Bi-directional spruce for fuselage skins

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Wagy59

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Alright...here is a VERY BASIC frame for the airplane I'm designing..I'm going to build a light frame and cover it it thin strips of "spar quality" spruce..Light frame meaning aircraft spruce using usual methods of construction..curved fuselage frame parts laminated spruce and line sanded once the frame is done..Then 2-3-4 inch wide strips of spruce SLOWLY glued on the frame...As designed so far, two layers running different directions....
572.jpg
 

SVSUSteve

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Just out of curiosity, where is the engine going on this aircraft or is it intended as a glider?

That said, I'd love to see more wood skinned aircraft flying. They are something of a lost art in this day and age.
 

wsimpso1

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Heard of the Pfalz line of airplanes made during the Great War? How about the deHaviland Mosquito during WWII? Or the Dr. Frati's Falco or George Pereira's line of airplanes. All made either of thin plywood or of veneer laminated into plywood, which is what you are proposing. Not new. Definitely a way to build an airplane. If you like working wood, it works great. Since I am an advocate of building in a material that you like working in, it could be a great thing.

Billski
 

SVSUSteve

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Since I am an advocate of building in a material that you like working in, it could be a great thing.
Amen to that. There's a reason why I was considering putting wooden wings at least so far as the ribs are concerned on my LSA. I love working with wood but decided to go with metal (with a fabric covering) for the initial version for the experience which is more useful toward future projects.
 

ultralajt

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Check this forum (plywood skinning)..diagonal.
It is not for real aeroplane, but just simulator cockpit, but it is shown, how to cover curvatures with strips.
Check also boat builders forums.. some boat skins are made from thin wood strips placed diagonally over the curves..one layer one direction, another layer another direction..
I think, that i also seen once how they did nose cone from wood strips for an vintage aeroplane replica.. maybe Airco DH2..
Plywood skinning

sking017.jpg
 

Wagy59

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Yes, that's the general idea exactly. I guess for me the draw back is the labor involved in it, since I would be carefully fitting each strip by hand work and doing as perfect a job as humanly possible, but I love working with wood anyway...here is a screen shot showing the bi-directional idea...autocad wont wrap it around quite like it will go in real life but this gives the general idea of 2 layers running opposite directions...580.jpg579.jpg
 

delta

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It sounds like you're essentially making your own plywood. There's got to be an easier and lighter way to go about it, but do what brings you joy. I assume by the title of the thread you're planning on crossing the strips at 90* for expansion stability. You might consider redwood bender boards at your local hardware if you're determined to proceed with this process.
 
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Wagy59

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well...in effect, I'm making my own plywood by running two relatively thin (3/16") layers of spruce, so the way I'm looking at it is I'll have a fuselage skin with identical mechanical & thermal properties as the fuselage frame it's bonded to...I don't see how you can get any better than that....And also, since the spruce skin will be way thicker than if I use birch or mahogany plywood, I doubt I would ever see wrinkles in the skin...I have some aircraft spruce laying around and already re-sawed some pieces to 3" wide by 3/16" thick and glued them up in a curve, just for for grins, to see what I end up with, and I'm already impressed.....just ridiculously stout for the weight....I was walking around in the garage bending on it and smacking things with it and it's darn near indestructible. I'll use West Systems marine epoxy, since they have good manuals and tell you how to do it right whether your laminating or what ever... I'm more worried about the layer of 1.5oz glass on the outside of it and what happens over time with expansion and contraction with THAT! and it has been suggested to go 3 layers instead of just two...Although I agree that should be considered, I'm still stubbornly resisting the idea
 

Wagy59

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How will you bond the two plies together? What adhesive and what will you use to apply even clamping pressure over the varying curves?
I've been thinking about that!...West Systems....Epoxy by the Leading Epoxy Manufacturer | WEST SYSTEM Epoxy
That's my answer..mostly:ban:

I've used this epoxy quite a bit. it is definitely superior stuff and relatively easy to use and has great literature coaching you on how to use it properly for various applications, whether structural, laminating, sealing, glassing etc....I had a couple wing ribs I built with it...Spruce truss type with birch ply gussets....found a big plastic tub storage container in the attic and stole it from that woman I live with and filled it with water and weighted down one of those ribs in it and forgot about it for 3 or 4 months, then finally took it out, let it dry, and then beat the crap out of it on my jointer (450lbs cast iron) until all i had left was chunks of spruce bits with bits of birch ply glued to em...glue joints were all absolutely intact...it's good stuff
 
