Torsional Rigidity: Polyspan vs. Silkspan

I have used silkspan frequently over a lot of years and am always amazed at the rigidity it imparts to a wing. I have not, however, yet tried Polyspan/Polyester Tissue. I have read a lot of articles and posts to this forum about applying and finishing Polyspan and really do not have any questions about that. But, I have been unable to find anything that clearly compares the torsional rigidity of a wing covered with Polyspan with one covered with silkspan. What has been your experience?
Thanks in advance for your observations.

It seems to me that the film of dope provides most of the rigidity, so the two should be quite similar. Dry dope is more rigid than highly plasticized. More coats stiffen the wing more than fewer. Epoxy over the dope renders the whole thing nearly rigid.

One of our club members, Ken Kaiser, just showed me a FF stabilizer that was double covered with Japanese tissue over Polyspan, 3 coats of dope over that and a spray of epoxy. He said the weight gain was very small for the durability and quality of finish. He did use colored tissue and clear dope, not colored.

I suspect that a light sanding and one coat of filler under the epoxy would have given a superb, glossy finish.

Tony Naccarato says that you can rub out Epoxy, but you better do it within the first week, it gets too hard later.

LAST EDITED ON Nov-13-03 AT 12:56 PM (CST)
 
I'LL NOT USE PLYSPAN AGAIN FOR THE FOLLOWING REASONS. THEY MAY OR MAY NOT BE GERMANE TO YOUR APPLICATION.

THE COLORS IN PLYSPAN SIMPLY DISAPPEAR OVER TIME. THIS IS ESPECIALLY TRUE OF THE BLUE WHICH WILL TURN COMPLETELY WHITE IN A MATTER OF A COUPLE OF YEARS. IF YOU'VE SEEN PICTURES OF DAVID FITZGERLD'S BLUE WINGED SHANGHAI WORLD CHAMPS AIRPLANE YOU WILL NOTE THAT WHAT WAS A DARK BLUE IS NOW COMPLETELY WHITE. THE YELLOW IS NOT AS BAD BUT THE DOCTOR IS DISTINCTLY LESS YELLOW ON THE TOP (FACING THE SUN) THAN THE BOTTOM WHICH IS STILL FAIRLY BRIGHT YELLOW

THE ORANGE AND BLUE TRIM COLORS (COLORED PLYSPAN APPLIED OVER A BASIC GREEEN SILKSPAN) ON MY ARES BUILT FOR THE FIRST VSC HAVE VIRTUALLY DISAPPEARED...AS DID THE ROUNDELS FOR THE INSIGNIA ON MY CHIEF BUILT FOR A LATER VSC.

SECOND, THE PLYSPAN APPEARS TO FAIL OVER TIME AND EXPOSURE. MY RED WINGED STUNTWAGON WHICH HAS MERELY BEEN HANGING ON THE WALL FOR LAST FIVE PLUS YEARS HAD THE COVERING POP A RIP ABOUT SIX INCHES LONG ON THE TOP OUTBOARD WING JUST OUTSIDE THE PLANKING. THE DOCTOR STARTED TO STALL ON A FLIGHT TWO WEEKENDS AGO WHICH SHOCKED ME...UNTIL I LANDED AND DISCOVERED ANOTHER SIX TO EIGHT INCH TEAR IN THE SAME LOCATION. MY GUESS IS THAT THE COVERING GETS BRITTLE WITH AGE AND EXPOSURE AND EVENTUALLY CAN'T STAND THE STRESS OF HIGH G'S...OR WALL HANGING!

IF PAINTED AND PROTECTED FROM THE SUN IT MIGHT BE OK BUT I'M NEVER GOING TO FIND OUT.

TED

P.S. AS USUAL, PARDON THE SHOUTING. WORK COMPUTER LOCKED IN ALL CAPS.

Ted,
Is Plyspan basically the same stuff as Polyspan?

If I remember correctly, Plyspan is a material SIG carried that looked similar to colored silkspan. I used it for a while and found the same fading problem as well as less puncture strength that silkspan.

