Grade A Cotton Testing

[1938BF50]

HELP!

My potential purchase is failing the fabric punch with readings as low as 45. Is there a different scale for Grade A cotton? All control surfaces are reading well above 60...fuse is above 60, top of wings are 45-50 range. The IA is going on memory with the "60" figure as minimum. Can anyone help me with facts!

[VictorBravo]

Re: Grade A Cotton Testing

Sorry, the fact is that you will be recovering a cotton airplane either very soon, or soon. Take the cost of recovering off of the price of the plane IMHO.

[robbie]

Re: Grade A Cotton Testing

I believe it can go to 40 lbs before it fails. Jump in guys.
Its not 60.

[Karl Rigdon]

Re: Grade A Cotton Testing

My first IA used 46 as a minium Maul punch test. You will not find a "approved" amount. It is used as a guide to test the fabric. The only approved test for grade A is the ravel or pull test. In this test you cut a 1 inch strip from the area that may be the weakest area. You have to remove all finish to the bare fabric. You then use a fixture (ravel tester) to pull to 60 lbs. If the fabric does not break or start to unravel it is OK. I have only seen 1 ravel tester and that was in the avation metalsmith shop when I was stationed on Adak while I was in the Navy. I am not a A/P so I am open to and will accept corection

[jgerard]

Re: Grade A Cotton Testing

I have pulled large pieces of flaking and cracked dope off fabric that you would swear it could punch over 40 by itself! Hard to judge, but I would also seem to think that the plane(or just the wings) would need some new fabric soon. I have also personaly looked at too many aircraft that are nice and shiny on the outside with a fresh recover or paint job and no cleanup or repairs made to the underlying structure. Just looked at a 90hp Super cub for a customer that fits that description. I am also weary of engines that have 0-SMOH. How about some test flying/break-in hrs there buddy?

[N96337]

Re: Grade A Cotton Testing

Victor Bravo is right on the money.... you're going to recover cotton soon or sooner. Sooner is best. If it's popping through at 45 with a Maule tester, it's not if, but how soon can you start recovering.
JH

[1938BF50]

Re: Grade A Cotton Testing

Guys,

Thanx for all the input. I knew I would be recovering soon but was hoping for a year or two of cheap flying. I made a nice pull test fixture last night for the AI to use per 43.13. According to 43.13 it has to test 70% of "new" which puts the tensile pull at 46 pounds. I tried it out last night with a cotton bedsheet test strip which withstood 6 gallons of water (approx 52#).

[high time cub]

Re: Grade A Cotton Testing

Hi 1938BF50,
The minimum tensile strength of approved grade A fabric in new undoped condition is 80 lb per inch (one inch wide).
The minimum permissible strength is 70 % of new strength, or 56 lb per inch.
Good Luck,
Jim

[1938BF50]

Re: Grade A Cotton Testing

Jim,

Thanx for the response! Can you tell me where you got your info as soon as possible? I want to make sure we are doing this right! I don't have the chart in front of me right now but recall that there were (3) categories of cotton and thought I had referenced the correct one.

[high time cub]

Re: Grade A Cotton Testing

Mike,
Try "Bed Bath and Beyond" for new bed sheets.

For airplane info.....

Here's a link:

http://www2.tech.purdue.edu/at/courses/at308/Technical_Links/Ac43-13-1B/CH2_1.pdf

Count the warp and fill.... Grade A fabric should have a thread count of 80 to 84 threads per inch in both length and width. (MIL-C-5646)

Jim

[alwaysoar]

Re: Grade A Cotton Testing

Jim--There's a distinction between Airplane Cloth Mercerized Cotton (Grade A) and Airplane Cloth Mercerized Cotton that has a different TSO. However, while "Grade A" requires 56 pounds, wouldn't an airplane with a wing loading under 9 p.s.f and a never exceed speed less than 160 mph still be legal (and safe, theoretically) at 46 pounds (70% of 65 pounds warp and fill)?

Ed

[1938BF50]

Re: Grade A Cotton Testing

Ed, Jim,

I tried posting a reply yesterday saying basically what Ed is stating. The IA is using the under 9# wing rating and Vne to establish the 46# test limit. Hope to have results this weekend using my fixture. It will be intersting to see if there is a correlation between the strip test results and the maule tester readings in the sane area.

[alwaysoar]

Re: Grade A Cotton Testing

By the way, many are saying recover now and there's logic to that. If you are satisfied all but the wing tops are ok, however, you can recover just the tops of the wings and get flying. That's not the best advice, however, for a bunch of reasons.

[Guest]

Re: Grade A Cotton Testing

Jim,

Thanx for the response! Can you tell me where you got your info as soon as possible? I want to make sure we are doing this right! I don't have the chart in front of me right now but recall that there were (3) categories of cotton and thought I had referenced the correct one.

Hi Mike,

look at section 2-4 and table 2-1 in Ac43.13-1B. on or near page 2-3

Dave

[1938BF50]

Re: Grade A Cotton Testing

For those interested......I built a fixture to tensile test the cotton on my plane. Previously the fabric had very consistantly "punched" around 45-50 (with the Maule tester) in the area that we took a "strip" to test. The result of the tensile test in this same area was 30 pounds (per inch width of bare fabric). Well below anything "legal". Interesting that the tests yield a 33% differential! I realize that the tests do not campare apples to apples, but at 45 on the Maule I was pretty close to legal minimums. Of course the plane is now unairworthy and I will be recovering. Thanx to all who responded with suggestions!