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Author Topic: Plastic Bird  (Read 3943 times)
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piecost
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« Reply #75 on: September 22, 2021, 02:25:20 PM »

Thanks for the idea Dave. I have a roll of 100g/m2 spread tow that I have been using for spars. Perhaps It can be spread further.

First I will try wetting the 10g/m2 carbon tissue then putting toilet roll above and below then putting in a food saver vacuum back for 5 minutes to see if it will give up more epoxy than rolling.

If I can get the epoxy weight down to , say, half the carbon weight then the carbon tissue should be lighter than using the lightest glass cloth, which I believe is 20g/m2.

Of course, balsa.is the solution, but we are not completely out of ideas yet.
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dosco
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« Reply #76 on: September 22, 2021, 04:52:19 PM »

Here's the RCG Thread: https://www.rcgroups.com/forums/showthread.php?2352182-30-inch-DLG-design

I haven't logged into RCG in a very very long time ... I'm sure if one were to search the OP's "view threads started by this user" there may be more content to be had.

Although the OP states that he used Mark Drela's method (copied and pasted, below):

1) I first prepare a hard, flat, alcohol-proof working surface. I use 3 mil Mylar taped down to 1/4" plywood, but other surfaces should work OK.
2) I lay down the tows side by side with the appropriate spacing. With 12K tow, a 10mm spacing will give a material weight of 78 g/m^2 (2.3 oz/yd^2) -- other spacings will change the weight in inverse proportion. I made wood sticks with many wire pins with 4mm spacing, which I use to easily set the spacing at each end of the board, either 4mm, 8mm, 12mm, etc. It's essential that the tow not have any twists, otherwise it will not spead out. The tows are anchored at the ends.
3) I pour a generous amount of isopropyl alcohol ("rubbing alcohol" in the US) on the tows, and immediately cover them with another sheet of 3 Mil mylar.
4) I roll the Mylar/tow/Mylar sandwich with a wide hard-rubber roller to spread out the tows. If the tows don't spread out well, adding more alcohol usually helps. You want enough alcohol so that the fibers can "float" and move around.
5) I carefully peel off the top Mylar sheet to let the alcohol evaporate. The sizing chemical which is present in the tow will remain after the alcohol evaporates, and help hold the spread tows together in one sheet.

Even though some sizing is present, the sheet is still very flimsy and falls apart easily. To allow handling and cutting, I've had to apply a wax paper carrier with an extremely light 3M77 mist before peeling off the carbon from the bottom Mylar sheet. This still leaves some free fiber "fuzz", but it's good enough to work with.

I think one reason why some fibers are loose is that they don't make enough contact with other fibers as the alcohol evaporates. On the next batch I'll try pressing the carbon sheet with porous teflon and vacuum as the alcohol dries. Hopefully this will result in a much more solid sheet. Another possibility is adding a drop of epoxy to the alcohol, or maybe more sizing chemical (whatever that might be).

The carbon tow I'm using is Toho HMA-7C, which has a modulus of 52Msi, versus 30Msi for regular carbon. I got it real cheap on EBay  I'm getting 10 gram DLG booms which are stiffer than the commercial 16 gram booms. I don't know how well they will hold up, though.
Last edited by markdrela; Jul 21, 2007 at 06:46 PM.
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OZPAF
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« Reply #77 on: September 22, 2021, 08:42:26 PM »

That's an interesting idea Dosco and would be worth a bit of experimentation.

I believe the hairspray would be a suitable sizing after the alcohol had evaporated - replacing the wax paper/3m spray.

Quote
Also interesting to note in that write up that the lower surface is apparently aluminum foil. Huh.
I think this is only shiny mylar used for separation.

It just highlights how much of a innovator Mark Drela is.

John
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dosco
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« Reply #78 on: September 22, 2021, 09:07:44 PM »

Quote
Also interesting to note in that write up that the lower surface is apparently aluminum foil. Huh.
I think this is only shiny mylar used for separation.


John:
I was thinking the same thing until the last couple of pictures, with the young man holding the glider. The reflection from the wing's undersurface is not glossy sheen. It's aluminum foil. I went back to the beginning and reviewed the pictures, you'll see the aluminum foil rests *on the mylar.* Odd. I'd think you'd want a torque box structure with the foil. I wonder what reasoning they used to select that approach?

Oh, yes, Dr. Drela is the man. Seems like he dropped off the planet as of late. I wonder what he's up to?

Best-
Dave
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piecost
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« Reply #79 on: October 02, 2021, 05:03:27 PM »

I am back from the first day of the nats. Chuck and catapult gliders were flown over lunchtime.

I got my wing 13 fitted to a fuselage employing a balsa tail and spent some time adjusting the CG. Discussion with the resident F1N expert confirmed that the wing was too flexible. The model was not flying well but I got the glide sorted and performed javelin launchs, loosing much height on the transition. I think I could see the wing twisting nose down on launch

I recorded a poor first competition flight and on the second a crack was heard as the RH lower spar cap deliminated between 12mm and 63mm from the root.

