What Hi-Start Bungee/Line for 2.5m Fantasia RE

[StraightEdge]

Picked up a used but almost pristine 2.5m Fantasia RE last week.  Unfortunately not legal for the 2m RES Challenge but looking forward to playing on my own.

What would be the ideal diameter/length of bungee, and what type/length of line for this?

HK does 10m of 6mm bungee for about £4.  Hyperflight does 15m of 8mm for £35 

[Gary B]

Hi,

It depends a bit on how strong your wing structure/joiners are and what conditions you want to fly in (windy/calm).

If it's the Balsa Cabin Fantasia then that's similar to my 100"  span RES wooden gliders (Sunrise/Albatross/Elan/Centiphase/Chieftain etc).

I launch them using 3/8th inch surgical tube (windy) or cotton covered bungee (calm).

The surgical tube is the one to go for if you can get hold of any. Typical length ratio is 1 to 4 rubber/bungee to line, maybe 100 to 150 metres in all but often much shorter.

There is a German company that sells high quality launching equipment, EMC Vega. This is their technical page on launching with rubber.

Sadly only available in German but you can run it through Google Translate. The graph shows the optimum (green range) for their products.

I have 'All round' which   I use for F3J gliders (60 metres total length ish) and 'HLG' for 1.5 metre span or less.

Line doesn't need to be anything special, fishing monofilament with a breaking strain only slightly over what you think the model can take.

Pop over sometime (Brackley) and you can have a look.

 Cheers

   Gary

[Tony]

Agree with Gary on the surgical tubing. I have a Vega set as well, their page can be a bit confusing if you e mail them tell you what you want it for and they come back to you within a day or so. I got a full set, tubing, end pieces, parachute and line etc, not cheap but top quality. Mines a bit of a monster as it can launch  an F3B (Mega rubber on the web site), I have used it on my Eliminator that's quite heavy at 70 oz and it flies up the line! 

[Whitmore]

Jon,

Hobbyking also do an 8mm x 10m: https://hobbyking.com/en_us/hobbykingr-8mm-silicon-rubber-bungee-hi-start-cord.html?wrh_pdp=1

It depends on the weight of the model really but a pull force of around 5:1 should still get you climbing ie 5kg pull should launch a 1kg model. You could put two together in 'series' with 100m of line, or if you need more force - double them up in parallel.

Jon

[StraightEdge]

Thanks for the feedback guys, really helpful.

Yes its the Balsa Cabin Fantasia.  Not sure of its weight yet as I had to go away the morning after my (ten quid!) win, but I'm guessing its AUW will be in the 500-750g (18-27oz) range?

I note the 2m RES rules limit the bungee to 15m (of I assume 8mm?), with models typically weighing 350-500g,  but if I get on with the Fantasia then it's feasible that I'll eventually build one of the RES kits for the 2m Comp.  In the meantime it seems sensible to get the right thickness/length of rubber to achieve good launch heights with the slightly larger/heavier Fantasia; plenty of good size open fields around here I can use.

Extracted from the (yes, confusing!) Vega website:

Megarubber made from UV-stabilized natural rubber (anthracite colour for extra UV-resistance)

Only available in 25 and 50 feet lengths, which we pair with connectors to length. 

It has a usable, maximum elongation up to 700% (break at 850%).

The optimum range of application is 2-5.5-fold stretch with a long-lasting, even pull.

Start your model with a starting train of 3-5 times the model weight. 

A good high-start set consists of: 22 - 30 m of rubber plus approx. 100 m of rope plus parachute.

A 22 m rubber 4 x stretched (88 m) has more energy than a 29.4 m rubber stretched 3 x (89.2 m) !!! 

The Allround rubber is 9.6mm diameter.  At the High Start lengths of 22-30m plus 100m line (for launch heights of 100-150m) for models from 1000 to 2500g, this produces forces from 7kg at 3x stretch to 11kg at 5x - which might be too powerful for the Fantasia?

The thinner HLG rubber might be more appropriate (specced for 300-1500g models, but no diameter indicated so can't compare to HK or Hyperflight 8mm alternatives).  It is available in a confusing range of different complete (anchor, rubber, line, connectors, flag/parachute) kits, simplified here:

• The 'Bungee Start' complete kit (Euros 46 + shipping) uses rubber 7.35m long - so a short powerful (min 5x, up to 10x) force?!
• The 'RES High Start' kits (between Euros 60-83 + shipping depending on type, either 100-A or 100-F whatever the difference?) use 14.6m of rubber.
• The '100m High Start' kit (Euros 104 + shipping) uses 22m of rubber - in the absence of advice otherwise I'm inclined to this.

Rubber on its own is cheaper.  HLG prices are Euros 20 for 7.35m, 40 for 14.6m and 60 for 22mm.

