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May 19, 2019, 01:25 PM
Registered User
Thread OP
Discussion

Fuselage longerons


Many fuselages have triangle stock longerons where the sides meet the bottom.

Forgive me for being pedantic:

In this case, the fuse side sits on "top" of the bottom of the fuse. If you imagine the cross section as an "L" shape, the bottom of the fuse forms the entire bottom of the "L" and the fuse side is the vertical part of the "L" but doesn't extend through the bottom of the "L".

The longeron in question goes where the vertical and bottom meet.

My question is, do you glue the triangle stock to the side of the fuse and then glue that as an assembly to the bottom or?

My worry is that I won't perfectly match the triangle stock face with the edge of the side of the fuse and I will have a gap when I go to assemble the side with triangle stock to the bottom.

Thanks!
Harjit
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May 19, 2019, 01:39 PM
Registered User
In order to resolve your concern glue the fuselage bottom and side together first if there are no instructions to the contrary. I would use Titebond and scrape the inside corner clear of any glue squeeze out. That will let you get the triangle stock (more of a filler than a longeron) tight into the corner. When you sand, you can sand until you have a radius that exposes a gair amount of the outside corner of the triangle stock for a nice rounded corner. Be sure to use plenty of good right angle supports or drawing triangles to make sure the side is precisely 90° to the bottom.

An alternative is to glue the triangle to the side and sand flush before gluing the side on.
May 19, 2019, 05:01 PM
Registered User
I think most of the time the sides and bottom are glued together with formers installed before the triangular stock is glued in. It is there to allow a nice rounded edge. A little filler, if you sand through, will fill any gaps.
May 19, 2019, 07:13 PM
Registered User
I've built a few that have the corners of the bulkheads chamfered to clear the triangle, too. Maybe he can let that help figure it out.

Harjit, what are you building? Chances are, someone here has already built one
May 19, 2019, 07:27 PM
Registered User
Quote:
Originally Posted by David2011
I've built a few that have the corners of the bulkheads chamfered to clear the triangle, too.
Yes, I have seen that also. I have also seen square longerons in the corners on some models. I think it would all depend on the building sequence and the plan. Gluing them to the sides and then sanding flat before gluing the bottom on should give a good edge. Some bottoms are cross grain and that might make a difference also. Wow, this just got complicated.
May 19, 2019, 08:30 PM
Registered User
DGrant's Avatar
Quote:
Originally Posted by Harjit
Many fuselages have triangle stock longerons where the sides meet the bottom.

Forgive me for being pedantic:

In this case, the fuse side sits on "top" of the bottom of the fuse. If you imagine the cross section as an "L" shape, the bottom of the fuse forms the entire bottom of the "L" and the fuse side is the vertical part of the "L" but doesn't extend through the bottom of the "L".

The longeron in question goes where the vertical and bottom meet.

My question is, do you glue the triangle stock to the side of the fuse and then glue that as an assembly to the bottom or?

My worry is that I won't perfectly match the triangle stock face with the edge of the side of the fuse and I will have a gap when I go to assemble the side with triangle stock to the bottom.

Thanks!
Harjit
To answer your question, generally what I've seen is the tri-stock is glued to the fusalage side when it's being prepped, and the bulkheads are generally corner cut to clear the tri-stock... Of course it does depend on the design, but by gluing the tri-stock to the fuse sides first, it serves the purpose of giving a larger surface to attach the bottom(or top) sheeting onto the fuselage.

Now... to answer your worry... Everything, every joint, every glue surface you apply glue too should be trued up with a block sander. You should be using a block sander on everything and everywhere when framing. You won't get anything perfect without it.

Generally speaking, the fuselage sides are prepped and joined with the bulkheads(if there's tri-stock involved that goes on the fuse sides before joining the sides)... Once the side are joined(usually starting with the bulkheads located at the wing-leading and trailing edge locations).. then you have the start of the fuselage "box".

Then the rest of the joining takes place after that center area is good and dry. Working over a plan is about the only way to get it true... and you don't want to end up with a banana fuselage... which is why you start with that center area first. From there, you'll probably join the firewall, and then the tail of the fuselage... and that will include installing any other bulkheads that are called for....

