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Online Motorman

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« on: November 15, 2017, 10:53:07 AM »
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« Last Edit: December 05, 2021, 09:20:08 AM by Motorman »
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Offline RandySmith

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Re: Foam Core Wing Center Joint
« Reply #1 on: November 15, 2017, 01:40:02 PM »
Quote from: Motorman on November 15, 2017, 10:53:07 AM
I'm looking at 2 fully balsa sheeted foam core wing halves for a full size stunt ship so, I have to glue them together. It's a butt joint that I'll make with epoxy after I get the bellcrank in. The spar is also butt jointed in the middle so, I guess it gets most of it's strength from the skin. My question is what size/shape/weight how much fiberglass cloth do I need over the skin at the center joint?

When I line up the wing with the trailing edge straight, the joint looks perfect from the trailing edge to the spar, then it starts to spread to a gap of 1/32" at the leading edge. Does that matter since it's getting glassed? I can either glue it together as is then fill the gap later or try to sand the joint and maybe mess it up, any advise on that?

Thanks,
Motorman 8)


How large is the wing ?  sq in  ?

Randy
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Offline Howard Rush

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Re: Foam Core Wing Center Joint
« Reply #2 on: November 15, 2017, 10:29:23 PM »
I’m no stress man, but here’s what I think. You don’t need much to hold the wings together except at the spar. The fuselage sides should get the loads to the spar. I’d add some spanwise pieces of carbon yarn at the spar that run between the fuselage sides and maybe some more yarn at the TE for good measure. Put some glass or carbon cloth between the fuselage and bellcrank axle so you can take a pull test.  Don’t worry about the gap at the LE. My first electric plane had the front 2” of wing cut out between the fuselage sides to accommodate a big battery. Our takeapart planes have nothing between the fuselage sides but three pins: one at the TE and one top and bottom at the spar. The wings fit into fillets on the fuselage sides.
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Offline john e. holliday

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Re: Foam Core Wing Questions
« Reply #3 on: November 17, 2017, 01:09:14 PM »
That is all you need.  When you put the covering on it will be even better.
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Offline Howard Rush

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Re: Foam Core Wing Questions
« Reply #4 on: November 17, 2017, 02:40:56 PM »
Quote from: Motorman on November 17, 2017, 12:32:51 PM
How do you put the tips on? There's nothing to glue them too except foam and a thin balsa shell.

I use balsa block tips.  Rough shape and hollow them, then glue them on with foam-friendly glue. Then put masking tape on the wing next to the tip. Sand the tip down to the tape, then remove the tape and finish sand. As Doc said, they’ll stay on.
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Offline phil c

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Re: Foam Core Wing Questions
« Reply #5 on: November 17, 2017, 05:22:01 PM »
Quote from: Motorman on November 15, 2017, 10:53:07 AM
I'm looking at 2 fully balsa sheeted foam core wing halves for a full size stunt ship so, I have to glue them together. It's a butt joint that I'll make with epoxy after I get the bellcrank in. The spar is also butt jointed in the middle so, I guess it gets most of it's strength from the skin. My question is what size/shape/weight how much fiberglass cloth do I need over the skin at the center joint?

When I line up the wing with the trailing edge straight, the joint looks perfect from the trailing edge to the spar, then it starts to spread to a gap of 1/32" at the leading edge. Does that matter since it's getting glassed? I can either glue it together as is then fill the gap later or try to sand the joint and maybe mess it up, any advise on that?

Thanks,
Motorman 8)

You probably want to put say a 1in. strip of glass/epoxy inside the fuselage to tie the wing and bellcrank mount to the sides better.  Howard's take-apart planes don't have cutouts in the sides, so they are much stronger.  They still probably need some reinforcement around the mount points to spread the loads. 
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Offline Howard Rush

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Re: Foam Core Wing Questions
« Reply #6 on: November 17, 2017, 11:59:38 PM »
Carbon mat has no strength, so will suffice for the seam where nothing is needed. Use something like glass and epoxy at the spar and bellcrank mount, though. 
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Offline Phil Krankowski

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Re: Foam Core Wing Questions
« Reply #7 on: November 20, 2017, 09:08:24 AM »
I would use a plywood biscuit or two to join the spars.  A couple grams in a very critical spot is not a loss.

Bias cut glass cloth, ideally, but glass cloth tape would be fine over the joint too.

