Fabric sag report

[astjp2]

Info on when your fabric sags between the ribs and how it affects flight

[PA1195]

Re: Fabric sag report

Interesting report Tim...but I wonder if the author every flew or observed a fabric covered plane in flight? Cold temps and improper shrinking can lead to excessive fabric sag between ribs when parked. Some is unavoidable but can be minimized when applied by technique.

In flight both the upper and lower fabric normally rise between ribs (due to differential air pressure) and likely maintain the airfoil's intended cross sectional shape. The higher the lift produced the higher the rise, same for wing sections closest to the fuselage subject to additional prop flow.

Edit: Here's some pics of the top of fabric wings. Note the raised fabric between the ribs. Sorry for the Cub...searched but didn't find a Taylorcraft with that view:



Gary

[astjp2]

Re: Fabric sag report

I think it was a wind tunnel testing, I need to actually dig into the article

[PA1195]

Re: Fabric sag report

I think it was a wind tunnel testing, I need to actually dig into the article

Yes it was derived via a sculpted model wing in a tunnel. Not sure what "sag" meant to the author. To me it means a vertical downward drop or subsidence to a lower level. To them it may have meant any change in level...up or down. The end result is that it is what it is so learn to deal with it as the effect can only be eliminated with rigid metal wing covering. Like metalized aircraft like Stinsons experienced.

Gary

[Hank Jarrett]

Re: Fabric sag report

I plan to read this report today and see how it might apply to the Taylorcrafts. It is from 1932 so the airfoil used in the report will have a lot to do with how it applies to us. I have seen a Taylorcraft with fabric that ballooned quite a bit and the owner never had any idea it was happening until another pilot was taking "Beauty Shots" of him in the air and got a really nice shot from above that showed the bulging fabric very clearly. The owner recovered his wings and found that teh fabric was put on 90 degrees out (with the long twill going front to rear instead of spanwise) which has a strong effect on the starved horse look on the ground and bulging in the air.
I don't think anyone should panic if this is happening (in the short term) to their aircraft and my own 41 has the fabric going in the wrong direction. I simply plan to put it on RIGHT when I recover.

Hank

One reason I am waiting was to get some really good test data points with the fabric on wrong, them recover and ballast the plane to match the prior weight and do the performance tests again to see if there is a noticeable change. Again, I am NOT worried about the safety of the plane, but I DO wonder about the performance changes. Don't hold your breath waiting for the results, I won't be recovering the wings until I have the 45 flying. ;-)

[3Dreaming]

Re: Fabric sag report

Hank, what is the basis of your contention that the fabric strength is different with 90° orientation? When you buy envelopes the orientation will be one way, and if you do the blanket method it will be the other. Both are approved by most STC's.
I understand the difference between warp and fill, but the thread count and size is the same for most aircraft fabrics. The amount of sag between ribs should be the same for either orientation.

[Guest]

Re: Fabric sag report

Hank, what is the basis of your contention that the fabric strength is different with 90° orientation? When you buy envelopes the orientation will be one way, and if you do the blanket method it will be the other. Both are approved by most STC's.
I understand the difference between warp and fill, but the thread count and size is the same for most aircraft fabrics. The amount of sag between ribs should be the same for either orientation.

I wondered about that also. I have not heard of directional dependence of fabric strength before (or if I did I have forgotten which is also possible )

[3Dreaming]

Re: Fabric sag report

I wondered about that also. I have not heard of directional dependence of fabric strength before (or if I did I have forgotten which is also possible )

I do know however that fabric orientation is very important with composites.

[Hank Jarrett]

Re: Fabric sag report

One of the things I have seen is the fabric sags more when the "grain" runs cord wise vs span wise. It isn't a strength issue, it is an elasticity issue. The fabric isn't going to tear sooner but it will bulge up more in flight, which not only changes the shape of the airfoil but the angle of the load at each rib stitch (or whatever fastener you use). I haven't found the reference that says to always put the warp span-wise but it is something I have "known" since high school. In fabrics the warp is a continuous strand and the woof (or weft or fill) is the back and forth strands and has a lower strength and stretch resistance.
I will need to do some more research to find out if there is a recommendation or requirement somewhere for fabric orientation.

[Hank Jarrett]

Re: Fabric sag report

I did find that Ceconite is supposed to be the same stiffness and strength in both directions. That does not explain why my 41 sags between the ribs.

Hank

It is covered with synthetic fabric with the old dope STC.

