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RE: Sailplane Wing Design Round II
Using Smelly Socks to Assemble the Wing Center Section
More progress today. I picked up from where I left off yesterday with the ribs. Drilled holes for the servo wires & sanded out the spar-cap notches... http://www.scipie.com/rc/vee-gull/20120707_0004.jpg I printed my center section plans on 8½" x 11" paper then taped them together... http://www.scipie.com/rc/vee-gull/20120707_0006.jpg Next I cut the CF spar-caps a little longer than needed. I also cut my 12mm thick shear-webbing making sure it was correctly tapered... http://www.scipie.com/rc/vee-gull/20120707_0008.jpg I created a jig for accurately cutting the shear-web into sections... http://www.scipie.com/rc/vee-gull/20120707_0016.jpg Here's the full set of shear-webbing for the center wing... http://www.scipie.com/rc/vee-gull/20120707_0022.jpg Pinned the lower CF spar-caps to the plans... http://www.scipie.com/rc/vee-gull/20120707_0024.jpg Alternating between rib, web, rib, web, rib, I started to assemble the wing... http://www.scipie.com/rc/vee-gull/20120707_0026.jpg Before you ask, yes I removed the paper rib template where the shear-webbing glues to the rib... http://www.scipie.com/rc/vee-gull/20120707_0027.jpg Ribs & shear-webbing installed... http://www.scipie.com/rc/vee-gull/20120707_0030.jpg Next I added the top CF cap-spar then placed our entire collection of dinner knives across them... http://www.scipie.com/rc/vee-gull/20120707_0032.jpg Next I added some socks, yes socks... http://www.scipie.com/rc/vee-gull/20120707_0035.jpg A piece of MDF board goes on top... http://www.scipie.com/rc/vee-gull/20120707_0038.jpg And the cherry on top, a 20 lb dumb-bell weight. I'll leave that to setup overnight. Hopefully no one will need a knife for their midnight snack... http://www.scipie.com/rc/vee-gull/20120707_0040.jpg |
RE: Sailplane Wing Design Round II
The new spar is looking good, feels crazy strong...
http://www.scipie.com/rc/vee-gull/20120708_0411.jpg In preparation for the Kevlar tow I made tick-marks along the spar so I could keep the wrap neat... http://www.scipie.com/rc/vee-gull/20120708_0416.jpg Although there's more wrapping to come I wanted to get the bulk of it done before adding the leading & trailing edges. Having the wing open makes life much easier... http://www.scipie.com/rc/vee-gull/20120708_0418.jpg With that done I added the TE strips... http://www.scipie.com/rc/vee-gull/20120708_0421.jpg I CA tacked the end ribs in place using angle templates I printed up... http://www.scipie.com/rc/vee-gull/20120708_0427.jpg My original wing was a really subtle gull-wing, almost flat, with barely any dihedral. The rather long wing rods & a thin airfoil greatly limited the angles I could use. This time around, using fairly short wing-rods, I'm free to use larger dihedral angles. After playing around on the 'puter I settled on 22° in the center & -9° for the outboard sections... http://www.scipie.com/rc/vee-gull/3D...ng_005_700.jpg Although the previous flatter wing was fine, this one should be significantly more stable. As you can see I'm also playing around with winglet designs. I maybe mistaken but this might be the first gull-winged sailplane to feature winglets *shrug* I know I've not seen one before, fun fun. Will have more progress tomorrow. |
RE: Sailplane Wing Design Round II
Great job and nicely done on the details.
