PARIS threatened by the bends

Solarfilm for model aircraft

Try This stuff... it's supposed to be really good http://www.solarfilm.co.uk/default.aspx

Wings....

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.

In days long ago

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.

Fold?

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.

Lester, I'm building a paper wing...

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.

Neil

preshrink

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...

Only Shrink using the dope?

If water shrinkage plus dope shrinkage is too much shrinkage, can you not just dope it, and go for only one lot of shrinkage -or am I missing something ?

Bah!

Would using solid card wings contravene the "paper" construction ethic?

Time for a monocoque rethink?

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.

Hm.

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.

A stiffening jig will do the trick

Get basswood (stronger but almost as easy to work as balsa) about 2x10mm and make a reinforcing frame to maintain a planar shape while the tissue dries.

Oh, and non-shrinking dope is a great idea.

tissue suggestion

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.

regards

Rick.

Stressed skin

Just a thought:

1. Make wing structure

2. Mount vertically

3. Glue paper all round

4. Spray a light coat of thinner all over with an airbrush (outdoors)

5. Leave to dry

Repeat 4 and 5 until you're happy with the tautness (or it's crushed itself).