Strength of Brodak Lines

[Matt Piatkowski]

Hello,
I have found the following data on the Brodak web site:

Wire Size    Engine Disp.    Line Length
.008              049                35'
.008              074                52'
.012              09                        52'
.015              19                        52'
.015              29                        60'
.015              35                        60'
.018              40                        60'
.018              49                        70'
.021              60                        70'
.024              74                        70'

What is the strength, in pounds, of these lines?

Also: some lines are braided, some are solid and there must be a difference in strength.

Is this difference substantial or negligible?

Thank you,

[Tim Wescott]

That looks like a chart from on old AMA rule book.  The rules go by model weight these days.

As for breaking strength, try contacting Brodak's directly.  All I know is that if I go by the rules, my lines will be more than adequate.

https://www.modelaircraft.org/files/2015-2016CLGeneral.pdf, https://www.modelaircraft.org/files/2015-2016CLPrecisionAerobatics.pdf.

[Mike Anderson]

There is a seller on ebay who has some spooled stainless 1 x 7 cable which calls out these test breaking strengths:

.012 ... 20 lb
.015 ... 30 lb
.018 ... 40 lb
.027 ... 135 lb

That agrees pretty well with what used to be listed in the McMaster-Carr catalog, to the best of my recollection. I wasn't able to find the info in the present M-C online catalog.  As for stranded versus solids, you don't want to use solids.  Nor do you want to mess with .008's in any form.  You can probably safely interpolate .021's to be around 65 lbs and .024's to be around 90 lbs.

Also, keep in mind that is "per (load-bearing) line" and applies to un-kinked and un-frayed lines.

[eric rule]

Thanks Mike, Your data is certainly correct.

If anyone wants more data just go to American Fishing Wire's web site. The stainless steel lines we use in control line all comes from the fishing industry. AFW has a lot of data on their site and their customer service people are well trained to answer any technical questions you may have. If you call them just remember that they are talking fishing not model aircraft so you may have to "translate" their comments into a form we can use.

[Steve Helmick]

Eric Rule (RSM) sells bulk spools of good cable. Can't say if he ships to Canada, but then again, he IS Canadian, so I'd expect that he'd be very willing. I INSIST on shiny silver (stainless) cable, simply because it shows up better on grass, concrete, and asphalt...in the pits.

The AMA website Tim posted will tell you what size to order, unless your plane is around one of the "thresholds", like 64 oz. It could be weighed at 63 one contest and 64.5 oz the next contest the next weekend. Ooops! In that case, opt for the heavier (.018" in this example), trim for them, and use them all the time.  Steve

[Paul Smith]

This is McMaster Carr's chart of 6/3/2014.

It lists the breaking strength of each size:

.012"  25 pounds
.015" (1/64")  40 pounds
.018" 55 pounds
.021" 80 pounds
.024" 100 pounds
.027" 125 pounds

The most recent chart has been revised from breaking strength to WORKING strength, which is about 1/5 of breaking strength.  The resultant lower values are (in my opinion) an attempt to shield the seller from any possible responsibility.  The drastically lowered ratings will allow the seller to claim that any failure is related to over stress.

Note that two .021" lines have a combined strength of 160 pounds.  Unless the pilot is an extremely heavy man with a very strong hand, the pilot, not the lines, will be the limiting factor for all larger sizes.  .024". 200 pounds of line strength.  To what end?

[Brett Buck]

This is McMaster Carr's chart of 6/3/2014.

It lists the breaking strength of each size:

.012"  25 pounds
.015" (1/64")  40 pounds
.018" 55 pounds
.021" 80 pounds
.024" 100 pounds
.027" 125 pounds

The most recent chart has been revised from breaking strength to WORKING strength, which is about 1/5 of breaking strength.  The resultant lower values are (in my opinion) an attempt to shield the seller from any possible responsibility.  The drastically lowered ratings will allow the seller to claim that any failure is related to over stress.

   A factor of about 5 between ultimate strength and working tension is pretty reasonable for many applications.   The current line size VS weight chart for stunt is based on a factor of roughly 4 and the pull test is roughly a factor of 2 over the in-flight loads. If I was going to list working strength levels, I would do roughly the same.