The use of the family supply of knives to bridge the gap was truly inspired. Shows a certain weakness in your sense of self preservation but inspired nonetheless.... :D |
RE: Sailplane Wing Design Round II
No Instructions Required
One of the things I love about building my own designs is the total freedom to screw-up totally, figure out what I did wrong & then get right back in there & build it better. Before attaching the end ribs I cut another set to be used as the first ribs on the outboard wing-panels. By drilling the wing-rod hole now I'm assured they'll align nicely later on... http://www.scipie.com/rc/vee-gull/20120709_0002.jpg Attached the leading edge... http://www.scipie.com/rc/vee-gull/20120709_0006.jpg On my last wing I butted the sheeting up against the LE which resulted in plenty of ugly & weak gaps. This time I pre-shaped the LE so that the sheeting will overlap... http://www.scipie.com/rc/vee-gull/20120709_0007.jpg I wanted to lock-down the Kevlar wrap so that it couldn't move, fray or whatnot. I considered using CA but that'd take a lot & that stuff isn't cheap. I heard enamel paint works well but the one can I have in my workshop was dried up. Instead I watered down some wood glue & brushed that onto the fibers... http://www.scipie.com/rc/vee-gull/20120709_0014.jpg It wicked into the wood nicely & sealed in the Kevlar. I only did this on the sides, I figure the top & bottom fibers will get entombed under the sheeting... http://www.scipie.com/rc/vee-gull/20120709_0022.jpg Wing Joint Box Now things are getting exciting. Time to join the two wing halves. Here's my chunky 8mm carbon fiber wing-rod. That thing ain't breaking even if an elephant sits on it. To save some epoxy I created hardwood space filling wedges... http://www.scipie.com/rc/vee-gull/20120709_0031.jpg To create the wing joint box I added 1/16" basswood shear-webbing to one side only. I eased the sharp edges so when its eventually wrapped it won't chafe the Kevlar... http://www.scipie.com/rc/vee-gull/20120709_0027.jpg Goop time! I glued the end ribs together with five minute epoxy & then started to fill the box around the wing-rod with 30 minute epoxy/microballoons goop... http://www.scipie.com/rc/vee-gull/20120709_0032.jpg With it about half filled I dropped in my wedges. As they sank the goop rose-up to fill the box to the rim... http://www.scipie.com/rc/vee-gull/20120709_0034.jpg With the glue still goopy I capped off the box with another set of 1/16" basswood webs... http://www.scipie.com/rc/vee-gull/20120709_0036.jpg Yeap she's starting to look like a wing now, liking the look of that 22° dihedral... http://www.scipie.com/rc/vee-gull/20120709_0040.jpg Time to stop for the day, let that setup good & strong over night... http://www.scipie.com/rc/vee-gull/20120709_0042.jpg |
RE: Sailplane Wing Design Round II
Time to work on the outboard wing joiner boxes. Unlike the permanent center joint these will be detachable. Brass tubing will support the CF wing-rod...
http://www.scipie.com/rc/vee-gull/20120710_0005.jpg Same system as the center joint, the box will be filled with epoxy/microballoon goop. Again I'm using a hardwood wedge to take up some of the space... http://www.scipie.com/rc/vee-gull/20120710_0003.jpg I needed a way to keep the epoxy from running into my wing tubes. Then it occurred to me, maybe epoxy in the tube is exactly what I need... http://www.scipie.com/rc/vee-gull/20120710_0006.jpg Let these setup for 10 minutes... http://www.scipie.com/rc/vee-gull/20120710_0009.jpg Trim the excess goop & I have two nicely capped tubes. I also roughened the brass with some heavy sand-paper to give the goop something to grip... http://www.scipie.com/rc/vee-gull/20120710_0016.jpg Installing the tubes... http://www.scipie.com/rc/vee-gull/20120710_0018.jpg In goes the goop... http://www.scipie.com/rc/vee-gull/20120710_0019.jpg Capped with 1/16" basswood... http://www.scipie.com/rc/vee-gull/20120710_0022.jpg Once that was set I wrapped both joints with Kevlar, this time closer wound as I figure this is a high stress area... http://www.scipie.com/rc/vee-gull/20120710_0028.jpg Speaking of a high stress area, I thought it'd be a good idea to join the spar-caps in the center using some CF ribbon... http://www.scipie.com/rc/vee-gull/20120710_0010.jpg The CF strips were epoxied top & bottom of the main joint. I used packing tape to help keep it in place while the glue dried. A thumb comes in handy too... http://www.scipie.com/rc/vee-gull/20120710_0012.jpg Here's the CF strips after the tape (& thumb) was removed. Something tells me my cap strips aren't going to pull apart any time soon... http://www.scipie.com/rc/vee-gull/20120710_0024.jpg And just to be sure everything stays together no matter what, more Kevlar wrapping... http://www.scipie.com/rc/vee-gull/20120710_0026.jpg So here she is with the completed spar & joinery. I must say I'm quite happy with the way this is looking. Thanks again everyone who offered advice... http://www.scipie.com/rc/vee-gull/20120710_0029.jpg Not sure how much I'll get done tomorrow as I'm flying most of the day, then entertaining during the evening. On the plus side though, my fancy tapered CF cap-strips for the outer wing panels are due to arrive tomorrow, fun fun. |
RE: Sailplane Wing Design Round II
A hint for you for next time. On the outer panel joints if you leave the brass tube full length and shim the face ribs apart with some thin card stock or balsa shim you ensure that the tubes line up 100% across the joint and avoid any glue making it's way in through the joint line. Once cured simply cut through the brass or aluminium slip tubes with a razor saw then file flush with the rib and de-burr. Doing it that way ensures that there is 0 misalignment potential.