Ted,

They are talking about POLYSPAN not PLYSPAN they are completely different animals. I have used POLYSPAN a lot and like it on open bays. It doesn't go around curves very well but you can work at it and get it around easy ones. I would use medium/heavy silkspan on highly curved tips however. The thing I don't like about POLYSPAN is that you must be very careful not to sand through it - that makes a big mess to fix. Secondly, the basic question is about torsional rigidity of POLYSPAN vs SILKSPAN. I find them to be similar but not exactly alike. Sometimes weather conditions can cause a slight slackening of the POLYSPAN but that can be fixed pretty easily with a heat gun - being very careful of course. Also you can pull it pretty tight when applying it and use a little less heat when shrinking it and it will come out a bit less untolerable to weather. POLYSPAN does have a significantly higher strength under puncture and ripping stresses than SILKSPAN does. Tom Morris and I have been using almost exclsively POLYSPAN since 1997 and I guess we would have a pretty good experience base with this product.

Jim Pollock

MEA CULPA, MEA MAXIMA CULPA.

I TOTALLY MISREAD PETER'S ORIGINAL POST.

I WAS SPEAKING OF SIG PLYSPAN NOT POLYSPAN.

MY EXPERIENCE WITH POLYSPAN HAS NOT BEEN TOTALLY POSITIVE ALTHOUGH I MUST ADMIT IT STOPPED SOMEBODY'S PEN OR SCREWDRIVER OR SOMETHING FROM GOING CLEAR THROUGH THE WING OF THE FINAL EDITION. IT IS VERY PUNCTURE RESISTANT.

ON THE OTHER HAND, AT LEAST IN THE COMPARATIVELY DARK COLORS THE FINAL EDITION IS PAINTED (PURPLE POND SCUM AS SWEETLY DESCRIBED BY BUBBA HUNT) THE POLYSPAN HAS BEEN DISCONCERTINGLY VARIABLE IN ITS SURFACE RIGIDITY. AT TIMES, IN HOT MOIST AIR THE TOP SURFACE WILL GET LOOSE AND THE RIGIDITY OF THE WING VARIES AS A RESULT. GENERALLY, AFTER A PERIOD OF EXPOSURE THE UPPER AND LOWER SURFACES WILL ADAPT UNIFORMLY TO THE AMBIENT TEMPERATURE BUT NOT ALWAYS. SOME INCONSISTENCY IN MANEUVERS IS LIKELY ATTRIBUTABLE TO THAT VARIATION IN STRUCTURAL RIGIDITY (COULDN'T POSSIBLY BE INCONSISTENCY ON THE PART OF THE PILOT, COULD IT?)

POLY SPAN ALSO REQUIRES A LITTLE DIFFERENT TECHNIQUE IN APPLICATION FROM SILKSPAN. YOU MUST BE VERY CAREFUL ABOUT SANDING IT TO REMOVE OVERLAPS ETC. BECAUSE OF A TENDENCY TO "FUZZ" UP AND NOT PRODUCE A SMOOTH CONSISTENT SURFACE. OTHERS HAVE MORE EXPERIENCE WITH THIS THAN ME SO I'LL GO NO FURTHER.

SUFFICE TO SAY, I'VE GONE BACK TO GOOD OLD SILKSPAN AND WILL KEEP MY FINGERS CROSSED THAT NOBODY BENDS OVER MY SHIPS TO TAKE PICTURES WITH THEIR SHIRT POCKETS FULL OF PENS, PENCILS, SCREWDRIVERS AND OR SLIDE RULES.

Peter,

I've used Polyspan on rubber model fuselages because its tough and it can be washed out (rubber lube inside fuse I mean). I have found however that it does not have the torsional rigidity of jap or silkspan. The reason is that like kevlar and exopy, the fibers don't wet. If you were to coat a polyester strand with dope and let it dry you could slide the strand out of the dope tube. Kevlar will do the same thing in epoxy which is why rigidity of kevlar layups is not the best. Because of this the fibers will actually move inside the dope coat and rather than be glued together as the silkspan fibers would be. To be fair I have never done a full blown dope finish over Polyspan so I don't really know the whole story. I have shyed away from it however for the reasons I site here. Polyspan is also nasty if you raise the fibers sanding because it won't trim off under abrasion. The only way to fix it is to trap the fibers under dope of filler, the finish can get pretty heavy doing that.

I know others have done beautiful work with Polyspan, I'm just relating my experience.

I have used Polyspan ONCE on an "I" beam impact. I have two other impacts of the exact same configuration, but with silkspan. All planes are within an ounce of each other. The wing with polyspan is CLEARLY more flexible in torsion than either of the silkspanned planes. For this reason, I will not use it again in this configuration.

