How to make spars for TD ship?

[jtprouty]

Hi All,

I'm contemplating scratch building a two piece TD wing for a multipurpose plane. I have a CNC foam cutter so making the cores is not an issue. I want to make the wings strong enough to withstand a heavy footed winch launch and I'm looking for information/suggestions on how to build the spar. I'd like to use CF wing joiners (also need a source for them).

What the best way to go about this? What do I use for wing tubes? How do I make the spar?

I've seem the Supra build article on the Charles River RC site but am a little confused as to the process and what all is needed to make a wing that's strong enough.

Any information would be greatly appreciated.

Happy flying,

Jimmy

[atjurhs]

Jimmy,

I am in the process of doing the same thing (building/bagging) a TD wing using foam cores.

I am going to attmept a new way of going about the construction, one that I haven't seen done before but should work very nicely, see the pic below.

NOTE: the CF cloth may or may not be overkill unless your rough on planes.

NOTE#2: Spyder Foam has a vertical grain to it, much like balsa shear webs, so I should be able to literally sew the kevlar thread around the spar caps, to hold them together. The distance betwen each kevlar loop as you traverse down the spar from root to tip should become greater because not as much force is aplied to the tips of a wing as is applied at the root.

Anyways, it's what I'm going to do.

[D_Ryan]

Jimmy;

Hope the folks at CST don't mind, but here you go...

http://www.cstsales.com/tutorials/buildcarbfiberspar.pdf

r/
Dave Stack

[GLIDERGIDER]

Jurhs,
What size of Kevlar thread do you plan for this sewn spar? I know you've used the fishing kevlar in the past, but that would require lots and lots of loops. Kevlar tow would require fewer loops, and reduce the number of needle pokes. Do you have a build thread for your idea? I hate to hijack Jimmy's thread should he want the more traditional method. Yet your idea is worth of the scrutiny of other builders.
Dave

[atjurhs]

Jurhs,
What size of Kevlar thread do you plan for this sewn spar?...
Dave

Ain't settled on the size just yet, but epoxing the cf spar caps into the foam would also be a necessity! I think I left that step out.

...Do you have a build thread for your idea?...
Dave

Not yet, but one will be forth coming...

...Yet your idea is worth of the scrutiny of other builders.
Dave

YEP! I'd love to hear from others on the idea!

[John Walter]

If you need Kevlar tow, see:

http://www.rcgroups.com/forums/showthread.php?t=587629

It's an old post, but Little Flyer still has lots for sale, and he needs to raise $ to pay for planes to fly at the F3J Worlds in Turkey next year.

[StevenatorLTFO]

If you need Kevlar tow, see:

http://www.rcgroups.com/forums/showthread.php?t=587629

It's an old post, but Little Flyer still has lots for sale, and he needs to raise $ to pay for planes to fly at the F3J Worlds in Turkey next year.


Make the boy get a paper route or something :D

SS

[John Walter]

He will need to deliver a lot of newspapers to pay for 2 more Supras!

[StevenatorLTFO]

Maybe put a pizza hot box on his bike LOL

See you guys Sunday hopefully.

SS

[Rusty Joe]

Carbon F should be thicker on the bottom spar, thinner on top spar - more stress on the bottomJimmy,

I am in the process of doing the same thing (building/bagging) a TD wing using foam cores.

I am going to attmept a new way of going about the construction, one that I haven't seen done before but should work very nicely, see the pic below.

NOTE: the CF cloth may or may not be overkill unless your rough on planes.

NOTE#2: Spyder Foam has a vertical grain to it, much like balsa shear webs, so I should be able to literally sew the kevlar thread around the spar caps, to hold them together. The distance betwen each kevlar loop as you traverse down the spar from root to tip should become greater because not as much force is aplied to the tips of a wing as is applied at the root.

Anyways, it's what I'm going to do.

[Fly2High]

Rusty,
I am no engineer but I think they put more carbon on the top than the bottom because carbon is not as good in compression as it is in tension, not the other way around.

Based on what I have read and have been told, I could be wrong.

