This realisation led to the installation of the exhaust fans but they did not help a lot. Cyno glue is barely tolerable inside or out for the same reasons.

I do have a portable air brushing booth that exhausts to the outside through a hole in the wall. This works but for the reasons given I only use water based inks and paints. Industrial grade denatured alcohol in shellac is tolerable in the work shop but I use it outside when ever possible. It appears the methyl alcohol is the culprit in shellac.

At the end of the day when my night time 24 hour antihistamine medication has worn off I am particularly sensitive and need to use an asthma preventative puffer.

I am not yet at a life threatening stage and am doing all I can to avoid going there short of giving up modelling completely.

Paul
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lincoln
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« Reply #19 on: November 08, 2019, 02:06:19 AM »

Yow! That sounds pretty serious. I'm guessing your polyurethane had isocyanates in it? I've certainly heard horrifying stories about that stuff. Used to work at a place that used foam that I think cured with isocyanates, and I wonder if the two part flexible urethane we used didn't have it. Either I wasn't there long enough to develop a problem, I was lucky, or our precautions were adequate. I don't know which.

At my last house, there was an exhaust fan in the basement, so I built a foldable hood to put over it when using contact cement.
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RolandD6
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« Reply #20 on: November 08, 2019, 05:18:56 AM »

Yes it did. Dangerous stuff. Old fully cured polyurethane castings are Ok for me to touch but I found that touching freshly cast stuff was a no no. It made my skin itch and tingle. I destroyed all my stocks by mixing the components in buckets and left it for months outside until I considered it safe to dispose of. I also developed asthma about the same time but the urethane was not necessarily the cause. It was probably going to happen anyway. What ever the cause, I need to be careful with chemicals, fumes, pollens, dry grass and so on.

The fine print on all of the urethane glues and sealers that I looked at here in Melbourne recently all carried some sort of warning that people with urethane sensitivity or a similar condition should not use the product. It is definitely worth seeking out the SDS or MSDS and reading it first.

Here in Melbourne we have experienced a new ( to us) phenomenon the authorities  call ‘thunder storm asthma’. I do not understand the mechanics of it but somehow the storm shatters certain grass pollens and the wind spreads it around. A year or two ago our hospital network was overwhelmed by people suffering from this type of asthma, some never having experienced asthma before. Some of them died. Now we get storm asthma warnings and people with the slightest suggestion of an asthma condition are advised to use their preventative medication as prescribed and stay indoors. I do not consider myself a bad sufferer but the condition is there and I try to avoid anything that may make it worse. I would rather die in a more pleasant way thanks.

Fortunately many years before all of this became serious, we had our house fitted with a ventilation system that pressurises the house with filtered air. I stay inside if the weather is declared or appears risky.

Our New Zealand members can now take a bow because the ventilation system came from NZ.

Paul


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OZPAF
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« Reply #21 on: November 09, 2019, 03:34:01 AM »

Don't give those Kiwi's too much praise Paul Smiley Clever little fellows that they are. Smiley
Good luck with your building and hope the coming summer doesn't cause any hassles.

Cheers
John
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alfakilo
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« Reply #22 on: November 09, 2019, 12:59:18 PM »

I'm trying out some of the tips and suggestions with a small (10") P-51, sort of a 'proof of concept' effort. Ribs are 1/32" sheet. As explained in this thread, the diagonal technique did indeed provide a substantive improvement in limiting twist. Left wing is basic construction. Right wing shows the diagonals sanded to shape. More later.

Fuselage keels temporarily held together with bracing that will be removed once the formers have been added.
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