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RE: Sailplane Wing Design Round II
Thanks, I'll keep that in mind for next time. I have a procedure in mind for this wing's alignment that should work nicely, too. Will get to try that later this week hopefully.
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RE: Sailplane Wing Design Round II
Sheeting
Not much to report today. I have the sheeting done on the center section. Here's how that went... http://www.scipie.com/rc/vee-gull/20120713_0034.jpg Unlike my previous fully sheeted center section, this time around only the area ahead of the spar will be sheeted. With the new kryptonite spar I don't see much structural need to slap sheeting all over, should help with the weight too... http://www.scipie.com/rc/vee-gull/20120713_0036.jpg I am sheeting the root some however... http://www.scipie.com/rc/vee-gull/20120713_0038.jpg To help support the diagonal sheeting I added diagonal ribs... http://www.scipie.com/rc/vee-gull/20120713_0043.jpg Sheeted the bottom & made a couple of holes for the servo wires to exit... http://www.scipie.com/rc/vee-gull/20120713_0053.jpg Pretty happy with the sheeting... http://www.scipie.com/rc/vee-gull/20120713_0049.jpg In other news, my nifty tapered CF spars for the outer wing sections have arrived. I should be able to start cutting ribs & putting those together this weekend. |
RE: Sailplane Wing Design Round II
Measure four times, cut once.
Time now to put the outer wing panels together. Before I can do that I needed to accurately measure the taper of my new fancy CF spar strips... http://www.scipie.com/rc/vee-gull/20120714_0002.jpg Not having a micrometer handy (I need to get one of those), I tried to use a ruler to measure the thickness... http://www.scipie.com/rc/vee-gull/20120714_0012.jpg Looks like the thin end is a bit less than one mm. Seeing as I'm going to base the depth of all the rib notches on these measurements, I wanted to be a little more accurate than that, so I used an old trick. Lets measure all four spars at once... http://www.scipie.com/rc/vee-gull/20120714_0007.jpg Looks like they total around 3mm... http://www.scipie.com/rc/vee-gull/20120714_0010.jpg Divide 3mm by four & we get 0.75mm. So that's my thin end measurement. I did the same for the thicker end... http://www.scipie.com/rc/vee-gull/20120714_0011.jpg That's a smidgen over 6mm. So 6.1mm divided by four equals 1.52mm. So I now know, with a fair degree of accuracy, that my spar strips taper from 1.5mm to 0.75mm. Oddly enough that's what the manufacturer said too but if you've ever brought wood at the Home Depot for example, you know to never go by the manufacturer's claims. Best to take real-world measurements. Anyway I'm going to fire up my CAD software & plot some ribs based on these measurements. |
RE: Sailplane Wing Design Round II
ATCHOO!
