Hey folks,

This is a continuation of my Scratch Designed/Built Sailplane build log. My design flew beautifully but...



...unfortunately my wing could use a rethink. Basically it folded in half mid-flight on the fourth or fifth flight...



The wing was of built-up balsa & spruce construction with carbon fiber reinforcing here & there...




Thankfully the damage was limited to the wing, not sure how the fuselage survived...



So my plan is to design & build a new stronger wing. This thread will document that process.

So to get started here's a fun YouTube video featuring the old wing...

YouTube Video - Wing Failure


I'll post details about my new design shortly & will give you folks an opportunity to see if I'm headed in the right direction. Thanks.

That's a shame, Nodd. Looking at the pictures I would think that a good first step would be rotating your main spars 90 degrees or going to something like 5/16 to 3/8 square. The way that your spars are oriented gives you no more strength than a 1/8 inch spar.

hey nodd

Vertical grain or 45 degree crossed shear webs epoxied between the spar caps( spruce or cf sticks) and kevlar or CF tow wrap will dramatically enhance your otherwise delightful design. It doesn't add a lot of weight and the strength increase is truly dramatic.

Remember, that is what prototypes are for...learning and improvement. From your video it looks like a new center gull wing panel is all you need. That should be a fairly quick rebuild.

Don't give up! You are so close to complete success.

Total encouragement -rjf

What ray said. I would still go for more vertical thickness of the spars but sheer webs are a must.

Just so we're all on the same page, here's a few amazing articles about shear loads, spars & wing design...

Shear Loads (look at pages 20 - 21)
http://www.rcsoaringdigest.com/pdfs/...SD-2005-04.pdf
http://www.rcsoaringdigest.com/pdfs/...SD-2005-05.pdf

Compression Load at Spar or Joiner Bend (look at pages 20 - 21)
http://www.rcsoaringdigest.com/pdfs/...SD-2005-06.pdf

I found those in this very informative thread, well worth a read too...

http://www.rcgroups.com/forums/showt...00#post6164407

That's just some of what I've been reading recently. Based on my new found understanding of wing spar science I'll have a few nifty design concepts posted shortly.

hi there from Toledo

Check out the builds of the Olympic III, the Houston Hawk, and the Bubble Dancer for real good designs and spar assembly techniques right here on RCG. They are a remarkable resource and at your finger tips.

ciao -rjf

The Old Wing


The New Wing


Okay so the obvious difference here is the 12mm x 2mm CF spar caps, no more wimpy spruce. After looking at a load of shear web options I've chosen to go with full spar width, vertical gain, balsa shear webbing. The CF & webbing will be epoxied together & then wrapped with Kevlar ribbon (not shown). That should produce an exceedingly strong spar.

I've eliminated the sheeting aft of the spar as I don't think its necessary, will save some weight there. Will probably fully sheet the first few bays around the center though. Speaking of the sheeting, it now overlaps the spar caps instead of butting up to them, should provide a much larger bonding surface. The holes in the ribs are for the servo wires. Lastly I doubled up the thickness of the TE material that the flaps/ailerons hinge against. The old 1/8" TE scolloped after I shrank the covering.

I'm looking into wing joinery options, have a couple of ideas in mind. Anyway that's the plan for now. Questions or suggestions? Please fire away.

I'm still a little doubtful about the total vertical thickness of the spars. I would think you would be better served by using, let's say, 4X6 spars set vertically. Another option would be to use the top spar as is but form an inverted "L" with a 4X6 or even a 4X4 or 2X6 bottom spar.

I'm not sure where you're getting this vertical Vs horizontal thing from but built-up wooden wings generally have the spar caps installed horizontally. A spar of this type gets its strength not from the vertical thickness of its spar caps but from the compressive force acting on the top spar & the tensile force trying to stretch the bottom spar. The shear webbing between them keeps the two from sliding past each other & thus keeps the wing rigid.

If this was a foam wing & you were adding just one stiffening spar then yes, mounting it vertically might be the way to go. This system is different from that though.

Originally Posted by ray foley

hi there from Toledo

Check out the builds of the Olympic III, the Houston Hawk, and the Bubble Dancer for real good designs and spar assembly techniques right here on RCG. They are a remarkable resource and at your finger tips.

ciao -rjf

Much obliged Ray. Yeah my current design is heavily influenced by the BubbleDancer. I'll check out the other two, thanks.

Here's a picture I stole of a BubbleDancer's spar wrapped in Kevlar, using full width shear webbing. Hopefully that's a little clearer than my diagram...

hi there from Toledo

Dr Drela designed his spars for serious gorilla full pedal ping launches from a 12V winch. For epower this fully wrapped spar may be overkill but still economical and seriously strong. For the gull wing panel, go for it. The tip panels are probably fine but shear webs "couldn't hoit".

again, total encouragement -rjf

The first thread is good and although different building techniques it is well worth the read.
Some very great minds sharing with us mere mortals.

Post #6 may pertain to this somewhat.
http://www.rcgroups.com/forums/showthread.php?t=815049

PS. I like building wings with foam so I am somewhat more educated in layups and this is one of my favorite testing threads too; by Phil Barnes.
http://www.rcgroups.com/forums/showthread.php?t=323741

These will take some time to fully understand, well for me anyway.

