Props4ever

I like to know, how can i reinforce already built in spar on my Arrow's wing that is fully sheeted also. Spar was made with construction grade plywood and now need's be reinforced with CF some how to in order to take aircraft's weight, take offs and landing stresses as i'm looking into adding retractable main landing gear on scale location. Spar was made as part of fuselage former but this former has been cut out at some point of this model's life as it's open frame now.

Spar ends in the middle of the each wing panel, just 6" behind the "Saw tooth notches."

Posted pictures of both front and back of this open frame spar former.

Any help will be appreciated.

ergocentric

Like a modern jet fighter, sheet the wing with carbon fibre.

flysrc

Sam,
In the 4th picture, Is that a ghost like spirit of the original avro from days gone by?
Dave,
As far as the spar goes. Peel the balsa back(remove the skin) about 1" on either side of the spar and the same the past the end of the spar. Inlay some carbon cloth and epoxy. Replace the balsa skin and sand flush. Do this top and bottom. This would give strength the the spar and surrounding material.

rcjets_63

My Feedback: (4)

Here is a technique I used when building the Yellow Aircraft F-18 (single) that was very simple.

The kit came with the foam wings presheeted and a 1/4" wide slot cut into the foam to form a cavity to receive the spars (which were part of the formers). I decided to do plug in wings which meant installing an aluminum spar. Using the edge of the slot as a guide, I simply cut away the 1/4" width of balsa above/below the foam slot leaving a 1/4" wide slot completely through the wing. The new spar was a laminate of the 1/8" wide aluminum plug-in spars and 1/8" balsa sheet. The wing was then aligned on the fuselage and the spars glued in place using epoxy. Then 1/4" wide balsa was glued in place over/under the spar and sanded flush with the skin. The skin was then glassed and painted.

In your case, I'd simply cut out the spar. You can use a fine toothed stiff blade (like a hacksaw or reciprocating saw blade) and cut the along the spar. The spar itself will act as a guide to the blade. You can cut then subsitute the existing spar with your new stiffer spar and then add the balsa top/bottom caps.

I suspect that you don't have to run the spar all the way out to the notch. A bit of extra reinforcement (eg 2" wide medium fibreglass tape) of the skin over the spar wouldn't hurt either.

Regards,

Jim

Props4ever

Dave,

Good suggestion there, i was thinking on similar lines too myself.

yeah you can say that again, , ,ha ha ha


rcjets_63 aka Jim,

From what i gather, you are suggestion to replace the present spar and insert newer spar by cutting along the sides of present spar to make a slot there and replacing with newer spar, , .

Well let say if i was to do that, then what about the former in the fuselage, it's part of the spar!. Removing this former wont be easy and it can easily get messed up also. I don't think i would want to use heat gun in there as whole fuselage could get distorted also due to heat. I do understand your suggestion and could have applied only if it was matter of replacing spars in the wings only.


Here is another suggestion 2 other guys suggested.

They would remove balsa capping width of spar, maybe bit more on each side then make a 2mm deep slot on each side of the spar lengthwise from root to the tip of the spar, layup CF strip in that slot laminating on both sides, fill the slot with epoxy and add top balsa capping while flushing it with rest of balsa sheeting. Then they would layup 1/2" wide strip of CF on top and bottom of the spar following with application of fiber glass cloth over the wing's surface.


What i would do if i took on these suggestions:

I would take off capping say about 1" wide length wise from root to the tip of the spar both from top and bottom of the wings, make 2 slots along lengthwise of the spar deep enough from top to the bottom and layup CF in those slots on both sides of the spar, once this is done, i would fill the slot with epoxy and milled fibers mixed together. Let it cure and then layup 3/4" wide CF strip on both top and bottom of spar and foam before re-capping the full lengths of this opening, then flush the capping with rest of the wings surface.

Then since i want to add main retractable LG on this model, i would add CF patch on top of the sheeting where main LG will be mounted in the wing, then i would layup 2 layers of FG on the wing's top surface covering CF patch also. I would do first layer from root to the tip of spar and second from root to the tip of the wing, both would be done with 3/4oz fiberglass cloth. I can also do the same on the bottom of the wing also if needed to be or just apply single layer
of FG cloth from root tot he tip.

Former reinforcement:

I would start at the bottom and layup 1" CF tape all around to the top of the former, this i would do on both sides of this former. I would also layup 1/2" CF strip on the fuselage side walls also as i do my layup. This way both former and fuselage will get reinforced. Once this is done, then i can add another proper 1/8" former made from aircraft grade plywood against the back side of this former while butt joining it with epoxy and silica paste to get structural strength from the glue bond.


What do you guys think?...


