Riggers - have you heard of...?

[councilman24 37]

Rob,

I agree that without practice it's easy to damage a canopy. 40lbs is also to high a force for 0-3 cfm fabric whose spec is ummm 46 I think. Some folks believe any load is partially destructive. When I came back from the 1989 Rigger Conference in Muskogee with my clamps I started practicing. The first couple, with the clamps not tight enough, they slipped and damaged the weave. A few more practice pulls and I was confident. And I had several phantoms and others that I needed to test routinely. I found a couple of Pioneer canopys that were weak.

BUT, I HAVE found a legitimately weak ram air reserve through pull testing. It's common for the coating on the inside of a Javelin free bag to transfer to the canopy. This makes the fabric somewhat stiff, at least it feels different. I'm not telling you anything you don't know. BECAUSE it isn't normal, I've routinely testing those areas to 30 or 35 lbs depending on what era it was. They had never failed for many years. Until I did one that was similar in feeling to all the others and it tore essentially as soon as I started loading the clamps, at 3-5 lbs. My first thought was what the hell had I did. So I did it again and it failed again at just a few pounds. It was a Laser reserve. And I still have it.

I tested other areas of the canopy and they held fine.

By the way FCI is fine with doing a thumb test. I don't have a clue how hard I'm pushing in a puncture mode doing that. If they said test with clamps but only to 20lbs I'd be fine. But to say never put a clamp on a canopy, to pack, test, or even hang up for inspection doesn't seem reasonable.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[JerryBaumchen 1,367]

Hi Terry,

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If they said test with clamps but only to 20lbs I'd be fine.

It has been ~ 6 yrs ago at a Symposium; there were some reps from the British fabric milling company ( I cannot remember the company name & I think that they are out of business now ) who said that the actual loading on a square inch of fabric in a round canopy, at the highest point of loading, was around 2 lbs/square inch. A rather low number I thought at the time.

I have an old Security 26 ft Lo-Po laying around in the other room so I took some rough measurements ( please remember that I am saying 'rough' measurements ) and the gore length is ~12 ft. If I consider that the upper 25% of the canopy takes all of the load ( and it doesn't ) and I then do some rough calculations, with some assumptions ( which is what us engineer types do all of the time ), and then ( just to get some rough numbers ) if I consider the canopy to be a 'flat' canopy & not a conical ( I do this only to get some easy to calculate numbers ), then the upper 25% gives me ~6.75 square feet of available fabric for this calculation ( remember, 'rough' numbers ) and the apex is 1 ft across which is 0.75 square feet ( which I must subtract out ). This yields about 6 square feet of fabric to take the load ( considering that nothing else takes any of the load ) and this yields 864 square inches of fabric. Now, if we consider that the canopy is going to be loaded to 5,000 lbs this then gives us ~6 lbs/square inch of load on the fabric.

For a lot of you, you are probably thinking that he sure is doing a lot of 'considering,' but that is what engineers do when they do calculations.

I know that this is just rough numbers ( I'm simply way too lazy to sit down and spend the next week trying to come up with the actual square footage of fabric in this canopy ) but it does point us into a direction for concluding just what might be a good number to test fabric at.

As for your 20 lbs; I always find it very interesting when someone just throws a number in the dark and they almost hit the bullseye.

I hope that I have not confused some of you with the above calculations & how I have explained them.

And if anyone is not in agreement with my assumptions, I am very OK with that.

Interesting, very interesting.

JerryBaumchen

[councilman24 37]

That's one way of considering what might be a 'low' test result that is acceptable. Plus some overage.

The other is what degradation from the minimum spec strength is acceptable. 80-120 cfm military fabric minimum was 42 lbs per inch breaking strength. Lo-Po is 50. F-111 was 45, and Exacta chute 47 lbs. (per poynter)

So 40lbs for 50 lb lo po was probably resonable but higher than necessary. 40 for 45 lbs F-111 is awefully close. The reason for using 30 or 35. Or even 20? Fifty percent degradation? Maybe too low?

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[JerryBaumchen 1,367]

Hi Terry,

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The other is what degradation from the minimum spec strength is acceptable.

To me, this is the sole purpose of the test. This, of course, assumes that the fabric when sewn into the canopy is at zero degradation; which is a fair assumption IMO.

So what is going to be on the agenda for the Rigging Comm. in Reno???????

JerryBaumchen

PS) Aren't you up rather late tonight?

[councilman24 37]

Yeah I'm up late.

Should be working on our project.

Don't know yet what's up for Reno.

