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Old 12-04-2012, 12:10 AM   #1
mclarkson
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Default Fiberglassing a wing for strength - how to proceed?

I have a GWS Pico Tiger Moth that I'm convinced came from a bad batch of foam. The tailfeathers were so soft I had to reinforce them to keep them from flopping in the breeze. And the wings ... the wings are apparently made of eggshells.

Any minor incident, such as a nose-over on landing, will snap the wings. I've repaired the wings 6 times in 8 flights.

So ... I'm gonna try reinforcing them with a little fiberglass. (And maybe CF.) It's that, or chuck the plane in the bin.

My plan is to glass the undersides of the wings, using WBPU and 0.55oz fiberglass cloth.

I've never worked with fiberglass at all before. At all. I am in no way handy or experienced. Any pro-tips?

I figure, if it doesn't work out, I'm no worse off than before and will at least have learned how not to fiberglass.


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Old 12-04-2012, 01:39 AM   #2
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It is a super light airplane no doubt but that is what makes it a great flying plane it is super light.

I think glassing this will be a HUGE mistake but hey that is what life is all about testing and trying.

I recommend using Water Based Poly Urathane (WBPU) as it will be lightest. Epoxy finishing resins take a great deal more work to keep them light - DO NOT use that (Epoxy) on this plane. Polyester resin (also heavy) can't be used on raw foam (it will attack the foam) but works really well on larger models. Epoxy and Polyester resin are MUCH stronger and harder but that is simply not needed here.

It is really simple:
  • Cut the glass oversize for the surface you are covering (an inch or so overlap)
  • Take the WBPU and a foam brush
  • Apply the liquid assuring you cover all areas
  • ASSURE (this is very important) that you coat the overlap too as this makes the glass MUCH easier to cut or sand later
  • Lay flat and let it FULLY dry 24 hours
  • Sand the glass lightly
  • Sandpaper will cut the excess edges off since you coated the overlap (cool eh!). Just hold the sandpaper block at a 45 degree angle and it will cut the glass!
  • Do another light coat of WBPU
  • Lightly sand
Works well - but you must always glass solid structures (no open wings for example). Foam airplanes obviously work very well. Do not try to "wrap" the glass on a small plane - it does not work until you get to larger planes.

I should note the more coats and sanding the more smooth the finish (and the more weight). On this plane you really can't do more than one or two very light coats.

Keep it light - again the Pico stick is not a good candidate IMHO as it will likely add 1-2 ounces (on this plane 20-30% more weight). On larger planes the weight addition is much more like 5%.

I don't think you have defective foam - it is always delicate but again that is why it flies so well IMHO.

Mike
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Old 12-04-2012, 04:13 AM   #3
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Thanks for the tips!

This is my first ever Pico Moth so I can't say for sure about the foam. On that other forum, some people suspect the foam, others are sure my aileron mods are to blame, or that I just don't appreciate how delicate these moths are supposed to be.

I can't say for sure. At AUW just under 10oz, including my beefier landing gear, it's heavier that some Pico Moths, but others report flying Moths weighing 12oz and over with no problems. One guy mentioned cartwheeling his a few times with no damage. Again, this one will snap a wing if I nose it over on a grass landing (or, indeed, if I look at it funny.)

I've had a couple dozen planes, from little ultra-micro Cubs and T-28's to a 400-size Tiger Moth, Radian, Depron pusher jets, scratch-built FFF planes, etc. etc. None of them have been anywhere near as fragile as this guy.

But, as I said, whether the factory screwed it up or I did, I must now either fix it or bin it. Alternative suggestions are welcome but I can't fly the thing like it is.

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Old 12-04-2012, 05:43 AM   #4
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Originally Posted by mclarkson View Post
Thanks for the tips!

This is my first ever Pico Moth so I can't say for sure about the foam. On that other forum, some people suspect the foam, others are sure my aileron mods are to blame, or that I just don't appreciate how delicate these moths are supposed to be.

I can't say for sure. At AUW just under 10oz, including my beefier landing gear, it's heavier that some Pico Moths, but others report flying Moths weighing 12oz and over with no problems. One guy mentioned cartwheeling his a few times with no damage. Again, this one will snap a wing if I nose it over on a grass landing (or, indeed, if I look at it funny.)

I've had a couple dozen planes, from little ultra-micro Cubs and T-28's to a 400-size Tiger Moth, Radian, Depron pusher jets, scratch-built FFF planes, etc. etc. None of them have been anywhere near as fragile as this guy.

But, as I said, whether the factory screwed it up or I did, I must now either fix it or bin it. Alternative suggestions are welcome but I can't fly the thing like it is.
Yeah, epoxy will add a lot of weight very quickly. Don't know if it would work, but wattflyer readers, would pieces of round small carbon fiber tubing help in this type of application

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Old 12-04-2012, 01:19 PM   #5
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I also feel epoxy is too heavy. Fiberglass cloth and Min Wax is good and easy to apply.
If you have trouble locating fiberglass cloth, I've found 5mm silk works fine too.
Silk is available from here
http://www.dharmatrading.com/html/eng/3374-AA.shtml

Apply the MinWax with a cheap foam brush from the center out, works great.