Sam

rcjets_63

My Feedback: (4)

Sam, you are quite right that I suspected that you would be changing the former as well. Most of the former appears to have been cut away and very little strength remains. The former appears to have been cut away at the top center and the sides. I marked up your photo with black to outline what I understand to be the shape of the former.

Applying a bending load to the spars will result in the top of the former being loaded in compression and the bottom of the former being loaded in tension. These combine to load the sides of the former in shear. The sides of the former will fail (in shear) approximately along the side red lines, the top will fail in bending at the center red line, and that will be the end of your Arrow.

You are increasing the strength of the wing, but a chain is only as strong as it's weakest link (in this case, it appears to be the former). There isn't much material left otherwise you could leave it in place and simply laminate reinforcement to it.

I don't understand your suggestion as to why you would use a heat gun to remove the former unless it's glued in with a hot glue gun (in which case I would definitely get it out of there. The tried and true (and messy) method of getting rid of formers is grinding them out with a Dremel drum sander. Install the new former with Hysol.

Replacing the former also gives you the opportunity to install plug in wings (to which will make the model easy to transport) and perhaps make a few other easy structural improvements or simpify the turbine mounting. I also attached a figure showing a recommended former shape. The grey areas are CF reinforcement.

Regards,

Jim

<edit: added holes in lower corners of former for wire/tubing routing>

Props4ever

Jim,

I see what you are saying and i understand what you mean failing sequence of this present weak former by replacing it with newer one. I mentioned using heat gun was to loosen up the epoxy it's glued on with, but i can always try to use hot knife technique also. No there is no "hot glue" anywhere on this model.

Other thing comes to mind is how to remove this former without grinding it or cutting it in pieces inside the fuselage first, but then how would i be able to install new former as spars will be part of it also unless i just make separate new former by itself and look into making removable wings. I don't think i can do that myself at this point that is cutting these wings off to make removable types, i want to work with what i have and i'm capable of doing myself easily.

Sometime ago i was thinking to make removable OB wing panels pass the notches to make this model easily transportable, but again it's not something i can do myself at this time since this is already built wing.


Here is another idea Jim:

How about i make a insert filler of this former and then glue it with Hysol inside the open section of this former. I can always cut 2 holes for ducting to pass through it while regaining all the strength by it's surrounding material area. I will use 1/4" aircraft grade plywood to make this insert. Once this insert is glued in, then i can always reinforce the former with laminating CF on both sides of the former and will do same on the spars.


These are just the ideas we are exploring, nothing is finalized as yet, so anyone is welcome to chip in with his suggestions and ideas, i like to do things in simplest of ways when possible.


Sam

rcjets_63

My Feedback: (4)

Sam,

The heat gun or knife probably won't make the job too much easier since it's epoxy which withstands heat pretty well (that's why it was used on heat shields for re-entering the atmosphere).

Your idea of laminating a new "filler" former would work if more of the current former still remained. Judging from the photo, you don't have enough surface area for a sufficiently strong glue joint. Additionally, you will likely have to grind away the fuselage/former epoxy fillet just to get the new former to butt against the existing former.

There are still a few ways to proceed. Let's take this off-line and see if we can come up with a solution.

PM sent.

Regards,

Jim

Props4ever

Yup replied to ur PM...


Sam

Edit: Arrow drawing posted for Jim.

Props4ever

I have figured out a way to reinforce this spar by the help of 2 other modelers, now my question is about cutting the spar shorter as it's 13"out into the wing. Since this wing is foam core there is no need for spar to be this much long. I am looking to install main landing gears in the scale location so that is why i want to weigh the option of cutting spar shorter say 3"-4" from OB tip end of it at the "Saw tooth notches", in order to install landing gears at the scale location without getting interrupted with spar in the middle of traction and retraction movement of the strut!!.....


Sam

L Turner

My Feedback: (3)

Sam,

Better lay out in rough drawing the locations of the retracts, retract bearers, struts and wheels on that wing, and consider the whole sructure from the strength viewpoint and how it might fail under both high G flying loads as well as on hard landings.

The Arrow has an unusually wide stance, sort of like the T-2 Buckeye conversion I did last year, as well as a severely swept wing. You may need an aft spar closer to the wing TE where the flap LE is, and carry that spar outwards to the outboard wing panels. You also need the strength of the forward spar to support the impact of the airframe weight on landing tending to bend the forwards wing/fuse joint as well as pushing the retract bearers up through the wing.

How thick are the wing cores at the area of the wing notches? How were the outboard foam panels joined to the inboard wing panel? Are there joiners? Is there an aft spar? Is the sheeting already glassed?