More fun sanding ribs to shape. Warning those with allergies may find the following disturbing... http://www.scipie.com/rc/vee-gull/20120715_0013.jpg After a rather dusty couple of hours I have a full set of ribs ready for the outer wing panels... http://www.scipie.com/rc/vee-gull/20120715_0021.jpg With the hard part done I'm hoping to get the outer wings together tomorrow. |
RE: Sailplane Wing Design Round II
Outboard Wing Sections
I printed up a set of plans & started construction by laying the tapered CF spar-caps down... http://www.scipie.com/rc/vee-gull/20120716_0019.jpg As with the center wing section, the ribs & shear-webs go down in alternating order... http://www.scipie.com/rc/vee-gull/20120717_0001.jpg Done adding ribs & shear-webbing... http://www.scipie.com/rc/vee-gull/20120717_0005.jpg Wanting to keep things light out at the tips I split the shear-webbing in the last four bays... http://www.scipie.com/rc/vee-gull/20120717_0008.jpg With the CF tapering off to a hair's thickness & hollow shear-webbing these outer panels should be light yet still plenty strong... http://www.scipie.com/rc/vee-gull/20120717_0010.jpg That's all for today. |
RE: Sailplane Wing Design Round II
To make sure there's no gaps between the shear-webbing & the spar-caps I created a custom width sanding block out of scrap MDF, contact-cemented the sand-paper to the block...
http://www.scipie.com/rc/vee-gull/20120717_0106.jpg Using the sanding block to fix any problem areas... http://www.scipie.com/rc/vee-gull/20120717_0105.jpg Epoxying the spar-caps to the wing... http://www.scipie.com/rc/vee-gull/20120717_0111.jpg Its random household item time again! On go the dinner knives... http://www.scipie.com/rc/vee-gull/20120717_0112.jpg This time instead of socks, I used foam pipe insulation to evenly distribute the weight... http://www.scipie.com/rc/vee-gull/20120717_0114.jpg On goes a piece of MDF board... http://www.scipie.com/rc/vee-gull/20120717_0117.jpg Added some weight... http://www.scipie.com/rc/vee-gull/20120717_0120.jpg And to top everything off, a cute baby penguin... http://www.scipie.com/rc/vee-gull/20120717_0131.jpg Once that had setup for a few hours I did a quick weight check, looks pretty good to me... http://www.scipie.com/rc/vee-gull/20120717_0134.jpg Lastly I wrapped the spars with Kevlar... http://www.scipie.com/rc/vee-gull/20120717_0137.jpg Tomorrow I go flying (I fly Wednesdays & Sundays). Its killing me to not have this bird ready yet, all these gorgeous Summer days passing by, skies filled with boomers. Soon enough she will fly again! |
RE: Sailplane Wing Design Round II
One thing for sure, it ain't gonna break in flight THIS time.... :D
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RE: Sailplane Wing Design Round II
Yeah I think I have the strength issue sorted. There's still one hundred & one other things that can go wrong though but that's our hobby. Speaking of things going wrong...
Its days like this that I ask myself, "why the heck am I scratch building?" and then I remember why... its fun! Well most of the time it is, just not so much today :-( Everything was going great, I added the leading & trailing edges... http://www.scipie.com/rc/vee-gull/20120719_0002.jpg Capped some brass tubes, ready to be installed... http://www.scipie.com/rc/vee-gull/20120719_0004.jpg While test fitting the wing sections together I noticed something ominous about the left side wing-rod... http://www.scipie.com/rc/vee-gull/20120719_0014.jpg It just didn't look right. Its supposed to be angled down at 9°. Instead it was sticking straight out at 0°, not good! I have no clue how that got screwed-up as I was plenty careful when installing the wing tubes... http://www.scipie.com/rc/vee-gull/20120719_0010.jpg After scratching my head for a bit I realized there was only one thing to be done. I'd have to completely redo the wing tube on that side & that meant major surgery, nooooo! So reluctantly I started hacking apart all my hard work... http://www.scipie.com/rc/vee-gull/20120719_0015.jpg My nice neat Kevlar wrap had to go... http://www.scipie.com/rc/vee-gull/20120719_0018.jpg All I can say is thank God for the Dremel tool... http://www.scipie.com/rc/vee-gull/20120719_0024.jpg Finally got the miss-aligned tube out of there, what a hassle!.. http://www.scipie.com/rc/vee-gull/20120719_0026.jpg Anyway, several frustrating hours after first discovering the issue a new tube was installed, this time at the correct 9°... http://www.scipie.com/rc/vee-gull/20120719_0031.jpg In the meantime, while the epoxy cures, I turned my attention to the other side of the wing... http://www.scipie.com/rc/vee-gull/20120719_0041.jpg I temporarily clamped the sections together, carefully checked the alignment & dihedral, then glued in the wing tube... http://www.scipie.com/rc/vee-gull/20120719_0034.jpg I'll let this thoroughly cure over night. Tomorrow I should be able to get the left side done too. A frustrating build session but even with today's major setback, I am at least still making progress. I have no clue how that got messed up. Fingers crossed that'll be the last unsolved mystery for this project. |
RE: Sailplane Wing Design Round II
Things went more smoothly today. Glued up the other side's wing-rod...
