Beibitoi

put a little weight on the trailing edge near center piece, the leading edge will be lifted like this.

i also have a handheld steamer. just in case that's gonna help.

TomCrump

If it were my wing, I'd cut the offending pieces loose, and re-glue them, making sure that the wing was straight. I'd use weights to hold the wing flat.

Beibitoi

thanks tom. what could be the offending pieces here?

mgnostic

Let's back up a little bit. A bent or warped structure is like a puzzle.Straightening a wing will take some work so lets make sure it isn't wasted. Is your building flat and rigid. I've had building surfaces that looked flat until you piled the kit and building supplies on then and then they bowed in the middle. One of the best disassembly tools I have found is a single edge razor blade. They are cheap and usually come in a box of 100. To respond to your question to Tom Crump; it depends. you will have to go through and see which joints are holding parts of the wing in tension and inducing the warp. You don't have to replace the cap strips just cut straight through them with the razor blade. Basically you are going to cut through glue joints until the wing will easily lay flat and then with the wing pinned and weighted down glue them all back together with CA glue. It is most likely that the warp was induced when you were putting on the upper leading edge sheeting. If you do try to steam the wing, not really recomended. Be sure it is completely dry before you try to move it.

Beibitoi

while deciding on what to do with the wing warp, I started doing the fuel tank..

upon looking at the finished product, it seems zip tie use was a little overkill.

how do you suggest I attach the tank to the board?

Beibitoi

ugh. just few minutes before your post, i started steaming and wetting the sheet upon being advised by a fellow club member. will wait after it dries before doing what you suggested. hopefully though, just the steaming would work out. had it steamed twice now. from half inch, i had it reduced to 1/4 inch just after the first steaming.

the table actually looks well levelled and flat. i had noticed as well that before I installed the sheets (only ribs and spars glued), everything was flat. it seems you guys were right when you said it must be the top sheet on the leading edge.

​​​​​​by the way, does the ammonia method work?

TomCrump

I suggest that you follow mgnostic's advice. To me, it looks like the warp is built in. I don't think that steaming will permanently solve your problem.

mgnostic

The ammonia method is good for relaxing balsa so that it doesn't crack while you bend it to shape against a structure. For instance curving the upper wing sheeting along the grain. Thin balsa sheet has a tendency to crack along the grain when you bend it across the wing ribs. Ammonia helps to relax the wood. Any moisture will do this to some degree. Even saliva will do in a pinch. It looks like the most likely thing is that some tension was introduced into the wing structure during construction. In my own experience this happens most often when trying to get to wing sheeting to conform to the upper surface. Since you have tried steaming I would suggest that you leave it pinned/weighted down on a flat surface for at least 24 hours. Full scale aircraft builders will often leave steam bent parts to dry in a jig for 48 hours or more. Steaming pushes a lot of moisture into the wood and then it gradually migrates back out. If you allow it to move around the warp may creep back in.
As long as they aren't snagging on anything a few zip ties aren't going to hurt anything. When installing a fuel tank I have seen everything from tanks flopping around loose to tanks that were solidly glued to the airframe. Best practice involves isolating the tank from airframe vibration with a bit of firm foam. Installation varies from airplane to airplane. Often if the tank is a close fit in the fuselage it is possible to wedge the tank in with nothing more that some reasonably firm foam. Since you mention fastening the tank to a board your best option may be wrapping the tank in some foam rubber and then anchoring it with zip ties. I've also seen velcro ties used and even anchoring the tank in its own box inside the fuselage. It is important that the tank should not be able to move around in the fuselage. It is a bad thing when a shifting fuel tank changes the balance of the airplane or it moves enough to kink or pull loose from the fuel lines.

buzzard bait

I have a method for getting rid of wing warps that worked on a Jr. Kadet wind that had D tube construction. This was a model bought from someone. Twisting with a heat gun (two man job) worked for awhile, but the warp crept back in, just as someone said above.

I stripped off the covering, filled a big bucket with water, and stuck the wing in it. Came back awhile later, pinned the wing to a building board with about an 1/8inch of counter twist. Let it dry over night. Next day, it sat pretty flat. Then I used 1/8th inch square sticks criss-crossed between each rib from the main spars to the TE. Used CA to glue them in, while it was pinned down flat. Re-covered, and that wing is FLAT and RIGID!