Frank

[rdwoebke]

NOTE#2: Spyder Foam has a vertical grain to it, much like balsa shear webs, so I should be able to literally sew the kevlar thread around the spar caps, to hold them together.

I'm no building expert, but if you are doing a bagged wing, I don't think you want to wrap the spar in Kevlar thread. I think you want to wrap it in a bias wrap of fiberglass cloth, like on the Supra/Aegea plans.

Ryan

[Twizter68]

"3-2. DAMAGE TO ADVANCED COMPOSITE
MATERIALS (ACM). Damage incurred by ACM is quite
different than that experienced by metallic materials. ACM
do not deform like metals. ACM either resist an impact force
and spring back or rupture. Due to the brittle nature of most
ACM, these ruptures can occur at rather low impact energies.
The damage resulting from a rupture produces cracks in
the matrix, delaminations between plies and broken fibers."

From NAVAIR 01-1A-21, Composite Repair Manual.

It goes farther in depth about extensive stiffness, compressive strength, etc, but we do use it to create structural components; however, the ACM used in those components are made of several plies of bi-directional weave, with alternating orientations to increase strength across the part.

[kablair]

Wrapping with Kevlar thread works extremely well - see my thread at http://www.rcgroups.com/forums/showthread.php?t=699630 .

Or see the original documentation from Harley's Genie pages.

This is an incredibly strong system. You will note however that after repeated full pedal launches, I did bend the 3/8" steel blade at the tip. Next time I'll use a 1/2" steel blade. But the center section barely bowed on even the inadvertent deep zoom.

-keith

[rdwoebke]

Wrapping with Kevlar thread works extremely well - see my thread at http://www.rcgroups.com/forums/showthread.php?t=699630 .

I agree, but if you search some of the posts from Dr. Drela on the subject of spars (I won't do the work for you all, but I assure you it is out there) he says that a spar wrapped with Kevlar is very critical on the spar to spar core bond and the wraps. He said that a spar wrapped with bias glass the bond is much less critical.

Hence the Supra with its "drop in spar" has bias glass wrapped spar and the Bubble Dancer/Allegro Lite, with its ribs into the spar is kevlar wrapped, since the bias glass is not really an option for that kind of construction method.

My 2 cents. As always, remember I'm a crappy builder/flyer.

Ryan

[atjurhs]

I'm no building expert, but if you are doing a bagged wing, I don't think you want to wrap the spar in Kevlar thread. I think you want to wrap it in a bias wrap of fiberglass cloth, like on the Supra/Aegea plans.

Ryan

Ryan,

If I wrapped the spapr as you state, that would completely mess up my build sequence that I have outlined. The beauty of this build method is that the spar is build right into the wing cores, so that I don't have to cut the wing panel into pieces build a spar, insert the spar, and then glue them all back together again.

I know that kevlar wrapping works really well. I have several "built-up" planes whose spar is wrapped with kevlar, and they are tough as nails on launch!

I also know that this build method is not traditional, dare I even say "revolutionary" ? Yes, I am attempting a whole knew way to construct a wing (well at least I've never heard of anyone building a wing in this manner). Nothing ventured, nothing gained, and if it fails, what have I lost in the end, a piece of foam, some epoxy, a few pieces of composite, and a bit of my time. I have a saying that I try to go by, but often fail, "what's the worst thing that's could happen, I make a mistake." The saying helps to keep me from being overly anal-retentive. My friends and co-workers know that I can use use all the help I can get in that area :D

But has all this de-railed from the original thread too far?

[rdwoebke]

My bad. Sorry for making the suggestion then and I did not fully understand what you are doing with the sewing. I wish you success with that.

Like I said, I'm a crappy builder (and a crappy pilot).... I built a Bubble Dancer in 2004 and had a spar failure on launch. Of course, due to my crappy building (I'm thinking I must have had a small gap or weak bond between the core and the spar). But since then, I have occasionaly thought a less susceptible to this kind of thing way to do a BD type plane for a crappy builder like myself (and I did build an Allegro since then, but via the standard method, aka the kevlar wrapping, although I have not really stressed it on the winch) is instead to do a bagged center section ala Supra with the glass cloth wrapped spar. Seems to me, such a setup would be a comprimise for crappy builders like myself, perhaps a bit more tollarant to crappyness that I impart.

