Carbon Fiber Door Construction Using 3D-Printed Joints

Source: 2025Q2 Beartracks, Matt Clark
I wanted one-piece gull-wing style doors similar to what Virgil did on his BH5. I considered welding the window and door sections together or building from scratch out of steel, aluminum, or carbon fiber. I wanted to use square tubing to make attaching the plexiglass and sealing the edges easier. I’m better with composites than I am with TIG but my initial problem with the carbon fiber idea was how to make the corners strong.
When looking at carbon fiber tubing online I found some manufacturers sold corner inserts at normal angles like 45 and 90 to make strong epoxied joints. So, I figured I could 3D print corner inserts like those at whatever angles I needed and then wrap the corners with another layer of carbon fiber to make a very strong joint. I used 3/4 square 0.045 wall thickness CF tubing (Dragonplate from Allred co.) and 3D printed corner inserts using ASA-CF filament. The ASA is good with epoxy and the CF infusion makes it stiff and dimensionally stable. I created a 3D model in Solidworks for the corner insert and printed a few (many!) prototypes in cheap PLA or PETG. The top two corners are the easiest because those 3 pieces (top, upper front, and rear) are in the same plane. I measured each corner angle and printed those top 2 prototypes first. I had to print a few versions to get the angles just right. To get the angles perfect on the CF tubes I cut with an abrasive wheel and then sanded to fit.
1. Prototype Corner Insert:

2. Checking Fit

3. Fits Good

Once I was happy with the fit, I printed the corners out of the ASA-CF. I had to adjust the dimensions of the CAD drawings to get the ASA-CF part to fit snugly compared to the PLA or PETG. I epoxied those top 2 corners in place by clamping them into the door openings with approximately a 3/16 gap all around.
Top 3 Pieces and Hinges Tacked:

With those 2 corners secure I next worked on the hinges using 3/8” long sections of 3/16”ID stainless tubing on either side of the fuselage hinge tube very similar to the window hinge design. The stainless barrels were epoxy tacked in place. I used a clear 3/16 PETG plastic rod through all the stainless to keep them aligned during the glue up. I did have to adjust the hinges on the frame to provide more standoff to clear the square tubing and reinforcement CF (two photos below). I cut the outer welds, bent the barrels down slightly and then re-welded. I used a 3/16 solid brass rod through both hinges to keep them aligned while adjusting.
Hinge Outer Welds Cut:

Hinges Adjusted and Welded:

Once the SS hinge pieces were tacked in place on the top CF tube, I used epoxy with micro bubble filler to smooth the hinges out before wrapping them with carbon fiber.

7. Micro filler epoxy:

8. Sanded Smooth and Painted:

9. Unwrapping Hinges:

To wrap the hinges, I used 3 wraps of 3k carbon fiber, peel ply, and flow material. I then tightly wrapped the whole joint with electrical tape working from the middle out and let it set for the whole curing process under a heat blanket. The electrical tape wrap is a good way squeeze the excess resin out into the peel ply and flow material for a part that is difficult to vacuum bag. And the vinyl tape doesn’t stick to epoxy. The lower doors are a little more complicated as the front lower piece is curved and creates a compound angle at the 2 lower joints. For the curved lower pieces I clecoed a piece of .025 aluminum to the lower front door former and used that to trace the curve onto a piece of ¾” MDF. I cut kerfs into the inside of that piece without cutting through the 4th wall and bent it around the MDF form to glue the kerfs shut. Then I wrapped and vacuum bagged those pieces.

10. Lower front piece bent around form. The cuts are on the gold lines through only 3 walls:

11. Vacuum bagged lower pieces:

The lower corners have to allow for a slight twist to allow the square tube to be parallel and flush to the bottom former, so one leg of the bottom corner inserts is round to allow the bottom tube to be on a slightly different plane than the lower front. The round section isn’t as strong because it’s less surface area to epoxy, but that lower front corner will get a lot more reinforcement to mount the door handles there.

12. Final corner inserts for the lower 3 joints in ASA-CF:

13. Epoxy for lower corners

Once I had it all aligned perfectly in the door opening, I epoxied all the corners with the doors clamped into the openings just like I had done the top three pieces.