http://www.scipie.com/rc/vee-gull/20120720_0005.jpg Its important that I match precisely the dihedral for both wing halves. My CAD plans called for -9° but this being the real world, that's not what I ended up with exactly. I learnt long ago that its usually best to take real world measurements rather than assume what you've built is accurate. I marked up my workbench so I could set the dihedral to what I measured off the other wing... http://www.scipie.com/rc/vee-gull/20120720_0003.jpg So unfortunately, with that needing to cure for a good long time, I'm done for the day. Will leave this be overnight... http://www.scipie.com/rc/vee-gull/20120720_0002.jpg Time instead to go edit some video for my YouTube NoddRC show. Will have more tomorrow. |
RE: Sailplane Wing Design Round II
Nicely done recovery. You didn't panic.... well likely you didn't panic MUCH :D
Generally I find I tend to call myself a lot of rather not nice names for about 2 minutes. Then I sigh and figure out how to fix it. |
RE: Sailplane Wing Design Round II
I think I know what I did wrong. When I installed the brass tube I placed one end through the hole in the end rib & the other end rested up against the top spar-cap. When I did the other side, apparently I rested the end of the tube against the lower spar-cap instead. All I knew was as long as its against the spar-cap, its going to be right... ehh wrong!
Of course if I'd used your full length brass tube method & cut the tube after it was installed, I wouldn't have had this issue. Anyway its fixed now & I learned yet another lesson. I won't be making that mistake again. |
RE: Sailplane Wing Design Round II
ORIGINAL: Nodd ....Of course if I'd used your full length brass tube method & cut the tube after it was installed, I wouldn't have had this issue. Anyway its fixed now & I learned yet another lesson. I won't be making that mistake again. AH HA ! ! ! ! ! ! I WIN! Is it a good time to say "I told ya so" ? :D .... sorry, I don't know what came over me to gloat like this and make fun of your trials and tribulations..... well, actually maybe I do..... :D I'm only making light of it because you fixed it up so neatly already. So it's time for the good natured smack talk... :D |
RE: Sailplane Wing Design Round II
This is way to late and someone in the thread may have said this before...but it also may help with future projects.
I studied stress/strain in composites as my Masters specialty and as a postgrad at Uni. I specialised in adhesive and composite fracture using FEA which is a way of computer modelling structures and designs to analyse their behaviour. This was also my first job so I *obviously* abused it to the max by using the £10000s equipment to test my wing and fuz designs. You were right the reason your wing failed was because of the carbon spar - but maybe not in the way you think. Most failures of RC wings built with sound materials start because ofan initial shear fracture that delaminates bonded components - wood bonded to wood or wood bonded to something else.It occursin the adhesive and then causes massive localstress whichfractures the component locally or maybe somewhere completely different because of transferred stress. The worstcase is when dissimilar materials are bonded together. Carbon is v. stiff compared to wood for example.Hence your wings failed because of the carbon and the way you designed it in, without it they would probablyhave been fine.The carbon was short and wouldn't twist as much as wood so it just transferred the shear stress to it's ends and 'bang' wing failure (see below). Also what adhesive did you use as this is very important. Dissimilar materials (i.e. material properties - stiff joined to bendy etc.) should be joined with a rubberised adhesive. Hard adhesives like untreated cyno or expoxy are bad and make things worse. Adhesives like rubberised epoxy and some of the "plastic" bonds are ine. The other thing that won't have helped is your spar webbing as it was not built into the skin and asymettrical so would have introduced extra shear forces(again see below). The final thing was the location of your servo hole at the same point as the carbonspar ended. Remember servos do form part of the structure and are very stiff in torsion compared to wood. http://www.jamesandtracy.co.uk/temp/sparstress.png If you're going to build carbon into wings scarf it (as in the diagram above)in or better yet use it as part of the skin. Despite all my attempts to come up with a more modern structural design using FEA, nothing beats the structure below (from the 50s I think!) for stiffness and lightness in trad built up structures. All the stresses are transferred to the skin which forms a box even in theopen sections. The webbing can now be asymettrical and not introduce adverse shear effects (see below). This structure outperformed all the others I tested both in the lab and on the computer - it changed the way I built my wings for LEG and I have never had a failure even on a tow line. The spar by the way is spruce or carbon thickness to suit loads, but with a skin of light balsa of equal thickness abutting it to form the box. http://www.jamesandtracy.co.uk/temp/...edskinwing.png You should be fine with your longer spar as the wing joiners will carry the stress onwards - but hopefully the above might help in future builds....or not ;) |
