Line Length

I serached the forums with "line" and "length" and, while I came up with *tons* of interesting posts, I didn;t come across what I was after. Admittedly, I didn't exhaust my hits. Regradless, I'll ignorantly forge ahead:

How do I determine an optimal line length for a model? I've always flown 60 ft lines with a 35-sized ship, 52 ft lines with a 15, etc., but I've never seen any disussion of how these lengths were determined. And, what about solid vs stranded? Even though solid lines can be smaller for a given size odel, do the smaller solid lines have less drag than the equivalent stranded lines (I'm thinking of the inherent "pebbeling" that stranded lines may have)?

And, what sized lines should reall be flown? I'd ever seen .015 and .018, but now I see lots of .012 lines, too. What uses the .012 lines?

Perhaps Really Good flyers use different line lengths based on conditions. Yet, I'd think that would play havoc with carefully practiced patterns, where predictable repetition is key.

I find that, upon returning to the hobby, I'm wondering more and more what the basis is for the things that "we all know."

Kim Elmore

AMA 56239
"All of weather is divided into three parts: Yes, No and Maybe. The greatest of these is Maybe."

60 feet is a nice round number for calculating speed. 7 laps + 1/2 mile. 70 feet is 6 laps for 1/2 mile. 52.5 feet is 8 laps for a 1/2 mile, 35 feet is 12 laps, etc. So there is nothing magic about these standardized lengths.

>60 feet is a nice round number for calculating speed. 7 laps
>+ 1/2 mile. 70 feet is 6 laps for 1/2 mile. 52.5 feet is 8
>laps for a 1/2 mile, 35 feet is 12 laps, etc. So there is
>nothing magic about these standardized lengths.
>
>Currell

As charted above, length and diameter MUST be eqalizied for SPEED, RACING, and COMBAT. For sport, scale, and stunt, use whatever feels good.

>
>And, what sized lines should reall be flown? I'd ever seen
>.015 and .018, but now I see lots of .012 lines, too. What
>uses the .012 lines?

Line *diameter* is dictated by AMA safety requriements, and is correleated with engine displacement. For control systems using two multistranded cables to perform aerobatics, the requirements are:

0.008" under .051cubic inches

0.012" from 0.051 to .15 cubic inches

0.015" .15 to .40 cubic inches

0.018" .40 to 0.65 cubic inches

The above is approximate and incomplete, but gives you an idea. The detailed information for CLPA is on the AMA web site:

http://www.modelaircraft.org/PDF-files/Rulebook/cline3.pdf

The general link is http://www.modelaircraft.org. Look under the Competition Regulations tab under the Publications tab. You'll find all the detailed rules for pull test, line specifications etc. Note that the requirements vary for different classes (Speed vs. CLPA for example) and that the requirements are different for solid calbes when compared to multistranded cables. Note also that the 70' maximum line length is simply the maximum length allowed under AMA rules.

Andy

Well, I finally used half a brain and found all the threads on line length. I'm embarassed to verify my suspicions: that I asked a question that's been answered, at length, more than once. I appreciate everyone's indulgence. Everything in the previous threads pretty much answered my questions.

Thanks everyone!

Kim Elmore

I may be wrong here, but I believe that every 2 feet added will give you .20 seconds per lap. ie. 60 feet equals 5.0 then 62 will give you 5.2 or there abouts. I use a longer line to give me lap times, blade pitch will do the same, but there is a point of no return there. Blade length gives the engine its load. Also at times a longer line length can help your maneuver size and shape. But not always, but practice sure will. It's a matter of trial and error.

>I may be wrong here, but I believe that every 2 feet added
>will give you .20 seconds per lap. ie. 60 feet equals 5.0
>then 62 will give you 5.2 or there abouts. I use a longer
>line to give me lap times, blade pitch will do the same, but
>there is a point of no return there. Blade length gives the
>engine its load. Also at times a longer line length can
>help your maneuver size and shape. But not always, but
>practice sure will. It's a matter of trial and error.

FYI, here's a thread where I published formulas for lap time, airspeed, line tension, etc. These are valid strictly for uniform circular motion (a "rock on a string") and do not account for aerodynamic forces such as lift generated by the fuselage. They are useful however for making ballpark estimates.

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=5919&forum=DCForumID1

Andy

The only unknown to me concerning lines is why we cannot get on the same page when describing length of same.

I mean, you ask a guy, "How long are your lines," and he responds with a measurement running from his handle to the centerline of the model. How dumb is that?!

Well, it's not really dumb at all, but it is annoying at times.