14. Curing in place within door formers:

15. Corners reinforced:

After the inserts cured, I reinforced each corner with an external wrap of carbon fiber using the same technique as the hinges, with the electrical tape compression. Once I figure out where exactly to mount the gas strut to the front upper tube, I’ll add 1 more wrap from the front hinge all the way to the gas strut attachment. To seal the front, bottom, and rear edges I’ll wet layup carbon and Kevlar around the edges just like Virgil did. Those pieces will extend ½” inside the frame to provide the lip for securing the window pane. This will also strengthen and stiffen the whole assemblies. To seal the top, weatherstrip will get squeezed between the top tube of the door and the fuselage tube as it closes. The lower front corners will get reinforced further to mount the door handles which will be the same as Virgil’s with the Z linkage to pin the front and rear. I’m planning to use a single pane of 3/32 Lexan for the windows. My door frames weigh 0lb 13oz compared to 4lb 14oz for the steel frames. Yes, there is more weight to add with the handles, windows, seals etc, but all those things get added to the steel frames too.

Making the Rear Left Window Removeable

Source: 2022 Q1 Beartracks, Jay Townsend
Firstly let me say I am not a “handy DIY” type of guy. I undertook this project as a complete novice.
There are probably easier planes to build, but none had the flying envelope of the Bearhawk, so it was an easy choice for me. I am very lucky to have a mentor who has built both a Rocket and a Carbon Cub along with all his tools. My local airport, Heber City, can easily be called a hot bed for experimental builders so I have a great group of guys to lean on for advice. That being said, the rear windows on the
Companion had us all baffled.
Most things building up to this point (getting ready to cover the fuselage) have been pretty straight forward. Ok, I may have called Mark a time or two! Rear Windows have been confusing. We collectively had a lot of discussions which resulted in a lot of head scratching. In the end both Bill (my building mentor) and I came up with a very similar plan the same evening, which is amazing as this had been analyzed for a good 2 to 3 months. I had sent Bill a text early one morning telling him I had figured it out, he texted back that he was looking forward to hearing the solution as he had an idea too. I was planning on installing and gluing in screws from the
outside of the outer frame, to allow the window and inner frame to be attached after covering. Bill had an odd look in his eye as I explained it. Turns out he had a very similar idea, but with a better solution for attaching the inner to the outer frame.
So here ya go, this is how I solved the problem. To begin with, I really wanted the window frame inside the door former, but this makes a potential window replacement without cutting into covering very difficult. First I drilled out the factory rivets to separate the inner and outer frames. I bought 70 Clickbond studs, #CB5000, but they are round, so I had to grind each one to allow the window to fit into the window channel (photo of the modified stud below).

The flat plate portion of the stud is adhered to the inside of the outer frame with special adhesive, creating threaded studs protruding into the cabin. The inner frame is attached using these studs, forming a sandwich with the window plastic. I used denatured alcohol to clean the frame and the back of the stud for bonding using the proprietary Clickbond adhesive that is super strong and fast-acting. Next I put down cling wrap on the inner frame and pushed the studs through the cling wrap. This protects the inner frame from the adhesive. This was Bill’s idea and a brilliant one to keep things nice and clean (photo below).

I put the Clickbond adhesive on the stud backs. The Clickbond adhesive is amazing and sets up in less than five minutes, so it is not really possible to do all the studs at the same time, so I did 1/3 of them at a time. Then I put the outer frame on very gently so as
not to mess up the orientation of the flat sides of the studs, and used clecos and clamps to allow the inner and outer frames to be perfectly matched (photo below).


I also used Knipex “Pliers Wrench” pliers (part number 86 03 250) to align the inner and outer frames perfectly by squeezing the edges so they align. These pliers are the perfect tool as the jaws move parallel to each other, squeezing evenly rather than on an
angle like conventional pliers. This is crucial in my view as you want the studs to line up when the inner and outer frames are aligned.
I also used JB Weld on the top and bottom of the outer frame to allow it to be riveted to the door former while remaining parallel to the door former. Otherwise the joggle of the outer frame would cause it to be at an angle.

