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156" Foam Wing Carbon Fiber Question

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Old 09-27-2010, 02:54 PM
  #1  
billk101
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Default 156" Foam Wing Carbon Fiber Question

I am back building a 1/4 scale P-38 (156" wingspan 30" root cord)

My tendency is to over engineer on the safe side so I am wondering if my proposed use of carbon fiber makes sense?

step 1 cut foam core for 3 piece wing

step 2 install 1" by 1" hardwood spar (spar will taper top 1/2" thickness on outer wingtip)

step 3 apply 3.7 oz unidirectional carbon fiber directly to foam core mirroring postion of cf on top and bottom (see attached cf layout diagram, picture 2)

step 4 sheet entire wing (except last 6" to 8" of wingtip) with 0.010 G10 fiberglass sheet

These are only the high level steps. I would appreciate any constructive feedback.

Regards,
Bill

UniWeb Info:
[link=http://www.acp-composites.com/home.php?cat=251]ACP Composite Product Info[/link]

UniWeb is a new and easy to apply type of reinforcement. It consists of a nonwoven sheet of unidirectional carbon fibers that are held in position by a fine spider web of polymer fibrils Iying on the surface. The special polymer used is compatible with epoxy and polyester resins. The fibril or web system that bonds the reinforcement fibers together allows the fabric to be cut easily, trimmed, or slit, giving clean edges with absolutely no fraying. The fibers lie flat and straight, and cannot shift or bunch up as often happens with uni-stitched fabrics. Carbon Fiber UniWeb weighs 3.7 Oz. / Sq. Yd. and is .006" thick.
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Old 09-27-2010, 04:31 PM
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jetjockey69
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Default RE: 156

I've built a couple of foam core, fiberglass skinned, vacum bagged wings for gliders with 145" span, and I capped the spars top and bottom with 7mil thick unidirectional carbon that tapered span wise. For your P-38, I would say that by my standards, your approach is just right.
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Old 09-28-2010, 05:41 AM
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Magne
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Default RE: 156

Bill,
Do you have a shear web in your spar? (I.e. connecting the upper and lower spars, or will all the shear forces be taken by the foam core?)

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Old 09-29-2010, 07:06 AM
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billk101
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Default RE: 156

Magne,
I am not sure if shear web is required. Does anyone have thoughts regarding shear web vs the foam acting as shear?
Bill

Suggestions and experienced input is always welcome.
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Old 09-29-2010, 09:03 AM
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Default RE: 156

The Foam should work as a shear web. If your going to run UNI-tow over your spar, i think the spar could be reduced in size a bit. I would probably do a 1" x 5/8" spar and have it stand on edge (so the 1" side is standing vertical). This should give it a bit more strength and save a few ounces.

In a wing that size, i would probably run an additional wing spar at the leading edge of the flaps and ailerons. And then a Wing rib at the tip of the outer flap/root aileron intersection to connect the rear spar the the leading edge.

with 3.7oz cloth and only .010" G-10, i would suggest you inlay the cloth down into the foam, as when you go to lay the G-10 skin down, you'll leave a ridge where the edge of the carbon is.
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Old 09-29-2010, 04:16 PM
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SCALECRAFT
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Default RE: 156

I would do what invertmas has suggested. However, I would make a laminated spar using four 1/4" inch aircraft grade plywood with unidirectional carbon sandwiched between the ply layers. Maybe even capped off with carbon as well on the ply sides.

I would make sure I used a tip rib or wood tip to tie the additional smaller spar all together

All clamped together and wiped of the residual epoxy resin.

If the natural wood spar has a hidden flaw in the grain, your wing may find it at the worst time. Then the unidirectional carbon strips are useless.

Steve
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Old 09-30-2010, 04:27 AM
  #7  
Magne
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Default RE: 156

My opinion/question:

Carbon and wood have very different properties. Carbon, particularly unidirectional, (loaded longitudinally) is stiff (high modulus).
In comparison, wood is flexible. What happens when you combine the two and expose it to a load, is that virtually ALL the load is taken by the carbon.
That is because it is so stiff, and the elongation of the fibres is very low. The wood on its own could potentially also take a significant load, but is not "allowed" to contribute, because the carbon is preventing elengation. (Until it fractures, then the wood takes all the load...)
Therefore, I think that if you are going to put carbon in the wing, there has to be enough there to take ALL the load. In which case the wood is not actually doing any job, other than maybe support the carbon a bit and prevent buckling in compression.

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Old 09-30-2010, 07:20 AM
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Default RE: 156

Magne,

I am not experienced in carbon fiber techniques but what you are saying makes sense to me. My understanding of this is that the under a vertical load the fiber on top would be resisting the force under compression and the fiber on the bottom would be resisting the force under tension (thus create a tubular structure).

If a spar is not providing any real value why add the weight? I have also considered using an aluminum 1/16" walled 1" by 1" square tube as a spar with cf as a top cap. The weight is very similar to that of a wood spar but is much stiffer.

I just thought of this: What if I wrapped the aluminum tube with CF thus creating a square cf tube?

Any thoughts? Anyone?