So I got my trusted wing 1 out and taped it to the fuselage. I removed all nosewight. This was much better; I could javelin launch higher even though the flaps were stiffer than the other wing. I achieved times of about 20 seconds until the fin came off! This was fixed with a dob of cyano and aluminium tape. I found that I could nearly hit the 28 foot roof underarm tip launching, without giving it everything. I launched using my right hand on the left tip aiming up about 30 degrees. This model glided left.

I got up to 25 seconds, but still lost the best part of 10 foot in the transition. On the last flight I hear a noise and lost about one square inch of trailing edge off the RH wingtip!

I only achieved a poor score but was pleased that I had a working model. On the drive home I thought of further developments employing foam, without resorting to using balsa. I have not quite given up yet! It is clear that more torsional rigidity is needed; So, I will try a straight leading edge with carbon tissue or glass cloth (as used on my first wing). I will experiment to see how I can soak up excess epoxy prior to vacuming the carbon onto the foam.

Wing 1 with fuselage 8 weighed 9.997g with a CG of about 50mm aft of the root LE.

I attach photos of the delaminated spar on wing 13 and wing 1 and fuselage 8 showing the part of the tip that broke off. Also note the aluminium tape helping to hkd the fin on.
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Re: Plastic Bird
Re: Plastic Bird
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piecost
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« Reply #80 on: October 12, 2021, 07:51:06 PM »

I did not attend the second and final day of the nats. I heard that I had achieved bronze, but I suspect that there were only three competators.

I am still motivated to take the all-foam (without balsa) experiments further. Especially since, elsewhere on this forum, are some excellent models using this construction. So I performed a quick experiment; trying to reduce the epoxy to carbon ratio when using carbon tissue. I cut three one inch squares of Duluxe Materials Superlight 10g/m2 tissue and applied excess Easy Composites laminating epoxy resin with slow hardener using a scrap of card. The temperature was 21 decrees C, and I made a complete.pigs ear of the mixture ratio.   Putting the wetted tissue on top of 2 layers of toilet roll and two more on top then pressing hard with a rubber roller reduced the epoxy/carbon ratio to 1.2 to 1.1.

I next put the first sample between 3 layers of tissue (each side) inside a food saver bag and put it through a pouch laminating machine, before it had time to warm up. Apparently the pouches fuse at 150 to 200 decrees C which might ruin the epoxy. The idea being that the rollers of the machine might squeeze the glue out . The machine made several graunching noises as the bag was too thick and it needed help to stop getting stuck. This longshot idea got the epoxy/carbon ratio down to 1.1 from 1.2. So, it did not work.

I next employed my food saver vacuum machine to vacuum the other two samples, between toilet roll, for 5 minutes and 30 minutes. The former got the resin content to 0.57 and the latter to 0.36. A brilliant success!

All three samples, on removal from their bags and tissue were vacuumed again onto a scrap of blue foam to check the adhesion. I.e. there enough glue to stick to the foam? I will find out tomorrow.

So, this test has given me enthusiasm to try a straight leading edge with a single piece of carbon tissue folded around the nose. The tissue folds easily. I will employ a mylar backing for the carbon cut and the leading edge and joined with tape to negotiate the sharp curvature.I will wax the mylar but not apply PVA release agent since on a trial run it stuck to the workpiece.

 If I can achieve an epoxy to carbon ratio of 0.5, say, then the carbon tissue should weight about half that of 20g/m2 glass cloth. Another benefit is that the tissue remains in shape when wetting, unlike glass cloth which distorts and I end up cutting to size after wetting. The cloth is a real fiddle to use.

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dosco
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« Reply #81 on: October 12, 2021, 08:47:03 PM »

So I'm not up to speed on how others are building their plastic birds. Are they using carbon tissue? The carbon tissue I bought a looooong time ago was basically an epoxy sponge with no strength. In comparison to a modern wakefield taco shell ... are you expecting torsional stiffness at say, 45 degrees to the LE? Is this a reasonable expectation wrt carbon tissue?

One way to get more out of a laminated foam structure would be to put pins through the structure that connect the skins (the "pins" would be something other than metal). I have a research paper from many moons ago (around 2005-ish) where some work was done that showed such an approach would be beneficial. Not sure that would "add lightness" ... and it would probably be a major PITA.

Best-
Dave


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piecost
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« Reply #82 on: October 12, 2021, 09:39:34 PM »

Hi Dave,

I have seen photos of central European guys using carbon tissue so it should work. However, it may yet not prove torsionally stiff enough. I really need to construct a torsional rigidity measuring rig so I can compare to a balsa leading edge wing. Regarding the pins idea; I have been using Rohcell inset into the XPS foam between the spar caps and will do so again. This will, sort of, close the torsion box.

Tim
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piecost
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« Reply #83 on: October 13, 2021, 01:08:07 PM »

Full disclosure; the two lowest epoxy samples did not stick to the blue foam. On removal from their vacuum bags they became loose. So, I need to find a balance between weight of glue whilst still sticking to the foam.
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