[astjp2]

Re: Fabric sag report

Hank, Yes it does, the longitudinal strands pull it down because it is trying to go the straightest path between the leading edge and trailing edge, without a rib to give it shape, it pulls tight causing the sag....Tim

[PA1195]

Re: Fabric sag report

Edit: Disregard my last suggestions. I called the fabricator and here's what he does. Pull the fabric as tight as possible before gluing down. When ready to heat shrink heat the area over the ribs and 4-5" either side in the three increasing temp steps Stits recommends (~125-350*). Ignore heating the middle area between the ribs for now. Once the area over and adjacent to the ribs is fully shrunk, heat the middle section between the ribs in the three steps. It results in minimal sag.

Gary

[N96337]

Re: Fabric sag report

That's almost the way I was always taught to shrink synthetic fabric, Gary. The only time I've ever seen any problems with sagging on synthetic fabric, is when they ignore the lower 2 temperature steps when shrinking. I go completely over the wing (or whatever) starting in the middle rib and working my way out, over each rib, then I shrink up the middle spans, then go 90 degrees to that, covering the whole thing again at that same temp. You don't want to take any part of the fabric up to a higher temp until that whole surface has obtained the tension that is achieved by the temp you're working at. To do so, will disorient some threads and result in a difference in tension that may show up further down the road. The whole reason fabric is shrunk in stages is to keep the orientation of the threads as even as possible. I've never seen a cover job that was shrunk properly that has any sagging....inversely I've never seen a cover job that was shrunk improperly that didn't have some sagging....especially when the temperature starts to fall!
There is no difference in the strength of the fabric in either direction. Warp and fill are the same thread, same spacing, same strength. Fabric orientation has nothing do do with it. In the old days, there were some different weaves of fabric used, and that probably had a bit to do with thread orientation.

[Guest]

Re: Fabric sag report

I did find that Ceconite is supposed to be the same stiffness and strength in both directions. That does not explain why my 41 sags between the ribs.

Hank

It is covered with synthetic fabric with the old dope STC.

Here is thought and it has happened to me in the past.

If you leave too much slack in the fabric in one axis (let's call that A) compared to the other axis (call that B) then when you heat shrink the surface A and B axis cords will shrink by the same percentage over area that you ironed.

However since the A axis has less slack then it will apply more tension to the structure in the A direction. That may cause something like trailing edge bowing for example and then you stop shrinking.

But when you stop shrinking the B axis cords are still too long to make a nice tension against the airframe in the B direction. It shrinks the same % on each axis but the B axis will only have a slight tension when shrunk by n% because the fabric required greater than n% shrinkage to get good tension in the B direction but you were limited to n% shrinkage in the A direction or you bend the trailing edge.

I have done it when using envelopes when I carelessly pulled the envelope on and then glued the open end to make the fabric nice and taught in that axis. But the other axis was loose because the envelope was made that way. So on shrinking I really bent the structure that I glued to and had to remove fabric, fix, redo.

Hope this makes sense.

Dave

[Guest]

Re: Fabric sag report

That's almost the way I was always taught to shrink synthetic fabric, Gary. The only time I've ever seen any problems with sagging on synthetic fabric, is when they ignore the lower 2 temperature steps when shrinking. I go completely over the wing (or whatever) starting in the middle rib and working my way out, over each rib, then I shrink up the middle spans, then go 90 degrees to that, covering the whole thing again at that same temp. You don't want to take any part of the fabric up to a higher temp until that whole surface has obtained the tension that is achieved by the temp you're working at. To do so, will disorient some threads and result in a difference in tension that may show up further down the road. The whole reason fabric is shrunk in stages is to keep the orientation of the threads as even as possible. I've never seen a cover job that was shrunk properly that has any sagging....inversely I've never seen a cover job that was shrunk improperly that didn't have some sagging....especially when the temperature starts to fall!
There is no difference in the strength of the fabric in either direction. Warp and fill are the same thread, same spacing, same strength. Fabric orientation has nothing do do with it. In the old days, there were some different weaves of fabric used, and that probably had a bit to do with thread orientation.

Hi John,

Didn't there used to be a rule of thumb about dropping a quarter on it and it has to bounce? That is what I recall and wonder if an improper job would pass that test?

I just shrunk an Aeronca wing used the hi temp 350-375 range I at last past. Feels funny with the water born Stewarts process after first fill cross coat. Not stiff like nitrate.

Dave