RE: Sailplane Wing Design Round II
I like your wing design, looks like it could be made nice & light. I'm a little leery however of the way the sheeting butts up against the spar-caps. How do you insure a gap free joint & what type of adhesive do you suggest there? My first wing employed butt joined sheeting & it was a disaster. Very hard to achieve a gap-less joint & even the areas that were accurately cut seemed somewhat weak with so little surface area between the two components. For my second wing I overlapped the sheeting which resolved that issue.
I'm also not sure how your one sided asymmetrical webbing works. Your design is more or less identical to my first failed wing with the exception of the shear webbing mounted on the front of the spar instead of the rear. Everything I've learned since my first design suggests that is bad news. You mentioned the sheeting but what supports the aft area of the spar-caps? Its obvious you're knowledgeable in this area & I'm always open to new or in this case, old ideas. Those two points kinda conflict with what others have suggested though. Would love to hear more. |
RE: Sailplane Wing Design Round II
I can see why you have doubts - it does seem counter intuitive. I should also say that you shouldn't build every wing for every application like this as whilst it would work in theory, practically you are limited by skin thickness as this must mach your spar thickness. Hencethis design is perfect for thermal sailplanes up to a certain size (~1.5m) if you're winch launching, all electric thermal sailplanes, light electric sports models. Basically it's perfect for LEG where weight is critical.
This is all about shear stress and the efficient transferral of stress to the wing skin. The best analogy is the way classic and then modern cars were designed and built. Your design is close the the classic car design. Classic cars had a seperate chassis to carry the stress. Stresscouldnot be transferred efficiently into the body shell - hence the designs worked, but had to be heavier as the body need to be strong enough to work as a body and the chassis had to provide the structural strength of the car. The chassis was mounted to the body via moving rubberised joints to prevent it shearing due to fatigue from the body during use. This was fine but handling suffered and the whole assembly wasn't rigid and cars were heavy. Roll on to modernmonocoque frames. Now the body and chassis are built as one item. The strength is provided by the body shell - the whole piece is designed to efficiently distribute stress across the whole bodyshell. This is the same principle that prevents you from crushing an egg in your hand. Now you can have a much lighter car and it's very rigid as there are no real adverse shear effects in play. This isthe principle ofthe wing design from the 50s I posted. It is also the basis for pretty much all high performance hollow moulded and foam composite wings. It's just the balsa bashing fraternity like me never caught on! You ask why your design isn't the same as the one I posted with respect to adverse shear effects and moments. You're right in that there is still some shear in the 50s design, but it is far less than yours as it transferrs it's stress to the wing skin efficiently by compression and tensile effects. This is because the spar is part of the wing skin. It is a monocoque design not a classic car design with a seperate chassis (see scribble below). Adhesives are particularly poor in shear, and by placing the spar below the skin in your design you're transferring all the stress to the skin via shear through the adhesive. (edit - just noticed that some of my arrows in the top diagram in the scribble below are going the wrong way, but you should still get the principle - transfer loads with compressive/tensile stress not shear stress) http://www.jamesandtracy.co.uk/temp/skinstresses.png You ask about what is the best adhesive. This is not a simple answer I'm afraid as it depends on the structure and materials that need to be bonded. However, as a rule of thumb use adhesives with rubberisers in shear stress locations if the materials are dissimilar. Dissimilar materials sort of act as stress concentrators so you want to graduate the change in material properties (i.e. hard to soft, stiff to bendy) as much as possible as this will allow a gradual