Bill

The 0.10 g10 skin will certainly be stronger than a traditional fiberglassed balsa skin but the biggest reason is an almost paint ready surface when done.
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Old 09-30-2010, 09:10 AM
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Default RE: 156

According to Aircraft Spruce and supply out here, epoxy is measured in modulus for stiffness (once applied and cured) and composites are measured in tinsel strength for stiffness.

All the load is not taken up by the carbon in this specific configuration, only a portion at a certain stage. Each supports the other.

Balsa and carbon has been doing the same for years.

Steve
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Old 09-30-2010, 09:20 AM
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billk101
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Default RE: 156

Steve,

What do you think of the concept of wrapping 1/16" wall 1" by 1" aluminum square tube as opposed to using wood spar? From what I calculated the aluminum would be similar in overall weight to wood but would not have potential flaws or weak points that you have mentioned in a prior post.

Thanks,
Bill
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Old 09-30-2010, 04:58 PM
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Default RE: 156

I like the idea. I would glue unidirectional carbon onto the aluminum and then wrap it with ??

Also, I would fill the tube with a 2 part expanding foam ,(variety of densities, you'll need to figure that out) to keep it from caving in on it self.

Also, consider hard points if you need to bolt something to it.

I'll think about it a bit more and try to remember what other things has worked for us.

Weight and design for success in the application.

Steve
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Old 10-01-2010, 09:35 AM
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dertjack
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Default RE: 156

A spar consists of an upper and lower cap and a shear web. In this case the upper cap is the Uni carbon. It's not clear, but I assume you will have two 1x1 spars, one on the upper surface and one on the lower surface??? As a spar cap the carbon will take the majority of the load away from the wooden spar cap.

I also would not rely on the foam to act as a shear web on a 156" span plane. You need to connect the upper and lower spar caps with a shear web. The wood spar would make a nice place to attach the shear web.
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Old 10-03-2010, 04:17 PM
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Default RE: 156

I would first like to see structual drawings of the actual aircraft?

The construction used on the actual aircraft would offer valuable information in your decision making.

Anyone else ever build a P-38 this size?
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Old 10-04-2010, 12:17 PM
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billk101
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Default RE: 156

Charles,
Is there specific areas you are looking for more detail? General construction info is as follows:

-foam core wings sheeted with G10 fiberglass sheeting
-spars top and bottom (cf topcap benefits is the point of thread)
-spar web tbd, there seems to be differnet opinions on the subject
-gondola and booms will be fiberglassed bals/ply standard built up construction
-outboard 4 feet of each wing will be removable for transportation (outboard flap and aileron only required non-permanent connections)
-there will be a 2" primary wing joiner as well as a 1" secondary wing joiner (per drawing attached)
-center section including: horizontal stab, vertical stabs, gondola, and booms will be single unit


dertjack,
Of course there would be an upper and lower spar since one is not good without the other.

Thanks in advance for everyone's input

I have also attached a possible spar config. This spar config would create a "pocket" that could be used as a joiner sleeve for the outboard wing thus removing the joiner tube mentioned above.

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Old 10-04-2010, 05:16 PM
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Default RE: 156

Bill,

Unfortunately, I cannot read your measurements.

Are they in inches?

Charles
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Old 10-04-2010, 06:19 PM
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billk101
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Default RE: 156

Charles,

The measurements are in inches with following summary:

Wing info:
*Root chord 30" (max thickness 4.75") NACA 23016
*Middle Chord 22" (max thickness 3.38")
*Tip Chord 9" (max thickness 1.2") NACA 4412
*Overall wingspan 156"
*Outboard boom to boom 60"
*Removeable wingtips 48"
*2 1/2 degree washout
*114 1/2 square inches aileron per side (229 square inches total aileron)
*229 1/2 square inches of flap per side (459 square inches total flap area)
*2932 1/2 square inches of bottom wing surface

Booms are 101 1/2" long
Gondola is 59" long

Have you looked at the first post? The measurement are pretty legible.
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Old 10-04-2010, 07:55 PM
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Default RE: 156


Yes,

I read your first post. No, I could not make out the measurements. That's why I asked.

Thanks for those measurements.

Where did the 2.5 degrees of washout come from? Will you fly off of grass or are you sticking to pavement only? Overall gross weight?

So, do you have structural drawings of the actual aircraft?

You must have something to go by because you are using scaled down measurements for your model. Correct?

As a former commercial pilot, any pilot for that matter, we are taught to pay attention to V speeds. In fact, that's a given for any piloted aircraft. Know the V speeds and you can probably fly the airplane. Guess at them and you can get into serious trouble.

Aircraft structures are designed with operational V speeds in mind. Extremely large models could be designed this way also. Maybe they are?

That's why a peak at the aircraft's structural drawings are important. At lease to me.

Be interesting to see where the most important structures are located, in relationship to each other?

I personally wouldn't rely on only my modeling experience to design a model that will be worth that kind of money, let alone the investment in time.

