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Mar 12, 2018, 01:11 AM
Brett
bjaffee's Avatar
Question

Can an Up-Start bungee launcher be used on a 100" sailplane


I'm looking at getting a Dynaflite Up-Start, which is intended for 2m planes, because the field across the street from my house is only about 450-500 feet across. Would this be enough to get something like a Sagitta 900 safely into the air? I realize the launch height will be limited, but even 100 feet would be okay.

P.S. I have other places to fly that would be more suitable for a full-sized hi-start, but this is for a field across the street from my house, and I'd just like something to mess around with there every now and then.
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Mar 12, 2018, 01:51 AM
Registered User
I don't see any reason why a short High Start would not work.

Sure, the diameter of the rubber on an Up-Start is smaller, so it is not capable of the same amount of pull as a High Start with larger tubing, but if you aren't worried about a maximum altitude launch, that Up-Start should get a 100" sailplane into the air.

If you are going to be buying the High Start, you can still get a Heavy Duty or Standard Class High Start, just use a shorter piece of fishing line, and/or don't stretch the rubber all the way out.
Mar 12, 2018, 05:00 AM
Registered User
The Dynaflite Heavy Duty hi start rubber is about the right diameter for an average 2 meter, so double up the rubber and use less line. Unless your 100 inch glider is very light. Or get honestly sized Hosemonster tubing, which is nice stuff and lasts longer.
Mar 12, 2018, 05:57 AM
Gary L

High Start Rubber Data


Here's part of a chart I built some years ago, I believe the data is still valid for the manufacturers listed. The objective of the spreadsheet was to get you in the ballpark with respect to sailplane weight vs tubing size.

As noted in the disclaimers at the top of the chart, high start labeling (2m, Heavy Duty, red, blue, etc.) is inconsistent between manufacturers (or non-existent in the case of the Theraband tubing) so it can be somewhat confusing... ergo lincoln's "...honestly sized" comment in the previous post.

The link points at the Dick Williamson CRRC article that provided the underlying math used in the spreadsheet.

High Start Physics
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Mar 12, 2018, 09:31 AM
Sagitta Fanboy
Double the rubber, and it will work fine. Make sure you toss your Sagitta hard enough that it's at flying speed as you let go.

My histart is an ancient Craft-Aire histart which is marginal for 2m ships. I've launched my ~54oz Sagitta 900 off it, and it was marginal (~180' launches from 100' of rubber and 350' of line). Doubled the rubber and now I'm getting solid launches off it with 50' of doubled rubber (my launches are 300-400')
Mar 12, 2018, 11:24 AM
The King Moonracer of balsa.
Windependence's Avatar
Quote:
Originally Posted by bjaffee
I'm looking at getting a Dynaflite Up-Start, which is intended for 2m planes, because the field across the street from my house is only about 450-500 feet across. Would this be enough to get something like a Sagitta 900 safely into the air? I realize the launch height will be limited, but even 100 feet would be okay.

P.S. I have other places to fly that would be more suitable for a full-sized hi-start, but this is for a field across the street from my house, and I'd just like something to mess around with there every now and then.
It really depends on the weight of your sailplane. How heavy is your plane? I can tell you that I can launch a 100", 32 oz Art Hobby Thermic just fine from my standard Dynaflite hi-start. My 25 oz two meter Riser launches from it very well too. I get around 10 lbs of pull at full stretch. The basic rule of thumb for a hi-start is you want 3x - 5x the weight of the sailplane in pull.

Wayne
Mar 12, 2018, 12:21 PM
Brett
bjaffee's Avatar
Thanks for the info guys! My glider (which is actually an Altezza, but it looks like a Sagitta), isn't quite finished yet, so I don't know the weight just yet.

When you say "double the rubber," does that mean to literally use two pieces of tubing? Or does it mean to double the length, or double the thickness?
Mar 12, 2018, 12:32 PM
B for Bruce
BMatthews's Avatar
Pretty sure that he means fold it so you have double tubing and half the length. It'll make for a rather strong pull and a very spirited launch.

Looking at the Upstarts they show I only see one for up to 72" (so rather light models) and another with 3/16 rubber for up to 100 inch span. You're buying the 3/16 version I would guess?

If you find that the 3/16 can't handle the weight and that you're getting poor results then buy one of the 1/8" versions and just pair up the two rubber pieces. That'll smarten things up nicely!

Or shop for some different rubber and make your own upstart. A screw in dog anchor for the end and some 1/4" rubber with 200' of line. Combine some heavy key rings and a bit of a flag for a drag item to let the line down more easily and you're off to the races in fine form with no doubt that it'll handle the model.
Latest blog entry: Garden Gliders
Mar 12, 2018, 12:43 PM
Brett
bjaffee's Avatar
BMatthews-

Yeah, I was just looking at the "2 meter" one as a starting point, given my limited field length.

