A temporary support structure used whilst the dope dries?
El Reg's Paper Aircraft Released Into Space (PARIS) team continues to work on the Vulture 1-X aircraft structure, while attempting to refine the skinning process. It's all been a bit of a palaver, as followers of our audacious space programme know. We've tried various lines of attack, and recently thought we'd cracked the …
I'm not a modeller but I suggest filling the straws with something during the drying process (sand?)
Whilst pondering this I also wondered if the straws and paper covering will "blowout" as it ascends, possibly in quite a destructive way unless you drill some tiny pressure relief holes...
perhaps a sandfilling, holes drilled/glue removed to release the sand will act as the pressure relief...
Suggest on your next trip to Qinetic you put some straws with glued up ends in the chamber to test this.....
Rather than the traditional ribs n' spreaders construction, use only card ribs angled against each other and interlocked with slots. The resulting small rhomboids should be firmer against the skin.
My ascii art really isn't up to drawing this one unfortunately, think geodesic meets those slot-together dinosaur models.
... is 'eggbox wing construction'.
This is ts a doddle to do in balsa. A set of balsa strips, whose depth matches the wing thickness, are slotted so they are glued together to form a rectangular grid. The strips will form the ribs and are at 45 degrees to the wing span with each strip having three or four intersections with other strips. When the glue has set you add leading and trailing edges and then sand the lot to the required wing section.
Years ago I heard of being able to buy a block of paper that could be shaped, rather like a single thick rib and then stretched sideways to form a sort of honeycomb wing: I'd think that's about the only way you'd make a paper egg box wing structure.
Recalling a period, many years ago, when I made model aircraft. The technique for applying the skin, was to glue it on DRY, then moisten it to shrink it. The application of dope, then was to increase it's resistance to impact damage. Well, minor impact damage, anyway. It may well be that skinning wet implies too much shrinkage.
It would seem the covering from the wings need a small crease or fold in them (Kinda 'M' shaped), which may smooth out to flat whilst drying, 'perchance' if the dimensions are correct.
I'm glad to hear the whole project literally 'hangs' on the efforts by splendidly co-operative QinetiQ. Them folks deserve a 'gong', esp. as my ex-boss is there - head of something or other.
I did tip him the wink in an e-mail about PARIS when a hypobaric chamber was called for, so maybe he pulled a couple of strings. He's like that.
I've made plenty of balsa/tissue planes. And once even made the mistake of covering a single side before shrinking the tissue; now you know the results too. But I agree with other posters here. You glue (with dope - on the balsa and then through the tissue ) the tissue dry to one side of the wing, and only glue the leading edge, trailing edge, and outermost ribs. You trim that paper with fine sand paper. Fix paper to the other side the same way, and trim. Now you have the wing fully covered but only fixed with dope around the circumference of the wing. Now, you steal your mom's best perfume sprayer empty and fill with water. Very lightly spray the tissue uniformly so that it shrinks uniformly and does not warp your structure. You may need to do this three to six times being patient to let it dry between spraying. Hang dry using a thread. The tissue will now be taught uniformly and the structure should be fine. At that point further dope through the tissue at the ribs to fix the tissue to the ribs. After that you can choose a 50/50 dope/thinner spray - again in mom's perfume bottle - to coat the whole wing. But I never did as it will add too much weight. Good luck.
but it's a slow process. It's designed (well, mostly ripped off from West Wings), a 50" wing which I can slap on top of an R/C Orion to see it it will do what it needs to, but I'm building the main spar up from seven sets of 6mm straws arranged in a hex pattern (spot the gratuitous computing reference). That means building it a bit at a time so the thing dries vaguely straight, as the aliphatic resin I'm using to stick it together (it's what was on my bench, OK?) takes a while to dry. A couple more to do, so it might be finished tonight.
I'm planning on a reinforced centre section - skinned both sides with 300gsm paper for strength to hold it to the plane - and skinned with perhaps 100gsm from the top of the spar to the leading edge. The leading edge is a set of three straws, as is the trailing edge, but the TE will have some folded paper around it to maintain its shape.
Then pin it to the table, skin it with tissue, and spray water then *thinned* *non-shrinking* dope.
It's not as strong as it might be, and obviously I don't want to do a destructive test, but it should fly an all-up weight of about 600g if it doesn't get any high-g manoeuvres.
For flat surfaces: apply tissue over a strong, open wooden frame, and dope it. When that's dry and flat, place model on it, after applying glue to the model struts where they contact it. When that's dry, cut tissue away from frame and neaten up the edges. There should be little or no excess tension in the covering that way (I'm guessing the root problem is that the tissue deforms the model frame as it shrinks; since the doped tissue is hardly elastic at all once it's shrunk). This method wont' be so good if the frame doesn't present a really good planar gluing surface.
For simple curves like the top of wing you may be able to cut the shrunk tissue from the frame, and drape it over the wing with weights to keep it taut while it glues.
And as someone already mentioned, make sure none of the struts are sealed or it may be destroyed by pressure changes...
Switching to thick paper (cartridge paper or thin card) and building a monocoque structure from that should work better than an open tissue covered framework. Judging by the photos, the straws simply aren't stiff enough to support a doped tissue skin.
You'd make a monocoque wing by wrapping a single thick paper skin round the prebuilt leading edge, spar and rib structure, gluing it to the spar and ribs, starting and finishing at the trailing edge and gluing the surfaces together there. The spar should be assembled from top and bottom straws glued between thick vertical paper or thin card webs. Use another straw to form the leading edge. Use the same web material to make ribs spaced at 0.3 - 0.5 chords apart, with tabs along the edges to give plenty of gluing area between skin and rib. With the skin glued to the ribs and spars you'd have a pretty strong wing that could be finished and made waterproof with non-shrinking dope.
Cover the fuselage by gluing the same heavy paper to a fuselage with the current straw structure. End the paper where it touches the lengthwise straws to leave rounded corners and use non-shrinking dope to stick tissue paper strips on to reinforce the corners.
Tail surfaces can simply be 2-D frames made from straws and covered the same way as the fuselage to give thickish flat plate surfaces, which will make them relatively stiff and plenty strong enough since they only stabilise PARIS. Only the wing, which has to carry the aircraft's weight plus payload, needs to be made thicker and must be built round a load-carrying structure.
Sandwich several layers of paper and glue together in a vice, then chop more traditional rib shapes out of them? That's probably stretching the definition of "paper", but you'd effectively have something with the hardness of wood. It might bend less!
http://p-38.corpcomp.com/Glueing%20wing%20spars.jpg <- bit like that.
What about putting something like dowel inside the straws while skinning? That would give the extra strength during the curing process, and then they could be removed.
Not sure if when you removed the internal support the whole thing would bend up. This assumes that you haven't sealed the ends of the straws up either. I know you'd have a hard job doing the support struts, but "fixing" the main external edges should provide the support.
Paris - dowel, internals...
If you want to use tissue, have you considered pretreating the material prior to affixing to the airframe?
Make a wooden frame slightly smaller than the size of the tissue sheet, affix with your preferred glue, spray with water as before then use low shrink dope (avalable from model shops) and when shrunk use the main centre area to cover the airframe. (edges will be wrinkly) It should remain flexible enough to affix and will retain its surface rigidity to give the frame additional strength. No extra weight involved and no frame distortion.