I've completed the assembly of a Flis Kits Goddard L-13 (I did the fins in basswood since I had to cut them out anyway.)

The question is: do these teensy fins provide enough stability or should I add weight? I surmise that the real L-13 had some kind of rudimentary gyro.

Thanks!

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My admittedly uncalibrated eyeball says yes, it is stable......

What happens when you load a motor in it and swing test it?

kj

She's stable as provided in the kit. Her long profile is a big help here, that's for sure! Not knowing the weight of the fins in bass wood, it certainly doesn't hurt to add a small nose weight to counter that, but my concern is low

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Thanks, Jim!

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As I've posted here before, I discovered that basswood weighs roughly three times the same amount of balsa. So I agree with Jim that additional nose weight will probably be needed, but again, the question is, "Exactly how *much?*"

Cheers,

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I interpret "couldn't hurt" as not being synonymous with "probably needed". Looking at the design I can't believe the small weight difference really would cause any stability problems unless it was really marginal to begin with.

Simming or swing testing would be the best way to answer the question, but roughly:

From pictures online it looks like the fins are roughly 4" by 1.5" triangles, so 3 in^2 area each or 12 in^2 total. 3/32" thick so 1 1/8 in^3 volume, or about 18 cm^3.

Density of balsa and basswood varies but typical values I think are 0.17 g/cm^3 for balsa, 0.5 g/cm^3 for basswood.

So the balsa fins would have weighed about 3 g and the basswood ones 9 g.

Then assuming the center of gravity is close to the geometric center of the model, the ideal amount of weight to add to the nose should be about equal to the amount added to the tail, which is about 6 g.

Again, that's just a rough estimate, but probably within a factor of 2.

And it's just about equal to the change in mass at the tail going from a B6 to a C6, to put it in perspective.

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I'll spin test it when I get home from Portland, OR.

__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

NAR# 94042
SAM# 0078

She is stable as designed. The aft end is really draggy with all the bits and pieces simulating the launch rails - built three with no complaints

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