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Author Topic: Doctor's paper VS Silkspan  (Read 2120 times)

Offline Brett Buck

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Doctor's paper VS Silkspan
« on: January 16, 2021, 02:06:59 AM »
In an off-line discussion, the topic of covering getting loose when it's damp outside and screwing up the airplane trim (and maybe the airflow) came up. Part of the discussion was about different types of covering and how they react - in the example case, the airplane used "doctor's paper" with what might be described as an "acceptable" weight-saving dope finish - and not to be uncharitable, good enough to not stand way out, but also, no threat to Windy's concours award. In several similar cases of morning fog/dew/condensation, the airplane performance degenerated dramatically, only to recover once it dried out. The underlying structure is a conventional D-tube with open-bays and "perpendicular" ribs.

    This is a very well-known phenomenon and the reason that FF guys, and most stunt fliers, use synthetic materials instead of paper products for covering. But I got to wondering about how the doctor paper might be different from the real deal silkspan. So I put my little USB microscope to work again:



 
Same lighting conditions and same magnification (about 150x). As you can see, the doctor's paper is definitely NOT the same as silkspan, it is amorphous short-fiber pulp made like classic wrapping/packing tissue, where as the 00 Silkspan (probably K&S, maybe from some other distributor that used to exist but I forget) is the traditional long-fiber strongly-linear-grained arrangement.
 
   I also just hand-tested the wet strength, and even though the silkspan is only the lightest normal OO strength, and the doctor's table paper is pretty solid and heavy, the silkspan had much better wet strength.

    So, I would not say the doctor's exam paper is unusable for our purposes, obviously, some pretty good airplanes have been made with it. But I am now not too surprised that our anonymous modeler had some trouble with it in damp conditions, the lack of directionality alone would seem to greatly alter the structural deflection, and with more pulp, I would expect to absorb more water.

    For my purposes, I think I will stick with polyspan, which more-or-less doesn't respond to water, for an open-bay structure. I think it doesn't make any difference over a solid surface, all you have to do is span the cells in the balsa.

   Anyway, food for thought.

    Brett



   

Offline Dan McEntee

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Re: Doctor's paper VS Silkspan
« Reply #1 on: January 16, 2021, 07:16:38 AM »
    Brett;
     Which brand of exam paper did you use? There are several brands that people have tried, and the stuff that I acquired is from a company called Tidi, and there were two different weights at the time I got mine. I would have to review the old threads to get all the details. Tidi had a huge variety different kinds of exam table paper, and it took some work to figure out what kind we wanted, and I think they called it crepe. The best testing I could do was comparing it to a known sample of the original K&S paper by weighing it and then see how it handled wet. Some were reporting at that time that the stuff they had didn't stay together when soaked but the Tidi paper held together as well as the K&S paper did. It also passed the "tear test" by having a tear in the paper change direction and follow the grain to the length of the roll. Simply holding a sheet up to a light showed it to be very similar. The differences in the two weights were not very far apart and I could only tell by weighing on a balance gram scale and by holding up to a light. I think the consensus back at the time all this went on was about 50-50 and opinions varied  Adding your microscope examination is pretty interesting also. I was just curious as to what brand of paper you were working with?
    In the mean time since then, I think K&S found another source of material and they have it available again? I don't know if that is true or not but I remember seeing it written somewhere..
   Type at you later,
    Dan McEntee
 
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Offline 944_Jim

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Re: Doctor's paper VS Silkspan
« Reply #2 on: January 16, 2021, 07:18:38 AM »
Mr. Brett,
Is your DocSpan the "everyday," or the "choice?" Could I ask you to scope a piece of the "everyday" for me? I'll mail a small chunk for the sake of collecting/comparing known variations of DocSpan to Silkspan.

I am so thankful for your efforts on this. I am drawn to DocSpan as a frugal/miserly option to finding real silkspan (I make change from a penny). The first few models coming off my mid-life crisis bench were indeed covered with gift-wrap tissue. This is acceptable on smaller 1/2A models which are typically flown on dry, wind-free days. The open bay areas on those models pulled down/swagged between the ribs. The tissue is tight and smooth, so I didn't think it was my application, and the planes looked swell compared to a flat plank wing. I blamed the gift-wrap tissue that has no real grain.
After a couple of 18" planes doing this, I started paying closer attention to my work. I suspect the swag came from omni-directional pulling across the ribs for a wrinkle-free finish, while also trying to "shorten the distance between the TE/LE by pulling down between the ribs.

This became most apparent on my Brown/Novotnik Mosquito (23" WS, 2x .020 Pee Wees). This particular model has a flat bottom wing with a significantly thick rib set. Since I was looking for the best possible tissue job, out came my DocSpan. When I put it on the wing (doped on edges only) and water-misted the paper, it pulled well across the ribs. In fact, it looked great!  But it wasn't drum tight yet. (Note: the unfinished plane is on RCG's CL forum.)