Curtis

My point, Nodd, is that the weakest and most flexible axis is also the axis of greatest stress. Even with CF 2mm just strikes me as being a possible concern. I would worry that the flex of the spars might exceed the strength and flexibility of the supporting structure.

Many many years ago I built a glider with 1/8X1/2 spruce spars and, while it didn't break, it sure did flap; especially on the hi-start! I built another wing with 1/4X3/8 spars and it was much, much, more rigid.

Great looking plane and excellent CAD and Sim work! Damn!

Your spruce spars may have been strong enough but your shear web was very poor. With only one web glued to the back of the caps the C-channel shaped spar is asymmetrically loaded so the caps will twist and then easily buckle. A somewhat better design is front/rear webs to form a complete box beam, but even better (and lighter) is a center web to form an I-beam. Kevlar wrapping is not usually very helpful in this design other than around the joiner sections.

Of course carbon caps can be far stronger and lighter. If you go this route you can use much thinner material and it should be lighter. You don't need a solid balsa core though, just weakly bond thin carbon caps to a lightweight blue/pink styrofoam core and wrap a single thin layer of fiberglass or carbon around the whole thing with a +/-45 fiber orientation. That Bubble Dancer spar can handle the tightest loops you've ever seen at 100mph, you don't need anything even close to that. The problem with a composite spar like this is that you can't put ribs thru it.

A good compromise may be to use the same spruce spar caps, with centered webbing, no wrapping, and just a paper-thin (.005") strip of carbon bonded to the outer faces of the spruce caps. Spruce will generally fail in tension and carbon generally in compression/buckling, so a hybrid spar can be very effective.

Originally Posted by ray foley

...Dr Drela designed his spars for serious gorilla full pedal ping launches from a 12V winch. For epower this fully wrapped spar may be overkill but still economical and seriously strong...

Yes his simple "bullet proof" design is what attracted me to it. After watching the old wing fold I'm understandably leaning towards the overkill approach.

Originally Posted by CloudyIFR

...Some very great minds sharing with us mere mortals...

Thanks Curtis for the links. There's a lot of info there to process, knowledge is good.

Originally Posted by peterlngh

...Even with CF 2mm just strikes me as being a possible concern...

I appreciate your input Peter, my problem is that it contradicts what everyone else is saying. For example Steve over on the Wattflyer forums had this to say:

"2mm x 12mm is massively over the top for the spars. The bubble dancer is 3m span and the wing is stressed to take 150lb load when on tow.. And it only uses a 12mmx1.5mm spar and that's only for the centre section, the outer panels go down to 6mmx0.375mm thick

For your model that doesn't require to take the huge loads of being towed then 12mm x 1mm on the centre and 6mm x 1mm deep in the outer panels would be still well in excess if what's really needed."

Vespa here on RC Groups went further & suggested:

"...paper-thin (.005") strip of carbon bonded to the outer faces of the spruce caps..."

Please don't get me wrong though, I totally appreciate your suggestions & agree that more "vertical meat" in the spar could be one route to take.

Originally Posted by vespa

Your spruce spars may have been strong enough but your shear web was very poor. With only one web glued to the back of the caps the C-channel shaped spar is asymmetrically loaded so the caps will twist and then easily buckle.

A guy over at RC Universe pointed out the same problem with my original C shaped spar. I agree a fore & aft webbed box design or a standard I-beam spar would have been better. Anyway that was the old wing, moving on with the new design...

Originally Posted by vespa

...A good compromise may be to use the same spruce spar caps, with centered webbing, no wrapping, and just a paper-thin (.005") strip of carbon bonded to the outer faces of the spruce caps...

I was impressed with the CF "tape" I applied to my center section spruce spars. Hybrid CF/spruce cap strips make a lot of sense. As I understand it though, while CF is better in tension than compression, CF of the same thickness as spruce alone is superior in both tension & compression. Seems simpler to ditch the spruce & just go with thicker CF. Still I can foresee applications were hybrid CF/wood might be the way to go.

Thanks again everyone for their input, much appreciated.

Spar Confusion
It took me a little while to realize there's two ways to go about building my new spar...

Continuous Webbing - Split Ribs (top cap strip not shown)

In this technique, the spar is completed first using a strip of continus shear web material. This is ideal for constructing spars wrapped with 45° bias fiberglass or various tube/sock wraps. The advantage is the ribs don't get in the way while you're doing the wrapping, they're attached afterwards.

Bay Webbing - Full Ribs (top cap strip not shown)

Using this technique the ribs & webbing are added in alternating order to the bottom cap strip. The advantage here is you can use full ribs, which on non-flat bottomed airfoils like the SD 7037, helps assure things stay aligned. Obviously trying to slip a tube wrap over the spar with all those ribs in the way isn't an option. However you can still wrap it using Kevlar thread or ribbon (tow).

I like both techniques but I think I'm going to go with the second. Seeing as this is a tapered wing, I'll need to construct a jig to accurately cut the shear webs but that sounds like a fun challenge. I'll be doing the same thing for the outer panels but with thinner/narrower CF. Will likely ease up on the Kevlar wrap out there too. Anyway I'm liking the direction this is going.