Jim

Beibitoi

thanks for all the info you shared. have tried redoing the steaming then drying and have finally reduced the amount of warp. as of now, the trailing edge near center piece is just about 5mm above coming from 15 to 20mm i think. I guess I'll have to redo the process until the wing comes flat. will save the disassembling method as my last resort. but, just how much is the acceptable amount of warp can I have?

mgnostic

Theoretically, acceptable warp is zero. Practically, if you have the same amount of wash out on both wings that can be a good thing. Usually wings with washout are built with tabs on the ribs or some sort of jig to ensure that the amount of washout is symmetrical across both wings. A cub will tolerate some things that a faster, heavier model won't. Just for clarity, "washout" means that the trailing edge at the wing tip is higher than the leading edge. I have seen some very out of trim airplanes being flown. It can be done but it isn't pretty. If the wing is mostly flat, you can dial in enough aileron deflection to make it flyable but ... in doing so you will lose some of the qualities that make the Cub a pleasant flying machine. It increases drag on one side of the airplane. The compensation will effect the airplane differently at different airspeeds and one wing will stall before the other, contributing to unintended spins. It sounds like you are reaching a point where you say to yourself "good enough" and move ahead with the build. Just note that with a few odd exceptions, a symmetrical airplane is a happy airplane. In the worst case, if when the airplane is completed, it cannot be trimmed to fly in a satisfactory manner, you still have some options. 1. treat the warped wing like crash damage, and repair it via dissasembly and new parts. 2. Go to the local craft store/hobby shop and buy enough wood to scratch build a new wing. The plans will probably show enough information to do this. 3. Go to the Sig Mfg web site and contact customer service. I haven't dealt with Sig specifically but some of the better kit companies will sell the die cut parts for a specific sub assembly.
I just went and looked at the Sig website and they do sell a replacement wing kit for the 1/4 scale cub. So if it turns out that your repair efforts aren't satisfactory you can try again.
This is just my opinion on the subject so your results may vary and other people may have better suggestions.

buzzard bait

Nobody believes me. Well, that's OK, we're anonymous to each other so that's reasonable.

But doesn't it make sense that steaming will not soak the wood as thoroughly as total immersion in water will? Of course, you have to be sure the glue is not water soluble, but that's true of steaming too. Completely soaking the wood and allowing to dry pinned down with a counter twist gets rid of a warp. Cross pieces will lock in the flat surface.

Actually full scale Cubs did have some washout, so I would do that on a model too. The cross pieces will lock it in. As I understand the problem, it is wash-IN, because the trailing edge is higher near the center of the wing. If it were higher at the tip, that would be wash-OUT and you could just put the same warp into the other wing and be done with it.

Certainly before going to all the trouble of building a new wing, try this. What is there to lose?

Jim

mgnostic

You know Jim, as long as you pinned/weighted everything down good, softening the glue might not be a bad thing as it could allow the structure to shift into alignment. One would just want to be sure to go back and reinforce all- every last one- of the glue joints with a shot of CA glue after it was dry.

Beibitoi

thanks Matt and Jim. Will then strive for that zero warp. I could have already started with the unsheeting but being a beginner with a little knowledge about balsa plane construction, I'm a little hesitant to do it.

the steaming seems to be working. just dont know if it will be permanent. if it doesnt work out, will have to submerge it in water as you advised. if not, will resort to the more invasive method of removing the sheet.

just a thought: wil cutting or making slits on the top sheet between the ribs help? once flat, will seal that slit with the foaming gorilla glue or anything that could seal those slits. something like this.

Beibitoi

here's what the wing looked like when I got home.

mgnostic

I would suggest popping the shear webs off of the rear spars and regluing them before I cut the the top sheeting. It will be easier and will have less impact on the appearance of the wing. Think of the rear spars as a parallelogram with the webs holding the sides in place. Right now the rear spars are not completely parallel with the front spars. Removing the webs will allow you to adjust that parallelogram and regluing them will lock it into position. All that being said, it looks like you have made a lot of progress. A less drastic measure would be to over correct the warp just a little bit, 2 or 3 mm and leave it to dry for another day. If you can get the bare wing to within a couple of millimeters of being flat then you probably can correct the rest of the warp by shrinking the covering and adjusting the wing struts. As an aside, what will you be covering the model with? Also as you store the wing until you get to the point of doing the covering be sure to place the wing so that it can lay completely flat or even compensate for the warp by a millimeter or two.

Beibitoi

I have finally removed the warp on the trailing edge. fingers crossed, it's going to permanently hold. BUT.... here's another thing I noticed just now. take note on the leading edge bottom spar. on the middle part of the wing, it seems the spar is lifted by more or less 3mm. same with the upper sheet when I invert the wing.


might have happened when I corrected the warp? what to do now? hopefully this wouldnt happen with the other wing. sorry guys these dont seem to be the orgasmic photos some of you expected. haha

​​​​​

Beibitoi

i know what my mistake was. i separately sanded W1s, W2s, W3s, and so on. now, i call that stupid. that to do now with the other wing? correctly sand the ribs? or leave it as it is now for the left wing to be even with the right wing?