I don't do big models like that. 90" is my largest. Doesn't mean I don't know anything or that I cannot offer advice. I am quick to give advice. [sm=bananahead.gif]

But I would really do my homework, in depth! Someone has built and flown P-38's of this size someplace. IMHO, that's the guy you want to talk to. Or guys.

I do have to tell you, guys like you, that take on projects like this, inspire all of us. Well, me anyway.

Kudos!

There's a guy in CT. Metamorphosis, (sp.) plating or something like this? His work is incredible. He has a large P-38, not sure how big, but it's large.

He's all over RCU. Has some secrete way to aluminum plate. His P-38 Thread is like nothing you'll ever see with finishing a model to look like it's aluminum. Check it out! By the looks of his work, he knows more than I'll ever know about models.

Can't hurt talking to this guy.







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Old 10-04-2010, 09:31 PM
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Default RE: 156

Personally i think ultizing aluminum U channels for the spars is a little over-kill for a model this size. Its not "overly" large compared to some other models that are being built with conventional techniques. (look at the 257" B-36 that utilizes uni-directional laminated spars).
Wrapping Carbon around an aluminum part is also just asking for trouble in the long run, as you will be setting up a situation for corrosion which would render the wing useless over time (if it was around long enough).

I still say that a 1" x 5/8" hardwood spar layed on edge w/ uni-carbon on the top and bottom of it and a secondary spar closer to the rear tied together utilzing a root, mid span (or close to it) and a tip rib would suffice w/ the use of carbon that you plan to use. Now that i know you are planning to use a 2" and 1" "wing tubes" type of connection for the outer wing panels, i would also suggest putting a rib connecting the spars and the outer ends of the wing tube sockets together. This should provide plenty of strength for the model (which i'm assuming will end up around 60-75lbs RTF).

I have built 130-140" extra's, caps, Edge's, etc utilzing a foam core wing with only 1/16" balsa sheeting (monokoted) with unidirectional carbon running on the foam above and below the wing tube and just infront of the balsa Trailing edge cap of the wing. The Outer wing tube was supported with a 1/8" 5-ply 1/2 chord rib as well, and of course the balsa leading edge. I never once had a failure in flight. And with the remains of one plane left, i took a wing panel and supported it between some cement blocks and stood on the wing and it didn't budge (i weighted 150-160lbs at the time). It only broke after i jumped on it 3 times.
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Old 10-05-2010, 09:05 AM
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Default RE: 156

I have a question.

What is the span of the outer panels at the engines. This is the point I have seen fail on some P-38s.

The P-38 is not your conventional airframe. Plus being a warbird, they tend to be heavier than non warbird models

steve
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Old 10-05-2010, 10:59 AM
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Default RE: 156

Steve,

The dimension of the center section, outside left boom to outside right boom is 60 inches (this is the point where each 48 inch outer panel detaches). I want to mention again that this center section will be built as a single unit.

The chord at the detach point is 22 inches long with max thickness of 3 3/8 inches and then double tapers to a tip chord of 9 inches with max thickness of 1 1/4 inches.

Is this what you were asking?


Avaiojet,
The washout is to try to minimize tip stall effect at slower airspeeds. This design is scaled from 3-view drawings using TurboCad and I have incorporated the airfoils and and dimension of the full size P-38. The full size used NACA23016 at the root and NACA4412 at the tip. Using Profili software is created the templates used for foam cutting.

This is not my first large scratch RC build; I have build a 1/4 scale P-51 (see attached picture) using traditional stick building methods. That said this is my first foray into large scale foam core building.

I have closely followed Metalmorphous's thread, he is plating a Ziroli 1/5.3 scale P-38 with a 114" wingspan and it looks amazing. The plane I am building is a scratch built true 1/4 scale P-38 with 156" wingspan. I wish I could swing the Metalmorphous process but it won't be needed as I am modeling my plane after "Thought of Midnight" which is painted.

I have also been playing with the overall design and build for 2 years. Here are some of the pictures of the overall design of the gondola

Thanks,
Bill
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Old 10-11-2010, 09:12 AM
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Default RE: 156

Just a quick update:

Foam cores have been cut using my foam cutting table. It takes a little setup time due to the double taper.

The table is designed to pull each end of the hotwire at a different rate so that the cuts are consistent and enter and exit the foam core simultaneously. My only job is to hold the bow as it is pulled through the foam. I my need to lighten the pivot portion of the table since in pulls a little harder than I would like but the overall all results are great. Just a light sand to remove the burrs and minor chatter marks.

Left Center Section
Left Outer Panel
Right Center Section
Right Out Panel
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Old 10-12-2010, 08:59 AM
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Default RE: 156

Cores look good. Is that the Feather Cut bow??

I think as you post pics of the construction, ideas will start to flow in with better understanding for all.

Steve
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Old 10-20-2010, 05:58 AM
  #23  
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Default RE: 156

Have you thought about using a medium strength vertical web spar and utilizing the wing skins for the strength?
I make composite wings and the spar is used as a spacer for the skins which take nearly all the loads.
A lightweight I shaped spar which has carbon at the top and bottom with vertical grain balsa between. Saves a lot of weight.
Only my 2 cents worth or Thai bahts worth.
Jim
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