Fixing spars to ribs in metal planes.

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StRaNgEdAyS

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I had to think for a while to decide where to put this, but since it is more a design question I thought I'd put it here.
I am working on the attachments for the wing spars to their corresponding fuselage ribs. I have drawn the 2mm (0.080") spar web to attach to the rib of the same thickness, via 2x 1mm (0.040") thick joiners riveted to each piece. The diagram shows only the wing side (and some of the main retracts, which I am in the process of at moving)
My questions is, will this be sufficient to support the wing and carry the plane on the main gear?
My reckoning is it will be, but I never discount the value of a second (or third etc.) opinion.
Next thing is to work on supports for the main gearin the wing. I am investigating this possibility to minimise CG shift, make the retracting mechanism easier to manage and give it a more stable base for taxiing.
 

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orion

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Clarification needed - are you contemplating attaching the wing to the fuselage bulkhead(s) through this .080" web? Is the bulkhead then supposed to carry the bending load of the wing or is there some type of other spar carrythrough?
 

StRaNgEdAyS

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Hey there, It's been a busy week for me, but I have done a litle work, and I hope to have some clarification done.
The 0.080" spar webs will attach to two 0.080" bulkheads with additional carrythrough of the spar caps. The inlet duct also is attached to these bulkheads, a portion of it is shown here to illustrate. There will of course be some ribs in there, but I left them out in order to make the concept of what I want to do clearer.
Cheers...:D
 

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pylon500

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I think it's time we had a real hard think about this project. :(
Your wing-spar web / fuselarge bulkhead / carry through cut out, is something I think even Boeing would be afraid to tackle. :eek:
Lets compare this to an R/C ducted job.
Say the model is about 6' long with about a 5' span, regardless of whether foam cored or ribbed, you would no doubt skin the wing with about 1/16" balsa (adequate even without a layer of glass).
Naturally you would want to put some kind of spar in the wing, the usual route would be something like 1/8" sheet lengthways (technically better if vertical) and if you like overkill probably 1/4" square balsa caps to give better joining surfaces. :rolleyes:
Now, to join the two wings to the fuse you're proposing to glue the web(s) to the bulkhead(s)? :mad:
Straight away we can see that it would be better to arrange to glue the CAPS to the bulkheads, that's the loadpath!!
So far, so good, now lets have a look at these bulheads.
Model's 'bout this big, powered by a .60 no doubt, and we're trying to keep the weight down to under....say 9lbs max? :whistle:
For this thing to fly with the spar carry through cut out like you suggest, you would probably have to make them from 5/16" hardwood ply (and probably set at 45º to get some grain around the corner!)
Now going back to you all metal pulse-jet powered jet, these bulkheads would probably have to be forged from about 1 1/2" to 2" Alloy, preferably 7075-T651 :roll: :eek:
If you're lucky they might weigh as little as about 25 Kilo's EACH!, and when you're flying through turbulence and see the wings flexing, you would have to ask yourself, 'Is that the wing spars and skins flexing, or those bulkheads stretching and (possibly) CRACKING!'
Solutions;
1/. If you can afford to make the wing thick enough, lower it and run the duct through the middle (you could flatten the duct a little)
2/. Move the wing up higher (like an Alpha-Jet)
3/. Lengthen the fuse even more to get the motor behind the spars, with a longer nose to balance.
4/. Build two smaller jets, lower the wing to the bottom, and make a scale PHANTOM! :ban:
Arthur.
 

StRaNgEdAyS

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This spar has been the biggest concern of the entire design.
I had a feeling that what I've got so far is dreadfully inadaequate, and it is certainly posing quite a challenge.
My immediate thought was to raise the wing, but then that means redesigning the entire fuselage to suit.
I think the best bet is to take a good hard look at the fuselage design itself, and try to figure out the best possible placement for the duct, possibly making use of curvature to induce low pressure in the intake, increasing the flow while reducing the size, allowing me greater room to run the spar carrythrough.
The real deciding factor in this is going to be positioniong of the engine itself. Due to the size, as you can see, it comes pretty close to the aft main spar, I don't have much room to play with. I have to have some sort of plenum before the inlet to absorb the back pressure generated when the valves close without greatly affecting the flow. The air coming into the plenum is also ducted down between the heatsheild and the engine to provide cooling. It is possible to move the engine back a little, but only a very small amount.
 

