24' glider spar.... - RC Groups
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Feb 19, 2007, 10:56 PM
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FairlyFaded's Avatar

24' glider spar....

hello everyone!
Im building a massive foam wing (24') and i need some advice on the spar. Im doing a D box structure as you can see, i plan on having two spars. one being the back of the D which will be most of the strength and another a foot or so behind the first one. The ribs are each 4' long. The entire thing is made out of white eps foam. The wing needs to beable to hold around 140 lbs ish. Most of the load will hopefully be on the spar/dbox. I did put the spar a little far forwards but hey theres no going back now! My mats are- 4yards of 1' 4ish oz uni directional carbon, and 4yards of 4oz glass. My original plan was to put a few layers of the uni directional stuff on the top and bottome of the 1 1/2" foam spar with glass underneath and above going a few inches over the edges of the spar, but i dont think this will be stronge enough.... I am now thinking of using some very nice light wood i have seen at the hardware store. It is 1 1/2" wide by around 1/4". It feels veryyy light and very rigid. Im not sure what kind of wood it is but i think it would make a great spar "cap". So now im thinking i will do one layer of glass, then the wood spar, then ontop of the wood 1,2 maby 3 layers if the unidir. carbon, then another layer of glass all over that. I think that would make for an increadibly strong and light spar. what do you guys think? thanks!

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Feb 19, 2007, 11:02 PM
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Why not go to Wicks Aircraft supply or Aircraft Spruce and order some aircraft grade spruce for the spar caps. It's good wood, light, strong and very close grained.
Feb 19, 2007, 11:11 PM
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picture sais it all.... and by spar to spar i mean rib to rib....
Feb 19, 2007, 11:16 PM
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thx for the link! i could use that stuff, it would probably be the best stuff to use. but shipping would be a huge pain for a 10' long section, or even a bunch of 7' sections. I think i may go with the stuff at the store, its cheap and looked really nice, if i get it ill post some pictures of it. What kind of grain should i look for? thanks!
Feb 20, 2007, 12:45 AM
Knight of Ni-wom
The wood would help to distribute any buckling loads across the foam, but won't do as well as carbon will as far as resisting bending loads. I personally would replace the foam spare cores with endgrain balsa (which will hold up to a lot more compression to resist buckling) and then stick with several layers of carbon for spar caps.

If you go with the wood caps ditch the fibergalss between the wood and foam; it's not going to do anything for you that the wood isn't already doing. Keep the fiberglass wrap though, that will help prevent the spar caps from buckling outwards during bending. Kevlar would be a much better choice for this than fiberglass though. Keep in mind that the entire length should be wrapped, and if you only wrap between the ribs (leaving gaps evenly distributed) the wing will fail at these unwrapped sections near the root of the wing.

Oh, and that is a HUGE wing!

Feb 20, 2007, 02:04 AM
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I agree completely that there would be better ways of making the wing spar, such as end grain balsa and kevlar. but im doing this on somwhat of a budget so that knocks alot of ideas out. So if i understand right you think the spar will lift off were it crosses over the rib because theres not cloth over that right? I agree so i will remidy that by putting a 2" strip running cross wise over the spar cap and then down the side of the guset. It is true that carbon is technicly stronger then wood, but i find when combined together then can be reallly strong, much stronger then either one of them alown, so i think my wood/carbon/glass spar cap will be plenty strong. One thing that comes to my mind with the spar is the fact that my foam will be much more enclined to depress then to have the spar cap lift off. For this reason i dont think i will really need much glass- or kevlar holding the spar down, unless i put it weblets and up the load greatly. I will run the glass all the way around the spar though as you sugested. I think i may want to add some weblets... agreed? or do you think this setup will beable to stand up to 150lbs? thanks!

P.S. any ideas on cool inexpesive ways of making some weblets?
Also i need an idea for the trailing edge.... Right now i just have the spars going down to about 1/4" on eacn end. I also added the middle spar a bit ago and it is all drieing right now. Do you think i could maby do a trailing edge just made out of a rod of carbon or glass. Iv seen this on some hlg's. It seems like it would be preaty flimsy. I could amagine after the shrink covering goes on it may get warped... but it has worked for some people...Anyong have any ideas?!
Last edited by FairlyFaded; Feb 20, 2007 at 02:16 AM.
Feb 20, 2007, 04:28 AM
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"The entire thing is made out of white eps foam. The wing needs to beable to hold around 140 lbs ish."

The shear load is carried thru the foam rib. So the spar design is the very bad because of weak foam at the rib is part of the spar. You must split the ribs so that shear web is not from root to tip so that the shear load is not interupted.

I don't think the wing is strong enough for is size. Why do you think the wing strength is to carry 140 pounds? Do mean 140 pounds of lift? If lift is 140 pounds, what red line air speed to generate lift? The wing will fold at very, very low air speed in my opinion.
Feb 20, 2007, 09:20 AM
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I think the opposite, I feel you will have more strength than you can imagine with two thin spar caps of spruce. Just think, a 16' 2X4 supported only at the ends can easily hold a 140lb person even on the 4" side. Sure it will bend, but it won't break. 2X4's are made of poor wood.

If you use spruce, and have spar caps of 1.5" X 3/4" dimension epoxied to the foam and wrapped as is with thick carbon tow (or similar), it will support 140lbs with ease.

