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Old Mar 01, 2010, 08:19 PM
kids ate my heli budget
jameotron's Avatar
Auckland, New Zealand
Joined Feb 2005
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Build Log
The Spatula - original scratch built TWF PNF madness

Hi everyone.

This is something I have been slowly working on for the last few months, I haven't gotten very far, but kind of the whole idea is to teach myself new skills as I go along.

I want to build it fully-moulded, which is way beyond my skill level (kind of the point). I have some 180 gsm carbon/kevlar cloth and 200 gsm glass and 50 gsm glass also.

Details:
  • 1400mm (55") span
  • 1100mm (43") length
  • 7% RG14 wing profile
  • SD8020 tail profile
  • TWF both front and back

There has been some discussion on rcsoaring.co.nz also.
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Old Mar 01, 2010, 08:34 PM
kids ate my heli budget
jameotron's Avatar
Auckland, New Zealand
Joined Feb 2005
1,782 Posts
sudo sand_it_round

Half an hour of fiddling with intersects in Sketchup and I had tab and slot pattern to cut out. I designed it with 2mm material, but the best I could get my hands on was 3mm MDF, so I had to revise the design.

I assembled the templates and laminated them with balsa, before proceeding to rough sanding, spackling and fine sanding. Then both halves were glued together and spackled and sanded again.

I'm ready for paint, but that has to wait for a couple of weeks
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Old Mar 01, 2010, 10:08 PM
just CHUCK it!
racingodds's Avatar
Ramona Ca.
Joined Jun 2003
2,091 Posts
Veaaaaady interesting!!!!!!! Nice concept!
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Old Mar 02, 2010, 12:44 AM
Family, wind, snow & fun
Mtntop's Avatar
Osburn Idaho
Joined May 2006
445 Posts
Interesting design. The TWF design on a fuse like this will probably make it slower than most other TWFs though.
I also like the horizontal lamination instead of verticle... helps with symetry left to right.
Anything TWF is cool.
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Old Mar 02, 2010, 08:48 AM
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Ward Hagaman's Avatar
San Diego
Joined Mar 2003
2,281 Posts
Yes, VERY interesting!

Are you planning pull-pull controls on the wings? The arms are fairly short. Are you limited by the fuse height, or could you extend them a bit?

How are you actuating the twisty tails?
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Old Mar 02, 2010, 01:02 PM
Vitruvian JART
Reed's Avatar
San Francisco
Joined Mar 2003
5,019 Posts
Nice concept and a very pretty plane.

How do you like Sketchup? Had you used other 3D programs before diving into this one?
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Old Mar 02, 2010, 03:41 PM
Firecracker!
BillO's Avatar
San Francisco & Santa Cruz
Joined Oct 2004
2,349 Posts
I used the laminated balsa technique for a plug myself, but everyone said to glass the balsa with at least one layer of 3 oz cloth. That's what I did. If you don't glass it, it is said that the plug won't be stable enough for the molding process...
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Old Mar 02, 2010, 03:53 PM
kids ate my heli budget
jameotron's Avatar
Auckland, New Zealand
Joined Feb 2005
1,782 Posts
Thanks for the feedback guys.

Quote:
Originally Posted by Mtntop
The TWF design on a fuse like this will probably make it slower than most other TWFs though.
You're right of course, but I have started to think that a lot of the planes (especially TWF) are starting to all look a bit the same these days and the planes I admire the most are ones that look a bit different (Jart, Talon, etc) so I thought it might be interesting to see how much difference it really makes to trade some of that performance for a slightly different look.

Quote:
Originally Posted by Ward Hagaman
Are you planning pull-pull controls on the wings? The arms are fairly short. Are you limited by the fuse height, or could you extend them a bit?
That was the idea, but I'm going to revise the design with input from the many excellent amateur engineers here on this forum and of course the fuse that ultimately pops out of the moulds at the other end.
There's not a lot of available vertical space within the fuse even with making the wing roots the widest point, so I'm not sure how I'm going to pull it off. I had started to think that I might be better of trying to squeeze in a couple of larger and more torquey servos in there and try for some sort of torque tube setup.

Quote:
Originally Posted by Ward Hagaman
How are you actuating the twisty tails?
I was thinking of using carbon control rods and a torque rod system with carbon rods extending into the stabs, but again any feedback I can get would be gratefully received. I also have to figure out how I should part the tail so that I can have a removable piece for installing the mechanism.

Quote:
Originally Posted by Reed
Nice concept and a very pretty plane.
Thanks Reed, that means a lot to me as the Jart is one of my favourite looking planes of all time.

Quote:
Originally Posted by Reed
How do you like Sketchup? Had you used other 3D programs before diving into this one?
Because I use a mac it's pretty hard to get my hands on anything that doesn't cost me a fortune so I make do with Sketchup (and QCad for 2D on an Ubuntu VM). Sketchup is pretty great for quickly throwing together a model (although there are some problems with creating complex surfaces, lack of arcs and splines, etc). It can be a real PITA when it comes to trying to take your model and part it out though.

