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Old Aug 30, 2012, 03:46 PM
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bungy

Good for you Dave,,, I see our little talk inspired you!


yeap, winches SUCK for F3J.

At the Worlds in South Africa there was no comparison between the performance of direct tows in the wind out there and wimpy launches with a under powered F3B winch at the USA team selects in 2011.
Especially for 2.5 to 3 second tows , its so easy to stall the resistered motor and thats a big no no!


See what you guys come up with. I'll be lurking! R
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Old Aug 30, 2012, 04:24 PM
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Originally Posted by Rich Burnoski View Post
Good for you Dave,,, I see our little talk inspired you!


yeap, winches SUCK for F3J.

At the Worlds in South Africa there was no comparison between the performance of direct tows in the wind out there and wimpy launches with a under powered F3B winch at the USA team selects in 2011.
Especially for 2.5 to 3 second tows , its so easy to stall the resistered motor and thats a big no no!


See what you guys come up with. I'll be lurking! R
The F3J community as a whole is inspiring - Everyone gets to take credit here

I think the over riding theme is still "let the best pilot win" and anything we can do to push that pilot development is good for that community.

If successful the bonus of this launch system and format in general is a reduced resistance to participaction in j like events. The f3j community in Canada is basically a core group of 20 pilots most of which I have not even met yet due to the logistics of living in such a large country. We need more pilots or its going to die simple fact - so im trying to do something positive here - maybe it will catch on if it works well.

Blue skies out the window - time to fly !
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Old Aug 30, 2012, 06:39 PM
kdt
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Mega or Medium

The Mega for quick launches is awesome. I have not been able to get the maximum performance out of my short Mega bungee very often . It sure gets the arm in condition though. The heavy material with short line would make for some interesting carnage in a tight corridor J style even with a dedicated helper. With a medium setup the risk to models and people would be less and might be better for a man on man contest.

But then a tall fast launch is a lot of fun!

Models would evolve into an optimization for this type of launch ...

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Old Aug 30, 2012, 08:01 PM
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2x 50' of 4 meter hose monster and 100 feet of line

Ok so I used some of my existing material which was 2, 50 ft 4 meter (.40 inch )bungees and about 100 feet of mono - I could probably come close to getting 6 seconds of tension if I had my throwing glove - the mono is hard on the palm of my hand for my style of throwing. I estimated I was getting 4 second and maybe 100 meters with about 40 lbs of tension. if I had my glove and went back to a higher tension or there was some wind (it was dead calm) I would expect some better launches. I would like to see a min of 150 meters...

ill get my glove and use a howhi and post some results by Sunday
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Old Aug 31, 2012, 12:45 PM
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Robert,

Not sure that I can provide a good answer on this one. So much of the answer is strongly dependent on the capability of the thrower. If the thrower can pull back to 90 lb reliably, the answer is a bit different than if the thrower can pull back to 60 lb reliably.

For most people, my bungee could be a potential disaster with a hair trigger at 90 lb of tension. There are so many ways that things can go wrong, and there is a LOT of energy stored in the bungee.




Interesting. I wonder what launch pre-tension you guys were using on Keiths bungee? For zero wind, the zoom needs to occur reasonably quickly for the best launch height. I have about 65 m of line on mine, and get about 130-140 m of height in zero wind when I pull back to about 90 lb of line tension prior to the throw. With a little bit of wind I get up to around 200 m of height. The launch height drops off quickly if one does not start with high tension.

It may be appropriate to use a smaller diameter bungee with more length to reduce the launcher requirement.


I do like the concept, and would enjoy flying with this format.
Joe, how are you measuring 90lbs of line tension? Do you use that fish weighing scale for checking your mono that you used in France?
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Old Aug 31, 2012, 02:43 PM
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$12 later

using a fish scale I might be underestimating what we were pulling the mega bungee at. 70 lbs is only a little more than a medium pull and no where near what a high tension f3J pretension is like. The extra 20 lbs is getting to an athletic requirement though and a lot of people would not be comfortable there. So ill go back to the field and measure the 2x 4 meter at 50 feet of rubber and see if I am at 70 lbs - i think ill be in that zone easily. I dont think its going to be enough power duration though.