    For a 63.9999 ounce airplane, the expected line tension of around 10 lbs/2.5gs, and you get a 40 lb pull test. In flight at the Nezeband wall, you get that 10 lb on one line. Hence the absolute strength is about 4x the working tension. The 40 lb pull test gives you 20 lb/line, which yields a *test* of about a factor of two over the flight loads *for a single line*. This, in turn, is about half the expected breaking strength, so you are testing it with about a factor of two margin as well, ensuring that you are not over-stressing the lines with the test and causing premature or unnecessary damage.

     This was not accidental or coincidence, it was based on standard practices in the aerospace industry.

    Brett

 

[Tim Wescott]

I would hope that the thinking behind line sizing is not just to cover the normal flight loads, but for that occasional event where your plane goes slack on the lines and then bangs when it hits the ends again (and you have to finish out the flight before you can change your shorts).

I've never had anything break when this happened to me, but I've seen a plane with an older set of lines have both lines come off at the handle end -- fortunately there was a convenient tree to catch the airplane.

[Brett Buck]

I would hope that the thinking behind line sizing is not just to cover the normal flight loads, but for that occasional event where your plane goes slack on the lines and then bangs when it hits the ends again (and you have to finish out the flight before you can change your shorts).

I've never had anything break when this happened to me, but I've seen a plane with an older set of lines have both lines come off at the handle end -- fortunately there was a convenient tree to catch the airplane.

  That's why it is a factor of 4, and a factor of 2 on a single line failure.

    In engineering, you have to make reasonable trade offs margin for performance all the time. I think you are somewhat familiar with controls engineering, and typical stability margin requirements are 9 db gain margin and 30 degree phase margin for normal variation of plant parameters. You can make it 20 dB and 90 degrees, or anything you want, and ensure even more margin, but give up performance.
 
    You could make everyone fly on .032 solids, and then you will never have an issue with failures. It also cripples the performance to the point that it becomes even safer when no one bothers to fly at all.

   We have some analysis from engineers in the aerospace industry, and 70ish years of practical experience to guide the analysis. There have been occasional failures like you describe (like Allen Goff at the 2003 NATS where two .018 lines broke in a worst-case accident) but not many.

    I am pretty comfortable that the current chart is about what you need to make a reasonable balance between safety and practicality.

     Brett

     

[Brett Buck]

This is McMaster Carr's chart of 6/3/2014.

It lists the breaking strength of each size:

.012"  25 pounds
.015" (1/64")  40 pounds
.018" 55 pounds
.021" 80 pounds
.024" 100 pounds
.027" 125 pounds

     Additionally, while this is perfectly in line with the industry standards for 1x7 stranded SS cable, I and others have found individual samples of wire that appeared be be far below this from time to time. In particular, in the late 80's, we found a batch of .015s that failed in several cases, and in testing, it was failing at around 20-25 lb pretty consistently. And the failure mode was almost inexplicable - it would sometimes fail in multiple spots at the same time. Someone told me about that and I thought it was impossible, but I tried it with the old WAM pull tester, and sure enough, at about 20 lbs, it failed at 3 places in the same line at the same time. It measured .015 but it was a bit lighter than other manufacturer's lines.

   These were all from the same supplier at about the same time. They were notified and later batches were fine like everybody elses, so I don't think I want to name them as the problem is resolved.

    I use the cable from MBS and it seems to meet or exceed the ratings listed above.

   Brett

[Paul Smith]

There's no need to base rules an a legend about a bad batch 35 years ago.  That is one function of the pull test.

I am more inclined to trust the metal products I buy from a national industrial supplier than a spool of plastic fish line bought from a bait shop.  The seller of the fish line is only liable for the value of the fish that got away.  An industrial supplier can get in real trouble if his product causes the end user's products to fail.

[pmackenzie]

     Additionally, while this is perfectly in line with the industry standards for 1x7 stranded SS cable, I and others have found individual samples of wire that appeared be be far below this from time to time. In particular, in the late 80's, we found a batch of .015s that failed in several cases, and in testing, it was failing at around 20-25 lb pretty consistently. And the failure mode was almost inexplicable - it would sometimes fail in multiple spots at the same time. Someone told me about that and I thought it was impossible, but I tried it with the old WAM pull tester, and sure enough, at about 20 lbs, it failed at 3 places in the same line at the same time. It measured .015 but it was a bit lighter than other manufacturer's lines.

I have had lines fail at both ends in Combat, as you say it seems iinexplicable???

Any theory/WAG as to how it can happen?