CF of the same thickness as spruce alone is superior in both tension & compression.

Yes, absolutely and 12mm x 2mm solid carbon is not entirely crazy for a 3m plane so you're likely on a good track but I'm sure 12mm x 1.5mm would be more than adequate, especially if you can wrap a little bit of Kevlar around it -- it doesn't take much. If you have blue foam I would suggest it as the shear web as it is much lighter and easier to work with. Glue it with a light coat of messy polyurethane glue which will foam to fill any gaps so the web need not be accurately shaped. I wouldn't bother with all that glass and external carbon at the center dihedral break, just a tiny strip of glass on the seam and a strong block of carbon internally joining the spar halves. You could use trimmings from your 12x1.5mm carbon to make a 12mm x 10.5mm x 80mm bar to join the halves with a lot of glue and external wrappings.

Foam shear webbing is an interesting possibility. Expanding polyurethane glue, that's cheating!.. I like cheating :-) Will probably stick to what I know though, balsa for the webbing.

I haven't yet put much thought into how best to shore-up the center dihedral joint but some combination of fiberglass, hardwood, CF, kryptonite will be used I'm sure. Lots of glue too :-)

This time its between the spars!



I'm not locked into using an 8mm CF rod but that sounds about right & according to my CAD software, should fit. Currently my plan is for a 7" (17cm) rod length, that will extend one rib bay into each wing panel. The hardwood block will be wrapped in plenty of Kevlar tow, epoxy, fiberglass & anything else I can think of to make sure it stays together. Will probably insert some sort of tubing into the block so that the rod fits consistently (I know humidity will wreak havoc with the size of the hole if I don't line it with something). Well that's the plan for now.

May I suggest ply webbing either side of the spar around the joiner area. One strip 2 bays (tapering over the second) and one 3 bays (tapering over the third bay). It does mean splitting the ribs for those bays but the strength gained is worth it. Bind over the ply webs for max strength. It helps taper the stress point the joiner block creates.

Ply webbing around the joiner sounds like a great idea. I don't mind splitting the ribs in that area I guess. Now when you say taper, do you mean I should actually taper the thickness of the plywood or are you referring to the length, 2 bay length, 3 bay length as the taper? I'm a little leery of sanding down plywood as it gets its strength from its layers running at opposite grains.

Taper the depth of the ply so that it goes from full depth of the section to nothing over one bay. I hope the rather rushed sketch gives you the idea. Should be used with full depth and width webbing between the spars as well.

Ahh now I see what you're saying, I should make a diagonal cut (taper) in the ply after the first bay, got ya. Then I guess for the other side I do the same diagonal cut but this one between the second & third bay. Yeah I like that, sounds like a plan. I'll see what the folks on the other forums think. Thanks for taking the time to illustrate the idea, much obliged.

The main loads at the joiner box are vertically pulling the spar caps apart. Assuming you wrap this area with Kevlar, plywood will only weaken the junction by confusing the wrap.

Look at the freely published plans for the Supra or Bubble Dancer to see how to make joiners and spars that are hundreds of times stronger than you could ever need with an absolute minimum of weight and effort. The Supra spar and all of the other mechanical (and aerodynamic) engineering of that plane are very near theoretical perfection and are the basis for which all other model sailplane designs are derived. It's worth studying the design approach and material choices but keep in mind the Supra is 50% larger than your model and is designed to routinely withstand 50 G's at 150 mph.

Ah-hah! Now I see that you will have solid wood between the spar caps. I was thinking you were using a "C" or box spar.

hi there from Toledo

I have reviewed the discussion since last I posted and strongly embrace the direction you are headed. This will seriously improve the gull wing center panel and the tip panels too.

Go for it! -rjf

Originally Posted by vespa

...The main loads at the joiner box are vertically pulling the spar caps apart. Assuming you wrap this area with Kevlar, plywood will only weaken the junction by confusing the wrap...

That's a new phrase for me, "confusing the wrap". There's a joke in there somewhere but I'll leave it alone *smile*

I did notice that the BubbleDancer's wing-rod block is nice & clean/simple without any additional structure on the sides. I suppose if I were inclined to add side bracing, as mhodgson suggests, it could be added after the wrap is in place. Then additional Kevlar could be wrapped around the whole thing. Truth is as you said, most of the load being vertical, its probably fine as is. Will do some pondering overnight & see what's what tomorrow.

Originally Posted by ray foley

...I have reviewed the discussion since last I posted and strongly embrace the direction you are headed. This will seriously improve the gull wing center panel and the tip panels too...

Thanks again for the continuing encouragement Ray. I'll take all I can get *grin*

Hi nodd really sorry to here about the wing fold. It really surprises me that with the original construction of the wing center section, that it would break like that. If i could offer my humble opinion , If you go with your second idea with using full ribs with blocks of balsa in between , with carbon top and bottom cap, BUT as a complete over kill could you not fabricate carbon shear webbing to the blocks before fixing top and bottom caps , and then wrap the whole box section with kevlar . I know this is a complete overkill but better be safe than sorry. You could even use blue core foam as you blocks in-between ribs and just solid blocks where the outer wing joiner goes? The whole design is beautiful and its a shame about the centre section. also i cant remember but did you glass the centre section top and bottom?