Again, I can't disclaimer enough, I'm a crappy builder (and a crappy pilot). Disclaimer, disclaimer, etc., etc.


But has all this de-railed from the original thread too far?

Welcome to the interweb! :-)

And, as always, best of luck with your project.

Ryan

[GLIDERGIDER]

Ryan,
I agree totally that the bias carbon tube is superior to the kevlar wrap, however its pretty expensive, and wrapping with Kevlar tow is nearly equivelent. At a cost of $17 per foot, you end up with a bunch of bucks in the spar, when I'd rather spend it else where.
I'm a Civil Engineer by schooling, and practicing mechanical engineer. I understand about shear flow in a beam. I also understand how beams of reinforced concrete are made. Generally, we can take some schooling from the structural analysis of a concrete beam, which has hoops of steel much like the wraps of Kevlar.

I know this subject is dear to many followers of Dr. Drela, and I know his carbon sock is a superior product. If you want to spend $17 per foot, you will get the best. If you'd like to save a few bucks, ( about a $100), then consider wrapping the spar in kevlar.

Again, I agree the sock is the best, but I will probably use Kevlar tow wrap in my next spar, because with a good wrap, you won't know the difference.

In my case, I got this huge spool of Kevlar for very cheep. But you can also buy it cheep from LittleFlyer, as others have pointed out.
Dave


I agree, but if you search some of the posts from Dr. Drela on the subject of spars (I won't do the work for you all, but I assure you it is out there) he says that a spar wrapped with Kevlar is very critical on the spar to spar core bond and the wraps. He said that a spar wrapped with bias glass the bond is much less critical.



My 2 cents. As always, remember I'm a crappy builder/flyer.

Ryan

[markdrela]

I agree totally that the bias carbon tube is superior to the kevlar wrap, however its pretty expensive, and wrapping with Kevlar tow is nearly equivelent. It's not equivalent. Not even close. The key difference is that the bias fabric wrap can take large shear loads, but the Kevlar tow wrap can not. With the Kevlar wrap, the spar core must take most of the shear loads, and it's nowhere near as strong in shear as the bias wrap.

At a cost of $17 per foot, you end up with a bunch of bucks in the spar, when I'd rather spend it else where. You don't need expensive carbon sock for the spar wrap --- glass is almost as strong/weight as carbon. And you don't really need a glass sock. Using just plain bias glass fabric works OK.

[GLIDERGIDER]

Mark,
Thanks for the clarification.

[rdwoebke]

Ryan,
I agree totally that the bias carbon tube is superior to the kevlar wrap, however its pretty expensive, and wrapping with Kevlar tow is nearly equivelent.

Hey Cousin Dave,

OK, I have to take some issue with you here. I admit, I did not really read properly/understand cousin atjurhs's plan for his sewing the spar, but where, where, oh where, do I ever talk about a carbon sock? I talked about bias glass. Fiberglass is really inexpensive. I referenced the Supra/Aegea wing plans. These plans also call for the bais glass cloth wraps:

http://www.charlesriverrc.org/articles/supra/all%20PDFs/sparbuild.pdf

The plans call for 1.5 ounce glass cloth to wrap the spars with. You can get a 1 yard by 1 yard square of this from CST for about $13.00. Unless I'm mistaken, a person could wrap all the spar for a Supra with that peice and still have some left over (aka small ends)...

I'm thinking, that you got this carbon sock idea from somebody else. I know a few folks have built Supras with carbon sock for the spar wrap (I think Tom Keisling does his that way), but that is not how I built my Aegea wing (that is crappy, remember disclaimer above...) and I did not reference the carbon sock.....

I guess while I'm giving disclaimers that I'm a crappy builder, and a crappy pilot, I should disclaim that I am really, really, really cheap. :)

Ryan

[GLIDERGIDER]

Ryan,
Post number 3 of this tread shows the Carbon sock. I'm glad to hear of the clarification about the fiberglass cloth. Much cheaper and it's good to know the carbon sock is not a demanded item.