BTW, Good night.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[pchapman 279]

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This yields about 6 square feet of fabric to take the load ( considering that nothing else takes any of the load ) and this yields 864 square inches of fabric. Now, if we consider that the canopy is going to be loaded to 5,000 lbs this then gives us ~6 lbs/square inch of load on the fabric.

But didn't you calculate 12.5% of the area? 6.75 sq ft is like a diameter of 3 ft, rather than a radius of 3 ft, which would be 25% of 12 ft gore length. But it's late and I'm tired so I might have misinterpreted something.

But let's for convenience stay with the final value of the 6 ft^2 at the top taking the shock load of a max allowed 5000lb, giving 6 lbs/in^2.

What I want to emphasize is that we would also have to look at gore width -- and that's where the stresses on the fabric end up looking a lot worse, and more difficult to pin down.

Just making up numbers, say that away from the apex, at the bottom of the area taking the load, the radial seams are 6" apart. So if thinking about little one inch squares, there are 6 in a row from seam to seam. So thats 6*6 lbs = 36 lbs, supported at the seam on either side. That brings the stress to the fabric to a total of 18 lbs per inch.

But that's an ideal case since the fabric won't be in a U-shape but pulled somewhat more taut, increasing the stress on the fabric. If one tried to pull the fabric absolutely straight, the sideways loads would go to infinity trying to react the outward force. In practice the canopy gores will bulge, so the actual force on the fabric will be between 18 lbs and infinity.

In the end I'm wondering if we end up with too many approximations here to guess at the actual fabric stresses. On the one hand even high opening shock forces end up as a pretty small pressure on any particular square inch, but the forces add up across the width of every gore, and without knowing the final bulged shape of every gore, one can't tell exactly what the fabric stresses will be.

[fcajump 164]

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I know that Flight Concepts prohibits pull testing and use of clamps when packing their reserves. Do you know of any others?

Don't know if they ever put it in writing, but an offline conversation a couple years ago with PISA said "NEVER" put a clamp on anything. When I asked, OK I've got one of your chutes and a suspect point... how do I test it??

Answers as I recall were "thumb" (though no one present seemed comfortable with the calibration of the average rigger's thumb) or "send it to us".

This does not leave the field rigger in a good position. We are required to certify that its airworthy without any approved means to test an aspect that has (on rare occasions) failed.

Most companies I've dealt with have said use the PIA standard test (some prefer 30-lbs) if you find a suspect area (discolored, stained, worn, etc).

JW

Always remember that some clouds are harder than others...

[firemedic 7]

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I know that Flight Concepts prohibits pull testing and use of clamps when packing their reserves. Do you know of any others?

.......................................................................

That is because Flight Concepts considers pull-testing healthy reserves to be too great a risk ... and i agree with their policy.

Performance Designs - is the only manufaturer of square reserves that - requires pull-testing to cover their butts in the event of unusual service wear, however, I can count on one hand the number of reserves I have patched and one of them was a PD reserve that a new rigger pull-tested a hole in. I patched the hole and pull-tested the canopy in a dozen other places and concluded that the most likely explanation was that he had pull-tested on top of an earlier pull-test and the the fault lay with the previous rigger who had forgotten to mark where he had pull-tested.
Remember that pull-testing first became popular when round reserves started suffering from acid mesh in the late 1980s.
PD introduced their new line of square reserves during the era when riggers were frantically trying to identify and ground all the round reserves suffering from acid mesh.

I have to concur with you. I don't see the need to pull test any canopy with out justification. On the other hand what is the reasoning for not using a clamp for any reason.

[JerryBaumchen 1,367]

Hi Peter,

As I said, it was just 'rough numbers' and they are no where near exact.

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In the end I'm wondering if we end up with too many approximations here to guess at the actual fabric stresses.

Absolutely, but we must start somewhere if we want some 'number' to test the fabric to with the clamps.

IMO the only other way is to ask the mfr and see what each says. FCI says 'don't test my fabric'. PD says 'test it like this'. Other mfrs say nothing.

I was only trying to get 'some' numbers to get the discussion going.

I doubt that we will ever:

1. Know what the actual stresses on the fabric are at the worst case scenario/location.

2. Agree as to what is the ideal number for testing. Way too many cooks in this kitchen.

I do appreciate your input; this is what makes for good discussions ( and not arguments ).

JerryBaumchen

[kingbunky 3]

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... On the other hand what is the reasoning for not using a clamp for any reason.

i was told that even when it's done properly, you are stressing that area of the canopy and even if there was no damage, you are creating a weak spot.

"Hang on a sec, the young'uns are throwin' beer cans at a golf cart."
MB4252 TDS699
killing threads since 2001

[councilman24 37]

This is the arguement. I've seen no data.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE