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Old Apr 01, 2014, 05:54 AM
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Bungee launched EDFs?

I currently have a starmax F35 64mm EDF, and after a long period of hand launches, quite often mixed with failed hand launches, i am thinking about buying hobbyking's bungee launch system, i would just like to know how:
A- safe it is (i.e how likely are you to crash)
B- could a foamy EDF withstand the launch

Thanks
Jacob
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Old Apr 01, 2014, 08:44 AM
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St. Mary, Maryland, United States
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A - Reasonably safe if you set up in to the wind and your hook is correct
B- No sorry, reinforcement would be needed and the rails are going to be hard on a foam wing
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Old Apr 01, 2014, 09:00 AM
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Some models can be made to hand launch much easier by simply making a good handle under the center of gravity. The shape of some planes makes it harder to hold and throw so a plywood tab glued under the belly near the CG will help. Sometimes, simply cutting open holes in the foam for finger grips works.
Trying to throw a model with your hand behind the wing (far behind the CG) can cause a crooked launch causing a crash.
i would avoid bungee for small models if possible.
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Old Apr 01, 2014, 10:22 AM
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its not too small, its over 1m long
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Old Apr 01, 2014, 10:37 AM
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It's a nice size and weight for hand launch (that's what I meant). Some folks hand launch 90mm models if they are built very light.
Cheers
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Old Apr 01, 2014, 11:45 AM
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ok thanks i'll try adding a handle or grip type thing soon
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Old Apr 01, 2014, 02:38 PM
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Quote:
Originally Posted by UKoctane View Post
I currently have a starmax F35 64mm EDF,
and after a long period of hand launches, quite often mixed with failed
hand launches, i am thinking about buying hobbyking's bungee launch system,
i would just like to know how:
A- safe it is (i.e how likely are you to crash)
B- could a foamy EDF withstand the launch

Thanks
Jacob
.. been there, done that .... :-)
I am in the same boat ...

here is a light 90mm mirage (~1.8 kg),
hand launch with 1:1 thrust to weight ratio
and an interesting, "nice" way to launch
FlyFly Mirage 2000 Gottenheim Industriegebiet (1 min 31 sec)
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Old Apr 02, 2014, 07:30 AM
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I bungee launch every single small EDF I own.

The smallest - the FreeWing F-35 64mm - has yet to have a failed launch. And it's powered by a Don's RC 60mm fan setup. And right at around 475 grams, with battery.

In fact, there have been zero failures, with any airplane, since abandoning hand launches for the bungee.

Alfa F-86, Alfa A4, FreeWing F-35, FMS F-5's, FMS F- 20, Dynam F-16, HobbyPeople EF-16, StarMax F-18......on and on. All have absolutely no trouble with bungee launches - as opposed to hand launches.

Maybe I just can't throw anymore. That's a real possibility. I do know that there's a real distrust, on my part, of how any throw is going to wind up, and that probably affects things big time. But faith in the bungee is established, strictly by experience. It works, and works every single time.

Do NOT be afraid of using a bungee, even on those little ones. Adjust the tension as you go, heavy to light. A little more than necessary only throws it up higher.
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Old Apr 02, 2014, 01:35 PM
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what kind of force would you use to launch a 530g plane, because i hear that you have to be quite precise
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Old Apr 02, 2014, 01:41 PM
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You want approx 5 times the weight of the model. So you would want about 2.5 to 3kg pull. I use a spring gauge to measure the pull.
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Old Apr 02, 2014, 02:23 PM
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Quote:
Originally Posted by UKoctane View Post
what kind of force would you use to launch a 530g plane, because i hear that you have to be quite precise
Mmmm.

I utilize the, "that feels about right" -o-meter. The lighter models much prefer a single strand bungee; these impart a longer, steadier pull than a doubled-up bungee. The longer pull allows speed to build up more gradually (that's the simplest way I can put it), as opposed to the immediate, violent blast the double imparts to a lightweight model.

The double is reserved for the 90mm F4, and pretty much anything up over the 1100 - 1200g. range. The single will launch the heavier models, altho it isn't a comfortable launch.

Honestly, you don't need to overthink this. SOME small models will need wing reinforcement, but then they need that even without a bungee. SOME small models may prove more sensitive to hook placement, but that manifests itself as a pronounced skyrocketing on launch. when too far aft. Too far forward generally just makes it shoot off level....but even then with the speed they have, a touch of up-elevator and away you go.

............Provided you don't run into the bungee anchor stake first. Ask me how I know that.

Is there wear on wing surfaces? - eventually, yes. Happens to the big ones too. Keep the rails clean and waxed, and the problem is reduced.

With the bungee launch system, I have never - ever - torn a wing off due to no spars, never torn off a stab, never sent one nose-first into the dirt. Some take to it easier, sitting on the rails flat instead of nose-down, but even the nose-down -balancing ones launch fine with proper hook placement.

In contrast, I have dorked virtually each and every one of my hand launches; the few that did make it to flying speed did so by virtue of being overpowered. To be honest, I never threw anything underhand, which appears to be a much, much more effective manner than overhand. Instead, I simply built a system for about $25 worth of PVC tube and bungee, and have never looked back.
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Old Apr 02, 2014, 02:45 PM
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Quote:
Originally Posted by PikeStaff View Post
Mmmm.

