XT-8F: The 8FT UAV Build

by joshuaparker | April 24, 2017 | (4) Posted in Projects

The Idea/Design:

 I started off RC Foam Fighters FF-Predator pdf file that I imported into Google SketchUp. I scaled the plan using the wings as my point of reference until one of the wing halves was 4ft in length. After the scaling was finished I began to 3D Model the plane, as you see below, to get a feel for how it would look with the correct thickness of each part. Once satisfied with all the parts fitting correctly, I then copied the faces of each part and laid them flat to print them as large pdf. 

FF-Predator Plans

 

***UPDATES V2***

If I remade the fuselage from the insulation by giving it 4 bulkheads instead of only two. I would move the two middle ones to where the wing slots in so the wing sits directly below it. As for the two new ones, they are located more evenly spaced between one bulkhead and the nose or tail section. For the foam board version, the fuselage has double the thickness of dollar tree foam board (9/16"). I also changed the wing slot so that it doesn't have that step and it is now flat with a 12/16"  tall opening.


Materials:

The plane was made out of 1/2" pink insulation foam, carbon fiber, and plywood. I used great stuff for filling gaps, mostly in the fuselage. Beacon foam tac glue was used for the nose and the wings. Lastly, I used hot glue for the servos, stabilizers and securing the nose and bulkheads in place. Be very careful with the hot glue if you are using insulation foam because if it's too hot it will destroy your foam. 

***UPDATES V2***

V2 has dollar tree foam board, tape, plywood and only hot glue as the adhesive. We used the nose from V1 that I got signed by most of the Flitetest crew.


Building:

I used a variety of methods to complete this build. For the fuselage, I used a box cutter from dollar tree. For the Nose, I used a hot wire foam cutter as shown below in the background, but you can use the same box cutter. For the horizontal and vertical stabilizers, I used a box cutter once again. The wings will be tricky without a hot wire foam cutter but to can sand the wing modules to shape. As for the spar holes, you can sand those as well. The wing modules were cut using the hot wire foam cutter that the fuselage is sitting on. I used the plywood wing ribs as the guides as I sculpted the wing modules from the foam block sandwiched between the two wing ribs. The same was done for the spar holes.  

***UPDATE V2***

The fuselage is two layers of foam board thick as well as stabilizers and their respective control surfaces. I taped 4 fuselage panels together in order to test fit all the fuselage pieces. Once satisfied, I then moved on to gluing.


Assembling and Gluing the Nose:

I used Beacon Foam Tac Glue I bought off of eBay for the nose and wing module assembling. The nose has about ten and a half nose cut out sections. Within the nose, there are about 3-5 nose pieces that have an additional cut out to be used as a battery bay.



Assembling and Gluing the Wing:

After gluing the wing module halves together, I put them on the spars to keep things aligned. I then glued the foam modules to the plywood wing ribs. In the second picture of the wing half, there are three modules that need to be joined to the wing however, I must mention that the last three have a flat CF spar on the bottom versus having two CF spars on the inside like the other modules. 

 

***UPDATES V2***

I used a modified Armin Wing technique from the YouTuber "Experimental Airlines" to make the wings. It uses a fold over technique with seven 6mm square carbon tubes as the "wing ribs". I laid a ~38" carbon fiber T-spar laying directly over top and glued it all in place with hot glue. I also laid two 30" x .5" foam panels on each side starting at the root chord and then 3-5" down I notched out a wooden meter stick and glued that to the base and spar to give the wing tip more support. I was LIBERAL with the glue to ensure the joining of the two spars was solid. To keep the wings together, I used short round carbon fiber spars. I cut a mark the went from the root chord edge of one wing to the other. After that, I cut along that line and used the spar to make the appropriately sized slot so I could bevel the foam to have enough space for the spar to sit. This next step was crucial to ensuring I didn't snap the wing in the middle. I got some scrap wood drilled out a hole the diameter of the spar and cut it to a small rectangle. Next, I rested the new spar holder with the spar in the slot and made relief cuts on both sides and layered up the foam accordingly and glued it together. I made sure to cut off the excess wood so that the top wood layer would sit flush. See the link below to see all that I have described.

Google Drive Pictures


Fuselage:

For the fuselage, I used an expanding foam called great stuff. You could get away with a foam safe epoxy or the Beacon Foam Tac glue but you would need to ensure the surfaces are flat and even. By even I mean not having dips and areas where the foam has been gouged out. I spread out the adhesive using a spare piece of foam or wood. Just a side note, putting tape on the back where the sides and the bottom meet will help make gluing easier.

***UPDATES V2***

Looking back, I would have added spars of some kind along the length of the fuselage to keep it from bowing in the center. Another way to strengthen the fuselage is to make an extra bulk heads piece out of thin wood, glue it to the foam and widen the bulkhead slots. Then get a circular spar and drill a hole in the wood to that size and glue the spar in the bulkhead. I would then installed the bulk heads into the fuselage bottom panel using hot glue. Then glue one of the side panels to the bottom panel and bulkheads with hot glue. Be mind full that you may need two hot glue guns to get this step done. Grab a friend to help or use a foam safe glue, like beacon foam tac, for the bulkheads to secure them in place. I glued the top panel on after the electronics were installed how I wanted them to be.


Installing the Electronics:

This part is pretty simple and required little effort. I installed the two elevator servos in opposite directions and connected them to a Y-harness so they move the same way. The rudder just had an extension cable to reach the receiver near the nose. The elevators and the rudder have push rods with coffee stir straws on them near their respective control surfaces to keep them from flexing too much. Each aileron has a 9-gram servo connected to a Y-harness with a flat carbon fiber spar running through them for rigidity. As for the nose, it has a servo that is in a mix with the rudder for a steerable nose wheel. You could probably use a Y-harness here too. Installing the steerable nose wheel is pretty simple. First, make an appropriately sized wood mount for the nose wheel axle to glue to the fuselage and one that will sit below it as shown. Second, place the wheel axle in the center up against the wood mount and pencil in where the holes are to be drilled for screws and the wheel axle. The bottom piece of wood is to keep it from destroying the foam too much.

***UPDATES V2*** 

I used the same electronics for everything. I used the same setup for the steerable nose wheel as well.

 

Finally, after all the building and gluing, I drilled some holes in the wings mounting plywood section for the screws that would secure the wheels to the wing.


Reaper Drone V2 (DTFB):

coming soon...


COMMENTS

Grenadapult on July 27, 2017
I would have preferred to see the original KFm airfoil (as usual! LOL!), but nice work. It looks great. I can't find the info on the steerable nose gear though. And how did it fly?
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joshuaparker on August 6, 2017
I will update this periodically because I am still building v2 in DTFB and I sometimes forget some steps. I will add that section in there when I have all that information for you. Thanks for helping me to make this article as detailed as possible.
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dexgeo on August 1, 2017
Greetings Friend Joshuaparker,
Congratulations on your project.
Your model aircraft looks really good.
Can you share his plan with us?
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joshuaparker on August 6, 2017
Thanks my rc club and I really appreciate it. I will add the link to their ff-preadator plans shortly.
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XT-8F: The 8FT UAV Build