Cheers

Jon

 

[StraightEdge]

23 hours ago, Gary B said:

Pop over sometime (Brackley) and you can have a look.

I'll take you up on that kind offer, thanks Gary! 

[Gary B]

No worries.

I think the 100M high start kit is what you would need, personally I would knot 1,000 rubber bands together before paying that much but I'm a skinflint!

Think I have a 7 metre length of the HLG, it's quite skinny.

It's fun to experiment and make your own, I got some rubber tubing from RS Components but it wasn't very stretchy.

Dog stakes from the pet shop and fishing line, cheap as chips. Line joiners and swivels also from the fishing tackle shop.

Cheap electrical extension reel from hardware shops, unwind the flex and plug, cut it off and use the reel for your line and rubber.

Occasionally second hand bungee sets come up for sale on here or eBay, have to strike quick!

Could always launch the Fantasia in a light wind from a slope, Uffington is just down the road. 

 

[StraightEdge]

Thanks for confirming the right overall length.  Yes, crazy money for the complete Vega set.  I've already got or can source most of the components, except I'll need to buy the rubber itself: two 10m lengths of the HK 8mm rubber would cost £12.  Just knot them to one another and to the line, or is there a cleverer way?

Uffington is indeed close by me.  I sometime play there with a foam Libelle in light/mod north-westerlies, and currently building an aileron-winged Middle Phase for the more usual conditions!

[Gary B]

What Graham said!

For the ends you can also use dowel, drill a hole (across the diameter) and add a strong keyring. If you use hairspray on the dowel it helps to slide it in then when it sets griptastic!

EMC Vega join their rubber much the same way with I think a couple of cable ties and a sheath over the top.

I might tidy the shed in a bit and see if I can take photos of my bungee stuff.

The tubing to get hold of is the beige coloured surgical tubing, I see it described as natural latex, what I have might be synthetic as it dates from the 1980s and isn't perished.

The EMC Vega rubber doesn't perish though I look after mine and keep it away from UV light when stored.

Launching with surgical tube in a fresh breeze is great fun, the rubber stretches more and the glider either climbs above your head or goes behind so you have to turn round!

The cotton covered stuff gives high power for a short time, when stretched it stops at a set point when the cotton hits its limit, the surgical tube can be stretched almost indefinitely but gives low power over a long time. 

[Gary B]

10 minutes and I have usable work top in the shed!

Some piccies:

EMC Vega 'All round' megarubber on an electrical extension reel, equal amounts of line and rubber for F3J low launches.

Airtronics branded reel with surgical tube.

Cotton covered bungee with plastic end fitting (a knot acts as the stop).

EMC Vega 'HLG' rubber with F3J winch parachute, they use a steel eye for end fittings. The rubber is about 6 mm diameter, about the same as the surgical tube.

[StraightEdge]

Gary, that detail and the photos are really helpful...

...so much so in fact that the picture of your Airtronics drum jogged my (very early-onset senility) memory

...and I suddenly remembered that a year or so ago an elederly lady who'd contacted my RC club had given me some kit her late father had from the 1990s

...so when I dropped by my house briefly late afternoon I checked the loft and couldn't believe that I already had

...a pristine Airtronics Super Start set with 30m of 8mm surgical cord, 120m of monofilament, parachute, etc!

I'm so stupid... but also rather fortunate it seems!

 

[StraightEdge]

I also checked the model itself.

All up weight, including battery and nose-lead is 1.3kg, which is much higher than I'd first guessed and more than double the current 2m RES gliders.

Wing area of 790 sq inches, so this gives a loading of 8.3 oz/sqft, again double that of the smaller RES gliders, but perhaps more useful for windier flat-field conditions?

 

[Gary B]

Imagine that!

No problem with the wing loading on the Fantasia, my 100" Chieftain is 10 oz/sq/ft and thermals well.

Assuming the hook and CG are in the right place it should launch itself (check for trimmed straight test glide first), a touch of up elevator near the top then a nudge down to release. 5 mph wind is ideal, the surgical tube doesn't work at all well without some headwind.

I stretch the Airtronics rubber about 100 to 120 paces.

The stake must be screwed all the way in.

You have a choice on whether to hook the model on to the line and stretch or stretch then hook on. I do the former but there is a risk (tripping over etc) that the model pings down the field out of control! Doing it this way you can gauge the pull better and with the bigger diameter rubber there comes a stretch point where the model can't be held, that's a logical limit.

If you are right handed and Mode 2 there is a short period with no fingers on the sticks on launch, it's an odd thing to do but imagine the model is free flight.

Please shout if you have any questions , looks a lovely model.