Phil
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Offline Fredvon4

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Re: Foam Core Wing Questions
« Reply #8 on: November 20, 2017, 10:26:24 AM »
MM I built several Two part combat and stunt foam wings...no sheeting

set the straight LEs in the shuck and like you, some gap in front,  but only 1/32" +- a tad... tried sanding it away but made it worse so quit

since I over worried about not perfect flat 100% fit ----I added in a short 2" carbon rod in the area of the gap, quick spritz of H2o, and then used foaming Gorilla type glue with wide Tape to control the out flow of setting glue

The spars and center glassing are gonna carry most of the loads... the very center glue section foam to foam is good enough that on the birds I doinked,  the foam broke at a fail point a away from the glue joint ( all profiles fuses) .... in fact it broke outboard of the center 4" wide glass/epoxy  area
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Online Crist Rigotti

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Re: Foam Core Wing Questions
« Reply #9 on: December 14, 2017, 08:31:41 AM »
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Offline Avaiojet

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Re: Foam Core Wing Questions
« Reply #10 on: December 14, 2017, 09:53:41 AM »
Quote from: Crist Rigotti on December 14, 2017, 08:31:41 AM
Post #45 and 46.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/legacy-40-electric-a-build-log/msg385715/#msg385715

Chris,

I forgot all about that Build Thread you did. They do get written over.

Did you repair that fine model you built?

Charles
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Online Brett Buck

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Re: Foam Core Wing Questions
« Reply #11 on: February 03, 2018, 10:09:35 PM »
Quote from: Motorman on January 29, 2018, 03:13:41 PM
Ok Crist, I got 3 coats of 50/50 nitrate down on the center section and I cut out 6 circles but I used polyspan instead of silk span, you think it will be ok with the polyspan? I also made the circles an inch bigger, seemed to fit this wing better.

   What "circles" are you talking about?  Are you tripling up on the covering near the center section?

    Brett
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Offline Fredvon4

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Re: Foam Core Wing Questions
« Reply #12 on: February 04, 2018, 10:42:04 AM »
Brett

up at post #12 from Chris R he links to some build where he used circles of some materiel for center reinforcement instead of the typical glass cloth LE to TE full width like most of us do

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Online Crist Rigotti

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Re: Foam Core Wing Questions
« Reply #13 on: February 04, 2018, 11:46:16 AM »
Quote from: Motorman on January 29, 2018, 03:13:41 PM
Ok Crist, I got 3 coats of 50/50 nitrate down on the center section and I cut out 6 circles but I used polyspan instead of silk span, you think it will be ok with the polyspan? I also made the circles an inch bigger, seemed to fit this wing better.


MM

I really don't know because I don't use Polyspan. 
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Offline Lauri Malila

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Re: Foam Core Wing Questions
« Reply #14 on: February 05, 2018, 09:50:24 AM »
Why did you not use glass cloth instead?
Polyester tissue is a bad choise especially on the compression side. L

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Online Brett Buck

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Re: Foam Core Wing Questions
« Reply #15 on: February 05, 2018, 12:45:01 PM »
Quote from: Lauri Malila on February 05, 2018, 09:50:24 AM
Why did you not use glass cloth instead?
Polyester tissue is a bad choise especially on the compression side. L

   I think this is on the outside, so presumably, you want it to be thin so you don't use an ounce and a half of filler on the weave.

   But I am not sure why you would need to reinforce it in this way. Windy and company used to do something like this with fiberglass between the balsa and foam, but that was because they didn't silkspan the entire wing, just up to the fuse sides. Silkspan (or carbon) the entire wing from tip to tip, and it is unnecessary.

    Brett
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Offline RandySmith

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Re: Foam Core Wing Questions
« Reply #16 on: February 08, 2018, 09:48:31 PM »
Quote from: Motorman on November 17, 2017, 12:32:51 PM
How do you put the tips on? There's nothing to glue them too except foam and a thin balsa shell.

You can take some very light balsa thin 1/32   , 1/20 or 1/16th  Then glue it to the edges of the tips, you will have a larger area to glue to if you need that, it is not heavy, some people also glue the thin  punk balsa  to the  root of the 2 cores  also, if you do this at the  root, SQUARE the edges up so the  wing is  dead level  when joined
I use sandable aliphatic resin to do this, and  also to glue the tips on, it is very strong, take very little and is  light

Randy
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