DW
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Old Aug 31, 2012, 04:03 PM
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Joe, how are you measuring 90lbs of line tension? Do you use that fish weighing scale for checking your mono that you used in France?
After I set up my bungee, I pull it back with a luggage scale (same as in France). At the limit of my scale (90 lb), I put down a marker on the ground. This is to calibrate my throw tension when practicing. The first time I pull the bungee back, it is easy to get to (or pass) that marker on the ground. By the 10th launch, that marker seems to have a lot more tension! If I am doing frequent launches, I have to work hard to get to that marker after a while.

Someday I'll put a tensiometer on the line to see what happens during the tow. I suspect that I am getting up to 150 lb of tension in the wind with the bungee (3-5 second launch). At least, I think I am getting around that value as based on various elements failing. The amount of energy stored in the bungee on a windy day launch is impressive.
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Old Aug 31, 2012, 08:54 PM
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90 lbs ?

I am seriously humbled - a measured 50 lb pull really seems like a lot of tension to me. Keith and I never really got past that mark with my 2x 4 meter bungees or his mega bungee at 25 feet + 100 meters of line.

I cant imagine 90 lbs nor could I anticipate that many pilots will be able to handle 50 lbs more than 10 times in a day.

I used a skytrace on the mega bungee launches at 50 lbs of tension - it looks like I got a consisten 300 feet at 50 lbs but the duration of power distribution was fairly agressive. I think everything was done in under 3 seconds which is asking alot of the majority of pilots.

When the wind picked up we kited straight up the line and got much higher and slower launches but I think everyone expects this.

Now what I think would be managable is a tension of possibly 40-60 LBS as options from the start of the safety corridor to the end. I would like to see the power dispersal over a longer period so that pilots dont need to react in 3 seconds or less. This gives the option to come off early or stay on for a longer ride.

I think something larger than 5/8 rubber but smaller than the mega bungee and potentially 70 feet each with 100 meters of line would be ideal.

ill do some more testing and logging and report back next week.
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Old Sep 01, 2012, 02:01 AM
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Tension

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Originally Posted by Joe W View Post
After I set up my bungee, I pull it back with a luggage scale (same as in France). At the limit of my scale (90 lb), I put down a marker on the ground. This is to calibrate my throw tension when practicing. The first time I pull the bungee back, it is easy to get to (or pass) that marker on the ground. By the 10th launch, that marker seems to have a lot more tension! If I am doing frequent launches, I have to work hard to get to that marker after a while.

Someday I'll put a tensiometer on the line to see what happens during the tow. I suspect that I am getting up to 150 lb of tension in the wind with the bungee (3-5 second launch). At least, I think I am getting around that value as based on various elements failing. The amount of energy stored in the bungee on a windy day launch is impressive.
The fish scale David used was attached to two spikes in the ground and attached to the bungee with a locking ring. One of us would observe the others launch and were quick to step back after the plane was released. I like the marker on the ground idea much better. As David points out we were pulling about 50lbs with both his 2x 4meter and my Mega Bungee. I have mine setup with two 3.5M mega bungees in series with 140M of mono. I have also used the two short bungees parallel with shorter mono. 60lbs seems to me to be about all I could ask for a practice launch. Now that we have a scale I will try to see how much I can hold before my arm gives out. Some serious damage could be done to both arm and model without some time on it. I'm sure we were getting more pull when we used the setup at Andersons Turf Farm where the ground was level and we could dig in better. The shorter mega bungee seems to be easier in the long run as you don't stay tensioned up as long as with the 2x 4M bungee.

The discussion Dave and I had considered the big difference trying for max tension fast J launch and what we could expect of others to do for a man on man contest. Most local flyers might not appreciate the time needed to get half way to Joe's level. Where do you set the corridor and what kind of bungee to choose is still a question for the man on man event.