When you load a wing with 140lbs, the center area of the wing supports the majority of the load, the tips do less than their share of work. Therefore, if your spar can hold 100 lbs while spanning only 16 feet, you will be all set.

Here is another option:

Skip all this complicated and time comsuming spar work, make a simple spruce cap spar wrapped with cheap flat strand cord, and add two 1/16" stainless cable flying wires below the wing.......similar to a hang glider.

Very strong, cheap, quick, and probably 20 lbs lighter.

Here is an ancient photo of an early hang-glider, but it is a great shot of the flying wires I'm talking about. (The assistant is holding one of them). Note, the hang-glider's span is about 24', and the "spar" (3" alumimum tube) is very weak compared to even the simplest spruce spar for your wing.


Good luck
Last edited by flystoolow; Feb 20, 2007 at 09:31 AM.
Feb 20, 2007, 09:27 AM
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140lbs !!!, is this being built to carry you Fairlyfaded?
Feb 20, 2007, 01:07 PM
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So what if it is : D.... 140lbs is a bit light, i think the wing should beable to suport 160lbs of lift, so around 150lbs of dead wieght. I agree with flystoolow that the spar will be pleanty strong- for example the 2x4. Olie you do have a good point that it may have been good to split the rib not split the spar, but i think both ways have their adv. With my set up when the spar goes into the rib the foam around it widens because of the gusets. So instead of the spar being 1 1/2" wide it goes to being 2" and then 48"s wide. Why i think this design will be good is also because the but joint wont be suporting any of the load. All of the load will be ontop and on bottom of the spar in the wood/carbon which will be continues across the wing. The foam isnt suposted to be stronge its just suposed to make there be a gap between the to spar caps- the larger the gap the stronger it is. Of course the foam needs to resist buckling which it does but the wood/possible weblets will increase the buckling resistance.
-fly, i think im going to use my original method. Im fairly compitent when it comes to composites and iv allready spent the money on them : D. I think the spar will come out being very light, id guess the wood was about 2lbs each so around 5lbs, then maby 1lb of carbon/glass.
p.s. can you imagine how strong a piece of 1/4" wood with a bunch of unicarbon ontop of it is interms of tension and compresion!! probably a few tons....
Last edited by FairlyFaded; Feb 20, 2007 at 02:26 PM.
Feb 20, 2007, 02:38 PM
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some more pics

Here is the second spar in place, put little foam booties on the trailing edges to keep them warm (safe). The wing wieghs 1 1/2 lbs right now!
So do you think i should put one layer of glass on the second smaller spar just to give it some rigidity? Also do you think i should put glass on the tops and bottoms of the ribs? it wouldnt add much wieght but would make them alot stiffer, the covering will add a ton of stifness to the structure behind the d box.
One last thing, any ideas on light clear inexpensive covering? i heard you can use laminating sheets??? thanks!
Feb 20, 2007, 04:30 PM
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So is this going to be for some kind of "Red Bull" human flying thing?
Feb 20, 2007, 05:06 PM
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Originally Posted by rdwoebke
So is this going to be for some kind of "Red Bull" human flying thing?

my thoughts exactly
come..... on fess up

probing minds want the full story

oh, and by the way...are the olds on holiday or something? not sure if I'd have won any favours doing that
on the dining room floor when I lived at home!
Feb 20, 2007, 05:25 PM
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rdwoebke's Avatar
I also think you are going about this all the wrong way...

You should have titled the thread something obscure like "here goes nothing II"

The photos are way too clear. We can clearly see that you are quite the crafstman.

Your posts don't have near enough spelling mistakes.

Because of the above, you are missing out on free parts/advice from highly frustrated posters. Of course, you also have a lot less chaff (except this post) in this thread...


Good luck with your project...

Feb 20, 2007, 05:27 PM
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lol the "olds" are at work.... I have this week off from school. Iv been building planes for 10years all over the house so they have gotten used to it : D. Im not to sure what a red bull human flying thing is, maby one of thouse events were people jump off a dock and try to fly? If so no thats not what it is : D It occured to me a few weeks ago that electric human flight is not only doable but also could be doen quite cheaply. I have found a nice sourse of li-ion 18650 cells (2.2ah). At only 1.75$ a cell i can put together a 1.6kwh pack for only 325ish$$$'s. With the price of chinese brushless stuff at its alltime low i can put together a very efective motor system for another cupple hundred. With that set up i could have bursts of 120lbs of thrust at 30mph pitch speed from a 25lb set up.
Yes it is true that this should be in the electric forums, but i think if i put it there theyd say to make it out of one huge piece of depron.... O.o
Also another thing that spured on my building of this plane is the fact that airplanes are really veryyy simple. I have found that i can make a plane from scratch in 2 hours and it works, all you really need is a decent SG/wing loading and some resonable control surfaces. Ill try and get over to the local ACE to get the spar cap wood so i can start building the spar, ill post pictures if you like, hahah who am i kidding who WOULDNT want to see pictures : D.
Anyone have any ideas for a trailing edge? thanks guys!
p.s. dont get to thinking "oh this guys young and foolish itll never work", cuz back a many years ago when i went to get a "fire bird" as my first rc plane the guy behind the counter said i was too young, boy was he wrong. Iv built alot of nice things like my composite dl50 and my cnc cutter so dont prejuduce me, not that anyone has been : D

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