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Author Topic: Mislabeled Dope?  (Read 1433 times)

Offline Howard Rush

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Mislabeled Dope?
« on: July 30, 2021, 07:23:45 PM »
I built a wing using the same construction and materials I'd used before, and it went bad.  Namely, the leading edge sheet between ribs has extreme dips, and a rib buckled between the spar and trailing edge.  I used clear Randolph nontautening dope I bought from Aircraft Spruce last year. My guess is that the stuff they put in the dope to make it nontautening didn't get put in this batch.  Has anyone else had this happen?  I'm not interested in blaming anybody, but I would like to know what caused the problem lest I make everything heavier on the replacement wing. 
The Jive Combat Team
Making combat and stunt great again

Offline Tim Wescott

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Re: Mislabeled Dope?
« Reply #1 on: July 30, 2021, 07:55:21 PM »
You're scaring me Howard.  Have you considered UltraCoat?
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Dan McEntee

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Re: Mislabeled Dope?
« Reply #2 on: July 30, 2021, 10:14:25 PM »
  Was it tinted a greenish color? I am of the understanding that they tint the non-tautening nitrate so that you can tell the difference from regular nitrate. The stuff I have in my garage is tinted. On full scale covering they use tautening dope up to a certain point and then they do the rib stitching and such. The strips that go over that are generally applied with non-tautening. Bill Byles has explained it here before and you should be able to dig it up. getting the tinted stuff will keep you from getting it mixed up with pother stuff. before you build the new wing, try some of what you got on a test frame on plain silk span. The do the old fashioned trick of adding some castor oil to the dope to plasticize it. f you have a pint of thinned dope f or brushing, add about a half to 3/4 ounce of castor to it. Use the medicinal stuff from the drug store of the food grade stuff from the supermarket. Do a second test panel with that and compare. Another option for plasticizing is get some Dave brown Flexall, but I think that is actually just medicinal castor oil.
   Type at you later,
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Offline Brett Buck

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Re: Mislabeled Dope?
« Reply #3 on: July 31, 2021, 01:49:20 AM »
I built a wing using the same construction and materials I'd used before, and it went bad.  Namely, the leading edge sheet between ribs has extreme dips, and a rib buckled between the spar and trailing edge.  I used clear Randolph nontautening dope I bought from Aircraft Spruce last year. My guess is that the stuff they put in the dope to make it nontautening didn't get put in this batch.  Has anyone else had this happen?  I'm not interested in blaming anybody, but I would like to know what caused the problem lest I make everything heavier on the replacement wing.

     Yes, something exactly like that happened on my previous airplane (the 2006 airplane). I grabbed what was supposed to be nitrate dope, same as I had used many times before, put one last coat on the fuselage, left it to try, came back that afternoon to find *extreme* "starved horse" look on the turtledeck. Ted suggested I put on another coat with lots of plasticizer, which relaxed most of it in about 15 minutes, but never went entirely away.   It was a new jar labelled exactly like the previous coats which caused no problems, one coat of the new jar shrank to an inconceivable degree in 5-6 hours. This was Brodak nitrate (supposedly).
 
    Brett

Offline Howard Rush

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Re: Mislabeled Dope?
« Reply #4 on: July 31, 2021, 02:09:22 AM »
Mine was butyrate, but if it can happen with nitrate, it can probably happen with butyrate. 
The Jive Combat Team
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Offline Howard Rush

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Re: Mislabeled Dope?
« Reply #5 on: July 31, 2021, 02:16:29 AM »
Sometimes if your balsa is too light it will do those things. Had it happen to a built up stab once. Looked like it got sucked in.

Could be the balsa.  I am trying to build a light plane.  It's for what's probably my last shot on the stunt team. I could use both heavier wood and new dope on the replacement, but if the dope is the problem, I wouldn't need the extra wood weight. 
The Jive Combat Team
Making combat and stunt great again

Offline Brett Buck

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Re: Mislabeled Dope?
« Reply #6 on: July 31, 2021, 03:06:49 PM »
Mine was butyrate, but if it can happen with nitrate, it can probably happen with butyrate.

   Mine was *supposed to be nitrate*, but I strongly suspect it was regular-shrink butyrate. They look about the same, and my receptors for solvents is so fried that I couldn't tell the difference in the smell, if any.

     Brett

Offline Massimo Rimoldi

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Re: Mislabeled Dope?
« Reply #7 on: August 01, 2021, 05:53:23 AM »
   Mine was *supposed to be nitrate*, but I strongly suspect it was regular-shrink butyrate. They look about the same, and my receptors for solvents is so fried that I couldn't tell the difference in the smell, if any.