I'm going to practice more with the hope of finding something closer to Joe's tension but my arm is old and I'm not too keen to drink a beer at the end of the day using my feet to hold the bottle
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Old Sep 01, 2012, 02:58 AM
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I suspect that the light bulb may be glowing now...

The interesting aspect to the bungee tension measurement is that it appears to be more difficult to pull back the bungee so as to mark the 90 lb tension point, than it is to pull it back with the plane hooked up to the bungee. I do not know why this is true! Possibly it is true because I am so used to pulling tension via holding onto a fuselage.

One very important caution. It is "easy" to pull back tension when the winds are not strong. When the winds are strong, there are some big risks with pulling significant tension.

An interesting thing for the analytical folk to consider/calculate. In zero wind, a quick launch (zoom from the throw) will provide about 100m of launch altitude with my bungee with my 4m Maxa (3.5m rubber, 40 kg of initial tension). How much of the initial bungee stretch is translated into altitude?
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Old Sep 01, 2012, 01:24 PM
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Light bulb glimmer

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Originally Posted by Joe W View Post
I suspect that the light bulb may be glowing now...

An interesting thing for the analytical folk to consider/calculate. In zero wind, a quick launch (zoom from the throw) will provide about 100m of launch altitude with my bungee with my 4m Maxa (3.5m rubber, 40 kg of initial tension). How much of the initial bungee stretch is translated into altitude?
It is clear high tension will bring a higher launch. What are the dynamics of the rubber stretch? Is it linear?

I have not yet used one of my 3.5M length bungees on its own. I also would like to see how it would work with only 65M of mono. Do you see this shorter setup being easier to get maximum tension over my 2x series 3,5 M mega bungees with longer mono?
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Old Sep 01, 2012, 05:44 PM
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Mostly, the short setup is to maximize practice time... with a byproduct of building up the arm.

A benefit of the shorter line is that the chute is less likely to wander too far. My set-up provides about 130m of launch height in calm conditions, and 160 to 200m with some wind. A very quick launch results in about 100m in calm conditions. The numbers in calm conditions drop quickly with a reduction in tension (E = 0.5*k*x^2). Dropping the initial tension from 90 lb to 64 lb cuts your stored energy by a factor of two.

It would be a bit of a grind to pull back the bungee if the rubber was twice as long. I have the vague impression that we may be pulling back a similar total distance, just that you have two pieces of the bungee in series. BTW, you have quite a large amount of stored energy available with your set-up. Some energy will get stored in the line, and a lot of amount of energy can be stored in your in-series bungees. When it is windy, the zooms must be pretty amazing!

The rubber load vs strain curve has a bit of an S shape. Near the end of its stretch, the load starts to increase quickly with a small increase in stretch. You are just beginning to venture into the non-linear region at 40 kg of tension. The initial part of the curve is only a little bit non-linear.
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Old Sep 01, 2012, 06:24 PM
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Originally Posted by Joe W View Post
Mostly, the short setup is to maximize practice time... with a byproduct of building up the arm.

A benefit of the shorter line is that the chute is less likely to wander too far. My set-up provides about 130m of launch height in calm conditions, and 160 to 200m with some wind. A very quick launch results in about 100m in calm conditions. The numbers in calm conditions drop quickly with a reduction in tension (E = 0.5*k*x^2). Dropping the initial tension from 90 lb to 64 lb cuts your stored energy by a factor of two.

It would be a bit of a grind to pull back the bungee if the rubber was twice as long. I have the vague impression that we may be pulling back a similar total distance, just that you have two pieces of the bungee in series. BTW, you have quite a large amount of stored energy available with your set-up. Some energy will get stored in the line, and a lot of amount of energy can be stored in your in-series bungees. When it is windy, the zooms must be pretty amazing!

The rubber load vs strain curve has a bit of an S shape. Near the end of its stretch, the load starts to increase quickly with a small increase in stretch. You are just beginning to venture into the non-linear region at 40 kg of tension. The initial part of the curve is only a little bit non-linear.
The zoom in wind is amazing with the in series 2x mega and 140 M of mono. Fun to launch fast and rip up the sky to do it again. The chute lands close with short launches. My arm gets tired pretty quickly even with 50lbs tension. Maybe I will start spending less time ripping it up to give my arm time to recover. Thermal skills need sharpening anyway. I'm surprised with the difference in stored energy between 64 lb and 90 lb. This is a good incentive to build up the arm. Next chance I get I will report on my findings with the short setup.