Beibitoi

I have finally removed the warp on the trailing edge. fingers crossed, it's going to permanently hold. BUT.... here's another thing I noticed just now. take note on the leading edge bottom spar. on the middle part of the wing, it seems the spar is lifted by more or less 3mm. same with the upper sheet when I invert the wing.


might have happened when I corrected the warp? what to do now? hopefully this wouldnt happen with the other wing. sorry guys these dont seem to be the orgasmic photos some of you expected. haha

​​​​​

Beibitoi

my apologies for the duplicate post. dont know how to delete it.

mgnostic

Double posts happen, it's no big deal. More importantly, good job on dealing with the warp. Persistence pays off. It sounds like you are describing that the wing, as viewed from the front is pinched in the middle. Is this correct? If this is correct, it is a lot less serious than a warp. My understanding is that you are building the 105 inch (2667mm) span 1/4 scale Cub. When deciding what to do there are two questions to ask first. 1. Does it hurt the way the airplane flies? 2. Does it hurt the way the airplane looks. Question one also includes a further question, does it impair the strength of the airplane. In this case if you carry on according to the plans the strength of the airplane will be unimpaired. To address the question of flyability, what the "pinch" will do is slightly thin out the airfoil. A slightly thinner airfoil will fly differently from the stock airfoil but, and this is an important point, across a 2.6 meter wing it probably won't make much of a difference. If you were flying precision aerobatics or racing then millimeters do make a difference. The Cub is a relatively slow flying, tolerant airframe. A straight airplane flies better, I'm just saying that there are places where you can get away with things not being quite to specification. This brings us to the second question, does it look okay? As a builder you will notice things during the build that no one else will know about unless you tell them. I would suggest the following steps. 1. Finish building the wing you are working on. 2. Build the other wing a accurately and close to plan as you can. It will be better to try to correct the first wing than to attempt to duplicate your errors. 3. Complete the kit to the point of being able to mount the wings to the fuselage including the wing struts but don't proceed to covering yet. 4. Mount and adjust the wings as you would to fly the aircraft. 5. Ask a fellow flier, preferably one who has not been following the build, to come look at the airplane from 2 meters away.If he can't spot the problem then you are probably okay.
All that being said you may want to fix it anyway. If you don't have one, go get a long sanding bar. 18 inches (450 mm) is a good length. These are very useful tools, inexpensive and you will wonder how you lived without it. Finish both wings. You will be comparing the first wing to the second. Go to where ever you get your hobby supplies and get the same sort of balsa wood that you used for the cap strips. 1.5 mm thickness is what you will want. Cut a new set of cap strips for the ribs. These will glue on top of the original cap strips, only on the places where the wing is undersized. You can then sand down the new material down so that it matches the adjacent ribs. You may be tempted to use filler. Avoid this where possible since it adds weight. When you do use filler, use the lightest possible. In the US we have a product called "drywall spackle" it is basically a plaster based filler that is spread on walls to smooth them for painting. It may be called something different where you live. There is a version that is sold as being lightweight and it does provide a considerable savings in weight. it is also easier to sand. After you have made your changes and repairs you will want to balance the airplane from side to side. This is another thing that will have less impact on a stable airplane like the Cub but it is a best practice. Sanding requires some care as it is easy to chase high spots until you have completely sanded away the piece of wood you were working on.

Beibitoi

Matt, seriously, if ever you're going to write a book about aircraft modelling/building, tell me, I'll be the first to buy. thanks for all the help.

I still have few spare capstrips here but dont really know what to do with it. just to clarify, the problem is on the leading edge sheeting.

notice the arrows on the pictures. the pointed gaps between the sheets and the table are not even.

Beibitoi

ok. so now I know how uneven the lengths of the right wing ribs are. as earlier said, my mistake was grouping w1s w2s and w3s separately then sanded them together so right w1=left w1, and so on. i was lacking some sleep when I did that I think.

so, here's what the right wing ribs look like. too uneven. not sure how to correct it or if it'll need correction at all. attached are pics of uncorrected and corrected wing ribs. notice that the top and bottom of the ribs are even though

mgnostic

I'm glad to help where I can. Before I retired one of my work tasks was managing the operations manual for my little chunk of a Texas state psychiatric hospital. Once I get started typing it is hard to quit..
With regard to sanding ribs, stacking them on some spar material as you have done in the pictures is the way to go. Spar alignment is the most important, then the leading edge. If I interpret the pictures correctly, the trailing edge is formed by the top and bottom sheeting that sandwich the rib between them so the aft tip of the rib isn't so critical so long as it doesn't distort the way that the sheeting lays. The leading edge, when completed, should have enough material for sanding to shape. A long sanding block will allow you to bring it to a straight line.

Beibitoi

keep on typing Matt, you're helping a lot here.

I have finally started with the left wing. double checked the table and it is indeed flat. I ensured the spars and ribs were straight with 0 gap on any part. when unpinned though, some parts lifted to more or less 1mm. not sure how it happened but most likely, the spars are a little bent. not much that it could cause a warp though. hopefully, once all the components are glued, it would come up close to perfect.

I'm suddenly becoming too amazed with those who build their own 3D and pattern planes. more so with those who work at the manufacturers of ARF planes.