change in stress and distribute the stress field.Hence in simplicity to give you the idea: carbon to balsa, you would want an adhesive that was stiffer than balsa butbendier thancarbon. This would distribute the stress/strain field as evenly as possible across the joint. The reason why penatrative cyno works well for balsa to balsa is that it peneratrates the balsa and gradually turns it harder as you progress across the joint. Similarly un-rubberisedthick cyno tends to lead to poor properties as it does not penetrate the balsa well so you end up with a soft-hard-soft sharp transition. Finally you ask how you construct the wing without leaving gaps. Very perceptive mate, gaps would be bad! However, it is a farely simple process as long as you do the steps in order. basically, lay the spars andrear leadng edge stripfirst, then the front top skin (cut oversizre then use the LE as a guide for trimming to fit). Next cut all your cap strips to length and fit the upper ones. Then use the cap strips as a guide to fitting the toptrailing edge skin (again cut oversize and trim to fit eg. sand on a flat big board). Now proceed to the bottom of the wing and repeat the above for the bottom LE and TE skins. You should have a perfect fit all round andit's very easy to do. Finally harden therear TE with penetrating cyno. hey presto - monocoque wing. See the scribble below for the top wing sequence. http://www.jamesandtracy.co.uk/temp/build.png The whole thing sort of looks like the scribble below once finished. You end up with two boxes even in partially sheeted areas of the wing: The front D-Box and the rear V-Box (trailing edge). Think of these two boxes as your wings main load bearing memebers (or spars if you prefer). The back-front or fore-aft loads are carried by the capping strips in open sections which form an 'Open' box and keep the front D-Box and V-box in position. Sorry, I don't have a great diagram, but see below. I hope this kind of explains why the whole thing is so strong and doesn't collapse? http://www.jamesandtracy.co.uk/temp/finished.png If you're worried that you can't do but sheering strongly then there alternative is to use offset shear webbing for and aft of the spars - this will only increase weight slightly, but keep the spars in the skin or you lose the monocoque and the strength. However, you should be able to achieve a good butt joint as it really carries no vertical load - only compressive, tensile and some shear as the wing bends. To do this glue the butt joint first and obtain a strong joint - take care and time. Once you have the butt joint in place and cured, bend the sheeting down onto the ribs and glue with penetrating cyno. Repeat for the bottom and then seal the box with shear webbing AFT of the spars instead of in front. When I started off this was what I did and as I improved I now use contact adhesive and rubberised cyno and fit the shear sheeting in front of the spars.<br type="_moz" /> |
RE: Sailplane Wing Design Round II
Interesting stuff, thanks for clarifying. Back to the build...
Busy week, sorry its been a few days since my last update, progress was made nevertheless. First order of business was to finish cleaning up last week's oops. In preparation for re-sheeting the end of the wing, I created a lip for it to rest on... http://www.scipie.com/rc/vee-gull/20120721_0002.jpg Installed the ridge... http://www.scipie.com/rc/vee-gull/20120721_0003.jpg Added new sheeting... http://www.scipie.com/rc/vee-gull/20120721_0006.jpg To celebrate getting that done I took the wing outside so I could stand back & have a good look... http://www.scipie.com/rc/vee-gull/20120721_0013.jpg Happy with that I headed back to the workshop & got down to sheeting the outer panels... http://www.scipie.com/rc/vee-gull/20120724_0002.jpg More Wing-rods To keep the outer panels from turning on the main wing-rods I needed to add a second smaller rod... http://www.scipie.com/rc/vee-gull/20120726_0002.jpg BMatthews you'll like this part... To keep things aligned, a brass guide tube was installed still in one piece. The idea here is once glued in, I'd use a hacksaw to cut the tube. Small shims were temporarily taped to the end ribs to provide a gap for the saw blade... http://www.scipie.com/rc/vee-gull/20120726_0012.jpg I should have done things this way for the main wing-rods too, would have avoided the misalignment issue for sure. Anyway it was cool to try this technique for these rods at least. While the glue setup I added gussets here & there to help reinforce things a bit. These rods shouldn't shoulder too much stress though so the construction can be fairly lightweight... http://www.scipie.com/rc/vee-gull/20120726_0013.jpg Once again I'll leave things to setup overnight... http://www.scipie.com/rc/vee-gull/20120726_0015.jpg |
RE: Sailplane Wing Design Round II
This looks scarier than it is.
