Building F3-RES Hi Starts - RC Groups
Thread Tools
Oct 16, 2017, 11:45 PM
Registered User
Randy Reynolds's Avatar
Discussion

Building F3-RES Hi Starts


Our club is going to build Evers land F3-RES Hi Starts with DJ AeroTech rubber. We are considering using lightweight braids line in fluorescent colors. The spec though calls far a specific monofilament diameter,etc. My question is does this monfiliment possess some characteristic to the launch in terms of stretch or otherwise that we lose with braided line?
Sign up now
to remove ads between posts
Oct 17, 2017, 12:18 AM
Team JR Air
tewatson's Avatar
Mono stretches and adds energy to a launch...the question is whether it does so enough to make a difference at the relatively low pull tension of F3RES. I would think the primary reason to use mono is the smaller diameter and smooth surface means less line-induced drag. Braided also picks up moisture and gets really heavy when wet.

Tom
Oct 17, 2017, 01:05 AM
Registered User
Randy Reynolds's Avatar
Thanks Tom, I hadn't thought of braided picking up water.

Also I had better check after this auto correct thing when it gets through with my posts. ...jeez. Thanks for being a mind reader.
Oct 17, 2017, 08:28 AM
Registered User
George Franklin's Avatar
Being an avid fisherman I can tell you that mono is significantly more stretchy than any braid. Depending on brand it will stretch about 25% at the rated strength. Fluorocarbon 30%.
So the added energy with mono is not insignificant.
Oct 17, 2017, 11:43 AM
Registered User
It may interest folks to know what we learned, when we recieved ten of the spec F3RES hi-starts from Zeller Modellbau, last spring. We use them often, and they work well. The launches are not as zippy as some of the other, Hose-Monster-type hi-starts we have around, but there have been no failures, and every tension check has registered spot on.

The quality and "bling" are very high. All you do when you get one is hook up snap links and knot the string. The rubber already has little eye-bolt type ends. There is rubber, string, carabiniers, rings, streamer, and spike.

It is important to recall that there is a minimum diameter specified for the monofilament, and that it is (compared to F3J monofilament) very thick, and very stiff.

When we set them up and tried the tension, we got the spec tension at the spec stretch. Then, presumably as the monofilament stretched, the tension would sag a bit, by about 3-5%. When the tension was released, the streamer would (of course) scoot along the ground until tension was spent. Then, for about thirty seconds, the streamer would slowly creep as the stretched monofilament shrank.

I believe that this monofilament is chosen to be as non-stretchy as possible. It does stretch, but has very little springiness. It stretches under tension, and slowly shrinks when tension is relieved.

I found when I was at a contest in Germany, that the monofilament was important because as lines crossed, they did not tangle. Also, the grass was very, very wet, and as noted in other posts above, it did not absorb moisture that I could tell. The streamers, on braided line, did tangle with each other on occasion, and were carrying moisture.

That's my experience. We are conducting a dress rehearsal for our November contest this coming Saturday, and will be setting up all our F3RES-compliant hi-starts. I will post if we learn anything different
Oct 17, 2017, 12:18 PM
Registered User
Don Stackhouse's Avatar
Yes, it's my understanding that the rules call for a minimum of 0.7 mm (0.028") diameter specifically to make sure that stretch in the line under tension is insignificant, so you cannot use line stretch to act like additional rubber and additional energy storage.
Oct 17, 2017, 12:26 PM
Registered User
John Lueke's Avatar
Mono line of the specified diameter works out to about 80lb test. Pulling on it with the spec 4.4kg is not going to stretch it.
Oct 17, 2017, 12:26 PM
Team JR Air
tewatson's Avatar
Quote:
Originally Posted by glidermang
It is important to recall that there is a minimum diameter specified for the monofilament, and that it is (compared to F3J monofilament) very thick, and very stiff.
Min diameter is 0.7mm, correct? F3J mono sizes run about 1.2 - 1.4mm. So this stuff is comparatively thin, and I would expect to be a bit more stretchy, although these tensions may not be enough to do so.

Tom
Oct 17, 2017, 12:48 PM
Registered User
Don Stackhouse's Avatar
Tension on an F3B winch or typical US longshaft-Ford winch is far greater (hundreds of pounds) than the 8.8 lbs of an F3-RES Hi-start.
Oct 17, 2017, 06:09 PM
Afflicted with DDD
Soarhead00's Avatar
Just to put a more exact number to the question:
If you can assume the tow line is normal Nylon as specified in the rules, then based on the Young's modulus of nylon (2.52 GPa) and the line parameters of 0.7mm dia and 100 meters long, then the stretch at 4 Kg will be 3.91 meters (154 inches).

Feel free to check my numbers. I was wrong (once)
Bob

P.S. for 1.2 mm F3J line at 100 lb load, stretch is ~616 inches
Last edited by Soarhead00; Oct 17, 2017 at 06:51 PM. Reason: added numbers for F3J tow line
Oct 17, 2017, 07:23 PM
Registered User
Don Stackhouse's Avatar
F3-RES:
154 inches times 8.8 lbs is 56 lb-ft of energy, assuming a linear buildup of tension during that 154 inches of stretching.

F3B:
616 inches times ~250 lbs is 6417 ft-lbs of energy, again assuming a linear buildup of tension during the stretch. That's 115 TIMES as much energy!