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delta

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well...in effect, I'm making my own plywood by running two relatively thin (3/16") layers of spruce, so the way I'm looking at it is I'll have a fuselage skin with identical mechanical & thermal properties as the fuselage frame it's bonded to...I don't see how you can get any better than that....And also, since the spruce skin will be way thicker than if I use birch or mahogany plywood, I doubt I would ever see wrinkles in the skin...I have some aircraft spruce laying around and already re-sawed some pieces to 3" wide by 3/16" thick and glued them up in a curve, just for for grins, to see what I end up with, and I'm already impressed.....just ridiculously stout for the weight....I was walking around in the garage bending on it and smacking things with it and it's darn near indestructible. I'll use West Systems marine epoxy, since they have good manuals and tell you how to do it right whether your laminating or what ever... I'm more worried about the layer of 1.5oz glass on the outside of it and what happens over time with expansion and contraction with THAT! and it has been suggested to go 3 layers instead of just two...Although I agree that should be considered, I'm still stubbornly resisting the idea
As tough as the stuff's going to be it looks like you could eliminate most of the internal framework or with glass in the picture you could almost build a lattice out of spruce like the old Wellingtons (I believe). I'm just trying to see your weight held down a little.
 

Wagy59

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yes I was thinking that too...but it seemed I was going to end up with an entirely different way of constructing the thing and I'm just not there yet...But I'm with you! I know what your're saying...That's why when I started moving towards the spruce fuselage skins and then started doing some basic calcs (mostly in my head) I realized that, well heck, i no longer need as heavy a frame so I went from 3/4 and i inch frame to 1/2"...which still makes it really really really strong..actually stronger.....I haven't done it yet but I can get really accurate weights of the structure out of autocad...I've done frame before at 3/4-1" spruce and was getting around 75-85 lbs of spruce for the fuselage....I need to do a few more things and I'll do a weight for the spruce shell and the frame and see what it comes to and report back
 

Jay Kempf

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yes I was thinking that too...but it seemed I was going to end up with an entirely different way of constructing the thing and I'm just not there yet...But I'm with you! I know what your're saying...That's why when I started moving towards the spruce fuselage skins and then started doing some basic calcs (mostly in my head) I realized that, well heck, i no longer need as heavy a frame so I went from 3/4 and i inch frame to 1/2"...which still makes it really really really strong..actually stronger.....I haven't done it yet but I can get really accurate weights of the structure out of autocad...I've done frame before at 3/4-1" spruce and was getting around 75-85 lbs of spruce for the fuselage....I need to do a few more things and I'll do a weight for the spruce shell and the frame and see what it comes to and report back
Yup,

Anything that has a volume can have a weight. Anything that has a weight can have a CG. Anything that can have a CG can be reconciled against a datum to lead to an overall weight and balance. By putting things in like different amounts of fuel, different sized occupants, engine oil lumps, etc... you can do all of the limits of the CG by just reconfiguring the model for each case. In Solidworks we have a suppressed state for any feature or component so that you can just turn things on and off and remeasure. It also gives you moments of inertia for sections, whole assemblies, radius of gyration, yadda so you can inspect and feed those numbers directly into spreadsheets to calculate anything you want. That's why it is worth it to suffer through the learning curve with this stuff. Most times I find a number I don't like it is because I input something wrong not because the computer is in error. Sometimes I have to fake it out for things like sandwich composites such that I don't have to detail the plys. I have made custom material specifications in the software for say 1/4" foam core with .010 of carbon BID both sides. I put in a composite density for that and shell the feature to that thickness and get a very accurate weight and CG. Go back and change the shell dimension and reapply the material for another thickness core and reanalyze the model and you can run through all kinds of cases in a hurry for optimizing structures. I also do first run strengths looking for structural hot spots using these fake models.

The moral being you don't have to put all the planks on to look at this. Just put a skin on. If you want a whole bunch of skin segment models just slice up the overall skin into pieces after you put the overall skin on.
 

Hot Wings

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That's why it is worth it to suffer through the learning curve with this stuff.

I'll second that!

It's so fast and easy to simply drag parts or sub-assemblies around in 3D and then double check the CG that this one feature is almost worth the time investment. Once you get a database of common parts built up even first round "what if" go pretty quick.
 

davidb

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Just some thoughts...epoxy is the obvious choice because you only need to bring the plies in contact with each other without a lot of clamping pressure. But, I can't help but think how difficult the whole process will be. Boat builders refer to this method as "cold molding". Seems they have two or three people working together and vacuum bag the laminations in small sections. Working alone and bagging doesn't seem possible. [I can't even imagine how you would bag it anyhow] I guess you could use temporary staples but seems you would have to staple into the underlying stringers for them to grip. Gosh, think about how many batches of epoxy you'll have to mix because (if you're working alone) you'll only be able to do small sections given the set time of the epoxy. Seems AS&S can provide Sitka spuce spar stock as thin as 1/8" thickness if you don't want to bother with resawing and planing. Have you considered using a different wood? You'll have a fortune invested in all that spruce.