Now.... You've got a fuselage, with the sides joined(and tri-stock already in place if it's called for).... Now..... BLOCK SAND the top and bottom of that assembly... using a bar/block sander with an appropriate grit... sanding both right/left sides of the fuselage assembly at once. That's how you will get that true, and ready to accept the sheeting.... If there's tri-stock it will get sanded true right along with the rest of it... and it will all make one nice flat surface ready for sheeting.

This above is a quick hypothetical example, but in general is how it's done for many/most balsa/ply type of airframes, regardless of size. A block sander is your friend... it's your best friend. I don't think there's any surfaces on any of my planes that don't get touched by my sanding bars.

I used the term "block sander", but reality is it's really sanding bars.. that's the key... I love my aluminum bars, and have several I keep a few different grades of sandpaper on, depending on what I'm doing, and how fast/aggressive I need for the application.

I hope this makes sense. It's just the tip of the iceberg, but it's something I've learned over years of building these things.

So what kind of plane are you building?? There's a really good chance some of us have built it if it's a mainstream kit... matter of fact, there's probably a 100% chance someone's built it if it is, or was a mainstream kit at some point.
May 20, 2019, 01:27 PM
Registered User
Thread OP
Thank you everyone!

I'm looking into building a glow powered, Jetfire 20. Something aerobatic but not too hot.

I had a similar plane but the battery died and we augured it in to tall grass and never found it. It was a low stress, fun plane to fly, so am looking for something similar.

All the best.
May 21, 2019, 08:23 PM
Owner of CFC Graphics
This model has triangle stock top and bottom.

It was applied to each fuselage side before assembling to the formers.

A thin saw blade, close cuts, helps on curves at the nose.

I built three of these 55" models before moving up to 64" pattern ships.
May 22, 2019, 08:28 AM
Sagitta Fanboy
Quote:
Originally Posted by Harjit
Thank you everyone!

I'm looking into building a glow powered, Jetfire 20. Something aerobatic but not too hot.

I had a similar plane but the battery died and we augured it in to tall grass and never found it. It was a low stress, fun plane to fly, so am looking for something similar.

All the best.
Make your life easy, head over to Manzano Laser Works and get the Short kit done by Andy Kunz. It's an update to the original design and a dream to build.

http://manzanolaser.com/Jetfire-20--...er_p_3398.html

You can see Gary Wright's and my builds of it in the Jetfire 20 thread in this forum, Gary's been flying his for a while, mine will hopefully fly in the next little while (I have a heck of a time finding bench time). While both of ours are electric, it comes ready for either electric or glow.
May 22, 2019, 01:10 PM
B for Bruce
BMatthews's Avatar
Harjit, some great advice in all this.

My own take on it is that when I build a model with the triangle stock like this to allow better shaping I glue the tri stock to the sides first. I aim for a nice flush fit of the outer faces to the edges of the sides. I then build the sides and formers up first on a building board. No top or bottom at that point. And depending on the shape of things I assemble the sides and bulkheads right side up or upside down. Then I'll dress the glue faces for the top or bottom sheet that is facing upwards with some sandpaper to even things up and assure a nice thin and clean easily sanded glue joint. Then the sheeting goes on. All of this while still secured to the building board so the trueness is locked in...... er..... be sure you remove ALL of the pins or other jigging from the insides first.... Don't ask......

With the fuselage alignment well locked in by having the three sides all in one piece lift it off, sand the other faces of the sides and tri stock and fit any internal stuff that needs fitting and glue on the other sheeting or blocks.

It really does help if you use glues for these tri stock joints that like being sanded. CA and aliphatic wood glue don't really like to be sanded and some special care such as fresh sharp garnet or the new high performance sandpaper along with a light touch and a good sense of feeling in your finger tips to dress down the resulting hard edges is a must.

Recently I've been using more Duco and Sigment on such joints. It requires a bit more patience to use it and wait between steps. But it pays off big time during the shaping and sanding by sanding fair with the wood like it's not even there. Makes the carving and shaping a true joy instead of an attention hogging exercise. And while it DOES take a lot longer to dry than some other options there's always some other part to work on, right? So with just a little planning to work around the drying time it need not slow things down much and maybe not at all.


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