kit I just bought uses 2 1/4" piece to make up the 1/2" balsa fuselage. What is the best glue to use to glue the two pieces of 1/4" together?

I would use slow epoxy.

By virtue of the grain mismatch the fuselage will be more twist/flex resistant.  If you put some very light fiberglass or CF in-between it will be even more so.  I would use a thin slow cure epoxy (Z-Poxy for example) and weight it down overnight. 

Ken

Nothing really stiffens if you put it on the central axis. You’d be much better off to cover the fuselage with that cf or gf. L

I was telling people that right up until the glue dried on the first fuselage that I used the technique on.  If you took two identical pieces of wood, with identical grain, and you put them together that way -- probably you'd see no change.  The key is the slight difference in grain between the wood pieces -- that seems to make all the difference in the world.

If you really want to get the full benefit, use two 1/8" sheets on the outside, with 1/4" in the center, and angle the grain by 15 degrees or so -- that should give you considerable stiffness.  Just go really light on the glue or whatever performance increase you get from a stiffer fuselage will get tossed out the window because of extra weight.

If you are trying to stiffen a profile fuselage in the lateral bending mode, then putting a higher modulus material right down the centerline (on the bending neutral axis) is the least weight efficient and least effective way to do that. That is what Lauri is telling you. If you added something--anything--then sure, you gained a bit of stiffness and strength, but in the least advantageous way.

Adding the material down the center works pretty well for vertical bending resistance because the outer balsa laminations support it and tend to keep the laminate material from buckling--and some of the higher modulus material is out near the beam outer fiber, but who has vertical stiffness issues with a profile?

This type of construction adds to torsional stiffness but is harder to quantify and explain. In other words, it takes engineering language and equations for a composite beam, which is a pain to calculate by hand. (I would be concerned about shear separation at failing loads if this were more important than just a model airplane.) But again, heuristically, using the higher modulus material as the outer fiber of the structure is the only efficient way to do this.

If these concepts are not appreciated, then there really is no purpose in discussing fiber orientation or other design details needed to understand composite structures....

Dave

I'm a big fan of west systems epoxy for that kind of operation myself. Dave's correct, adding CF in between the balsa plies won't do much for the actual stiffness. A better way would be to put 1/2" caps on top and bottom. Even better would be to uses some super light fiberglass, .75 OZ/yd, on the sides. Glass has a similar stiffness to wood and would tend to not delaminate from buckling under load. Although adding carbon fiber definitely hits the coolness bragging rights factor up a notch. That alone makes it worth doing.

If you have two 1/4" sides to laminate, you have a golden opportunity to cut a trench and lay a CF tube in there.  That will stiffen things.

Ken

Build a full bodied fuselage.

There are many valid reasons why he might be building a profile, so not sure how this suggestion  helps
You might just as well have said "Buy a Yatsenko". Good advice perhaps, but completely off-point.

It's not off point. Everyone is giving advice about "stiffness."

Save some weight also.

It's not off point. Everyone is giving advice about "stiffness."

Save some weight also.

     Again, proof positive that your reading comprehension skills absolutely suck, or you are just fond of your own "voice" or both!

    Type at you later,
   Dan McEntee

In an X pattern.

Motorman

What size carbon tow are you using for your X reinforcement?  I have some 48K tow that I might try on a profile.

Hi Phillip,
I don’t think Phillip asked about carbon fiber, he was asking about gluing to pieces of 1/4” balsa together to form a fuselage.  This works perfectly with slow dry epoxy or Slightly thinned Tite Bond glue (hell, I would bet you could use Gorilla glue to get a good bond).   With the Tite Bond glue it has to set up under the weights a lot longer. 

Note:  Place the T Pins through the edges so as not to in the way when placing the second sheet in place.  Good Luck Phillip on building your model.

Mike Pratt

Gentlemen:

Back to some serious thread drift!  A SIG Skyray kit build for my Winter project, might want to stiffen the fuselage so suggestions needed. Recently, Ken has suggested youth fiberglass arrow shafts embedded top and bottom of half-inch fuse. Not sure how to interpret Lauri's ideas, but am intrigued.

What say ye all?

Dave Mo...

Dave,
The Skyray is good the way it is, it’s not that long of a fuse, unless you want to drive post holes.  Most of the crash’s I’ve seen the Skyray’s fuse were pretty much intact, the wing takes the beating for the most part.  A strip of CF down each side of the fuse can help but not really needed.  I’ve seem all kinds of mods to the Skyray 35 some mods work pretty good, and the others not so much, they just added more weight.  When I first designed the Skyray 35, it was to use up the short balsa & plywood that were in the saw room in the plant.

     I have to agree that the plywood wing is suboptimal. On the first significant crash with mine, just about every single plywood part was shattered into multiple pieces, I had a nice monokote bag full of scraps. The fuselage was just cracked lengthwise and the tail was perfectly intact. The balsa version I made to replace it was was *6 ounces* lighter, which only slightly improved the performance with the 20FP, but survived many crashes - at least 50 of varying severity - only finally requiring a serious repair after the monokote had turned to glass and shattered in a mild pancake crash.

