Fabric sag report

[N96337]

Re: Fabric sag report

I remember hearing about that test Dave....I'm not sure if it'd pass or not! I also agree with your post above about too much slack....it'll only shrink so much!!
Yeah, it's a little different when it's soft and flexible!! I always loved the sound of the Polyfiber process when you were rib stitching...popped like a drum!
John

[N96337]

Re: Fabric sag report

I also recalled this morning, a video a friend showed me years ago, of the famed "cub doctor, Clyde Smith" at one of his seminars, showing the people how he left a BUNCH of slack in the fabric to keep it from distorting the structure when it was shrunk. Bad advice!!!! I can see where something like that could create all kinds of messes in the air and on the ground.
John

[astjp2]

Re: Fabric sag report

the heat you shrink the fabric will keep from damaging smaller structures with dacron fabric. Tim

[Guest]

Re: Fabric sag report

the heat you shrink the fabric will keep from damaging smaller structures with dacron fabric. Tim

I don't follow Tim.

[N96337]

Re: Fabric sag report

When you're over a delicate structure, don't go all the way to 350. Less heat = less tension. Unless you're using dope, as in Ceconite, then it's always going to keep tightening no matter what.
John

[Ray36048]

Re: Fabric sag report

From the Poly Fiber manual....Very lightweight aircraft, use the iron at 250* to 300*. It notes the fabric will not be as tight as at 350*, but is OK for ultra light aircraft. With Poly Fiber process the fabric is the only thing that shrinks and brings the fabric up to tension.

Process that use dope, the dope also shrinks, even non tautening dopes. So, if the fabric is very tight and you add dope, strings could be deformed. The manual says the thumping and bouncing of quarters are old aviation wives tails.

[Guest]

Re: Fabric sag report

From the Poly Fiber manual....Very lightweight aircraft, use the iron at 250* to 300*. It notes the fabric will not be as tight as at 350*, but is OK for ultra light aircraft. With Poly Fiber process the fabric is the only thing that shrinks and brings the fabric up to tension.

Process that use dope, the dope also shrinks, even non tautening dopes. So, if the fabric is very tight and you add dope, strings could be deformed. The manual says the thumping and bouncing of quarters are old aviation wives tails.

LOL - or in this case "old men's tales"

Dave

[Guest]

Re: Fabric sag report

Actually I recall reading it in manual so I looked thru my old manuals and i did find it in a 1996 Randolph manual.

It is there along with mention of using a heat gun which Stewarts forbids.

I think that originally I read the coin drop method earlier perhaps in the 60's or 80's but finding it at all made me stop looking.

Scan of the pages is attached.

Dave

[Ray36048]

Re: Fabric sag report

It is interesting to note the Randolph manual refers to an iron at 250*. The fabric that is used in these processes are the same. Poly Fiber shrinks to 350*. The Randolph manual allow for additional shrinking by the dope.

[astjp2]

Re: Fabric sag report

That is because the non taughting nitrate dope still tightens over time...

[N96337]

Re: Fabric sag report

Exactly! Non taughtening dope will taughten forever....just not as much as regular "Taughtening" dope.
A heat gun has NO way of accurately controlling the temp, so when you go over the 375* (I can't remember the exact degree...we tested it in shop, but 375 will do for now) point, the fibers release again and your fabric becomes saggy....and it's not repairable. the only way to make the job airworthy again is to pull the fabric off and start over. Not to mention that a heat gun will quite often spit little chips out from the elements...they make great little holes in polyester fabric. Use of a heat gun voids the STC for Stewarts.

[Ragwing nut]

Re: Fabric sag report

you have to remember that old analysis is done with grade a cotton and tautening dope. You do not want cotton to tight, as tautening dope shrinks at roughly 3% per year its entire life. Non-tautening is around 1%. If cotton is too tight it will deform structures and break wing ribs. If you have a modern system and it is "loose" or "ballooning" in flight then its one of 2 problems. Either fabric was not shrunk properly or too light a fabric was used in covering process. People trying to save weight by using a lighter approved fabric can create more parasitic drag in flight and may actually be slower in flight.