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StRaNgEdAyS

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OK I think it would be much easier to lengthen the fuselage.
I need only stretch it by a foot, and that should allow me to clear the spar with a redesigned duct. The up side of this is a smaller inlet which gives me a little more ground clearance, the down side is in order to keep the pitching moment in the right place, the Cl has to go back, which means the wing has to go back, probably a good 6 inches, to compensate. The down side of this is the **** spar moves back with it, so in effect, I only gain 6 inches of extra clearance. Still I think it will be enough to allow me sufficient room for the plenum, and give me the space I need in the bulkheads to make the carrythrough the full depth of the spar root.
To tell you the truth, that cutout bothered the heck out of me too....
 

orion

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A bulkhead carry-through, such as you are proposing, is actually very common as virtually every fighter out there is flying with one or more. The problem, as "pylon" alludes to, is that the load has to be coaxed from the wing root and around the duct. The mechanism for doing this can be tricky and can lead to a realtively complex and/or heavy piece of structure.

One approach is to make the wing root gradually a bit thicker and do sort of a blended wing/body design where the root spar and carry-through bulkhead gradually blend into each other and thus the load path accross is clean and relatively smooth, needing only little intermediate reinforcement to stabilize the webs and caps.

The second approach is your curent layout, although in most cases the designs incorporate a more sizeable wing root fitting, which is then pinned to an equal fitting mounted to the bulkhead (rather than a plate). The difficulty here is that the load still needs to go around the duct. As such, the bulkhead has to be able to react the bottom of the wing spar (which is pulling the bottom of the attachment, outboard) and so the vertical part of that bulkhead in the area of the wing root interface now has to be designed like a beam, which is then capable of taking the load of teh wing attachment around the bottom corner and accross.

The same argument holds for the top of the bulkhead except now, for positive maneuvering, the upper structure must be designed so that it minimizes the chance of failure in crippling, which will most likely be the failure mode of a spar cap that has its loading applied out of plane.

In short, this is all possible but far from simple. However, making guesses as to how much it will weight is still a bit early. This will require a very detailed analysis, most llikely through FEA due to the involved complexity and load paths.
 

StRaNgEdAyS

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I was originally intending on riveting the duct to the bulkheads, so the load is also carried through the skin of the duct, as well as being distributed around the outside of the plane via it's skin as well.
I had in mind a kind of two way load bearing technique.
The Bulkheads transmit the load, the stress coming from the bottom of the spar is absorbed mostly by the duct and the lower fuselage skin in conjunction with the somewhat thinner bulkhead section, the upper stress is taken by the thicker section of bulkhead and the upper fuselage skin.
Because there is two main spar carrythrough sections, the load should be spread out somewhat along the ducting and outer skin.
So maybe there is merit in refining my current ideas for the spar and maybe I should consider a more robust method of attaching the webs to the bulkheads other than a pair of plates on either side of each one?
 

orion

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Hard to say off the top of my head without doing any actual analysis. Do however keep in mind that due to plate stability theory, only a very locallized part of the outer skin and duct skin will act in conjunction with the bulkhead. Even so, loads do not like going around corners so this is still not a simple design casse.
 

StRaNgEdAyS

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One thing I probably should add, is that apart from the primary spar and it's brace, there are two additional webs running the length of the wing. The rear spar, which is simply a 0.040" piece bent to a "C" section following the taper of the wing, which runs at 30% of the chord from the TE, and a set of webs placed between the ribs at 10% from the LE. Both of which are indirectly carried through to the root of the strakes, a further 270mm (10") in from the main wing root, which is what is pictured here, and in the case of the rear spar, could easily be attached to a body rib in a similar fashion as the two main spar carrythroughs.
The front webs will remain as individual pieces riveted between the ribs, as they are more to provide support for the skin over the LE than anything. However they will contrubute to the support of the wing, albeit in a small capacity.
I'll put up a pic to illustrate this later, as I'm headed back to bed, now I've gotten that out of the way...:whistle:
Don't you just HATE it when you get woken up by an idea at 4am :confused: :zzz:
 
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