I will attach the outer frame to the tabs on the fuselage with solid rivets. I will have to drill the inner frame to allow for the shop head of the rivets. Next I will cover the interior cargo area and glue the fabric to the inside of the outer frame. Then I will cover the fuselage and wrap the fabric over the outer frame, and glue the exterior fabric to the the inside of the outer framer. This will allow me to install, but more importantly in the future, to replace a rear window all the while having nice clean lines of the window frame inside the door former. I will be using acorn nuts to hold the inner frame in place.
This method also makes sliding the window into the channel a non-issue. There have been multiple threads on the Bearhawk forums about the difficulty of sliding the windows in the window frame channel. This also allows you to determine the thickness of the “glass” rather than the channel dictating the thickness you can use, personally I will definitely use a slightly thinker window than the original
frame channel would have allowed.
I know I spent a LOT of time trying to come up with a plan that was pleasing to my eye, hopefully this will save others time contemplating a plan and make life easy should I need to ever replace a window. I am trying to think of anything down the road that may need to be done and how I can best set up the plane during the build process to allow for the needed maintenance.
Editor’s note– Bob likes to slide the window in from the front, but says folks are always free to innovate!

2019 Kit Upgrades, Skylight Detail

Source: 2019 Q4 Beartracks, Mark Goldberg
Over the last year and a half, we have made a number of changes to the kits. The purpose of this write up is to detail the most recent changes we have made in the Quick Build kits.
But first, let me go back to May 2018 to begin. Bob Barrows made the trip to the kit factory with me back then. We successfully showed the workers some new tricks. Really three things:
1) Bob showed the guys how to drill the wings to the fuselage and drill the wing struts.
2) Bob showed the workers how to make up the fuel tank straps and install the fuel tanks in the wings with the filler neck sticking out.
3) Bob calculated the lead weight needed for the elevator’s counter weights when the elevators are covered with the lightest fabric, Oratex. The lead is melted and installed on the elevators. Room is left for more lead/epoxy slurry if a builder uses a heavier fabric process or sprays a lot of paint.
More recently, we made two additional changes to the kits. Ever since kit production began in 2001, many builders have modified the ceiling for a skylight instead of a fabric ceiling as Bob intended. Our intention was to come up with a ceiling modification where a skylight could be installed with no modification but fabric could continue to be used for those of us suffering through the heat of summer down South. Bob designed a system using 5052 aluminum of .050 as used on the fuel tanks.
As you see in the pictures, the very easy installation is to slide the Lexan in from the side for the two outer panels. The Lexan is captured on three sides by the channels made from the aluminum. This ceiling retains some of the “hump” in the middle so the fabric will behave if an all-fabric ceiling is installed. The aluminum channels are held by three “stand offs” welded to the tubing as opposed to the two that previously connected the ceiling to the tubing. After sliding the Lexan into place, it is screwed down to the square tubing above the upper longeron that follows the shape of the upper wing skin. It is recommended to use bigger holes (like 3/16”), then bush a #6 screw with nylon or rubber line. This helps prevent cracking as the Lexan expands and contracts differently from heat and cold than the steel or aluminum around it.
It should be possible and easy to install a Lexan panel on the center section also. A builder would use two additional pieces of the channel but facing the opposite direction of the existing channels running front to back.
At the factory in December, per Bob’s instructions – I showed the workers how to final drill the aileron and flap hinges to their spars after placing the ailerons & flaps in their correct positions.
Detailed photos of the new skylight accomodations:

Bearhawk 4-Place Door Installation

The doors are one of the features what make the BH a uniquely useful airplane. Besides giving numerous ways of getting in the airplane, the doors let you load some really large, ungainly cargo and carry it with ease. They also hold the key to making the airplane into a flying motorhome.
On your quick-build kit, the doors are easy to mount. Clean the paint out of the hinge holes with a 3/16 drill, and insert either an AN3 bolt or 3/16 Clevis pin, depending on your preference. There’s no need to install cotter pins yet, you’ll be taking the doors off and putting them back on several times.
The Latches
The kit comes with the latches as shown in the plans, and this is the best reference on how to install and tune them. An alternate latch is available from Aircraft Spruce, PN 05-04520/21, which some builders prefer. This latch allows closing and latching the door by just lightly slamming it, which is a great convenience. If using the alternate latch, be sure to provide a thin stainless steel sheet striker plate on the door sill where it will be hitting.

Skinning the doors
Temporarily hang one of the doors in the fuselage to get an idea of what you’re trying to accomplish. The door skins must go out far enough past the frame that they chin themselves on the outside door frames. They then also provide a surface to which weather stripping can be attached. A gap has been purposely left between the door and the frame so it can be sealed properly.
Sealing the gap is necessary to make the airplane weather tight. It also cuts down on the wind noise and increases the airplane’s efficiency. Sealing can be left until the airplane is flying.
.025 2024-T3 is the usual choice for skinning the doors. It is light, tough and easily worked. You’ll also use it for making the outside frame for the top half of the front doors. Windows can be held in place with small “L” shaped strips, or they may be sandwiched between the aluminum skin and steel tube (more detail further down).

