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Old Mar 09, 2012, 09:45 AM
G_T
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I wonder if a two-sided core could be solvent bonded. That would eliminate the weight gain.

Gerald
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Old Mar 09, 2012, 09:02 PM
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Tom,

Maybe I'm not getting my sanded cores close enough to the airfoil shape before using them. I've only been using HL 25, and it seems that is compressing in the molds fine, but maybe expanding a bit after it comes out of the mold. Different materiel perhaps, might work better.

Gerald,

How much additional drag might there be from a vertical shaped like the Polaris, as opposed to the double compound curves in some verticals?

Gary
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Old Mar 10, 2012, 01:07 PM
Father of Fr3aK, DLG Pilot
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If you're getting springback from your foam (not rare in XPS) then you probably need to get it closer to the finished size. I was able to take square 1/4" stock and press it into a tail shape, but over a couple of days time it sprang back quite a bit.

I start with a block cut to the outline shape of the surface. The thickness should be just a bit thicker than the finished part's thickest point. I then start by sanding from the high point in the airfoil to the center of the trailing edge, making the TE fairly sharp. Flip over and repeat. Now you should have the shape pretty close for the aft half of the airfoil. Now I gradually round down the forward part of the airfoil. All sanding is done with 80 grit on a foam block, then I finish sand with a piece of loose 220.
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Old Mar 11, 2012, 09:25 PM
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I just pulled a horizontal tail out of my molds (different from Fr3aK molds, tad less area). Layup was 2mm depron, cut to the outlines shape of the surface, then pressed into each mold half, vacuum bagged and rolled flat. Post bagging, I taped off the mold flange and used a sanding block to sand each mold half core flush. I then wetted each core out with Pro-Set, and filled in the LE gap (intentional) with a Pro-Set+West Systems 410 mixture. Squeeze mold halves together, hot box for 12 hours and out comes a part.

Skin layup was as follows:
1) 1.2oz glass 0-90
2) 12K tow each side

Weight: 8.9g pre-trimming, expecting 8.7g post-trimming. Heavy but its a start. Can probably pull 1g of epoxy out in the core joining step.

Quality: Superb for a first pull, clean LE, clean TE etc.

Stiffness: Umm, I don't like it! Its weak in torsion (expected), and not as good as my H60 tails in bending. A glass shear web could fix the later, and some disser might help the former. Really I think I need stiffer foam. I might be able to use 25 psi pink or blue foam in the molds, and really hard roll the stuff into the mold. I might give it a try with my H60 I have thin sliced, or some other white stuff that's thin sliced. Thoughts?

Will post a picture later.
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Old Mar 11, 2012, 11:42 PM
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Sam,

Haven't done it yet, so these may not even be appropriate:

1. Shear the fabric?
2. A Super Gee type spar with weblets?
3. Modifying the planform, ie. more squat, think Polaris like?
4. CF veil?

Were your HL60 tails bagged?

Gary
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Old Mar 12, 2012, 07:58 AM
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XPS bagged into the mold halves then hotwired and joined will produce usable tails but I could never get them under 7.5g each. Maybe you'll have better luck. I didn't persist too much.

Weblets help alot and IMHO are necessary in a DLG tail. Surface (cap) spars aren't enough for the harder throwers anymore unless you're using alot of carbon and have higher compressive strength foam. They seem to last a couple of contests but then you develop little stress cracks and eventually they crease. For me that usually comes on the first launch after a landout with a bit of breeze. When I add doublers, they just crack and crease at the end of the doubler... unless of course the doubler is half the span of the tail, then it's heavy.

Disser on the tails can help alot, but weight is a concern there. I'd rather just jump up to a 1.2 glass or even 1.6 than do a .75 with disser. The stiffness is about the same, weight is really very similar if you're careful, and you don't have to string the frame.
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Old Mar 12, 2012, 08:45 AM
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Gary,

1) Definitely going to try shearing the fabric, either 0.75oz or 1.2oz
2) I need to go look at how the weblets are done, I've never used them
3) Modifying the planform isn't an option, I already have the molds!
4) CF veil might be, I think I have 0.5oz veil which will likely be heavy and not as strong as sheared glass

My H60 tails were bagged and I can build a tailset under 13g. I haven't noticed the stress cracking issues Tom is discussing but I switched to solid carbon rods in my verticals and have been using a carbon d-box as well.

If I can get down below 7.5g for a bagged molded tail with H60 or H40 skins, then I will be a very happy camper. Right now I'm not seeing the light at the end of that tunnel, trying to pull nearly 2g out and switching to heavier foam at the same time...
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Old Mar 12, 2012, 08:52 AM
Father of Fr3aK, DLG Pilot
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Solid carbon rod = weblet, just shaped a bit differently

Flat weblets can be made by producing stock of 2-3 layers of carbon between waxed plates of glass. You can then just razor off thin strips and press them into slits in the foam. You can also use carbon rods in the slots in the foam, or even just splooge. Carbon is best for strength... rods for convenience, splooge for weight.

There are lots of ways to make good tails. Don't be afraid to experiment.
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Old Mar 12, 2012, 10:36 PM
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Thanks Tom for the encouragement! I'll try the rod spars next, if I can get something stiff enough to fly at sub 15g I'll be a happy camper.
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Old Mar 13, 2012, 04:18 PM
G_T
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Quote:
Originally Posted by GaryO View Post
Tom,

...

Gerald,

How much additional drag might there be from a vertical shaped like the Polaris, as opposed to the double compound curves in some verticals?

Gary
Drag "should be" higher during launch but possibly a touch lower in straight line flight. The straight line portion can be readily computed once one has the designs. Once one knows the drag tradeoffs then one could compute the result over the whole assumed flight and then make appropriate design decisions.

However...

Verticals are sized for their effectiveness during launch. It is hard to design equally effective verticals. The software we have (affordable software, that is) isn't capable of modeling the dynamic behavior during launch. Change the shape, change the results. Then one would resize the vertical as needed. THEN one can see what happens to the drag in straight line flight and in normal turns.

It is not a trivial problem.

Gerald
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Old Mar 19, 2012, 08:15 AM
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Saw a hollow molded, lightweight vertical this past weekend. I think that method is very doable and cost effective. Won't work on the horizontals I don't think. Depron will work there structurally and with some effort the weight can be brought down in the sub 8g range, which is decent but not sub 6g like the milled solid cored tails.

I pulled a vertical and a horizontal out this weekend, both are overweight (9.6g and 8.2g respectively), and even worse the Depron at the TE wasn't sanded enough so there is a definitive "bubble" in that section.
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Old Mar 19, 2012, 09:49 AM
Father of Fr3aK, DLG Pilot
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My horizontals with Depron cores are usually coming out about 6.5g using milled cores (2.6g core) and about 7g using hand shaped cores (3.0-3.1g core)

Rohacell 31IGF is only about a tenth or two different.

I have never been able to get usable hollow molded tails in the past. I'm gonna have to snoop his methods and see what I'm doing wrong.
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Old Mar 20, 2012, 12:14 PM
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Aren't the Blaster 2 tails hollow molded? Who are y'all referring to?
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Old Mar 21, 2012, 07:03 AM
Father of Fr3aK, DLG Pilot
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Phil made some hollow molded tails in a set of Fr3aK molds.

The blasters are solid core, like most of the other DLG tails.
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Old Mar 21, 2012, 09:19 PM
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Tom,

What about "cellfoam" for tails? better or worse than Depron?

Gary
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