    I agree the fuselage is perfectly fine as is, same with the tail surfaces. I recently passed my original model to the next generation. I plan on making a slightly modified replacement, the only consequential change being about 1.5" shorter tail moment, with a correspondingly larger stabilizer to get the same TVC. I have always been running into the pitch rate limit long before I got a stall, so I want to increase the max pitch rate slightly.

      Brett

   

How about some poetry?

This drift brings out the big guns
I'll build it as is...
So let it be written, so let it be done.

Thanks to all!

Dave Mo

  have to agree that the plywood wing is suboptimal. On the first significant crash with mine, just about every single plywood part was shattered into multiple pieces, I had a nice monokote bag full of scraps. The fuselage was just cracked lengthwise and the tail was perfectly intact. The balsa version I made to replace it was was *6 ounces* lighter, which only slightly improved the performance with the 20FP, but survived many crashes - at least 50 of varying severity - only finally requiring a serious repair after the monokote had turned to glass and shattered in a mild pancake crash.

    I agree the fuselage is perfectly fine as is, same with the tail surfaces. I recently passed my original model to the next generation. I plan on making a slightly modified replacement, the only consequential change being about 1.5" shorter tail moment, with a correspondingly larger stabilizer to get the same TVC. I have always been running into the pitch rate limit long before I got a stall, so I want to increase the max pitch rate slightly.

      Brett 

Brett:

What do you use for the control system?  3" or 4" belcrank?  Horn size?

The reason I ask is that I finally finished a bad-luck Peacemaker I started many, many years ago.  It has a stock control system and it flies awful.  I would like to build one that flies well (if that is possible).  It's a sentimental thing....

Thanks!

Brett:

What do you use for the control system?  3" or 4" belcrank?  Horn size?

   It's a 3" SIG bellcrank straight out of the kit, and a nylon CG "Large" RC control horn, in the second hole (maybe 3/4" from the hinge line). Stock 3/32 music wire pushrod, unbushed and crimped leadouts.

    The original intent was to build this kit (which I acquired to give me a guide to rebuild someone else's airplane - after I crashed it...) for my work buddies to fly at a picnic, and the expected lifespan was <<1 hour. So I did absolutely nothing other than cut out a new fuselage (which I had used in the other rebuild) and stick it together exactly how it came. Picnic never happened.

    Dixon 40FP, which I never even *ran* before I gave it to someone to use on their Twister, later Fox 35 (disaster, although I beat David in a contest with it), sat for 2-3 years,  L&J Fox 35 with bypass mod at least ran until it started shaking itself apart, next weekend OS 20FP/Circus Hobbies 9.5-4.5, miraculously better. This takes us up to *1996*....

    You get the idea, the notion that it would be flying with more-or-less stock moving parts after closing in on 30 years was the furthest thing from my mind.

     I can't see any compelling reason to use a 4" bellcrank, the control loads are nearly non-existent. It won't hurt anything, of course.

    Brett

Well from experience putting my Primary Force together,  Gorilla Glue is the way to go.  Make sure you have a roll of Parchment paper(found in the Baking section of the store) on both sides of the laminate on a very flat surface.  The Gorilla Glue will find every worm hole in the balsa and once it cures the excess can be sanded off and is the lightest glue I have found.  Draw back is the glue does not have a very long shelf life.  Also I think I would break the laminate before it bends.

is this the stuff that expands when it dries...like foam

Yes Gorilla glue expands. If you don't clamp the parts you'll have a balsa/foam sandwich.

The carpenters Gorilla glue doesn't expand like the regular Gorilla.  It acts just like Titebond only stronger.

Ken

Usually when I laminate between Balsa sheets I use epoxy and a top flite wood pecker tool

not sure who sells them any more Tower is a worthless site not for any building stuff

Ebay seller is out of his ever loving mind for one 
https://www.ebay.com/p/2255975934

A brief comment on the original gorilla glue, someone said it doesn't have a good shelf life, and that can be true, bur doesn't have to be.  I use a ton of it in the guitar building process and lose very little to shelf life.  The key is to squeze as much air out of the container as possible before capping.  It goes and goes and.....

Yeah, but Ambroid had "that smell"....

    The best smell in the world! And also, great for chewing off your fingers for the next few days. CA is definitely second-rate in that regard.

      Brett

  Ambroid, it brought out the best in my Mother:  "OPEN THE G.. D... Window".  How many kids do you know that got a window fan for Christmas?

So now we have CA and it brings out the best in my wife: "OPEN THE G.. D... Window".

Ken 

Ken: Good one!!!  When I was a kid I got in real trouble after I dropped a jar of clear dope on the floor (oh yeah, it broke!!)

A brief comment on the original gorilla glue, someone said it doesn't have a good shelf life, and that can be true, bur doesn't have to be.  I use a ton of it in the guitar building process and lose very little to shelf life.  The key is to squeze as much air out of the container as possible before capping.  It goes and goes and.....

I'm going to try that!  Storing it top down works too.  The glue hardens in the cap but you can clean that easily; the stuff in the bottle lasts much longer.