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Aug 14, 2019, 09:35 AM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Question

Fuselage master construction for 2.5m aerobatics glider


Hello all - I'm taking my first foray into composite mold fabrication and have a question. I intend to create a fuselage master for a 2.5m aerobatics glider. The fuselage is 1.5m (60") long and about 8" tall and roughly 2" wide. It's a large fuselage, in other words.

My intention is to use a plywood keel of around 1/8" thickness down the center of the plug, with bulkheads at stations along the length to provide guidance for shape and width. In between these bulkheads, and laminated to the plywood keel, I will affix either blue foam or balsa blocks (depending on what I can source locally) with the plan to shape these to suit. Once complete, I will glass the entire master, and ultimately (after what I'm assured will be a lot of sanding, filling, polishing and similar) produce a part sufficient for creating a mold from.

Is this a reasonable way to approach a 1.5 long, relatively large fuselage master?

Should I be considering a thicker plywood than 1/8" for the center keel?

My main concern is obviously avoiding bends and twists during the shaping process.

Thanks in advance for any advice and suggestions. I've been kicking this project around for years, and always been hung up on this first step. I'm ready to take the leap, but I'm just seeking a small sanity check first.

Here's an example of what the plane which inspired this project is capable of:
Excalibur VTPR Slope Aerobatics at the Ménez-Hom (3 min 46 sec)


Here's the "too much talk not enough action" project thread in the Slope forum:
https://www.rcgroups.com/forums/show...Glider-Project

Thanks in advance,
Steve
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Aug 14, 2019, 05:16 PM
Entropy is happening!
Jim.Thompson's Avatar

Plug construction.


The choice is somewhat personal and subjective; there is no text book way to make a fuselage master (plug by another name).
But you no doubt already know this...................

The plywood keel with sub half formers and spacers filled in would work, but I would worry about keeping it straight during the assembly and shaping/covering stages. Plywood bends easily, especially when you start gluing anything to it.
What would I do?
I would use a solid piece of suitable well seasoned (preferably recycled) close grained timber and shape it out of that. I have used Meranti which is ok for this, but there would be any number of timbers types that might be suitable.
A variant on this method is to laminate up thin strips of timber (or MDF later stabilised) to provide a guide line for symmetry during later shaping.
Start by making two templates; one for the plan view and one for the elevation (side view). Then mark the dressed rectangular section of wood top and side.
Cut the side view out with a bandsaw.
Fix the off cuts back in place using hot glue or tape or some other method.
Then cut the top view symmetrical shape.
The rounding of the plug can be marked out first to 45 deg. lines and then planed down to the marks. I use a power planer then a hand plane.
If you are still with me and have not ruled this method out completely, I will explain further (if necessary).
Aug 14, 2019, 05:43 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Yes, the plan and side view method is what’s used to cut out EPP fuselages, something I actually have a little experience with. Makes shaping a lot easier.

Another thought that occurred, since this is a master, would be to epoxy a piece aluminum angle bracket along the length of the plywood (making sure to do this bonding on a perfectly flat surface). That would seem to help stabilize the plywood in the nose to tail dimension, and I’m not as worried about the belly to canopy dimension. The foam would need to be relieved to fit over the aluminum, but that wouldn’t be too hard I expect.
Aug 14, 2019, 05:52 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by surfimp
Y......................... a piece aluminum angle bracket along the length of the plywood (making sure to do this bonding on a perfectly flat surface). .............................
The fin would be the most concern in terms of trueness.
How would you treat that? It is too thin to accommodate ali strips as above.
Aug 14, 2019, 06:17 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Quote:
Originally Posted by Jim.Thompson
The fin would be the most concern in terms of trueness.
How would you treat that? It is too thin to accommodate ali strips as above.
The location where the rudder post will be glued - along the rudder hinge line - will need to be molded with an opening, so I'd plan to face that area with another piece of plywood shaped to fit the opening, and epoxied in position butted up to the end of the keel. The foam would be shaped down to match the size of that piece of plywood on either side, and care would be taken to ensure the keel was perfectly flat / unbowed when the rudder post facing was laminated to it.

When viewed from above, the main keel and rudder facing would meet in a T intersection.

Does that make sense?

This build describes a pretty similar process as what I'm describing for the master, with the exception of using a horizontal reinforcement rather than a vertical: https://sites.google.com/site/pierri...eurs/oxid-vtpr

Seemed to work out OK for that builder!

The main challenge now seems to be finding a piece of 60" plywood 1/8" thick that is reasonably flat.
Aug 14, 2019, 07:03 PM
Entropy is happening!
Jim.Thompson's Avatar
Ok Steve, you seem to have it worked out well enough.
What do you need from us readers?

Jim.
Aug 14, 2019, 08:33 PM
Registered User
Ward Hagaman's Avatar
Steve, even 1/8" thick steel wouldn't necessarily be flat...just too thin. You need a 3 dimensional shape to be straight in both directions I think. How about an aluminum extrusion with a "T" shape? You could cut a slot into your foam and bury the "standey out" part.
Aug 14, 2019, 10:57 PM
Themadartist

Another proven method ...


Steve,

If you haven't already seen this, have a look at Kev's Afterburner build - particularly videos 1 and 2 with regards to fuselage plug construction.

Cheers, Steve.

https://www.rcgroups.com/forums/show...70&postcount=1
Aug 15, 2019, 06:35 PM
supreme being of leisure
ZAGNUT's Avatar
glass over foam sounds like a real PITA to me...actually glass over anything else as well. you'll need super human body shop skills to get anywhere near symmetrical.

epoxy soaked MDF is easy to make straight, true, and solid... especially if you have access to a table saw.
Aug 15, 2019, 06:37 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by zagnut
glass over foam sounds like a real pita to me...actually glass over anything else as well. You'll need super human body shop skills to get anywhere near symmetrical.

Epoxy soaked mdf is easy to make straight, true, and solid... Especially if you have access to a table saw.
+1!
Aug 19, 2019, 09:15 AM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Met up with a friend on the weekend who'd created a MDF master and can concur with what's been written above - looks like a great solution.

Thanks very much to all of you for the input, it's most appreciated!

Steve
Aug 20, 2019, 03:16 PM
Registered User
surfimp,

I used that exact method for a 2 meter slope racer I designed and built back in the 80's. I used 1/8 lite ply for the spine and formers, and added balsa between the formers. I carved and sanded to a final shape covered with light fiberglass/epoxy, filled and painted. As Jim Thompson mentioned, maintaining straightness is critical. I have no recollection of just how I accomplished that, but I'm pretty anal about things like that and the plug did come out quite straight. This fuse was about 41" long. I no longer have the plug, but here is a pic of the mold.
Sep 10, 2019, 12:45 AM
UAS Test Pilot
SpeedMaster's Avatar
For the fin, you can laminate the plywood with carbon on bias for both sides, keep it weighted down to something straight while it hardens. For the rest of the fuselage, aluminium square tubing is one solution.


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