Poly-C - any weight penalty v dope?

[Bob Cotsford]

I'm building a vintage APS Eros, an 84" cabin model.  The fuselage is 1/16" sheet planked over a 1/4" sq frame.  The original used a doped tissue finish, but as I've got dodgy lungs I would like to try Poly-C and tissue instead.

 

Instead of planking I'm using sheet molded by soaking in dilute ammonia solution, which is turning out easier than I expected, even round really tight radii edges and mild 3d curves.  It will soon be time to actually attach the pre-shaped skin, so I want to surface-finish it as quickly as possible to reduce hangar rash while construction continues.

 

So - how much weight does Poly-C add in comparison to sanding sealer and dope?  Has anyone any experience and suggestions?

 

Bob

Edited By Bob Cotsford on 16/03/2010 12:32:15

[Myron Beaumont]

Can anybody tell me whether this poly C stuff actually shrinks whatever its spread on like dope does

PS-- does it smell as nice ? 

[Bob Cotsford]

that smell is just a memory now, last time I used cellulose I was gasping for breath all day afterwards.  I doubt if Poly-C would shrink, at least not much.  If it does I'll need to find something else as I don't want the starved-horse look.

[Myron Beaumont]

Bob

Say no more, I shan't be using it even if I could buy it locally The starved horse effect comes about by stretching covering over stringers to formers  etc when you've not quite got it right !

[Bob Cotsford]

I've also seen it in the past on thin skinned surfaces from wings to fuselage tops.  Usually after a few coats too many of shrinking dope.

As the top and bottom of the Eros fuselage are 1/16" sheet (or planking) over semicircular formers at roughly 3" spacing, it will probably be a candidate.  I've added a 1/4*1/8 spine top and bottom to provide some support, but I'm still expecting a degree of bowing in between formers.

[Myron Beaumont]

Bob

 Any covering will shrink to the least line of resistance .Look at any rib/spar wing especially without LE sheeting !  

[Myron Beaumont]

PS

Goodness knows what the actual wing section REALLY is ?

[Tim Mackey]

No it doesnt shrink like dope does, it also smells a heck of a lot better IMO - but that of course is purely subjective.

[Myron Beaumont]

Timbo

You're too young to remember involuntary sniffing before it became a scenario for our up & coming young generation !

Actually ,it might be the cause of my present mental state

[Bob Cotsford]

Posted by Myron Beaumont on 16/03/2010 13:52:36:

PS

Goodness knows what the actual wing section REALLY is ?

 I think most of my early single spar glider and rubber powered models probably had a wing section best described as 'triangular'.

 

The real question is still how much does Poly-c and tissue weigh in comparison to dope and tissue?  How many coats are needed to get a reasonable surface finish where the tissue is adding to the balsa's strength?  With dope, I'd expect to use a couple of coats of thinned sanding sealer, a coat of thin dope to attach the tissue, then another couple of coats to bond it and fill the surface, say a minimum of 5 coats (most of them flatted back though).  I've seen suggestions of up to 10 coats of Poly-C with glass cloth, but I'd hope it would need less than that for tissue.

[Stephen Jones]

Hi ,

I can tell you that Poly C does not shrink tissue but for some strange reason it does shrink brown paper see my build on covering a guillows mustang with brown paper .

http://www.modelflying.co.uk/forums/postings.asp?th=37460

My Guillow`s P51 Mustang

Edited By Stephen Jones on 16/03/2010 15:54:48

[Matt Halton]

I don't perceive any weight penalties when I have used it (lots of times) The thing you need always to bear in mind, is it will warp balsa applied directly.

 

Curiously enough, I always use a thin coat of non shrink dope on sheeted areas before Poly C'ing. 3 thin coats of PC on tissue, sanded between each coat will suffice.

 

I suppose it doesn't matter what you seal the wood with, but I haven't tried anything other than dope, so I guess you won't completely get away from the smell...

 

 

 

[Tim Mackey]

Posted by Myron Beaumont on 16/03/2010 14:05:19:

Timbo

You're too young to remember involuntary sniffing before it became a scenario for our up & coming young generation !

Actually ,it might be the cause of my present mental state

 

Oh I wish that were true Myron - on both counts

[Bob Cotsford]

Posted by Stephen Jones on 16/03/2010 15:50:20:

Hi ,

I can tell you that Poly C does not shrink tissue but for some strange reason it does shrink brown paper see my build on covering a guillows mustang with brown paper .

http://www.modelflying.co.uk/forums/postings.asp?th=37460

My Guillow`s P51 Mustang

Edited By Stephen Jones on 16/03/2010 15:54:48

 Thanks Stephen, I'll have to make sure not to leave any wrinkles if they won't self heal.

Posted by Matt Halton on 16/03/2010 15:54:52:

...

Curiously enough, I always use a thin coat of non shrink dope on sheeted areas before Poly C'ing. 3 thin coats of PC on tissue, sanded between each coat will suffice.

 

I suppose it doesn't matter what you seal the wood with, but I haven't tried anything other than dope, so I guess you won't completely get away from the smell...

 

  

Kind of defeats the object of the exercise.  
It looks as though I need to make up some test pieces to see how it behaves on thin sheet without a cellulose sealing coat.

[Stephen Jones]

Yes, i made a test frame before i used the brown paper and as Poly C is a relatively new product there will be much expermenting needed to find out what it is best used for.

Edited By Stephen Jones on 16/03/2010 16:24:55