BTW, curiosity got the best of me and I weighed all my Altezza parts. So far it's at around 41oz, including radio equipment and battery, but with no covering or nose weight. Given that the Saigitta 900 was specified as 42-46 ounces, I think I will be in the ball park when finished.
Mar 12, 2018, 01:01 PM
B for Bruce
BMatthews's Avatar
Closer to 46 I suspect. And perhaps more if you need much in the way of nose weight.

It's amazing but just the covering film on a model of that size is likely to be up around 2.5 to 3 oz. I've never weighed my airframe on the bigger models before and after covering. But on a little 36" speedy sport model I did. Covering with a mix of Monokote and Ultrakote resulted in 1.6oz increase just from the covering. This included the base covering, some ironed on trim and making the control surface hinges from the covering material. But that's a small model. So that's why I'm thinking more like 2.5 to 3 at least on your glider.

If your plan is to fly a lot from that park you may want to look into the idea of one of the F3RES style models. A touch smaller at 2m span and a whole lot lighter. These would rocket up on the lighter of those two upstarts and fly well in the somewhat confined area. And being smaller and lighter the turn radius for a given bank angle would be far smaller too.

If you often get a lot of wind then perhaps a model built in the F3RES style but with flat wings and ailerons plus a bit of built in space to accept some ballast. The ailerons would prove to fly smoother in the wind and give you more immediate response and the ballast could be used to up your speed range by some amount to let you move around more easily in the wind. Yet even with ballast you'd still be no more heavy than a typical kit built Gentle Lady.
Latest blog entry: Garden Gliders
Mar 12, 2018, 01:24 PM
Sagitta Fanboy
When I say double the rubber, I mean I untied the rubber from the stake ring, ran it through the ring and tied it to the line end ring, so I have two sections of rubber, each half as long as the normal setup.
Mar 12, 2018, 01:54 PM
B for Bruce
BMatthews's Avatar
The trouble with that is that on the Upstart he's only starting with 25' of rubber. That's going to make for a very short pull distance.

I was a bit shocked to see that the upstarts are that short.
Latest blog entry: Garden Gliders
Mar 12, 2018, 04:30 PM
Registered User
My choice for a Sagitta 900 in a short space would be 50 feet of the 3 meter Hosemonster rubber, but it will require a bit more arm strength than the other options. I think most guys could handle it, but I'm not a great judge, since I'm stronger than most. You can still use a 2 meter with that. Or, at least, I've launched a bunch of 2 meter models on that size rubber. I don't know if an F3-RES model would be ok with that or not, but a well built Gentle Lady would. Wipe the sunscreen off your hand before launching and, for a light two meter, don't let the nose get too low. (Or too high, in which case the snap roll would be awesome.) It's not as bad as it sounds.
In any case, I'd use 50 lb test braided fishing line to minimize drag, and would use no more than a medium sized chute, to minimize altitude loss from drag. Particularly since, with strong tubing, you might be able to get a bit of a zoom off the end. I've seen a drawing someplace of a trick sometimes used with RC glider launches where you hook up the ring for the top of the chute, but also then hook up a ring from a little lower down. This lets the collapsed chute drag backwards instead of in line with the rest of the tow line. Might reduce drag some.
If you can find a Onewinch system, or you're handy and can put one together, that might be the best option short of an electric winch. Might even be cheaper than the Hosemonster, which I see is getting expensive. I notice that McMaster has some comparable prices on latex tubing, with more sizes available, though I expect the Hosemonster tubing is a bit better.
Mar 12, 2018, 06:00 PM
Brett
bjaffee's Avatar
Yeah, the Hosemonster is really pricely, at least compared to something like an Up-Start....it's like 5X more expensive. I'm sure it's worth it, quality-wise, but the Up-Start is cheap enough just to mess around and expiriment with, I think.

The local thermal club (Thousand Oaks Soaring Society) has a new dedicated flying field just a few minutes from my house. So, I'm planning to join the club, where I'll have access to a much larger field and winches and stuff (or at least space for a full sized hi-start).
Mar 12, 2018, 06:21 PM
B for Bruce
BMatthews's Avatar
The thing is that if that other field is just a casual few minutes walking to reach then it's awfully tempting to take advantage of it.

Lots of things occur to me for using a local field that will find you sharing it with sports types and other user such as dog walkers and the like. A 100" model in such a setting might be tempting fate a little in terms of risk of injury to other folks in the area. A slightly smaller, somewhat more nimble and very much more light model such as those F3RES designs or something along that same structural and weight example would greatly reduce that by allowing you to use a smaller piece of "personal space" in that area.

Another option would be a DLG set up with a hook to allow it to hi start up and soar around. Great fun for learning to pick thermals and the smaller size would tend to keep you over the park instead of drifting too far out over the surrounding properties. Or a small and light DLG sized electric and then you don't need a hi start at all?

Going over with the big stuff for some test gliding and a short lift up to extend the test glide is certainly an option. But it's surprising how much room a 100" model will cover in a longer flight. And really due to that ever present risk of some manner of failure I always have this sense of respect about not flying over houses or factories. But I'm not above the idea of a shared field with others. I just do my darndest to be considerate with them.
Latest blog entry: Garden Gliders


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