Thanks Joe!
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Old Sep 01, 2012, 09:18 PM
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Originally Posted by Joe W View Post
Mostly, the short setup is to maximize practice time... with a byproduct of building up the arm.

A benefit of the shorter line is that the chute is less likely to wander too far. My set-up provides about 130m of launch height in calm conditions, and 160 to 200m with some wind. A very quick launch results in about 100m in calm conditions. The numbers in calm conditions drop quickly with a reduction in tension (E = 0.5*k*x^2). Dropping the initial tension from 90 lb to 64 lb cuts your stored energy by a factor of two.

It would be a bit of a grind to pull back the bungee if the rubber was twice as long. I have the vague impression that we may be pulling back a similar total distance, just that you have two pieces of the bungee in series. BTW, you have quite a large amount of stored energy available with your set-up. Some energy will get stored in the line, and a lot of amount of energy can be stored in your in-series bungees. When it is windy, the zooms must be pretty amazing!

The rubber load vs strain curve has a bit of an S shape. Near the end of its stretch, the load starts to increase quickly with a small increase in stretch. You are just beginning to venture into the non-linear region at 40 kg of tension. The initial part of the curve is only a little bit non-linear.

In terms of J launch setups and training solutions they both work pretty well but I think the experience would be asking too much for the general contest public to get into what is required for a decent launch especially in dead conditions.

So for the purposes of a contest launch that most people can accomodate in terms of both tension and power curve I think something a little easier spread out over more time is going to be better.

While I am positive you will be walking a longer distance to gain 45-50 lbs the result will be a longer launch that moderate pilots can anticipate. The last thing I want to do is invite pilots out to a contest and have them find out that they just cant get used to a launch system that forces them into the bucket at 2.5 seconds. I also want to avoid requiring them to do super human tensions. That seems like a recipe for misfires and broken planes...

Over the next few weeks I might even get closer to 60 LBS with practice but only the truly comitted are going to do this. I cant expect anyone to arive with that level of arm ready for 10 launches in a day...I really want to try and hit the masses here with something.
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Old Sep 02, 2012, 12:53 PM
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Mega or middle of the road?

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In terms of J launch setups and training solutions they both work pretty well but I think the experience would be asking too much for the general contest public to get into what is required for a decent launch especially in dead conditions.

So for the purposes of a contest launch that most people can accomodate in terms of both tension and power curve I think something a little easier spread out over more time is going to be better.

While I am positive you will be walking a longer distance to gain 45-50 lbs the result will be a longer launch that moderate pilots can anticipate. The last thing I want to do is invite pilots out to a contest and have them find out that they just cant get used to a launch system that forces them into the bucket at 2.5 seconds. I also want to avoid requiring them to do super human tensions. That seems like a recipe for misfires and broken planes...

Over the next few weeks I might even get closer to 60 LBS with practice but only the truly comitted are going to do this. I cant expect anyone to arive with that level of arm ready for 10 launches in a day...I really want to try and hit the masses here with something.
The origional idea of this thread was for a bungee format for a Team Select. With this in mind, do we allow pilots to use their preferred bungee or do we make it a standard size to fit all? My feeling is the medium size would be fine for club events but for a Team Selects we should allow for a higher level of expertise. We were lucky to have Arend throwing for us in SA. He was able to put more tension into our launches then we would have been able to do on our own on a consistent basis. If a two man tow is not an option in a Team Select then the Mega Bungee should be an option for the pilots that want to take this to a higher level.

The launch in F3J is critical and the tactics involved cannot be duplicated with a winch or a bungee. My preference is to continue to use 2 man tows for a Team Select regardless of the difficulty in getting it off the ground

A bungee launch J style format would be a great for club events and I am all for it.
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