My old wing had several major design flaws including a weak C shaped spar, poorly attached webbing, gappy butt-joined top & bottom sheeting & areas of high strength that ended abruptly causing stress points. Failure was inevitable but on the up side, I've learned from my mistakes. The new wing will employ proven tech, should be way stronger & hopefully lighter too.

CF webbing is something I might consider for an aerobatic plane or a big winch sailplane. Trying not to jinx my self here but yes, as you suggested for an EP glider CF webbing might be way overkill. Others have also suggested blue foam for the webbing. I'll look into that but for now the plan is to stick with good old balsa.

Appreciate the input.

BMatthews over at the RC Universe forums had some great suggestions. One of which was to lose the hardwood wing joiner block in favor of an epoxy/microballoon mix...

BMatthews wrote:
On the bays with the joiners I don't use webbing between the caps. Instead I mix up a batch of epoxy thickened with phenolic or glass micro balloons to thicken the resin and pack it in between the spars. Before it can run out I cap and clamp 1/32 ply outside "webbings" in place to provide a dam to hold the thickened resin in place. When it cures the resin itself becomes the full webbing for those rib bays as well as ensuring that the bending loads are correctly transferred to the carbon or spruce caps.

I like this idea because it avoids the hassle of needing to accurately drill holes in hardwood blocks at exacting angles. A nightmare to get right using wood but with Matthews' liquid method, all I need do it place the two wing panels together, insert the tubes & wing-rod & pack in the goop.

Yes, that's the way I would do it too. It's messy to do with joined wings though so what I would suggest is to join the panels, tack the tube with CA or 5min, then disassemble and pack with a very light/dry microballon/flox mix. It needn't be very strong, ideally not much stronger than the neighboring regions.

I’m going to build a replacement wing for my Pulsar 3.2 REF – see reason below. I’m going to copy the airfoils and planform exactly. I will simplify the construction as follows:

Spar will be 1/8 x 3/8 spruce caps with individual single web pieces. D section will be 1/64 ply (3 ply) lightly sanded to bend around sharp nose of ribs. The 1/64 ply will cover the spar caps. Wing joiners will be 1/8 ply (6 ply) and Kevlar thread wrapped for two rib bays. The only carbon will be the 5mm x 0.5mm trailing edge. I’ll cover the D section with Ultracote simulated Kevlar to mimic the original Pulsar construction. Additional colors will be transparent purple and transparent fluorescent yellow. The spar construction will be similar to my 3m Paragon. Should be strong enough even though ribs and spar are shorter in height. Paragon spar is 1-1/8” and Pulsar is ¾”. I suspect that my construction will be heavier than the Pulsar. Should be a fun build...

Holy thread hijacking Batman!
Sorry to see your Pulsar in pieces. I know that feeling all to well. Sounds like you have a solid plan for the rebuild though. Best of luck there.

Originally Posted by vespa

Yes, that's the way I would do it too. It's messy to do with joined wings though so what I would suggest is to join the panels, tack the tube with CA or 5min, then disassemble and pack with a very light/dry microballon/flox mix. It needn't be very strong, ideally not much stronger than the neighboring regions.

Temporarily tacking the wing tube in place is an excellent idea. Yeah I'm a tad conflicted on just how strong the goop needs to be. On one hand I want that tube in there super solid, on the other hand I want to avoid sudden changes in spar strength, want to avoid creating any stress points. Somewhere in the middle is where I should aim I guess.

A HUGE THANK YOU to everyone for helping me redesign this wing.
Enough talk though, now its time to build, woohoo!

I spent a few hours tonight working up the new design in my CAD software. Here's a few teaser views...







I'll probably order the CF & wood early this week, can't wait to get started building or should I say re-building :-)

Check's in the Mail!
I ordered the supplies for the new wing, should have those later this week.

Some of you maybe interested in how much scratch building in wood costs. Here's my shopping list for the new wing...



This order was placed with RCFoam.com. I've dealt with them before & like their level of service & products. There's lots of other online retailers for the this stuff too. Oh & of course don't forget to support your LHS (local hobby shop), I try to buy my parts there when I can.

Balsa in a Box
The wood & CF for my new wing arrived today, woohoo!..



This is the second time I've ordered from RCFoam.com. Once again I'm impressed with the packaging & the quality of the wood. The carbon fiber looks real nice too.

Now that I have the parts in hand I can finalize my plans. I wanted to wait until I could take actual real world measurements from the wood & CF. Unlike the first, this wing will be tight & precise. Here's an initial layout for the ribs...



I'm headed off on a fishing trip this week so won't be able to build until later in the week, will post more when I get back.

Nice! Looking forward to seeing her in the Air Again...

Thanks.

A quick update, its been a few weeks since my building supplies arrived. Unfortunately they're still sitting on my workbench. I've been a tad distracted by a DLG project but that's almost complete so I should be able to get back to the wing shortly.

Oops!
I just noticed, looking at my supplies, that I only ordered four CF spar-caps. This will be a three part wing so I need a total of six, not four, grumble, grumble, grumble. Turns out my over-site might be one of those happy accidents though...