If it is going on a built up "D" tube wing (and my built up "D" tube wings have a "tube" in the front as well as the back), then this reduction in rigidity might not be a problem.

The one significant thing Polyspan has if it's puncture resistance. The previous Impact mentioned ("I" beam with Polyspan) flew through a hail storm for 95% of a flight (the first 5% was not hailing). The only damage was some cracking of the paint where the hail hit the covering, but not one tore through. There is no way a silkspanned wing would have survived that encounter with mother nature.

I agree with Ted's (the person, not the lean airline's) comments regarding use over compound curves and sanding into (don't do that!!).

It occurred to me that Ted is working so hard after retiring that he may not be aware that an airline has been named after him. Also in keeping with the subject at hand- my Impact is polyspan and the wing seems to be as torsionally rigid as my silkspanned SVII. However, there is a hole through the wing on the SV left by a photographer's Cross pen while the Impact has fended of an attack by a dope brush! Put me down for polyspan.

LAST EDITED ON Nov-13-03 AT 04:31 PM (CST)
 
I can't speak in detailed quantitative terms, but I can tell you two things:

I got my Polyspan from Model Research Labs, a free flight support outfit:

http://modelresearchlabs.com/polyspan.htm

In their discussion of how to use Polyspan, they say "Be very careful when applying more than three coats of dope as Polyspan is very strong and can tighten to the point of destroying the strongest model framework" They are talking about free flight models, not stunt, but even so, that ought to tell you that the stuff is strong--and will shrink. Which ought to give you plenty of torsional rigidity on a stunt wing.

I have covered a Berkeley Orbit Ace wing in the stuff that MRL sells. I shrank the bare Polyspan with a heat gun prior to putting on 4 coats of thinned Aerogloss clear dope. This wing is NOT twisting and I have not even gotten to silver and color dope. Grabbing both balsa wing tips and attempting to twist them along the axis of the main spar, which is buried in the center of the ribs, as hard as I feel sensible to do, which is quite hard, results in a torsional deformation that is imperceptiable to my eye. No wrinkles appear on any of the fabric and flopping the wing up and down showe that the hinge remains free and clear on the elevons.

For all practical purposes there is no torsional distortion on an open bay structure that is essentially a combat wing covered with MRL Polyspan and doped until a bit of a sheen appears.

Not science, I know, but certainly this stuff is as good as any silkspan ever was and I think a fair amount better. How much better I can't tell you because I'm not prepared to twist the wing to distruction to find out. I suspect that the covering would help absorb torsional loads right up to the point where they overwhelmed the whole structure and if a tear occured the structure would shatter.

I have similiar results with a Nobler and Dolphin wing, although they have a D-tube structure the Orbit Ace does not.

As long as I can get it I'm using MRL Polyspan on my wings.

My two sense worth.

Cy

PS:

Reading over other comments, I need to add that I took considerable care to get the right side down, tack a corner of the material down with clear dope/thinner over a clear doped structure and then pull the material as tight as possible while applying dope around the edges to attache. The Polyspan was quite evenly applied and taunt to the touch when all the edge dope dried before I attempted to shrink the cover with the heat gun. The heat gun put some tension on the surface and several coats of 50/50 dope/thinnner applied days apart gradually filled in the material without inducing any sagging. I'm surprised to hear others say that they find Polyspan less satisfying than silkspan in terms of getting tight and staying tight--although there may be differences in actual material and application technique at work here.

What I can agree with is the fundamental difficulty getting Polyspan to stretch around compound curves like fuselage tops and wing tips. I did a couple of wingtips using a series of overlapped small sections of Polyspan, which came out OK, but I decided that was more work than was sensible and will use Silkspan on wingtips and other compound curved areas from now on.

And no, you don't want to sand it before the weave is filled with dope and very, very gently there after. It is not at all as organic and pleasureable to work with as Silkspan.

But the result, on the open bays of a wing or tail surface, is in my exprience, superior to silkspan.

My thanks to all of you for your comments. You have provided a lot of very useful, on point information.

Since Jim Pollock and Tom Morris convinced me to try polyspan I have become a great user of same. However, it is not as rigid on "I" beamers. Ask me how I know. But on a rigid C or D tube wings, it's great. It is more forgiving of punctures, will not "pop" and tear from the dope drying out. Keep the shiney side up, don't get agressive with the sand paper. It will also fill much quicker or with less dope than silkspan, about 1/3 as much. It has a smoother surface to begin with.