Quote:
Originally Posted by BillO
I used the laminated balsa technique for a plug myself, but everyone said to glass the balsa with at least one layer of 3 oz cloth. That's what I did. If you don't glass it, it is said that the plug won't be stable enough for the molding process...
Thanks for that Bill. I was planning on just doing ultracal moulds because I'm not making many parts - do you still think I need to glass it? I need to figure out how to convert 3 oz (is that per foot?) to gsm (grammes per square meter). I have some 50gsm glass which would give a nice smooth surface, would that be strong enough? Would I have to vacuum bag the plug or just brush on, cure and sand?
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Old Mar 02, 2010, 05:05 PM
Twisted and Confused
flyonline's Avatar
Joined May 2003
4,296 Posts
Metric to imperial - as far as I know, unless stated imperial weights are in oz/sq yrd so:
25 (gram / (m^2)) = 0.737338117 oz / (yd^2) or your cloth = 1.5oz.

Twisty tails! I'm thinking you might be making a whole heap of trouble for yourself. Why do you want twisty tails? The roll rate will be high enough anyway - I ran an f15 that I had split tails coupled to the ailerons and while it did increase the roll rate it wasn't really worth it in the end.

Just brush the resin on your plug, lay the cloth as smoothly as possible and make sure it's completely wet. Sand, fill, paint, sand etc. until smooth.

Steve
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Old Mar 02, 2010, 05:09 PM
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Ward Hagaman's Avatar
San Diego
Joined Mar 2003
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You could pivot the slotted cam ahead of the wing pivot, similar to the Rotor, Samurai, Shrike, ect. That allows for more angular travel of the cam, and you can use more of the servo's available travel. I would suggest the same concept for the tail, but it is going to be a little trickier in that small area. Someone once put up a design for an angled slot in a blade which fit into a box-shaped rectangle with a window in it-here is a link: http://images.google.com/imgres?imgu...26tbs%3Disch:1 maybe this would fit into a tail easier?
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Old Mar 02, 2010, 05:16 PM
Twisted and Confused
flyonline's Avatar
Joined May 2003
4,296 Posts
You could use something along the lines of these bellcranks that I used for either the wings or the tails. No slop and they don't require a slot in the cam to be cut. These clamp down on the pin so they don't need a collar either, but the pins can be held by a collar instead if you can't machine the clamp.

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Old Mar 02, 2010, 05:54 PM
kids ate my heli budget
jameotron's Avatar
Auckland, New Zealand
Joined Feb 2005
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Thanks guys. In my profession I've learned the hard way not to say things like "how hard can it be?" but apparently I'm still game for crazy stuff when it comes to my hobby. I've been thinking about this a lot, and I've dealt with a lot more monkey motion in the past (anyone who has ever set up a flybar on a helicopter knows my pain) and I'm sure there are going to be good solutions to the problems.

My main concern about the tail is adding a lot of weight up the back with mechanics that then need a lot of weight up the front to balance out. Original design calls for a 5mm carbon tube as the pivot for each tail stab and I was wondering what's wrong with epoxying a horn (maybe push/pull) to the end of each within the fuse? A small nylon bolt through the stab could hold it on. How much force is likely to be exerted on the tails? Each tail stab is 211cm2 surface area (each side, obviously).

Hardware up the front is a little easier, but I'm worried about bending stress on the pin, and thinking that the cam may have to be a little more "3D" to move the pin at the wing root but have torque applied closer to the centre of the fuselage. I'm pretty good with the lasers, so I'm sure between us we can come up with something.
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Old Mar 02, 2010, 07:34 PM
Firecracker!
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San Francisco & Santa Cruz
Joined Oct 2004
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OK, I didn't realize you were going to use ultracal for the mold. I did one of those too, with a plug made of blue foam. Even pulling a plug out of a plaster mold stresses the plug. I'm wondering if the spackle will just pull right off the plug, leaving paint stuck to the plaster. Is that a water-based spackle like is used on EPP foam? I don't think that is very strong.

IMHO you would be better off rolling some epoxy over the plug to fill the grain and make it waterproof. You probably don't need any glass, but epoxy is going to stick to the wood much better than spackle.
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Old Mar 02, 2010, 07:56 PM
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San Diego
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It just occured to me that any drive pin would have to have its "smiley" slot run out way into the beautiful wing fillets. Are you familiar with Winchdoc hardware?

Ward
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Old Mar 02, 2010, 09:14 PM
kids ate my heli budget
jameotron's Avatar
Auckland, New Zealand
Joined Feb 2005
1,782 Posts
Quote:
Originally Posted by BillO
OK, I didn't realize you were going to use ultracal for the mold. I did one of those too, with a plug made of blue foam. Even pulling a plug out of a plaster mold stresses the plug. I'm wondering if the spackle will just pull right off the plug, leaving paint stuck to the plaster. Is that a water-based spackle like is used on EPP foam? I don't think that is very strong.

IMHO you would be better off rolling some epoxy over the plug to fill the grain and make it waterproof. You probably don't need any glass, but epoxy is going to stick to the wood much better than spackle.
You're right that it's just water based spackle. I think you might be right that I should glass it. I was starting to think that if I lay up two layers of 50gsm glass then I should get a nice smooth finish and a good seal before I paint.

Quote:
Originally Posted by Ward Hagaman
It just occured to me that any drive pin would have to have its "smiley" slot run out way into the beautiful wing fillets. Are you familiar with Winchdoc hardware?
You're right. Current design only gives +/- ~6 degrees pitch on the main wings. The torque rod idea is really starting to work for me. I've seen photos of the Winchdoc hardware, but you still need a guide pin, right?
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