Time to cut through the brass wing-rod tube... http://www.scipie.com/rc/vee-gull/20120727_0004.jpg As I was hoping, both wing-rods aligned perfectly, sweet!.. http://www.scipie.com/rc/vee-gull/20120727_0008.jpg Sanding time. I usually dread hand sanding the leading edge, get it wrong & you've got real problems. As with last time, things went well... http://www.scipie.com/rc/vee-gull/20120727_0019.jpg A little more sanding with the wing assembled to make everything match up nicely... http://www.scipie.com/rc/vee-gull/20120727_0010.jpg And then I chopped bloody great holes in the wing. In keeping with my stronger/lighter aspirations I decided there was some further lightning that could be done. A series of holes were cut into the bottom sheeting... http://www.scipie.com/rc/vee-gull/20120727_0022.jpg I know what you're going to say, "that's messing up the "D box" & you're correct. Still though I'm only doing this to the outer sections of the wing where stresses should be less. I would have liked to do this to the top sheeting too but I didn't want to compromise the airfoil by hacking holes in its curviest part... http://www.scipie.com/rc/vee-gull/20120727_0025.jpg Its possible my fiber-glassing skills are improving. I discovered the wonders of thinning the mixture so its not so gooey. Dabbing off the excess is something new to me also. Masking the area off so I get a nice clean edge should help with the aesthetics. Oh yeah & spraying the wood with contact cement first to hold the glass in place, ahh wonderful stuff. Learning new skills, gotta love it... http://www.scipie.com/rc/vee-gull/20120727_0030.jpg |
RE: Sailplane Wing Design Round II
Some more center section fiber-glassing...
http://www.scipie.com/rc/vee-gull/20120728_0003.jpg Control Surfaces My last wing had sheeted flaps & ailerons. I'm hoping to save some weight this time around by making built up control surfaces. The down side of this is lots & lots of very small riblets... http://www.scipie.com/rc/vee-gull/20120728_0033.jpg An hour or so later I have a set of trailing edge riblets ready to go... http://www.scipie.com/rc/vee-gull/20120728_0036.jpg Tomorrow I hope to put these to use making my new ailerons & flaps. |
RE: Sailplane Wing Design Round II
Razor Sharp
I spoke with the RC guru at my LHS about methods for creating a really nice sharp trailing edge. He suggested using some insanely thin 1/32" plywood, said that's how he does it. So taking his advice, here's what I'm planning... http://www.scipie.com/rc/vee-gull/3D...ing_II_004.jpg A sheet of this fancy 1/32" ply long enough to span my wing sections was going to cost a small fortune. The hobby store guy sold me a smaller piece instead & suggested I create my TE by gluing strips together using angled joints. It was going to be laminated to 1/8" balsa anyway so combined it should be fairly strong. Here's how that looked... http://www.scipie.com/rc/vee-gull/20120729_0004.jpg No I'm not getting bored with this project but reruns of Star Trek Voyager playing on my Kindle Fire does help pass the time. I never tire of looking at Seven of Nine even if she is a Borg... http://www.scipie.com/rc/vee-gull/20120729_0002.jpg The left flap... http://www.scipie.com/rc/vee-gull/20120729_0006.jpg Seeing how it looks in-place... http://www.scipie.com/rc/vee-gull/20120729_0016.jpg Here's the ailerons... http://www.scipie.com/rc/vee-gull/20120729_0008.jpg The control surfaces look to be a tad flimsy but I have some reinforcing in mind. Will get to that tomorrow... http://www.scipie.com/rc/vee-gull/20120729_0018.jpg |
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