I utilize the, "that feels about right" -o-meter. The lighter models much prefer a single strand bungee; these impart a longer, steadier pull than a doubled-up bungee. The longer pull allows speed to build up more gradually (that's the simplest way I can put it), as opposed to the immediate, violent blast the double imparts to a lightweight model.

The double is reserved for the 90mm F4, and pretty much anything up over the 1100 - 1200g. range. The single will launch the heavier models, altho it isn't a comfortable launch.

Honestly, you don't need to overthink this. SOME small models will need wing reinforcement, but then they need that even without a bungee. SOME small models may prove more sensitive to hook placement, but that manifests itself as a pronounced skyrocketing on launch. when too far aft. Too far forward generally just makes it shoot off level....but even then with the speed they have, a touch of up-elevator and away you go.

............Provided you don't run into the bungee anchor stake first. Ask me how I know that.

Is there wear on wing surfaces? - eventually, yes. Happens to the big ones too. Keep the rails clean and waxed, and the problem is reduced.

With the bungee launch system, I have never - ever - torn a wing off due to no spars, never torn off a stab, never sent one nose-first into the dirt. Some take to it easier, sitting on the rails flat instead of nose-down, but even the nose-down -balancing ones launch fine with proper hook placement.

In contrast, I have dorked virtually each and every one of my hand launches; the few that did make it to flying speed did so by virtue of being overpowered. To be honest, I never threw anything underhand, which appears to be a much, much more effective manner than overhand. Instead, I simply built a system for about $25 worth of PVC tube and bungee, and have never looked back.
thanks for great info.
could you give some more details, general rule for proper hook placement.
Normally people said it is about 2/3 distance from the cg to the tip of the nose,
or about 1/2 distance for straight trajectory .

and the length of the bungee cord? 10 meters? 5 meters?

and how to fix the problem that the airplane seat nose down on the rails, it
is kind of scary though

thx.


.
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Old Apr 02, 2014, 02:47 PM
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Hey now,
I have serious rotator cuff issues so I don't like to hand chuck anything except slopers. So I use a bungee system "stomp launcher" every time.
On my smaller planes like my Phase III EF 16-S the S stands for super. I used a Het 6904 fan with an ARC 28-48-1 and Ice lite 100amp esc on three cells (3300-3s T.P. cells). It goes like stink but I can't launch it. So on tye bungee with 7/16" surgical tubing pulled twice its length. Every launch a perfect launch. And every flight turns mt pants brown
I also launch my twin 70mm Mig 29 composite from the same rail with double tubing and a two and half times pull. Same perfect launch every time and a much more sedate fligh.
RobII
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Old Apr 02, 2014, 04:42 PM
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Quote:
Originally Posted by flylow2011 View Post
.....could you give some more details, general rule for proper hook placement.
Wish I could. I know that several in these threads swear by the 2/3 forward of CG, and that probably works...never really measured any of mine. I pick a spot that looks/feels good. Some times it needs to go farther forward, sometimes farther back, but usually it works OK. I haven't found placement of the hook to be terribly critical. Remember that closer to the CG will result in the plane climbing on the pull much more steeply....farther away from the CG will make for a flatter trajectory.

and the length of the bungee cord? 10 meters? 5 meters?
5 meters is fine; more is always welcome but not really needed.

and how to fix the problem that the airplane seat nose down on the rails, it
is kind of scary though
For these, I move the hook back a little (3 - 6mm) from my normal location....just a little assurance that the nose will positively rise with the pull.
Quote:
Originally Posted by elfwreck View Post
Hey now,
I have serious rotator cuff issues so I don't like to hand chuck anything except slopers. So I use a bungee system "stomp launcher" every time.
I have tried to throw with the left hand so that the right stays on the sticks...and the throw is pathetic. Worse than a girl. The bungee is the ticket to certain flight.

On my smaller planes like my Phase III EF 16-S the S stands for super. I used a Het 6904 fan with an ARC 28-48-1 and Ice lite 100amp esc on three cells (3300-3s T.P. cells). It goes like stink but I can't launch it. So on tye bungee with 7/16" surgical tubing pulled twice its length. Every launch a perfect launch. And every flight turns mt pants brown
Oh, yah, LOVE the 28-48-1; best motor they made. Send me your address, and I'll send over some Depends

I also launch my twin 70mm Mig 29 composite from the same rail with double tubing and a two and half times pull. Same perfect launch every time........
And there it is again. The bungee makes the launch just right. I like your method for tension; easy to do and it works.
There probably are as many preferred methods for bungees as there are guys using them. The best part is they all work and they all are pretty simple to use.
Try a few, find the one you like best, and enjoy a new way to practice this passion.
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Old Apr 03, 2014, 06:32 AM
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Excellent advice from Pike.

And like he said, don't over think it.
On lighter models you don't need a ton of pull, just a little shot will get it going.

Agreed that hook placement isn't that critical. Just about anywhere (within reason) ahead of the LE of the wing will work.
If you wondering about what your take-off trajectory will be, hang you model by the hook and that will give you a pretty good idea of the jet's attitude on launch.
(see pic below)
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