[StraightEdge]

Good to hear the loading is in the right ball park, and thanks for the gen on launching.

Whilst walking the dog on the Ridgeway before sunset, I paced the combined line and tube length of 150 meters, which was about 200 paces (at roughly 3m per 4 paces).  So another 120 paces to stretch = roughly 90 meters which implies a 3x stretch on the cord.

Looking forward to getting going once the weather permits.  Will let you know how I get on, and/or with more questions.

Thanks again for all the helpful advice!

Jon

[StraightEdge]

So, ready to test the Fantasia - the forecast for tomorrow Friday shows the middle half of the day SW backing S, 7mpg, gusting 14mph, sunny, clear at first becoming slightly cloudy later.

There is about 40 degrees eyeballed max deflection on the rudder before things bind, and 8mm up/down on the elevator.  I assume this is about right, and I've added 20% expo on both controls to soften the sticks slightly.

I cut off the original weak and ugly bungee-to-ring knots and managed to insert 9mm dowels into each end using PVA as a lubricant/glue, then cable-tied.  Through mishandling things on an empty stomach, I then managed to turn the 120m of monofilament into a birds-nest right in the middle.  Two frustrating hours later I simply cut it to unravel!  The join now is a double-sheet bend with blood-knots in each tail, all faired down tight with some insulation tape.

[StraightEdge]

Hurrah!   She launches well high and she flies very nicely.

Couldn't always judge when the Fantasia was at the top of the launch, and wasn't always successful in ditching the towing-ring the first time (at least when I could see the tiny  fluorescent pink parachute billow out) so sometimes lost height unnecessarily by having to repeat the dip.  Is this because I'm trying to ditch it too early or too late?

There wasn't much thermal action really.  When I arrived at the patch around midday and was laying out the bungee and setting up the glider, I spotted half a dozen red kites circling well high in a lovely column, but once I was ready to launch they'd all gone and never came back.  Best 'dead-air' flight times were about 2mins, but my best flight was 5:07 from release to landing - which for me was a real thrill first time out!  Less thrilling was hiding most of my face behind the TX held high up in front of my nose... because all the thermal action was directly between me and the sun!

One thing I found was that the Fantasia - with its relatively thick wing and no flapperons to alter the camber - struggled to penetrate upwind.  With a more modern soarer I'd have been more confident following the few whisps of thermals  (I'd merely stumbled into) downwind, knowing I could get back without radical loss of height.  But, hey ho, an awful lot of fun for a tenner, a twenty quid RX and a spare battery!

One other thing:  I thought the bungee would be a faff, but it totally wasn't!  Once it was laid out, which only took a trice, all I had to do each time was land back at the pink parachute, re-connect and pull back a hundred or so paces to re-launch.  At going-home time, just wound the whole caboodle up and that was that.

Jon

[StraightEdge]

Cheers Pete

The FAN...I mean FLOATER did fairly well again yesterday, best of three launches was just under 4 mins in fairly cold brisk conditions with mainly layer cloud.

Solved the problem of not being able to ping cleanly off the line by increasing the elevator throws from original 8mm to 12mm.

Bring on the balmy times 

[StraightEdge]

Just a question about CG on the Fantasia:

When doing dive-tests in dead air this (cold) afternoon there was a fairly immediate pull-up each time, but adding any down-trim wrecked level flight, which had previously been fine.  The solution normally would be to remove a bit of the nose-lead to move the CG further aft and re-set the elevator trim accordingly.   But it then struck me that maybe the relatively thick 'floaty' wing-section simply isn't designed to be flown too fast (as it would be in a 45 degree dive)?  Also, would it be counterproductive to have too twitchy a thermal soarer (intented to be flown high and difficult to read) by moving the CG back?

Jon

[StraightEdge]

Cheers Pete - that seems to confirm what I suspected.  Might experiment with removing just a smidgeon of nose-weight and adjust trim accordingly to make the dive-test pull-up a weeny bit more gentle (rather than remove it altogether), and then see how stable level and turning flight is in dead-air conditions.

All good fun to explore - and a good prelude to learning to handle the much lighter, thinner-winged  X-RES (also pre-owned but a modern design only built last year) which I'm due to collect in a couple of weeks time.

[Whitmore]

13 hours ago, StraightEdge said:

Might experiment with removing just a smidgeon of nose-weight and adjust trim accordingly to make the dive-test pull-up a weeny bit more gentle (rather than remove it altogether), and then see how stable level and turning flight is in dead-air conditions.

Jon, try adding a gram or so of blu tack on the tail to shift the CG back without too much commitment

Also by moving the CG back AND a click or two of down trim you'll end up with the same trim speed but a little less stability in pitch. Which might be nicer to fly (or not - it's partly personal preference...)