Line Size

According to the AMA rules for control line stunt, one is permitted to use 0.015" braided lines for motors with displacements of up to 0.4028 cu. in./6.600 cc. Above those displacements, 0.018" lines are required.
My OS 40 FP is claimed (by OS) to have a displacement of 6.49 cc (0.396 cu. in.). At least for now, this motor will be used on a Ukey 40 and a flapped Magician (about 36 ounces).
I have 0.018" lines to use with this motor and these planes, but I am now wondering whether/why I should them if 0.015" lines are permitted and, presumably, considered safe by the AMA.
I would appreciate any thoughts, comments or precautions that anyone might have regarding line size for these motor/plane combinations.
Thanks in advance for your help.

Certainly the .015 line size is adequate for this size engine/plane combination so long as they are properly terminated and cared for. The FAI rules require the passing of a pull test, only, and many have flown 60 size planes on .015 lines in FAI competition. However,

There are some, including myself, who think we should review the current AMA regulations and, perhaps, go to a different, more stringent requirement. If a modern 40 pipe ship has more power than an old ST 60, why do we permit the pipe ship to fly on .015 lines when the 60 (or even ST 46) must fly on .018?

I flew my 25 and 40 size ships for many years on .018 size lines even when I was only flying for fun. They added an extra safety factor and lasted me a long time. So, here is what you must weight.

The .015 lines are lighter and produce less drag. That is the good part.

The .018 lines will stretch less and provide more positive control. They will be a bit draggier, but with a good, modern engine, that becomes less of a factor. They do provide that extra margin of safety, especially if you get a loose plane that you have to "catch", or as the lines get older and deteriorate in quality. (If you see noticeable defects, do replace them anyway.)

Leonard Neumann

I agree with Len, Thicker lines will give better feel even if a little draggier. Drag doesn't matter if you have a modern powerful engine. Fathom this, I had to use my 66' .018 cables on a Cavelier with one of Len's OS .35's in it. To date, this is the most powerful engine for it's size that I have ever used. Oh yes, I was using an 11 & 1/2 inch 3 blade Eather slight undercamber prop on it too at 4.7 pitch. This A/C engine combo was able to complete 5 consecutive hourglasses in a 20 Knot wind with gusts to 31 Knots. I have the freestyle trophy from St. Louis in Feb 99 to show for it too. Of course, with the wind the way it was, no one else but myself would attempt to fly!
Since then I accidentally destroyed the Cavelier, but I am in the process of building a new one and it will be powered with the same engine, except I will probably use 65' .018 19 strand lines and a Bolly 3 blade 11.5 by 4.5 prop in it.

I recently completed a scratch built Skylark,foamie,for .46 engines with 56" wingspan, weight 52oz before first flight, with Silver Fox 40 BB. Southwick plans, dubbed "Skylark II". Power seems adequate with the Fox and Zinger 12/5 cut to 11/5. Needs tail weight and tank adjustment but shows promise.

This is a large stunter, to me at least and the heaviest I have flown. I decided to use .018's at 62 feet to start. With only two trimming flights to date, the .018's seem OK. I'll probably also try it with some .015's for comparison as line tension is solid but not really heavy.

The decision toward .018's was somewhat influenced by the maiden flight loss of another stunter as some crimped, older .015's chose that particular unfortunate moment to break. The Skylark got a double stout pull test prior to first flight and the lines are well wrapped. Perhaps I have become a little paranoid about maiden flights.

I noticed that a couple of Dallas area stunters were apparently lost at the Nats due to line failure, details largely unknown. My thoughts are that maybe .018's provide a slight safety margin in the event of overlooked line flaws, unexpected excessive stress, etc. particularly when the airplane is somewhat marginal for .015's to begin with.

Extra saftey margin, especially important on rough fields, not a bad thing. Keeping other folks from walking on them is a major deal, dragging them across asphalt is also something to avoid.
On the negative side, it may lead you to be careless with your lines as you have that extra safety margin, NOT a good habit to get into. And don't forget to increase the tipweight to compensate for the heavier lines. Steve

Just a quick comment. The AMA rules that you refer to are for contests--although not a bad suggestion to follow in any case. In addition there is the AMA safety code for all cl flying which says that you must pull test your plane before flying it (that day I guess).

Peter --

I got a little beat up in a previous thread about using .012s, which I like. No matter what you use, line maintenance is the key.

Larry, I'm with you...I would feel ok with using .012's on a Fox .35 or FP .25 powered Banshee or such, however, if I thought I would ever enter a contest with it, I'd go for the .015's simply so I would not have to add tipweight for the contest. Steve