wholds

I have been into powered flight for quite awhile, but at the urging of a friend who flies slope gliders I thought I'd try unpowered flight so I bought a GP Fling2M. I have setup up a High Start using 25' of 5/16" (1/16 wall) surgical tubing along with 250' of light gauge flor-green poly line that I found at Home Depot. Results are pretty good, but I was wondering if there would be an advantage to using Kevlar thread. I have found a source and the price seems reasonable enough, but I'm not sure what size to buy or for that matter if I'm going to gain any great advantage.The Fling weighs approximately 22oz. The other question is aside from stretching the rubber beyond 200-300% is there an optimal ratio for launch load? I've read anywhere from 4:1 to 10:1? Any help would be appreciated. Thanks

rscarawa

Not sure about the kevlar issue. The kevlar and poly green line have about the same stretch ratio with the poly line having a little more give. As far as the proper ratio of tubing to string goes. I think 4:1 works well in most cases. If you are launching into a good head wind, you can up the ratio depending on conditions to something closer to 10:1.

wholds

When you're talking 4:1 your talking about my preload on line at launch which in my case would be (4*22) =88 or 88 ounces is that correct or are you talking about rubber to line ratio which in my case I'm currently using 10:1 or 25' rubber to 250' of line. Thanks for your help Scot.

JRinAZMN

I'd go to your local sporting goods store and buy some "Spectra type" fishing line. It comes in assorted colors (orange works for me) and is thin & tough (I use 90 lb test, but 60 works well)! It is much thinner than mason's line and because of that feature, gives the launch significant additional height.
Cheers,
JR

wholds

Thanks JR, I'll give that a try.

BMatthews

Yep, the smaller Spectra line will have a lot less drag on the luanch. It's not outside of the realm of reality that the line drag with the builder's line is actually more than the drag from the model so line size counts for a lot.

The tubing will work best when stretched to between 3 to 3.75 times it's rest length. Much more than 3.75 to 4 times and you risk permanent damage to the rubber. Also I'd suggest that 5/16 rubber for a sub 30 oz model is a little overkill. You'd get a softer and easier to manage launch for such a model by going for 35 feet of 1/4 inch. As it is right now the model must take off from your hand like a marble from a slingshot.

wholds

Thanks for your input on the Spectra line. I work at a hospital and 5/16" rubber was available so that's what I'm using for now. If it becomes a problem I'll probably "bite the bullet" and buy some1/4" rubber. I'll pick up some of the Spectra line and give it a try this weekend. I'm also using a fish scale to get an approximate idea of how much preload ( I'll stay conservative, maybe 4:1 or so) I'm getting.

AMA-69405

My Feedback: (22)

I'm going to disagree a little with the previous post... I don't think you're that far off on your rubber size. While your current stuff may be a wee bit oversize for the Fling, that's far better than too small; you can always adjust the stretch to match the model weight. At a "full" 300% stretch (25 + 75 = 100 ft stretched length), you should be seeing about 9 lbs load. That might be a bit much for a 22 oz plane, but stretching somewhere around 200-250% should put you in the ballpark, e.g. 6-7 lb pull for your 1.4 lb Fling.

wholds

Thanks for the input. Iv'e gone ahead an ordered a 50' spool of 1/4" OD. The way I see it there is no "exact science" to this launch system, only general guidelines to get you moving in the right direction. I'm actually kicking myself for cutting the 5/16" tubing in half so that I would be working with 25'. I discovered that I have plenty of room for running the 50' length at the park where I fly. I'm thinking about splicing the two sections back together. Gotta say that I'm getting a "kick" out of this "silent flight" thing. The beauty is when wind conditions aren't good for my "powered flight" planes I always have something to fly.

AMA-69405

My Feedback: (22)

Actually, there is a fair amount of science here with a little "art' thrown in as well, but still... physics is physics. I think you'll find the 1/4" (assuming 1/16th wall) is a little weak for your plane, unless you have a bit of a breeze in your face. I can't upload an .XLS or .DOC file here, or I would post a page of high start data that you will probably find useful. Go here:

[link=http://www.rcgroups.com/forums/showthread.php?t=1185805&page=3#post14285344]High Start Rubber Specs[/link]

wholds

Thanks for the link to the info, this will help me out quite a bit. So do you think that I should probably stick with the 5/16" and skip buying the 1/4"?

lfinney

My Feedback: (44)

rename the xls file to .txt then upload, the user can download and rename to .xls simple workaround

Fleet

Keep at the silent flight thing. In little time, you'll realize just how much better pilot you have become. Especially in the powered plane department. Energy management usually means a better chance of making the field at a dead stick.

If you haven't already, when you hook up with your first thermal ,you'll come to understand what the sailplane is talking to you about. Like surfing, you'll be looking for that perfect wave/thermal.

Enjoy.

AMA-69405

My Feedback: (22)

I would stick with the 5/16"... it's just a matter of not over-doing it on the stretch. And actually, you'll probably find yourself using all the power available, once you get enough launches behind you and the conditions are right (little or no wind).

wholds

Just wanted to thank everyone for all of the information you have provided. One more question. I'm going to be in Cary North Carolina for a week on business and I thought I'd bring my sailplane. Can anyone direct me to a internet good site to find a good place to fly down there? Thanks again

BMatthews

I also agree with splicing the extra 5/16 into your histart. If I'd known that you had the extra rubber it would have been the option I would have suggested.

Much of my reasoning for switching to a lighter rubber was the short distance at the correct tension you'd be working with for such a light model. 35 feet doesn't allow for a decent length of pull for anything other than a short line length. With a long line and short rubber you end up running out of sufficient tension part way up the launch. And further shortening the length of pull to limit the tension to a degree more suitable for a light model just makes this worse. But by putting all or much of the extra rubber back in you'll be able to achieve a better lenght of pull where the tension is acceptable for your model for the whole launch. And it'll be good for models up to the 100 inch region that are heavier for the future.

From this you may be figuring that there's an optimum line length for the amount of rubber used. And you'd be right. Too much line and the histart runs out of good tension part way through the launch despite how carefully the flyer meters the tension. THis will obviously show up with larger and heavier models moreso than with a lightweight model on the same higstart. Too little line and the model zips up and pings off the end of the line like it was shot from a cannon. This extra speed can make for a nervous launch on a lighter model. BUt such a short high start may work well with a larger model for shallower launches in a confined flying site. Like most things in life there's a happy medium. For histarts it's a fairly broad range of line lengths to rubber lengths but it's good to be able to recognize the symptoms if you're seeing either a too energetic launch or one that lugs it's way up.

wholds

Thanks for the additional information. I'll be trying the 50' rubber with 200' of 50# spiderwire this weekend. I'm lucky to be flying att a sight where overall HighStart length isn't an issue. Being in the electronics field I equate the the stored energy in the tubing to that of a capacitor. What you've explained to me makes perfect sense. Thanks