A local guy once overheard PW and I talking about line length back when he was first fiddling with four-strokes and using a converted Impact. When Paul said, "Fifty-seven foot lines," I know the dude got it wrong...

The most accurate description of line length has frequently come from Rush: "Full length." Note this applies to both his CL Combat lines and his Stunt lines.

>The only unknown to me concerning lines is why we cannot get
>on the same page when describing length of same.
>
>I mean, you ask a guy, "How long are your lines," and he
>responds with a measurement running from his handle to the
>centerline of the model. How dumb is that?!
>
>Well, it's not really dumb at all, but it is annoying at
>times.
>

what it is is, it's by the book: From CL of handle to CL of model. Necessary, as stated before, for speed, combat, racing; I suppose force of habit for stunt. Oh, also add into that list somewhere above, 42' lines = 10 laps/half-mile.
--Ray

>>The only unknown to me concerning lines is why we cannot get
>>on the same page when describing length of same.
>>
>>I mean, you ask a guy, "How long are your lines," and he
>>responds with a measurement running from his handle to the
>>centerline of the model. How dumb is that?!
>>
>>Well, it's not really dumb at all, but it is annoying at
>>times.
>>
>
>what it is is, it's by the book: From CL of handle to CL of
>model. Necessary, as stated before, for speed, combat,
>racing; I suppose force of habit for stunt. Oh, also add
>into that list somewhere above, 42' lines = 10
>laps/half-mile.
>--Ray

Actually for stunt that is the way to measure line length as well. When it is stated that 70 feet is the maximum length allowed, that is the measurement from the center of the fuselage (nose, prop, whatever) to the handle. At the Nats there is a tape layed out at the point of the pull test which will show if someone exceeds that length. (And this is done for safety purposes, so that our airplanes do not fly outside of the designated circles.)

LAST EDITED ON Oct-27-04 AT 04:08 PM (CDT)
 
>How do I determine an optimal line length for a model? I've
>always flown 60 ft lines with a 35-sized ship, 52 ft lines
>with a 15, etc., but I've never seen any disussion of how
>these lengths were determined. And, what about solid vs
>stranded? Even though solid lines can be smaller for a
>given size odel, do the smaller solid lines have less drag
>than the equivalent stranded lines (I'm thinking of the
>inherent "pebbeling" that stranded lines may have)?

Since nobody seemed to answer this part of the question, I'll try. Basically I'd recommend the shortest lines that are compatible with doing a nice pattern. The line length is another trimming tool, just like an adjustable handle or tweaking the flaps. Start with a nominal length, as you said, 60 ft. for 35-40, 50-55 for 25's engines, 65-70 for 60's. Fly and trim the plane as best you can. If the lines are too long you will see them whipping the plane around in sharp maneuvers, and the plane will continually have problems with poor tension in the high maneuvers. Try lines a couple feet shorter and re-trim as needed to see if they help the tension problems. The line tension should get better with shorter lines. When you get the lines too short the plane will pull like a rock, but you'll have a hard time fitting the maneuvers in due to lack of space. It's all a compromise anyway. You have to find what suits you and the particular plane.

Two feet is a pretty big change in line length. Be prepared to cut a new set of lines in between. Some folks keep a set of lines in 6 in. increments for trimming. Once the plane is well-trimmed a good pilot can probably feel an even smaller difference if he's paying attention.

Forget lap times when you are changing line lengths. Measure the length(handle to prop nut) and calculate the nominal circumference of the circle(pi X diameter) and use that to calculate the model airspeed. See Swordsman18's thread. Try and keep the airspeed the same when testing. Usually the motor/prop wants to run particular airspeed anyway. A couple feet of linelength won't make a huge difference in airspeed. It isn't much drag, but the length will make a big difference in lap times.

I'm not sure if I am agreeing or partially disagreeing with Phil C. Line length is a useful trimming tool. I think an airplane/engine combination has an airspeed at which it flies best. I don't think it is a good idea to try to make the airplane fly greatly faster or slower than this airspeed. For the usual flier, line length is a major component of tension and lap time. So the best line length is one which gives both enough line tension and slow enough lap times to suit you.

The theoreticaL is that you put some lines on the airplane and fly it a bit. You decide you need lines longer or shorter. You make up a new set of lines and stash your old lines away. You do this until you get lines to suit. Now you have a stash of lines of different lengths which you can fiddle with on future airplanes. This implies that you buy lines in large lengths and figure out how to make up a set of lines quick, easy and accurately. That's a useful skill.