For the straight wing Buckeye, I extended the forward and aft spars out about 1-2 inches beyond where the retract mounts insert into the underside. I connected the two spars with a large 1/8th 5 ply retract plate, about 4 x 6 inches, milled into the foam core, laminated with another 1/8th plate about 3 x 3 with the retract opening cut into it. To diffuse the impact of hard landings, I used .007 unidirectional carbon cloth (Aerocomposites) about 6 x 8 inches, under the top wing sheeting and vac bagged it between the foam cores and balsa sheet skins. The retract plates are tied through the wing to the top skin using 6-8 dowels epoxied perpendicular up through holes drilled in the plate.

If you cannot remove balsa skins already attached to the cores, you might consider adding a thin layer of additional balsa sheeting (?1/16th) with grain more parallel to the wing TE, with CF or glass reinforcement between the layers where needed (over retract area and new spar slots). You could also add 1/4 to 1/2 inch wide .007 CF strips from the root out beyond the wing notches. Sand the contours down so the airfoil is only slightly thicker, then reglass the outside of the skins with 0.5 oz cloth. Would put most of the reinforcement top and bottom of the spars and arround the main gear areas.

Props4ever

Good Idea, it's better i plan this out before cutting anything. I will make a drawing showing all of these LG mounting points.


There are TE spars in the wing structure but they end at the side walls of the fuselage, there is no former or spar brace constantly going through the fuselage to carry any loads of the TE spar or flap and ailerons. This is another area of concern that needs to be looked at after main spar reinforcement is complete.


This is how i'm planning to add 7" to 8" LG mounting plates behind the notch area as it's pretty thin chord thickness in this area. Chord thickness is 1-1/8" behind the notches where LG will be mounted. I will be making whole gear bay first and then reinforce inner top of the top skin with CF cloth while CF laminated ply strips as gear mounts. How OB wings are joined with inner wings, i have no clue, if there are wing joiners or not under the balsa sheeting. I can do some digging with pin every 2" from LE to TE to find out though.

Hmm, i do want to add 2 layers of 3/4oz fiber glass on top of the wings, i am also going to add 1/2" CF tape on the top of the spar under the sheeting and then do re-capping of the balsa skin and flush with wings top surface. I will add CF strips on TE spars and also on top of LE from root to tip before i do full wing top fiber glassing.


Sam

Props4ever

I roughly drew all the gear doors location on right lower wing. This does give good idea how landing gear assembly will be mounted. All has to be on the inner side of the wing fence, just behind the notches. Airplane CofG point is just behind notch but in front of the gear bay.
As i was saying before to cut the spar 3"-4" at the tip end of it, well now that it will have to be cut about that length in order for easy operation of the strut and gear mechanism.


Sam

invertmast

My Feedback: (23)

Sam,
I think you will be ok removing the spar material from the forward spar. BUT you are GOING To have to reinforce the rear spar and make a carry-through bulkhead to connect the 2 wing panels together and transfer the forces through the fuselage via a bulkhead. IMO this bulkhead should have its wing spar "kick outs" go atleast mid span into the wing panel.

Props4ever

Alright Thomas, i will cut it short before or after i reinforce the main spar with CF plates from the behind while passing right through the fuselage. I will also make 4 square notches at both fuselage spar junction joint on top and bottom of the spar and add CF square rods say about 10" long in there. This way my spar and center fuselage will become very rigid and will be able to take a lot of loads and stresses.

As for TE spar, i'm thinking to add a former in the fuselage connection both spar tabs on the side of fuselage and then add say 10" flat CF strip equal to wing thickness on both wings, then bolt them to the former directly. I will get Hysol epoxy and epoxy all the formers in the fuselage, but before i do this, i will cut 2- 3-1/2" dia holes for thrust tubes to pass through.

This is my plan for TE spar, if anyone has another suggestion, then jump in and let us know. [sm=tongue_smile.gif]


Sam

Props4ever

Ok, i can't find CF plates for reasonable price, can i use 2x 1/16" G10 FG sheets laminated together with CF unidirectional cloth on both outer sides instead for this application?. How strong laminated G10 is compared to laminated CF.....i'm talking for 1/8" thickness.


Sam

invertmast

My Feedback: (23)

make your own CF sheet from cloth. Lay up the cloth on a 0/45/90/45/0 bias then sandwhich it between 2 glass plates. weight down and let cure.. You should be able to get 3k carbon sheet scraps off ebay for under $40.

Props4ever

Well i don't shop on Ebay anymore, making my own CF sheets is a good idea. Thomas layup you mentioned is same as how i made fiberglass sheet few days ago. I can certainly make CF sheets also. Results were excellent. Here take a look.