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Cox Engine of The Month
September-2018
Davenz13's

"Cox Special MKII .15"



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Jumping Bean

Post  oldguy on Thu Mar 24, 2016 12:40 pm

Hello guy's,did they make a Jr. .15 size jumping bean?
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Re: Jumping Bean

Post  Cribbs74 on Thu Mar 24, 2016 1:57 pm

No, but you could always scale the 1/2A one up or scale down a Shoestring.
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Re: Jumping Bean

Post  oldguy on Thu Mar 24, 2016 5:09 pm

Cribbs74 wrote:No, but you could always scale the 1/2A one up or scale down a Shoestring.

Thank you, that's what I thought.Had to ask.
Jim
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Re: Jumping Bean

Post  kevbo on Tue Mar 29, 2016 11:13 am

They did make a Shoestring racer for goodyear racing, intended for a .15   Similar lines, but this was a solid wing, asymmetrical airfoil go-fast plane, not a fat airfoiled stunter like the .35 or .049 models.

On Edit:  Sig made the kit,  not Goldberg.

http://beseenontheinternet.com/VAHS/images/Catalog/CL13.jpg
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Re: Jumping Bean

Post  GallopingGhostler on Tue Mar 29, 2016 8:10 pm

afaik, ron cribbs is correct. li'l jumping bean was a scaled down cg shoestring, more of a sport stunter. sig goodyear racer more of a speedster. was impressed w/ ljb as it had nice built up wing.
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Re: Jumping Bean

Post  getback on Wed Mar 30, 2016 6:46 am

Well that would bee cool a scaled down Shoestring , I didnt know the Jumping bean was a dissident of the SS . I have the .29 size although I haven't flown it, I really need to get it down and start the repair of being moved around for 40 yrs. Shocked getback
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Re: Jumping Bean

Post  Cribbs74 on Wed Mar 30, 2016 10:51 am

The Bean is loosely based on the SS. The wing and tailplane are near identical. The Fuse is a little altered.

The best way to re-create a Bean in .15 size would be to scale up the Bean. The problem lies in the fact that no actual plans exist.

Now, you could take the Goldberg plans for the wing and scale it up, you could also trace the fuse and tailplane and scale to match the wing. It's possible to do, just time consuming.

Now that I think about it, Brodak may have full size Jumpin Bean plans. Something to look in to.

Ron
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Re: Jumping Bean

Post  kevbo on Wed Mar 30, 2016 11:39 am

For a .15 I'd want to add a spar to the wing. The 'bean relied on just the LE and TE. It would be strong enough in flight with just that, but I'd want a little more strength to handle less-than-graceful landings. Also I'd want to copy the original CG laminated wing tips rather than the sheet tips of the Brodak version. The laminated version has the grain running around the tip.
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Re: Jumping Bean

Post  pkrankow on Wed Mar 30, 2016 3:45 pm

kevbo wrote:For a .15 I'd want to add a spar to the wing.  The 'bean relied on just the LE and TE.   It would be strong enough in flight with just that, but I'd want a little more strength to handle less-than-graceful landings.   Also I'd want to copy the original CG laminated wing tips rather than the sheet tips of the Brodak version.  The laminated version has the grain running around the tip.

Yes, and yes.

I would add top and bottom spars such as in the Flite Streak instead of any version that goes through the center of the wing. Using a couple vertical grain shear webs in select positions would be a good idea too, but probably not needed. If you are still crashing then the shear webs would be better left out so the wing is more flexible.

Building a multi-piece wing tip is pretty quick and easy.

Scaling up to about a 30 inch wing would be similar to a ringmaster jr., 31 inch would be like the flite streak jr and jr lightening streak.

Phil
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Re: Jumping Bean

Post  getback on Wed Mar 30, 2016 8:51 pm

pkrankow wrote:
kevbo wrote:For a .15 I'd want to add a spar to the wing.  The 'bean relied on just the LE and TE.   It would be strong enough in flight with just that, but I'd want a little more strength to handle less-than-graceful landings.   Also I'd want to copy the original CG laminated wing tips rather than the sheet tips of the Brodak version.  The laminated version has the grain running around the tip.

Yes, and yes.

I would add top and bottom spars such as in the Flite Streak instead of any version that goes through the center of the wing. Using a couple vertical grain shear webs in select positions would be a good idea too, but probably not needed.  If you are still crashing then the shear webs would be better left out so the wing is more flexible.

Building a multi-piece wing tip is pretty quick and easy.

Scaling up to about a 30 inch wing would be similar to a ringmaster jr., 31 inch would be like the flite streak jr and jr lightening streak.