While poking around the Net looking to order another two 12mm x 2mm CF strips I came across some really cool tapered CF. These strips are 12mm wide & start off 1.5mm thick then taper off to 0.5mm over around a meter. So I ordered four figuring I'll use these for my outboard wing sections, tapering off as we approach the wing-tips. I'll use my non tapered CF for the center wing section.

I'm liking this, should save me some weight as surprisingly, 12mm x 2mm x 1m CF does actually weight a fair amount. Using tapered spar-caps should be pretty slick. Now of course I have to wait for them to arrive. I should be able to get going on the center wing though. We like happy accidents.

Construction Begins!
Enough talk, enough delays, time to get this wing built! Today I started cutting ribs. First I printed them onto 8½ x 11 sheets then contact cemented them to my wood...



Next I rough cut them on my scroll saw...



Here's the ribs for the wing center section. At this point they don't need to be accurately cut...



Next I head to the belt sander where the shape is refined...



Here's the center wing section rib set. I still need to cut the spar notches & the servo wire holes but its a start...



Feels great to be putting this bird back together. Can't wait to have her flying again. Summer is rolling by fast, I want to get back to enjoying her while its still nice out.

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...



I printed my center section plans on 8½" x 11" paper then taped them together...



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...



I created a jig for accurately cutting the shear-web into sections...



Here's the full set of shear-webbing for the center wing...



Pinned the lower CF spar-caps to the plans...



Alternating between rib, web, rib, web, rib, I started to assemble the wing...



Before you ask, yes I removed the paper rib template where the shear-webbing glues to the rib...



Ribs & shear-webbing installed...



Next I added the top CF cap-spar then placed our entire collection of dinner knives across them...



Next I added some socks, yes socks...



A piece of MDF board goes on top...



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...

Nodd, if anything you are one of the more innovative scratch builders I have seen. Very nicely done.

Innovative or slightly out of my mind, not sure which one *grin*

The new spar is looking good, feels crazy strong...



In preparation for the Kevlar tow I made tick-marks along the spar so I could keep the wrap neat...



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...



With that done I added the TE strips...



I CA tacked the end ribs in place using angle templates I printed up...



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...



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.

Awsome!!!!

hi there from Toledo

Now this is the kind of turn around we all were hoping for, BRAVO!

+1 with plane_tech, this kind of creativity breathes new life into the world of scratch building.

One question/ concern though, most current designers (Drela, Womack, others) use vertical end grain medium to hard balsa as shear webs, one also uses laminated +/- 45 degree balsa shear webs. I noticed you sliced horizontal grain balsa as your shear webs. So what is the understanding here? Are you doing this because it is easier to build or what? It certainly is not stronger in shear nor compression ? Your thoughts? You may want to reconsider this, seriously. I know this is really late in the process but I would be remiss If I didn't mention this.

cheers -rjf

ps: I like the knives, socks, board, and dumbell method, I am still giggling, truly inspired, improbable but inspired. -rjf

I noticed that too but wasn't going to say anything since those sparcaps are so crazy strong that they'd surely be fine with no shear web at all.

Good catch, I was wondering why no one was giving me grief for the webbing.

From what I've read, the vertical grain requirement for shear-webbing is somewhat of a myth. The shearing forces are diagonal so neither, horizontal nor vertical, is superior. As you mentioned an ideal shear-web is made from 45° cross-grained material but this isn't a competition winchable wing. I've actually had folks recommend using foam for the webbing so I'm pretty sure balsa will be fine.

As for the thinking behind using horizontal grain, given that it's apparently a non issue, yes I did it that way because its much easier. I just cut a tapered length from my ½" stock, then chopped it into sections, done.

The spar feels like an iron bar, I'm really impressed. Light weight yet total overkill for an EP glider. Liking this lots.

Somewhat of a myth, but only somewhat. By far the greatest authority is Dr. Drela and he has addressed this topic as well as any other.

Page 20 here discusses why the grain direction does not matter.

And page 20 here discusses why the grain does matter.

The key difference is the intended function of the web:

• If the web is handling the shear loads then grain orientation doesn't matter because the web will fail in shear long before it fails in compression.
• If the web is handling only compressive loads then grain should be vertical. This would require something else like a 45deg composite skin to handle shear loads.

So, since you're not doing a dedicated shear layer, the horizontal grain is perfectly suitable.

hi there from Toledo

Well, good luck anyway, we are all rooting for you.

ciao -rjf

That's funny you linked to that article, that's where I first heard about the grain orientation issue (I'd actually already linked to that article earlier in this thread). Great info, we like Dr. Drela, he's helped pull us out of the dark ages many times over.

Originally Posted by k.smith459

Awsome!!!!

Originally Posted by ray foley

...Now this is the kind of turn around we all were hoping for, BRAVO!

+1 with plane_tech, this kind of creativity breathes new life into the world of scratch building.

ps: I like the knives, socks, board, and dumbell method, I am still giggling, truly inspired, improbable but inspired...

Thanks guys. I really apreciate the support. Its comments like yours that make this so very rewarding. Cheers.

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...



Attached the leading edge...



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...



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...



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...



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...



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...



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...



With it about half filled I dropped in my wedges. As they sank the goop rose-up to fill the box to the rim...



With the glue still goopy I capped off the box with another set of 1/16" basswood webs...



