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  #21  
Old 09-23-2007, 10:43 AM
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Bohica Bohica is offline
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Quote:
Originally Posted by barone
Hey Bo....distance between rings? Looks like an interesting build....


Wellll...let me measure it real quick....OK...heres a quick sketch...
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  #22  
Old 09-23-2007, 10:49 AM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by Bohica
Wellll...let me measure it real quick....OK...heres a quick sketch...


And I see I got the rings "close". All I did was match the ring location to the simulated shroud where they would touch.
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  #23  
Old 09-23-2007, 06:43 PM
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Quote:
Originally Posted by CPMcGraw
Your version using BT-20 simulated out to nearly 400'. Lighter design, but still highly stable. Both designs look like stable mates next to each other.

Like minds thinking alike...

Thanks for checking it for me. However, I'm curious... what engines did you simulate it with? I was intending to build it for 18mm... but I can see that the lower mass of 13mm engines might contribute positively to the stability.
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  #24  
Old 09-23-2007, 11:05 PM
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Bohica Bohica is offline
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Quote:
Originally Posted by Solomoriah
Thanks for checking it for me. However, I'm curious... what engines did you simulate it with? I was intending to build it for 18mm... but I can see that the lower mass of 13mm engines might contribute positively to the stability.


Now I have not put the components on a scale but bear with me for a second here...

If your bird is a T-20 using 18mm motors the difference in stability vice a 13mm motor in a T-20 tube should be negligible.

Now...assuming friction fit...For the 18mm version your gonna have the motor and motor block. For a 13mm version (still using a T-20) your not only gonna have the 13mm motor and motor block, your also gonna have 2 centering rings and a motor tube. I think the weight difference will be very small with a minimal effect on stability.

I have an 18mm version around here somewhere...I just can't find the freakin' thing.

Let me just grab the scale instead of guessing....

OK...13mm Estes A10 + 2 CR0520 + motor tube = .3oz

18mm Estes A8 + motor block = .6oz

Well....more difference than I expected but I believe .3oz is negligible in a bird this stable. Tell ya what...I'll find out the easy way tomorrow afternoon...I'll just throw one together and let er' rip.

Bo
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  #25  
Old 09-23-2007, 11:28 PM
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Quote:
Originally Posted by Solomoriah
Thanks for checking it for me. However, I'm curious... what engines did you simulate it with? I was intending to build it for 18mm... but I can see that the lower mass of 13mm engines might contribute positively to the stability.


I just revised the Spike (Solo's Nail) plan, adjusting the centering ring sizes to match the shroud diameter at those locations. See attached...

Motors are the A10-3T and A3-4T; the A3 is the better motor overall with a Dv of 8 FPS.

Margin with the A10 is 1.56, and with the A3 is 1.04. The shroud could be tweaked a bit more to get a higher margin.
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  #26  
Old 09-24-2007, 07:43 AM
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Hmm. Honestly, if it's that close, I'd just throw a few grams of noseweight in and call it good.

Thanks!
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  #27  
Old 05-18-2014, 03:43 PM
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Rich Holmes Rich Holmes is offline
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Bumping an old, old thread to show off my just-finished version of Bohica's Dead Ringer. I built it with an 18mm motor mount, because something down in my reptilian brain refused to believe it'd go far on a 13mm motor; reptilian brain is probably wrong as usual, but it wouldn't shut up so 18mm it was.
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