Phil
 I agree with the spars upper and lower but I am not sure what a shear web is ? is it a 1/2 rib ? Help me out please . getback
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Re: Jumping Bean

Post  pkrankow on Wed Mar 30, 2016 9:03 pm

A shear web goes between the top and bottom spars to create a quasi-I-beam arrangement spar. It is typically vertical grain because it is really resisting the movement of the top spars relative to each other in shear due to the ribs becoming out of square to the spars.

They are not needed between every rib. Some people build a truss arrangement between the top and bottom spar to serve the same function with less material.

Omitting shear webs tends to make the wing will be more crash tolerant since it will be able to flex more.

Phil
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Re: Jumping Bean

Post  getback on Thu Mar 31, 2016 11:10 am

I got you Phil kind of structural bracing for wing flex / Thanks I will keep that in mind when building and probably ask again lol Eric Airplane
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Re: Jumping Bean

Post  kevbo on Thu Mar 31, 2016 12:30 pm

pkrankow wrote:A shear web goes between the top and bottom spars to create a quasi-I-beam arrangement spar.  It is typically vertical grain because it is really resisting the movement of the top spars relative to each other in shear due to the ribs becoming out of square to the spars.

This is an area where my understanding has changed over the years. I used to agree with the above, and would make V trusses between the ribs...seemed like a better and lighter way to deal with shear forces. I'd put a V on the front of the caps, and an upside down V on the other, or just put a diagonal each way. Then I figured out the real reason why the vertical grain is used:

In spite of the common name, it isn't really for shear. 1/16" vertical grain balsa won't take much shear, and there is very little load that direciton...how it really works is to prevent the spar caps from buckling and failing in compression. The spruce or basswood is very strong in tension, but less so in compression. When a bending load is applied to the wing, it puts one cap in tension and the other in compression. The vertical grain web links the compressed cap to the one in tension, so it can't buckle.

For the usual wing, this is really only needed in the center 1/2 or 2/3 at most of the span. Out near the tips, the loads are less, and you can omit the web and save weight.

It is important to get a good fit and decent glue joint to the spar caps. But the joint isn't all that stressed..glue fillets are a waste of wight here. It doesn't take a lot to keep the compressed cap straight. Also, a little gap near the ribs doesn't hurt anything...The buckling force goes like the 3rd or 4th power of the unsupported span, so a short gap is no problem. The main thing is to get a good fit between the caps. For a constant thickness wing, I will use a balsa stripper to cut the shear webbing, testing the fit before cutting the whole batch. For a tapered wing, I measure each end if the rib bay with dial caliper, and cut a piece to fit. I make them about 1/4" too short, so if cut perfect, there will be a 1/8" gap at each rib...but if it is a hair off, I can move it one direction or the other and make a perfect fit due to the taper. Only if I get the taper wrong, or really undersize do I have to cut another, and I can use the bad one on the next bay toward the tip, so there is little waste. And that is why I always work from the root outward.

I will also put a 1/8-1/4" piece of 1/16" vertical grain on each rib just in front of, and just behind the spar. This helps keep the ribs from splitting, both due to crash loads on the spars, and also from shear in the covering when it crashes. It is very common for the cambered front part of a rib to remain attached to the covering in a crash. This also serves the same function as the shear webbing. I cut these just a little short so they don't blemish the covering.
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Re: Jumping Bean

Post  RknRusty on Thu Mar 31, 2016 12:55 pm

About shear webs:
http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/rc_aircraft_design/shear_webs_in_model_aircraft_wings.htm


For our sport purposes they can be face mounted to make it easier. And they may pop off in a crash, possibly saving some wing damage.

Go to that site's home page and bookmark it. This is one of the best resources on building planes I have ever found. When in doubt, this is where I go.

Rusty

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Re: Jumping Bean

Post  getback on Fri Apr 01, 2016 7:28 am

You got that right I have been there a few time and they do have every thing you need to know // some times even more than I can comprehend. Thanks for the explanation kevbo, when I start fling larger planes I am going to need some extra strength , My father 77 YY said they would make there planes from plywood because there was no dought they would be dog fighting and doing tricks and all , I can imagine what engine they were using he can't remember Sad but he still likes seeing me building and playing with my toys . getback Very Happy
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Re: Jumping Bean

Post  Jim Howell on Sat May 14, 2016 9:30 am

Cribbs74 wrote:...
The best way to re-create a Bean in .15 size would be to scale up the Bean. The problem lies in the fact that no actual plans exist.
...
Now that I think about it, Brodak may have full size Jumpin Bean plans. Something to look in to.

Ron
Ron, et al,

I'm probably a day late and a dollar short, but you are correct, Brodak did create a CAD plan of the LJB.  Because the Goldberg kits were my favs in the '60s, and the LJB was the last of them that I built before heading off to college, I had to add a Brodak LJB kit to my stash once they became available a few years back.  Send me a PM or straight email if interested.  I haven't built it yet, but had decided to do so for the club 1/2A day.

Jim Howell
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Re: Jumping Bean

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