Yeap she's starting to look like a wing now, liking the look of that 22° dihedral...



Time to stop for the day, let that setup good & strong over night...

hi there from Toledo

Very nice and well documented too. Again innovative use of fairly common materials in slightly unorthodox ways, delightful truly.

ciao -rjf

... Continuing to be impressed with your build Nodd... And thanks for the on going inspiration to continue to work on my design

Never give up, never surrender!

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...



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...



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...



Let these setup for 10 minutes...



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...



Installing the tubes...



In goes the goop...



Capped with 1/16" basswood...



Once that was set I wrapped both joints with Kevlar, this time closer wound as I figure this is a high stress area...



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...



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...



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...



And just to be sure everything stays together no matter what, more Kevlar wrapping...



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...



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.

I might have to steal your idea for the joiner tubes

Nodd,

What an interesting build! I'll certainly be stealing some of your ideas on my future builds.

Many of the links in your posts don't work for me, such as in post #50. Do you, or anyone here know what I can do to fix that?

Russ

Originally Posted by ulpilotrmh

Many of the links in your posts don't work for me, such as in post #50. Do you, or anyone here know what I can do to fix that?

Russ

Try using this button. Sorry for the intrusion Nodd. Russ PM sent.

Nodd, someone once told me to try and learn something new every day. Well since round one, I have been quietly following alone and I must say, every day that you post on your build, I learn a number of new things. Keep up the good work.

Glad you folks are enjoying the build. Hoping to get this thing back in the air soon.

Sheeting
Not much to report today. I have the sheeting done on the center section. Here's how that went...



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...



I am sheeting the root some however...



To help support the diagonal sheeting I added diagonal ribs...



Sheeted the bottom & made a couple of holes for the servo wires to exit...



Pretty happy with the sheeting...



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.

Awsomeeee!

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...



Not having a micrometer handy (I need to get one of those), I tried to use a ruler to measure the thickness...



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...



Looks like they total around 3mm...



Divide 3mm by four & we get 0.75mm. So that's my thin end measurement. I did the same for the thicker end...



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.

ATCHOO!
More fun sanding ribs to shape. Warning those with allergies may find the following disturbing...



After a rather dusty couple of hours I have a full set of ribs ready for the outer wing panels...



With the hard part done I'm hoping to get the outer wings together tomorrow.

Nodd I hope you’re wearing a dust mask so your lungs don’t look like your arm. I find if I sand wood in the house, the dust goes to all the electronic.

What’s the thickness of the ribs?

I'm sure you have already stated somewhere, but what software are you using to draw up the plans? I’m looking at getting my own drawing program, there’s a lot to pick from.

Thanks.

Brian

Originally Posted by scallopking

...Nodd I hope you’re wearing a dust mask so your lungs don’t look like your arm...

Yeap it only takes one unprotected sanding session to learn that balsa dust is zero fun to breath, cough, splutter, sneeze, bluh!. I picked up a bag of surgical style masks. They're probably not the best protection but because, unlike bulkier masks, they're comfortable & light I find I'm more likely to toss one on even for small sanding jobs. My previous masks often stayed on the bench because they were uncomfortable & claustrophobic to wear...

Originally Posted by scallopking

...I find if I sand wood in the house, the dust goes to all the electronic...

You're right about the dust getting into electronics. I try to keep my workshop door closed but invariably the dust finds its way into the rest of the house. I use a shop-vac hooked up to my belt sander on big or harmful jobs.

Originally Posted by scallopking

...What’s the thickness of the ribs?..

I'm using 1/8" balsa for the ribs. That's a tad thicker than maybe it needs be but because I'm using exponential rib spacing (they're closer together at the root, further apart at the tips) the total rib count is relatively low...

Originally Posted by scallopking

...I'm sure you have already stated somewhere, but what software are you using to draw up the plans? I’m looking at getting my own drawing program, there’s a lot to pick from...

I do CG (computer graphics) & animation so I have some fairly spendy CAD tools. For the airframe work I like 3D Studio MAX. That software is way overkill for this stuff though. Check out Google SketchUp, Blender, AC3D. Also if you're interested in aerodynamics X-Plane's Plane Maker is really good too.

Outboard Wing Sections
I printed up a set of plans & started construction by laying the tapered CF spar-caps down...



As with the center wing section, the ribs & shear-webs go down in alternating order...



Done adding ribs & shear-webbing...



Wanting to keep things light out at the tips I split the shear-webbing in the last four bays...



With the CF tapering off to a hair's thickness & hollow shear-webbing these outer panels should be light yet still plenty strong...



That's all for today.

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...



Using the sanding block to fix any problem areas...



Epoxying the spar-caps to the wing...



Its random household item time again!
On go the dinner knives...



This time instead of socks, I used foam pipe insulation to evenly distribute the weight...



On goes a piece of MDF board...



Added some weight...



And to top everything off, a cute baby penguin...



Once that had setup for a few hours I did a quick weight check, looks pretty good to me...



Lastly I wrapped the spars with Kevlar...



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!

hi there from Toledo

I am certainly impressed with the new wing structure and the penguin makes it all perfect.