     Brett

Hi.
 Nitrate and Butyrate are respectively "Nitrocellulose" and "Cellulose Acetate Butyrate".  Probably the easiest way to recognize them is by exposing a tiny sample (completely dry and free of residual solvents) to the flame of a match.  Nitrate is extremely flammable and will ignite immediately.  Butyrate will burn with more difficulty, giving off the characteristic vinegar smell.  Both products are soluble in the same solvents, so any difference in odor cannot be considered definitive.

Massimo

Offline Brett Buck

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Re: Mislabeled Dope?
« Reply #8 on: August 01, 2021, 05:40:44 PM »
Hi.
 Nitrate and Butyrate are respectively "Nitrocellulose" and "Cellulose Acetate Butyrate".  Probably the easiest way to recognize them is by exposing a tiny sample (completely dry and free of residual solvents) to the flame of a match.  Nitrate is extremely flammable and will ignite immediately.  Butyrate will burn with more difficulty, giving off the characteristic vinegar smell.  Both products are soluble in the same solvents, so any difference in odor cannot be considered definitive.


   That's interesting - I knew that nitrate was much more flammable (although I have flown about 500 model rocket boost gliders with nitrate/Jap tissue with no flamers yet), but  didn't know about the vinegar smell.    In the case in question, it was 15 years ago+- a month, so it's too late to verify my theory. The residual "starved horse" scallops are still there but haven't gotten any worse since that day, now, they have 15-year-old *rock hard* 2-part epoxy paint on them, I think it will probably be that way forever.

    Brett

Offline wwwarbird

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Re: Mislabeled Dope?
« Reply #9 on: August 01, 2021, 09:31:38 PM »
Could be the balsa.  I am trying to build a light plane.

 I'm betting that's the problem.  y1

 FWIW, I've used nothing but butyrate taughtening dope on every wing I've ever built, forever. I only recall one, maybe two, that even began to hint at the starved horse look once finished. My bet on those is the L/E sheeting was too soft/light. Another point of note is that I always pull my covering as tight as possible while attaching it, which, if anything, would increase the odds of the issue.

 I've still never figured out why anyone ever bothers with nitrate. Thinning butyrate clear to a desired level for whatever the task at hand works perfectly IMO and greatly decreases the potential for problems. K.I.S.S.  :)
Narrowly averting disaster since 1964! 

Wayne Willey
Albert Lea, MN U.S.A. IC C/L Aircraft Modeler, Ex AMA member

Offline L0U CRANE

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Re: Mislabeled Dope?
« Reply #10 on: August 02, 2021, 01:30:37 PM »
For many of us, models don't last long enough to show this...

Mine do, probably due to long times between flying occasions, and fewer flights, total. My Allen Aldrich  1/2A Envoy, some may have seen, is over 20 years old. No puckered LE sheeting, (no LE sheeting!) or 'starved wing ribs', but the fuselage sides are now concave by about 3/32" in around 1 1/4" height. The upper decking -curved soft sheet- is not puckered.

Don Holllfelder's WAM Rules-inspired .15 stunter. Sirocco (misspelled Sirorcco on the mag plans) used a nice trick - not for this purpose, but for strength.
 
Flat, soft sheet is easily deformed. - But, curved to a 'sorta-tubular' form, such soft flat sheet is mechanically stronger, and the curve will also likely prevent - or reduce-  the effects of different dope shrinkage between the inner (undoped) and the outer (doped) surfaces.

The curvature's improvement in strength might allow us to use thinner or softer balsa for things like sheet fuse sides. With the reduced  availability of good balsa, this might be a way to use what is available. It doesn't take much curvature. Test this with a 3X5 card? Try to stand it on edge. Ha! Roll a curve across it and if it's enough, it WILL stand on edge.
\BEST\LOU

Offline Brett Buck

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Re: Mislabeled Dope?
« Reply #11 on: August 02, 2021, 06:38:20 PM »
For many of us, models don't last long enough to show this...


   Mine showed up in a few hours.

     Brett

Offline Dennis Toth

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Re: Mislabeled Dope?
« Reply #12 on: August 03, 2021, 06:52:49 AM »
One option that has been discussed by Bob Hunt and others is to use harder but thinner balsa that maintains the lower weight but is more resistant to deforming. Some additional soft balsa false ribs under the LE sheeting can insure they don't sag and extra fuse formers keep the flat sides in line. Lou's tip and the slight curve in the fuse side also adds that extra sag resistance (only need about 1/32" curvature). I did this with 1/64 ply sides on a built-up profile and the rigidity it added was like having 1/4 sides, works well.

Best,   DennisT


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