Of course all of that is academic. The rules specify 0.7 mm (0.028") min diameter, so whatever the reasoning of the folks who established the rules, that's what everyone active in this event up until now has agreed to use.
Oct 17, 2017, 08:30 PM
Cognitive dissonance
kcaldwel's Avatar
Monofilament comes in a lot of grades these days, with E from 1.3GPa to 8Gpa. Many common ones seem to be down around E = 1.3GPa. A 1.3Gpa monofilament will stretch a lot more, over 7m, with a lot more stored energy. The stretch and return is not really linear though, and time dependent.

https://prezi.com/dbcyndhbp4af/fishing-line-test/

http://www.toray-mono.com/english/product/pro_c002.html

Kevin
Oct 17, 2017, 08:39 PM
Afflicted with DDD
Soarhead00's Avatar
Another interesting factoid:
When I was looking up data on nylon line stretching, I ran across a test one fisherman performed to test fishing line (similar to what we use). He found that fishing line took from 25 to 30 seconds to return to its pre-stretched dimension. The end result for us would be that you would get the benefit of a longer launch line (by ~ 150 inches) because our time on launch is only 3-4 seconds.

Something just occurred to me. If I remember correctly, Nylon also stretches slowly in the test noted above. (on the order of 25-30 seconds). If we are only loading the line for a few seconds then the line may
not stretch anywhere near 3 - 7 meters before we launch. Maybe its time to do some tests. Who wants to take this on??? Who cares???
Bob
Last edited by Soarhead00; Oct 17, 2017 at 09:01 PM. Reason: added info on time to stretch nylon.
Oct 17, 2017, 09:16 PM
Registered User
John Lueke's Avatar
If we were using low strength line where the 4kg load would get us close to the nominal 25% elongation, then we could play games with strain energy in the mono. And we could wait 30 seconds to launch to let the mono stretch, But since we can only stretch th whole thing 30 meters, then the rubber would have to relax as the mono stretched. Best to just meet the spec and go fly. I suspect the guys that wrote the rules went through all of this and picked some specs that made the high start rubber the dominant factor in the launch.
Oct 17, 2017, 09:18 PM
Registered User
Don Stackhouse's Avatar
I think we may be overthinking some of this Hi-start stuff.

The rules specify rubber forces and stretch, and line size and length. All we have to do is meet those specs, as cheaply and conveniently as possible.

Importing special fishing line all the way from Germany? Even if it's Mercedes Benz fish line, it's not going to make any difference when you launch your plane with it. Ditto the imported rubber. As long as the free length is 15 meters, and at 45 meters stretched length it's less than 8.8 pounds (4 Kg) pull, then it doesn't matter where it came from.

OTOH, if one of us manufacturers / suppliers is going to stock it, we have to have a reliable and consistent source, so that we can be sure whatever we get this time will be the same as what we get next time, and that our source can give us consistent lead times and costs.

In any case, it should not be all that difficult, other than making sure the rubber issues are under control.

Joe and I have been wrestling with this whole Hi-start source issue since not long after we started working on the Chrysalis Lite last fall. We figured that we weren't going to be able to sell our beautiful new kit if folks could not find Hi-starts for it.

After considerable hunting, correspondence and testing, we nailed down a supplier that we can count on. We have rubber in stock, and we can get more, and it will be consistent.

There was the question of the other fittings and components. A search of fancy end fittings didn't turn up anything that we liked. However, a trip to our local Lowe's builders supply and some experimenting came up with something equivalent to what Seta-Tech sells, but for far less cost and trouble. Get a pair of 8-32 x 1 5/8" eye bolts, two 8-32 acorn nuts, a small bag of nylon tie straps, and a 1/8" (wire diameter) steel quick link. Also pick up a small tube of Loctite and one of those corkscrew dog stakes.

Loctite the acorn nuts to the ends of the eyebolts, then push them into the ends of the rubber, almost to the eye of the eye bolt. Put two tie straps around the outside of the rubber between the acorn nut and the end of the rubber. Trim the tie straps, then put a piece of 3/4" shrink tubing over the last 2" of the rubber at each end so the tie straps won't get snagged in the grass.

I found a very nice reel at WalMart for $7 in the hardware / automotive section. I think it's for winding up extension cords, but it works fine for Hi-start rubber.

Use the quick link to attach one end of the rubber to the ring on the dog stake.

So, what about line? Today I found an easy answer to that, again at WalMart, this time in the fishing section. Get a pack of heavy duty snap swivels, and a roll of Zebco "OmniFlex" 50 lb test clear monofilament. It's 0.029" diameter (only 0.001" bigger than the minimum in the class rules), comes in spools of 150 yards (137 meters, so you will have 37 meters left over), and a spool of it cost me this afternoon only $1.57 (that's the regular price, it was not on sale).

A scrap of left-over fabric makes a nice streamer (Joe uses a piece of left over fabric from the wedding dress that Karen made for her daughter).

I've attached pics of all of these components. As far as the 3/4" wide shrink tubing, I picked that up at Radical RC, but there are other places as well, such as McMaster-Carr.

Yes, we can put together a complete package of all of this that you can buy off our website when you are ordering the rubber (and if enough of you express an interest in that, we probably will), but it's all stuff you should be able to get yourself if you'd rather do it on your own. The tricky part was finding the rubber, the rest of it is not a big deal.
Last edited by Don Stackhouse; Oct 17, 2017 at 09:30 PM.


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Build Log "Fresh" F3 RES build log dafische Thermal 13 Dec 06, 2017 02:22 PM
Build Log F3-RES FRESH Build by Richard rnshagam Sailplane Talk 45 Sep 19, 2017 11:03 AM
Discussion Hi-Starts For F3-RES dharban Thermal 193 May 26, 2017 06:17 PM
Discussion F3-RES Hi-start Setup shinck Intermountain Silent Flyers 4 May 12, 2017 08:29 PM
Discussion First F3-RES Build donsinger1 Thermal 20 Apr 04, 2017 09:59 AM