I promised someone I'd post the CG of a Jumpin Bean. Don't remember who or what thread

[RknRusty]

It was a fairly new member, a few days ago. I can't recall who, or what thread.

To whom it may concern, if you're still needing it, the CG on a Brodak Li'l Jumpin Bean is 1" behind the LE at the root. And the landing gear with the supplied heavy rubber wheels are an important part of the balance.

Sorry I forgot to get back to you.
Rusty

[Cribbs74]

It wasn't me. However, I cut the nose off my Bean amd need to rebalance so you helped me in the process.

[Cribbs74]

This got me to thinking......

The Brodak kit has a long nose. So when I balanced it originally was I using the CG based off the original bean config or the Brodak config? I had to add a big chunk of lead to the tail initially to get it to balance. I have since cut down the nose to see if I can take some weight out of the tail. My thoughts are it may hve been improperly balanced from the get go.

It probably has nothing to do with line tension or coming in, but it would make a difference in how it flies for sure.

[pkrankow]

I think it was me. Thank you! I am going to drive a large screw into the wood behind the motor mount when I get it sorted out. My wheels have been missing for a long time.

The people on this forum are tops coming through with the good information. Thank you Rusty

Phil

[RknRusty]

Yep, it was definitely you, I remember the big screw now.

[RknRusty]

This got me to thinking......

The Brodak kit has a long nose. So when I balanced it originally was I using the CG based off the original bean config or the Brodak config? I had to add a big chunk of lead to the tail initially to get it to balance. I have since cut down the nose to see if I can take some weight out of the tail. My thoughts are it may hve been improperly balanced from the get go.

It probably has nothing to do with line tension or coming in, but it would make a difference in how it flies for sure.

Yours and mine were the same Brodak kit. I cut the beam off of mine too just like you did, so yours was probably balanced right.
I was looking through the remains of my BFS kit the other day and realized the CG on the plans was different from the CG in the instruction book. The one in the book was the right one.

[pkrankow]

Yep, it was definitely you, I remember the big screw now.

Well, the nut I wired underneath the motor did pretty good. The LE fractured, and poking about I split the covering so I have the LE repaired, and no matching covering.



https://i.servimg.com/u/f44/17/84/87/39/11130210.jpg



https://i.servimg.com/u/f44/17/84/87/39/11130211.jpg

Phil

[pkrankow]

I located an extra heavy counterweight spinner for a Cox engine in random stuff from an ebay auction. With a short run tank it is not quite enough. (I sorta want to put the original engine back on it.) I haven't tried with an extended run tank, and the engine pictured with the #10 wing nut tied to it I took apart to put the "black widow" stunt tank I ordered on. I have another extended run engine set up to try later.

Phil

[Cribbs74]

Nice Bean Phil! It's a real one.

I have an extended tank on mine. It does play with the CG some as I have used both on my BRM and it went a little nose heavy with the larger tank.

Ron

[Cribbs74]

This got me to thinking......

The Brodak kit has a long nose. So when I balanced it originally was I using the CG based off the original bean config or the Brodak config? I had to add a big chunk of lead to the tail initially to get it to balance. I have since cut down the nose to see if I can take some weight out of the tail. My thoughts are it may hve been improperly balanced from the get go.

It probably has nothing to do with line tension or coming in, but it would make a difference in how it flies for sure.

Yours and mine were the same Brodak kit. I cut the beam off of mine too just like you did, so yours was probably balanced right.
I was looking through the remains of my BFS kit the other day and realized the CG on the plans was different from the CG in the instruction book. The one in the book was the right one.

I had no beam mount cutouts just a solid fuse on mine. No insructions were provided to modify anything. I initially never cut down the nose. I just slid the plastic radial mount on.

[pkrankow]

I was 8 when I built this with my Mom (Dad helped me with my Lil Wizard when I was 6) I think Dad covered it as his first experiment with Monokote.

That would be 1984

I flew the heck out of it, the wood around the landing gear went soft and tore out, and I flew it quite a bit more without rebalancing it. Somehow I was able to fly it as a teen like that!

I am going to try blocking out the short engine with one or two pieces of 1/8 inch plywood and see if the extra-heavy spinner is adequate.

...unless someone has real reason not to use the extra heavy spinner. I am concerned about spinning the extra mass, but it is very concentrated so I don't expect a load on the engine parts. The stud seated neat all the way down, so I feel comfortable with it.

Phil

[RknRusty]

...unless someone has real reason not to use the extra heavy spinner. I am concerned about spinning the extra mass, but it is very concentrated so I don't expect a load on the engine parts. The stud seated neat all the way down, so I feel comfortable with it.

Phil

I had been restraining myself from mentioning that, but since you mentioned it, the worst place on a plane to add weight is to the rotating mass, and even worse is in front of that point. Of course it is the way to add the least weight to the plane, but unless you already have a weight problem, the closer you can compromise between adding weight and getting it close to the CG is the way to ensure the best aerobatics as possible. You will add more by placing it a bit farther back, but the physics work better when the plane's moment of inertia is closer to the CG.

If you only wanted to fly flat laps it wouldn't matter. But for loops and even quick cooperation saving it from the ground at any time, it needs its moment as far back as possible. Like I said before, you'll quickly get bored going round and round without any excitement, so you might as well design it to be responsive to your input. It will still be a good trainer, but will be ready to bust out into fun at any minute

[pkrankow]

I am building other planes to bash with, I want to be nice to this one, childhood toy and all. I plan to do loops, wingovers, overhead 8's (I did all that with the wingnut strapped on)

I suppose I can make up a plywood doubler for the motor mount and sandwich some washers in. I just cut ply for the motor mounts of the zingers I am building, which other than long noses are very similar to the 'bean

Phil

[RknRusty]

I had to make a lead necklace and hang the weight right under the tank on my original Bean. I pounded a stick-on weight to shape it flat and curved and wired it on. The wire went tightly around the crankcase between the cylinder and tank.

[pkrankow]

Rotating mass be damned! I am using the heavy spinner, on an extended tank engine, and I built a 1/4 inch spacer to push the engine out more forward.

I had almost an ounce hanging on the bottom of the short tank setup and it still wasn't balancing!

While I was at it I put an aluminum pickup in the tank and the hose on the nv and bored out the back plate (plastic back, I have spares). This piston is a #1 with SPI, I just checked. I just wish I had a stunt tank for this one.

I fly when I have a nice calm day without commitments. I also have 40 ft lines made up on a second handle...I might need them.

Phil

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