It turns out that Atjurhs and I both built balsa wings with a .010 thick carbon top and bottom. We wrapped in Kevlar $8.00 and to this day we can stand on the winch. These airplanes only have a 100 inch wing span, so the internal moment is not as great as one with 120 inches.

Another little point that I didn't make clear is my use of balsa in the spar which is stronger then a foam core. Here's a link to one spec for balsa shear strength (http://www.balsasales.co.uk/technical_info.html). In this spec it varies between 270 and 730 psi. Not too shabby. So if a foam core spar is going to be built, then a biased cloth is a must.

I do understand that the balsa is my strength limit, regarding shear resistance, and the Kevlar tow will not take shear.

Regarding your reference about you being a crappy builder and Cheap, well, that's what I like about you. :) I'm right there with you.

Dave

Hey Cousin Dave,
.....
I'm thinking, that you got this carbon sock idea from somebody else. I know a few folks have built Supras with carbon sock for the spar wrap (I think Tom Keisling does his that way), but that is not how I built my Aegea wing (that is crappy, remember disclaimer above...) and I did not reference the carbon sock.....

I guess while I'm giving disclaimers that I'm a crappy builder, and a crappy pilot, I should disclaim that I am really, really, really cheap. :)

Ryan

[kablair]

Dr. Mark,
I was planning on following Harley's instructions for my next Genie, and what I did for my last one, but it sounds like you would recommend using the bias glass instead.

It would sure be a WHOLE lot easier to make - I spent 4 solid hours wrapping the spar with Kevlar thread the last time. My eyes couldn't focus on anything else for the rest of the day. Would 2 layers of 2 oz glass be sufficient?

Thanks,
-Keith

[markdrela]

it sounds like you would recommend using the bias glass instead. For a composite wing, the bias wrap is probably easier, and will give a much stronger spar.

Would 2 layers of 2 oz glass be sufficient? 3 layers in the center, linearly tapering off to 0 layers at the tip. See the Supra plans.

[kablair]

For a composite wing, the bias wrap is probably easier, and will give a much stronger spar.

3 layers in the center, linearly tapering off to 0 layers at the tip. See the Supra plans.


Doh! No wonder I couldn't find it - I was looking through the Allegro forum.....
Thanks,
-Keith

[jfrickmann]

How about wrapping with some 1 or 1.7 oz aramid fabric instead of glass fabric - would that work and be even better?

[MarkusN]

How about wrapping with some 1 or 1.7 oz aramid fabric instead of glass fabric - would that work and be even better?
Nope. Aramid is tough stuff, takes lots of energy on impact, and is relatively light for strength. But its difficult to get to adhere to the matrix and terrible in compression, which sholuld make it so-so in shear.

[atjurhs]

It would sure be a WHOLE lot easier to make - I spent 4 solid hours wrapping the spar with Kevlar thread the last time. My eyes couldn't focus on anything else for the rest of the day.


Keith you got off easy!

When I wrapped the 100inch wing that Dave refers to I spend almost 25hours wrapping mine! I didn't know about kevlar tow, so instead I used kevlar thread from Bass Pro fishing supplies, and I wrapped it thread next to thread next to thread, all the way from root to tip. It's probably overkill, but I tell you, she's busted more than one tow line on launch :D

BTW, I do NOW understand how my "sew it together spar" won't work because SpyderFoam will only withstand 60psi and I need at least 5 times that do handle the forces involved (thanks Dave for your engineering background!). So I guess I'm back to building it the more traditional way - so much for inovative thinking :rolleyes:

[rdwoebke]

So I guess I'm back to building it the more traditional way - so much for inovative thinking :rolleyes:

Really, it is not that bad. In your case, since you use the high density foam you can just slice out the spar area, sand the spar cap thickness off, then glue the caps to the part you sliced out and then do the bias wrap. Not too bad and it sure seems to have a nice result.