To work the aluminum, an inexpensive sheet metal apron break like that available from Harbor Freight will work fine and you’ll find yourself using it for many other applications as well. One of the commonly available shear/break/roller combination tools is slightly more expensive, but makes working all of the aluminum pieces much easier and more exact. It should be noted that all of the cutting can be done with hand shears while two C-clamps and a couple pieces of angle iron can do the bending. Having the brake, however, makes neater bends. The skinning process is the same for all of the doors except the rear cargo door, as that requires several slight bends in the middle to conform to the fuselage/stringer shape.
The skin can be attached to the door frame with common 1/8” “pop” rivets although it is suggested that a filler be put in the rivet head holes to weather proof them and to improve appearance. Also, for appearance sake, some builders have used counter sunk pop rivets by lightly machine-countersinking the skin, which is actually too thin to counter sink, so it cuts slightly into the tubing underneath. This is okay because the doors aren’t primary structure.

Sealing the Door Frames
Some of the earlier kits have open tubing in the door frames and it is suggested that wood or plastic plugs be epoxied into the ends of those tubes to make them weather tight.
Door Sill Protection
Although it isn’t necessary, some builders will fabricate an aluminum or stainless steel scuff panel for the bottom door sills to keep careless feet from wearing down the fabric. This will naturally need to be done after covering.

You’ll want to devise a method for holding the front windows open. Here is one: https://bearhawk.tips/2147, here is another: https://bearhawk.tips/1637, and here is another consideration: https://bearhawk.tips/1742.
The front rear door can be held open with a small gas cylinder, with most builders using one rated for 20 pounds. The rear rear door will stay open thanks to gravity, but you’ll need a short length of cable to keep it from opening too far. A spring will help dampen shocks to that cable.
The Wing Strut “Mouse” Door

The little flap on the bottom of the front doors is necessary to get additional travel in the door for entry. It is nothing more than a piece of .025 aluminum attached to the door with a piano hinge. No spring is necessary as it automatically closes when you close the door. A piano hinge is attached and the mouse door skin is lapped to the inside of the main door skin, so that when the door is closed, the mouse door is automatically sealed shut. Note that there is a slight break at the edges of the door skin to give a better seal.

Note: to protect the paint on the wing strut, put a piece of clear teflon tape where the door flap touches or it’ll eventually scuff the paint. Bonding on a nicely trimmed and polished piece of .016 stainless would look good too.

If you prefer to install decorative indoor trim pieces, they can be screwed or riveted directly to the door frame directly, just as the skin was on the outside.

Installing the Windows
Make the windows out of .080-.125” Acrylic. Allow for expansion and contraction by drilling the holes oversize, using a special drill bit intended for acrylic. When installing them for the last time, run a small bead of caulk at the edge where they contact the door.
If you’ll not be sandwiching the clear panels between the skin and tubing, first make the outside frame cover and rivet it to the frame. Then make up some 3/8” x 3/8” angles using .025 aluminum. You’ll need to cut enough that you can run these angles inside the door window frames all the way around the window. Attach them with small sheet metal screws (PK screws) into the door frame but NOT into the acrylic. The acrylic is just clamped between the L-strips and the door skin.
Before setting the side/rear windows in place for the last time, put some sort of sealer into the channel or where the window will sit. The butyl strip used for sealing corrugated roofing works extremely well. If you’re going to use a caulking material be advised that if you use silicon, it is also a glue and could be a mess to clean up and get a clean edge if you aren’t careful during installation. Mask the windows leaving just a small area for the caulking and it’ll clean up much easier .
Note: There are two primary materials available for the windows, polycarbonate (Lexan is a name brand) or acrylic (Plexiglas is a brand name). Speaking broadly, acrylic is less strong, but more fuel resistant. There will be a fuel quick drain right above the front windows, and some builders have found the Lexan more likely to be damaged by the fuel exposure here. In the original (not model B) Bearhawk, the rear windows are bent slightly, and some builders have found that acrylic crazes at the bend points over time.