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Old 12-29-2006, 09:26 PM
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CPMcGraw CPMcGraw is offline
Join Date: Mar 2005
Location: Mobile, Alabama
Posts: 5,357

Originally Posted by James Pierson
Craig, have you considered a swing test on the built prototype?

PS, Craig I played with the Andromeda design a little to simulate the CG back the 3 1/2" from original location. I change the engine hook mass to .75 oz. which moved the CG south about that distance. All flights and static margins still look good.

James Pierson
NAR# 77907

I may do that tomorrow instead of power-testing. As I was sitting here with the model, I played with additional weights on RockSim and even tried to reverse-engineer how much weight had been added in the construction to those fins (I don't have a scale to weigh the model on -- something I really should pick up...). I came up with a figure of about 1 oz overweight, which means the model is about 145% what it should be, and all of that is in the tail. Your estimate of 0.75 oz back at the motor hook to achieve the balance point I found sounds OK, as you moved the weight as far back as you could; I tried to estimate the weight center of the fins, which is a couple of inches ahead of your position.

Bottom line (like we need puns right now...) the model gained weight on these fins in the worst possible way. RockSim says the fins shouldn't weigh more than about 0.28 oz. Fat chance... (There he goes again with the puns...)

Here's what I did with the skins: After trimming out the open areas, I saturated the paper and the edges of the fins with thin CA. This "plasticized" the skins and hopefully bonded the paper across the balsa. Now, I realize even CA has a bit of weight, but it would have taken fully half a 2 oz bottle of CA to add this much weight. I can't see the paper skins adding that much, either, as there really isn't that much paper on those fins. When the CA had cured, I sanded the surfaces to smooth them out.

That leaves the weight of Titebond II for attaching the fins and shrouds as one possible player. I suppose being a resin it could have introduced additional weight. Another possibility is the database of RockSim components may be out of whack on the balsa weight. Probably all of the above...

It really means I will have to carefully keep the next prototype on a strict diet (There he goes again!) and get an electronic package scale to track the total blubber (aahhhrrrrggghhh...) that winds up on it.
Craig McGraw

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