Ryan

[atjurhs]

It has occurred to me that MAYBE the spar construction methods that Dave and I are advocating with the Kevlar wrap have a different core material than the spar construction methods that Mark and Ryan are advocting. Dave and I built spars with balsa shear webs. And I THINK Mark and Ryan's spars have foam in between the spar caps. MAYBE this miscommunication is why Mark's comments of "It's not equivalent. Not even close." are not in contradiction to Dave and my experience with the Kevlar wrapping method???

[rdwoebke]

It has occurred to me that MAYBE the spar construction methods that Dave and I are advocating with the Kevlar wrap have a different core material than the spar construction methods that Mark and Ryan are advocting. Dave and I built spars with balsa shear webs. And I THINK Mark and Ryan's spars have foam in between the spar caps. MAYBE this miscommunication is why Mark's comments of "It's not equivalent. Not even close." are not in contradiction to Dave and my experience with the Kevlar wrapping method???

Yes and no. I made reference to the Supra, and the Supra does have foam spar core (spar core also known as shear webs). In this case, I think Dr. Drela says that that the foam without the bias glass is barely sufficient in compression but the bias glass wrap helps add to that strength. But I also reference the Aegea (that actually being a plane I built, poorly) and that design was pre Dr. Drela's using the high density foam as spar cores (In my build, I did not use high density foam for the wing cores, so I pretty much had to go with the balsa cores), it calls for vert balsa on the spar cores.

I'm not advocating not using the kevlar wrapping. And, for a wing like the Allegro Lite or Bubble Dancer, where it is rib, core, rib, core, rib, core setup, the wrapping is pretty much the only way to go. What I'm saying, is that Dr. D. has said that in a kevlar wrapped balsa core spar, good bond betwen the core and the spars is critical and good wrap is critical. I am a crappy builder (as I have pointed out many times, especially in this thread, so everyone always remember that) and I think that if a person is *not* doing the rib, core, rib, core, rib, core setup (aka on a non built up wing), it would behove them to instead of wrapping the spar with kevlar thread to consider bias glass. AKA, if I were to ever build a Genie (not out of the question), I'd wrap the spar in bias glass. But that is just me...

Ryan

[atjurhs]

Dammn, that thing looks crappy :D

[Les.Akers]

Jimmy,
Here are some wing rods. Depending on the wing span, I would use the 1/2 or 5/8 carbon. The spar can be carbon tubes or you can build them with tapered carbon laminates top and bottom with endgrain balsa shear webs.
See posts 7 and 9 here.

http://www.rcgroups.com/forums/showthread.php?t=443397&highlight=lovesong+spar


http://nesail.com/detail.php?productID=1103

[soholingo]

I was about to start a thread with the same theme... But here is my question, which is the better spar design for a composite laminated foam wing?

1. Carbon, Foam Carbon sandwich wrapped in glass.
2. Carbon, vertical Balsa, Carbon Sandwhich wrapped in glass.
3. Carbon tube.

The all have their merits but which is easier, stronger to build?

Thanks,

J

[Les.Akers]

Carbon tube is the easiest. It's already manufactured. All you have to do is select the appropriate size and install in the cores.

[rdwoebke]

I was about to start a thread with the same theme... But here is my question, which is the better spar design for a composite laminated foam wing?

1. Carbon, Foam Carbon sandwich wrapped in glass.
2. Carbon, vertical Balsa, Carbon Sandwhich wrapped in glass.
3. Carbon tube.

The all have their merits but which is easier, stronger to build?



I'd say most difficult to build is 2, since if you have a tapered spar you have to carefully cut each shear web. But, it is not terrible to do, Dr. Drela made a set of plans for a mitre box for doing this (I had to do that with my crappy Aegea).

[soholingo]

So which of the three designs is the strongest?

[rdwoebke]

I'd have to say 2, based on the fact that in the Supra, in critical spots Mark has you replace the foam with wood (center section). but, I'm thinking that in general the high density foam is plenty good enough (else I'm sure Mark and Tom would use wood in the Supras).

I'm sure the tube spar is great (heck, I own a plane with that setup), but there has to be a reason that Mark went with the spar setup he chose on the Supra...