Perhaps the penguin could become your transmitter mascot, velcroed to the antenna or to the carrying handle might work.

ciao -rjf

ps: perhaps you could christen the sailplane "the flying penguini" or some such sillyness. -r

pps: how about a penguin like motif for the finish, could be fun. -r

*smile* yeah I'm thinking a penguin might not be the best mascot or name for my new plane, given they're not exactly known for their flying prowess.

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...



Capped some brass tubes, ready to be installed...



While test fitting the wing sections together I noticed something ominous about the left side wing-rod...



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...



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...



My nice neat Kevlar wrap had to go...



All I can say is thank God for the Dremel tool...



Finally got the miss-aligned tube out of there, what a hassle!..



Anyway, several frustrating hours after first discovering the issue a new tube was installed, this time at the correct 9°...



In the meantime, while the epoxy cures, I turned my attention to the other side of the wing...



I temporarily clamped the sections together, carefully checked the alignment & dihedral, then glued in the wing tube...



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.

Ouch! What a bummer! Good save though!

hi there from Toledo

Remember, it is not a case of whether there will be adversity or not, there will be. The important thing is how adversity is handled and overcome.

Actually that was fairly minimal as yuck ups go; annoying to be sure, but with the right tools, pretty easy to repair. Nicely handled though. Carry on. Success is imminent.

ciao -rjf

Encouragement much appreciated, thanks guys.

Things went more smoothly today. Glued up the other side's wing-rod...



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...



So unfortunately, with that needing to cure for a good long time, I'm done for the day. Will leave this be overnight...



Time instead to go edit some video for my YouTube NoddRC show. Will have more tomorrow.

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...



Installed the ridge...



Added new sheeting...



To celebrate getting that done I took the wing outside so I could stand back & have a good look...



Happy with that I headed back to the workshop & got down to sheeting the outer panels...



More Wing-rods
To keep the outer panels from turning on the main wing-rods I needed to add a second smaller rod...



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...



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...



Once again I'll leave things to setup overnight...

Nodd,

Did you saturate the kevlar around your spar with epoxy and then remove the excess and allow it to cure prior to applying the sheeting?

Curtis

I used epoxy to secure the top-n-bottom sheeting to the spar so the Kevlar between is epoxied in place. For the front-n-rear of the spar I used thinned wood glue to lock the fibers down to the wood shear-webbing (see post #54).

In addition to your epoxy method I've also heard that enamel paint works quite well.

This looks scarier than it is.
Time to cut through the brass wing-rod tube...



As I was hoping, both wing-rods aligned perfectly, sweet!..



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...



A little more sanding with the wing assembled to make everything match up nicely...



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...



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...



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...

Some more center section fiber-glassing...



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...



An hour or so later I have a set of trailing edge riblets ready to go...



Tomorrow I hope to put these to use making my new ailerons & flaps.

Sweeeeeet, Nodd. I wouldn't worry about the holes in the leading edge because your covering will recreate the D-box with the stress along the strongest part of said covering. Just make sure it's really stuck and really shrunk.

Nice work on the glass too. Another trick is to use the contact cement, then use a really thin first coat of epoxy taking off most of the excess with paper towels, and then a thicker second coat to get a smooth finish. I like to mix in some talcum powder on the finish coat because it lightens it up a bit and makes it much easier to sand. The talcum powder also seems to help the covering stick and/or accept paint better.

In any case it looks like the bird will be flying with plenty of summer left and that's what it's all about.

Talcum powder aye? I might have to give that a try. I'm probably going to leave the wing's fiber-glassing as is but will consider that for other applications. The big question is, do your glassed planes smell like little old ladies? :-)

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...



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...



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...



The left flap...



Seeing how it looks in-place...



Here's the ailerons...



The control surfaces look to be a tad flimsy but I have some reinforcing in mind. Will get to that tomorrow...

Nodd, my trick for the razor sharp trailing edge is laminate three piece if wood like you did, but put the 1/32 or 1/64th ply in the middle. Then sand the outer bales layers to the required taper. The ply will give you a very nice, sharp edge. I have done this for my nitro speed planes on the LE and TE with great results.

Yeah I'm thinking this is going to work nicely. Not sure I need another piece of balsa on the bottom for this particular application but I can see that might be the way to go in some cases. Good to hear I'm on the right track, thanks.

Do your Catchphrase and whack in some CF, CF fixes everything ! (so does duct tape! )

Hi, Nodd. If you use enough talcum powder it does smell while sanding it but it makes a good, cheap, filler. It's not very attractive if you don't paint it though. Fortunately it takes paint quite well and sticks to covering as well as straight epoxy or a little better, which isn't that great as a rule. It's the type of thing you want to experiment around with. In general I mix it a little stiffer than one would think is needed but it flows for a few seconds when it hits "B-stage".

Hi there from Toledo!

+1 on the Borg babe, 7 of 9. Woof!

ciao -rjf

Zig-Zag
I've seen diagonal ribs used in countless airframes & there's no doubt they add a lot of strength to a structure. So to give my control surfaces some serious rigidity I added diagonals...



Where the control horns will eventually be installed I beefed things up some...



After some serious sanding the ailerons now flow seamlessly from the wing out to a nice razor sharp trailing edge...