I'm a bit of a woodworker myself and have built a wood strip canoe, so I share your passion but I'm overwelmed by the thought of this project. Have you considered just one layer of stripping with fiberglass skins?
 

Wagy59

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Yup,

Anything that has a volume can have a weight. Anything that has a weight can have a CG. Anything that can have a CG can be reconciled against a datum to lead to an overall weight and balance. By putting things in like different amounts of fuel, different sized occupants, engine oil lumps, etc... you can do all of the limits of the CG by just reconfiguring the model for each case. In Solidworks we have a suppressed state for any feature or component so that you can just turn things on and off and remeasure. It also gives you moments of inertia for sections, whole assemblies, radius of gyration, yadda so you can inspect and feed those numbers directly into spreadsheets to calculate anything you want. That's why it is worth it to suffer through the learning curve with this stuff. Most times I find a number I don't like it is because I input something wrong not because the computer is in error. Sometimes I have to fake it out for things like sandwich composites such that I don't have to detail the plys. I have made custom material specifications in the software for say 1/4" foam core with .010 of carbon BID both sides. I put in a composite density for that and shell the feature to that thickness and get a very accurate weight and CG. Go back and change the shell dimension and reapply the material for another thickness core and reanalyze the model and you can run through all kinds of cases in a hurry for optimizing structures. I also do first run strengths looking for structural hot spots using these fake models.

The moral being you don't have to put all the planks on to look at this. Just put a skin on. If you want a whole bunch of skin segment models just slice up the overall skin into pieces after you put the overall skin on.

You made a mistake talking about all that...Now, it's likely I may bug you with questions about certain things...LOL...solid works being one of them (ive been interested with it)....but ya know, I've resisted getting involved with other cad programs and that sort of thing because it takes up so much of my limited brain resources...nevertheless...you be speaking my language!...Don't be surprised if i ask ya foe help on something in the near future...hope ya don't mind...:nervous:
 

Jay Kempf

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You made a mistake talking about all that...Now, it's likely I may bug you with questions about certain things...LOL...solid works being one of them (ive been interested with it)....but ya know, I've resisted getting involved with other cad programs and that sort of thing because it takes up so much of my limited brain resources...nevertheless...you be speaking my language!...Don't be surprised if i ask ya foe help on something in the near future...hope ya don't mind...:nervous:
Any time. Putting aircraft into non aircraft specific modelers takes some creativity. Most modelers have similar command sets so normally you can take a technique and port it over to another platform. I know what works for me which is some evolved tricks and techniques that produce stable models. Catia and PROe think they have a lock on this stuff. But any modeler will work if you play to it's strengths. Solidworks lets you do a loft and then slice it up any way you want. And the edges of the slices are real geometry when done. That is not the approach for making manufacturing models and drawings but it is a good approach for doing what you are currently doing which is doing proof of concept models before detailing.
 

fly2kads

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yes I was thinking that too...but it seemed I was going to end up with an entirely different way of constructing the thing and I'm just not there yet...But I'm with you! I know what your're saying...That's why when I started moving towards the spruce fuselage skins and then started doing some basic calcs (mostly in my head) I realized that, well heck, i no longer need as heavy a frame so I went from 3/4 and i inch frame to 1/2"...which still makes it really really really strong..actually stronger
What you are touching on here is something that I find very interesting. At one end of the spectrum, you have a load bearing frame with an aerodynamic fairing, a smoother version of the old fashioned fabric covered structure. At the other end of the spectrum is a monocoque shell with local support for cutouts, and perhaps a few formers to resist buckling. In between, there is a large middle ground of semi-monocoque structures where the load is shared between frame and skin. There is a lot of opportunity to optimize the structure for weight/strength, materials, and build process (e.g. Skin laid up over a temporary mold vs. doing it in-situ over frame/stringers). There are a lot of factors that go into making a decision on where to land along this continuum. I think it starts with your goals for the project, a fully developed set of load cases, the materials you want/have to work with, and an assessment of your fabrication capabilities. This construction technique allows for a lot of flexibility.
 
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