By the time I started doping between bays, the swag started coming in. I really laid into the dope hoping to get the paper to do what I always expected dope and tissue to do. In fact, I even pulled out the hot air gun and hit the wing with more dope. By the time I really understood  what was going on, I nearly wrecked the wing. The ribs that hold the engine nacelles had pulled apart (open, untissued so the nacelles would slip in), and the outboard TE turned up a bit.
This is a plane I plan to revisit later (36" reproduction Frog Mosquito with 2x .09 Medallion), so I really need to know and choose the appropriate tissue.

My roll of DocSpan is Tidi's low-end "Every day crepe," PN 98-1004. I am trading a square for square with a gentleman in VA for his mid-grade "Choice crepe" because he says it has an observable grain. I hope he is right. If it is, then I'll relegate my "every day" to covering sheeted areas and smaller models, and buy a roll of the choice for larger open bay models.

I probably need to learn how to plasticwrap a wing. My Tom Tom and BHM Challenger deserve my best efforts since they are my gateway planes away from 1/2A toys.

Feel free to PM your mailing address if you care to check a known sample of "everyday."
Thanks much!

Offline 944_Jim

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Re: Doctor's paper VS Silkspan
« Reply #3 on: January 16, 2021, 07:25:20 AM »
PS
Yeah, what Mr. Dan says...he gave me my first piece after I read the two dominant threads here. ISTR a bunch of PN chatter, so I went with the cheapest option listed (any Tidi-1002 is18" wide, any Tidi-1004 is 21").

https://stunthanger.com/smf/open-forum/world-shortage-of-silkspan-ended-frank-d-nominated-for-'hero-of-the-reich'/

https://stunthanger.com/smf/open-forum/butt-tissue-paper-question/

While not intending to spread erroneous info, ISTR the McKesson paper was reported to bear the same "last four" (or word-for-word description) as they were reported to be Tidi resellers.
 

Offline Brett Buck

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Re: Doctor's paper VS Silkspan
« Reply #4 on: January 16, 2021, 12:53:59 PM »
Which brand of exam paper did you use? There are several brands that people have tried, and the stuff that I acquired is from a company called Tidi, and there were two different weights at the time I got mine.

   It was TIDI brand, in this case the "light" kind from the 18" roll. Here is the "heavy" paper:



Again, I make no judgements about it either way, but what I see is certainly consistent with the observation that it loosens up a lot when it is damp.

    Brett

p.s. Of course, if it is going extremely limp and then stiffening up later, it very strongly suggests that it is also changing stiffness (and maybe alignment) when the conditions change over smaller ranges. I assure you that being stiffer or softer over even small ranges *makes a huge difference* in how the airplane flies. I think no one really noticed the problem until the engines got so consistent in the late 80's, but afterward  we (Ted, David, Bill, and I, among others) spent *immense* amounts of time trying to figure out what was going on, and why some airplanes seemed so prone to it, and others not. My airplanes in particular were noted for never changing unexpectedly- it might be good, it might be bad, but it would be good or bad until I made an intentional changes. Some others were definitely not that way, requiring constant trim changes and flying spectacularly well sometimes, and not-so-good other times, and, why the foam-winged airplanes seemed to be far more consistent from day to day and generally flew better despite some of them weighing more, or much more, than the built-ups.

    Note also that we have had to contend with a much larger range of conditions than many people - foggy/dewey and 50 degrees in the morning, 110 degrees and 8% humidity the same afternoon, and also midwest muggy after travel. That gives us a very wide range of conditions to evaluate over a very short time.

    Mine have all (since the 1989 airplane, which means only 5 "serious" airplanes) been built in a way to resist changing characteristics, essentially regardless of the weight increase. My airplanes are not particularly heavy - the current one from the 2020 NATs is 61 something ounces, and appears to require a small design change to optimize the performance at the "under-target" weight - but they are clearly 4-8 ounces heavier than I could probably build it with extreme weight-savings.

    One of the things I am struggling with now is that this appears to be unwise with electric, where saving weight seems to be a critical matter due to limited power.

 
« Last Edit: January 16, 2021, 01:27:25 PM by Brett Buck »

Offline Dan McEntee

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Re: Doctor's paper VS Silkspan
« Reply #5 on: January 16, 2021, 01:22:39 PM »
  Thanks.   Now I just wonder why they are made so differently??!!! Like I mentioned, the stuff I have passes the "tear test."  You can determine the grain by tearing it. I have toured a paper plant once and have a small understanding of how paper is made.  I thought that the grain direction was a by product of the process when the pulp is poured onto the calendar, which is moving at the time, and that action sets the grain direction like we usually find it, along the length of the sheet on the roll. Like we have learned, this type of paper is made and used for a lot of different reasons and uses. I'm willing to bet that the paper made for K&S and others back in the day was also used for many other things. Some one who posted back during these other threads that lived in South Africa, I think, says they used it for making tea bags, and there was a company near him that produced tea bags and he was able to get all the rejected paper that he could use!!  And as for the aspect that it loosens up when the humidity gets up, I have seen that happen with silk span covered models even before doc-span came along, just never paid much attention to it. Tissue paper also sags a bit if in really damp air. I just chalked it up to being the nature of the beast.  Oh well, time marches on and just need to build up some models for further fun and testing purposes!!
    Type at you later,
   Dan McEntee
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