Paul
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Old 12-04-2012, 01:34 PM   #6
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Originally Posted by mclarkson View Post
some people suspect the foam, others are sure my aileron mods are to blame, or that I just don't appreciate how delicate these moths are supposed to be.
I suspect that is a big issue.

Originally Posted by mclarkson View Post
I can't say for sure. At AUW just under 10oz, including my beefier landing gear, it's heavier that some Pico Moths, but others report flying Moths weighing 12oz and over with no problems. One guy mentioned cartwheeling his a few times with no damage. Again, this one will snap a wing if I nose it over on a grass landing (or, indeed, if I look at it funny.)
That is primarily your issue - glass will add another 2oz and you will have a 12oz bird. WAY too heavy!

Mine is at just under 8oz and flies wonderfully including my super light custom gear mount. My buddy did a nice scale paint job and heavy gear and such his is 10 or so ounces and flies like crap.

He always marvels how well mine does and I remind him that it is 25% lighter than his - a really big deal!

It is fragile - no question (especially the tail feathers) but again that is why it flies so well.

Mike
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Old 12-07-2012, 03:54 PM   #7
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Something that needs to be considered .... beefing up or strengthening a structure can in fact be it's worst enemy ... when it crashes - the damage is usually far harder to sort.

There is a way to impart stiffening to a wing without too much additional weight ......... a tapered section UNDER the wing ... using Scree cloth ... which is a very light weight glass cloth that is more like tissue. It's used to finish of top surface of GRP items ... if not gelcoated. Best way to describe it ... it looks like you split the two layers of a Kleenex into two single layers ..

Instead of covering whole wing area ... you start at full chord at root and taper down to a point at wing tip ... this results in 'glassing area' of approx 40% of what a full side of wing would be ...
I used this method on my balsa IC powered FW190 years ago ... funny thing was 10yrs later - only part left was the wing !! The fuselage had failed ....

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Old 12-07-2012, 05:37 PM   #8
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How about a using a strip of reinforced packing tape along the bottom of the wing? Or covering the wing with light, low temp film like Ekonocote or Solarfilm?

Or try a 1/4 x 1/16 lite ply "spar" glued on the flat under the wing. I built a mini Blue Baby out of poster board foam a while back. As an experiment I flew it first without any kind of wing reinforcement, and it didn't take long for the wing to fold. I then added a strip of ply as above and it could take anything I threw at it.

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Old 12-07-2012, 09:08 PM   #9
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Thanks for all the suggestions, guys. Still not 100% sure how I'm gonna do this...

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Old 12-07-2012, 11:12 PM   #10
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How about taping a short length of maybe 2mm cf rod to the leading and trailing edge of the wing? Use some 3M bi-directional filament tape. There's another name for it, just can't remember it right now. About $7 I think at Lowe's. The fibers run in two directions so it's very strong. But I think short lengths of carbon fiber rod taped right on the LE and TE would stop the breaking. You must be pulling some real high G loops!
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Old 12-07-2012, 11:43 PM   #11
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It never breaks in flight; it's plenty strong enough for that. But give it a nose-over on landing - which applies the load from back to front - and SNAP!

This video, posted on that other forum, has convinced me that my wing foam is just extra-brittle. This guy crashes the plane hard enough to break the fuselage and the wings survive!
YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.



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Old 12-08-2012, 12:07 AM   #12
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I don't have much experience yet with foam planes. I have a couple of unbuilt kits on the shelf. Been mostly fiddling with balsa stuff. I wonder if age of a foam plane is a factor? Does foam get brittle after a certain amount of time?
Regardless I think some cf rods taped to the wing would be a quick and easy fix.
I have an unbuilt Sig Jenny that I think is very similar to yours. Just bought it a short time back. I think it had been on the seller's shelf for many years. I've bought a few small diameter cf rods for strengthening projects, might be a good idea for me to make more use of them. (at least till I become a better pilot!)
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Old 12-08-2012, 12:34 AM   #13
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I've thought about the CF rods. The problem I see is that, with the wings' backward slope, I'd have to use two, angled pieces of rod and the join would be very near where the breaks are always occurring. I have thought of running some CF rod straight through the middle of the wing.

But, for now ... I'm going ahead with the glass. The good news is that, after glassing and sanding one coat, the lower wing has only gained 2.2 grams. I'm using 0.55oz fiberglass cloth and MinWax WBPU. I'm only covering about 1/2 of the wing, leaving the outboard portions un-glass.


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Old 12-08-2012, 12:58 AM   #14
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Good job at keeping the weight down.
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Old 12-08-2012, 01:39 AM   #15
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Thanks. Second coat of of WBPU applied and sanded for a final weight gain of 2.7 grams.

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Old 12-08-2012, 04:23 AM   #16
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Crap! I messed up and forgot to weigh the top wing so now I'll never know how much weight I added.

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Old 12-08-2012, 03:43 PM   #17
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I simply paint with Latex. Lay down a super light piece of Dacron weave about 1" wide full length on the BOTTOM ONLY. Then the 1 top coat to the plane. I never do a outside loop.
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