[Paul Smith]

It is possible that the two lines were wrapped together several turns when one line broke at one place.  Then, when the plane stated to fly away, the wrapping momentarily stopped the flyaway, then broke the line at the second place when the wrapping took hold.

Bizarre, to say the least. So much for the weakest link principle.

[Brett Buck]

There's no need to base rules an a legend about a bad batch 35 years ago. 

   We didn't, it was a side story.

   The current line size requirements for stunt are based on engineering and the industry-standard published breaking strength ratings.

   Brett

[Chris McMillin]

I followed your post Brett, it was perfectly understandable.
Chris...

[dave siegler]

The requirements we all use are based on steady state line pull, which I have to wonder is real.

Personally i have never had a line fail in flight, except after a midair in combat.

The only time I have seen a line fail in flight was when the model free flights across the circle and hits the end of the lines.  

I know from impulse momentum theorem the load on the lines can get pretty high.  

for example a 60 oz airplane flying at about 65 mph (4 seconds/ lap on 60 foot lies) cutting across the circle at and changing direction in .25 sec ( is this too small?) , will load lines to about 43lbs or about 2x its normal flight loads. This assumes the 2.5 seconds is for a 90 degree turn, and forces are constant during the event.


I know there is some elastic effects in the lines, arm and shoulder that may make the number less. 
So do the numbers in a pull test allow for impulse loading?

Did someone back in the day, calculate this and put a load factor in the pull test?

[Brett Buck]

The requirements we all use are based on steady state line pull, which I have to wonder is real.

Personally i have never had a line fail in flight, except after a midair in combat.

The only time I have seen a line fail in flight was when the model free flights across the circle and hits the end of the lines.  

I know from impulse momentum theorem the load on the lines can get pretty high.  

for example a 60 oz airplane flying at about 65 mph (4 seconds/ lap on 60 foot lies) cutting across the circle at and changing direction in .25 sec ( is this too small?) , will load lines to about 43lbs or about 2x its normal flight loads. This assumes the 2.5 seconds is for a 90 degree turn, and forces are constant during the event.


I know there is some elastic effects in the lines, arm and shoulder that may make the number less. 
So do the numbers in a pull test allow for impulse loading?

Did someone back in the day, calculate this and put a load factor in the pull test?

   The reasoning is shown above. Be advised that "back in the day" was about 3 years ago!  Several people (primarily Paul Walker, with some input from me and a few others) corrected some of the previous line size break points to be consistent. The factor of roughly 4 over the static load is intended to cover the  dynamic cases. But bear in mind,  "absolute safety" is not possible or desirable nor is it the goal. All safety decisions are trade-offs and if you wanted to cover the worst case in all conditions then you would just shut down the event.

   Note that if you calculate the "jerk" as an impulse, the load approaches infinity as the time of application approaches zero. It's not really a simple fixed load over a finite time so you can't really pick a fixed time of application like your 1/4 second. However, if you *do* you use 1/4 second, using typical numbers (4 lbs (0.124 slugs) at 80 fps or ~10 lb-sec) you get about 40 lbs of force - which curiously is exactly the pull test for the same airplane, AND, the breaking strength of an individual line at the 63.9999 ounce weight for .015 lines. If you think that is really the worst case, you still have a factor of 2.

   The two main categories of line size (.015 and .018 stranded on 35-60 sized airplanes) have very extensive history dating back to the 40's and have proven adequate for the task. The only real questions were about where the 0.021 and to a lesser extent .012 fell. I had been concerned about the smaller line size since the previous change which permitted .012 stranded on 25-sized airplanes ( like a 25VF-powered Nobler weighing up to 48 ounces ). Others had been concerned that the .021 break point was set too low and unnecessarily crippled performance for airplanes slightly heavier than standard for which .018s had long been considered adequate. The last go-round made the margins consistent with each other across the board.

   Brett

[phil c]

Regarding a jerk on the lines- if, for whatever reason, a plane cuts across the circle and hits the end of the lines it is very likely to snap both lines.  If it happens, and you can think fast enough(or are prepared), stick your hand up overhead.  When the plane hits the end of the lines your arm will give enough to cushion the shock and prevent the lines from breaking.  The worst thing to do is just stand there with the handle tightly in your hand.l  I once did that with an 18oz plane going about 70 mph(Fox 35 speed) that cut across the circle.  It broke two .018 lines instantly.

Try not to do likewise,

Phil C