My sanding arm feels like a wet noodle so time for call it quits for the night. More tomorrow...

With the sanding complete I attached the fixed, non-aileron portion of the trailing edge...



And with that the wing's airframe is complete, woohoo!..



Winglets
Well okay maybe the wing isn't totally complete. I've been studying sailplane winglet design & I think this bird could benefit from a pair...



Among other source material I came across this article by Peter Masak. The above design is based heavily on what I learned from his experiments. So that's next on the to-do-list, make up a pair of bad-arse winglets.

Looks cool but just so you know, winglets are aerodynamically undesirable in every possible way. Their primary effect is simply increased drag but secondary effects are equally troubling -- reduced yaw stability and increased dihedral. Nevermind the cumbersome factor.

A well designed winglet can potentially mimic some of the effects of equivalently increased span and AR, but only at one speed. The major benefit is that they do this without increasing wing bending loads or runway clearance, thus they are often used on aircraft that are severely span limited and fly mostly at one speed (e.g. sailplanes and jetliners). So unless you're worried about navigating taxiways at LAX or breaking your wings due to excessive span (moment), winglets are nothing but bad.

A possible compromise however is subtle upswept tips which also look pretty cool. These are not intended to provide any of the benefits (or drawbacks) of winglets but instead they just soften the tip vortices.

I've been hanging around scale sailplane guys too much it seems. Most of their sexy looking birds all sport winglets...



I guess the key here is, as you mentioned, if they're "well designed" they're of benefit. Otherwise yes I'd agree they're probably not going to do much more than look cool at the expense of performance.

Mr Masak's article gives some pretty specific design recommendations as in a 30° sweep, 2° wash-in, 60% taper, 3° toe-out & PSU 90-125WL airfoil. His figures take a lot of the guess work out of the design process although he gets a little vague when it comes to tip chord percentage & length of the winglet.

I've seen lots of examples of "vorticity tips" like in your example. I could take that route although I'm not I'm willing to give up on the winglet idea just yet. Another compromise might be to reduce the size of my winglets & blend them into a vorticity tip design. Here's something I mocked up in CAD real quick...



These are small enough to not cause all that much drag or mess with the yaw stability, yet have enough airfoil to do a fair job controlling cross flow.

Nodd, I have always been a fan of winglets. Like you and the article states, if they are properly designed, they can be beneficial. Why would commercial airlines spent more than $100k per plane for a winglet mod if it didn't do anything. However, they are optimized for a specified speed range. Nonetheless, a set of winglets on a 767 can result in a savings of several million dollars of fuel over the course of a year per plane. So I would say they work...

Regardless, here are a couple more papers on the topic you might be interested in.

Wow thanks. That's quite a bit of reading there. Nice to see at least one person is somewhat for this idea, I'm getting hammered on my mirror posts for mentioning winglets. Who knew they were so controversial *grin*.

Thanks for the PDFs. I'll take a look.

Hi, Nodd.

I say go for it though I don't know how one would cover them and paint gets heavy in a hurry.. You probably won't see much difference in performance either way and you should go with what you want. Perhaps you could make removable wingtips/winglets and try both? I would think you could come up with a way to temporarily secure them with small nylon bolts. Then, after trying them out, you could glue on the tips you like best.

FWIW; my foam ASW 28 has winglets, soars very nicely, and looks great on those low passes.

Yeah I'm thinking something that's removable, mainly for transportation & storage reasons. But like you said, I guess that'd have the added value of allowing me to try different things.

Weight is always a concern, especially out on the wing tips where I'd like things light for thermal signaling. As I'm now leaning towards fairly small winglets I'll probably carve them from a solid block of balsa, maybe a little lightweight glassing to make them ding-proof.

I agree, covering them what with all those crazy compound curves would be a nightmare. A light dusting of good old Home Depot RustOleum shouldn't add too much weight.

Anyway I have some reading to do. Will see how I feel about winglets after I've done ed-u-ma-kate-ed me-self some more.

Servos
I did some work today getting the wing's servos installed. I'm using a different method this time around...



The servos are attached to the wing instead of to a hatch. I think this will produce a more solid mount & should avoid any unsightly bumps on the wing's under-surface, not to mention being cleaner aerodynamically...



More tomorrow hopefully.

I've had this roll of sheet aluminum ever since I can remember. A thousand uses, here's one more...



Cut some into small pieces, drilled a couple of holes...



Made my self some nice low profile straps to keep my servos in place...



Simple, strong, lightweight & easy to service. I'm liking this.

A few guys over at RCUniverse have been giving me some grief about my rib spacing at the wingtips. After sleeping on it I've decided they're correct, 3.75" between my outer ribs is pushing it a bit. My fancy high performance airfoil is sagging into all sorts of nasty shapes & as they also pointed out, it doesn't matter how strong the main spar is, flutter will kill this wing in an instant if it wants to. So today I took steps to fix both these concerns.

Lets get diagonal baby!
I hit my CAD software again & spat out a set of diagonal ribs...



I was worried about weight but by using thinner balsa for these there's not much to worry about...



Here they are installed...



Hmmm, nine feet of pure sexiness...



Oh we're getting somewhere now. Stay tuned kiddies, more to come soon...

Cool thread! very pic heavy which is always great