Ryan

[John Lueke]

Jay

I built my first Genie wing spar using the 3/8 carbon spars with 3/8 balsa out six inches from the centerline and transitioned over three inches to the foam. It was wrapped with kevlar. This is the balsa sheeting wing "easy" wing. I have done full-pedal launches moderate zooms in 15-20 mph winds with no problem. But the real test occurred on Saturday in a Cincinnati Soaring Society contest. I entertained the crowd with a really bad launch, wing hit back of my head and the Genie is headed left in knife edge flight and cartwheeled before I could get off the pedal. Sequence, left wingtip, nose right wingtip then right horizontal tail. I doubt anyone thought that the plane would fly again anytime soon, Ryan was there he could comment on the carnage, but after an hour and half of work it was ready for Sunday and again doing full pedal launches in 15 mph wind. So I think either construction, foam or balsa webs, wrapped in glass will handle anything you plan to do. Wrapping in glass may give more long term durablility than kevlar in addition to being stronger from the beginning. On the second Genie wing, I used all 3/8 balsa shear webs because I had it and it was not much more work than using a longer strip of foam.

[Bryan Quick]

OK, I've read up on spar design terminology and calculating loads.

If you're using carbon spar caps, for a 2M wing of about a 1/2" width, the caps will handle a load greater than the bending moment of a glider wing as long as they are .060" or greater thick and the cross section of the core is greater than 0.8" (roughly).

Balsa vs. Foam Core

The core handles the compression, the balsa is about 200 psi. The foam is about 60 psi for the high load/foamular 600. Your spar has a greater chance of buckling using a foam core. So, if you look at the Supra spar plans, Dr. Drela recommends using balsa shear webs in the center of the wing to handle the higher compression loads due to the bending moment in the center of the wing.

Shear
He's telling you that the Kevalr thread wrap won't handle the shear loads as effectively as a fabric with the fibers oriented on a 45 degree angle in two directions. Makes sense to me.

Check out his methods and test results here:

http://www.charlesriverrc.org/articles/allegro2m/markdrela_allegro2m.htm

I'm not sure I want to spend five or six days reinventing the wheel.

I built a spar for a 2m ship using 1/2" carbon tapered caps and balsa core with fiberglass wrap. I'll let you know how it does on the winch. I'm thinking it will bend lots but won't break. So far it holds 100 lbs. in the center balanced on the joiners, flexes about a 4.5". I'm a sissy, don't want to go the full 160 lbs.

[atjurhs]

2. Carbon, vertical Balsa, Carbon Sandwhich wrapped in glass.


I'm wondering about the way that Dave and I beefed-up our 100inch OLY2 wings with 0.01 CF epoxied atop Spruce spars that came with the original kit, then wrapped in Kevlar thread/tow?

I understand the shear strength of Dr. Drela's Supra methodolgy, outlined by Soho and the Supra build plans, but I'm wondering if the success that Dave and I have had with our "Full-Pedal Launchable" OLY2s is because of the added (relative to the Supra build method) Spruce spars between the balsa shear webs and the CF?

Thoughts???

[rdwoebke]

One thing to bear in mind (and remember, I'm not a mechanical or aeronautical engineer, so this is just my EE's way of trying to understand this stuff) is the thinner the profile, the more critical the spar. On thicker profiles (like on an Oly), you have more distance between your spar caps.

Wood cored carbon capped spars wrapped in Kevlar tow works great. There are a number of designs that use that method (Bubble Dancer, Allegro Lite, etc.). Remember, as Mark has said a number of times before, on this kind of construction method bond between the spars and the cores is critical. But, if you have the option (say like on a bagged wing), I never understood why a person would kevlar wrap rather than bias glass wrap. AKA, I could never figure out why on the Genie Harley would suggest the kevlar wrapping over bias glass wrapping. Seems the bias wrap glass would be a bit of insurance for poor building (like I'd be likely to do).

Ryan

[rdwoebke]

Ryan was there he could comment on the carnage,

John,

I saw that, but did not realize it was a Genie. I would have liked to inspected your Genie a bit closer to admire your project, had I realized that was what you were flying...


Ryan