UPDATE 1 of the build below
I wanted to share my first Talon build (and my first fixed wing):
(also posted on rcgroups here:http://www.rcgroups.com/forums/showthread.php?t=1977612&page=75)
Once it will be built (might still take a while), I intend to use Pixhawk as I am now deep into it my multicopter side of the force.
it will be useful to get comments and maybe help others building their own fixed wing. This build thread will consists of more than one posts but these posts will come in sequence when there is progress to show in the build process (especially when some parts are still being ordered and being shipped).
Here we go for this first post,
I ordered the Talon kit , in white, @ www.fpvmodel.com. Unfortunately I saw two weeks later the extension kit for the wings + landing gear + new dome which is oriented toward the ground. So I ordered this extension kit which is still currently blocked at customs...
I have never previously built a fixed wing UAV, this is my first. So bare with me.
First question I wondered was by which step to start : can I glue parts in any order ? Do I risk to be blocked later by assembling the fuselage before organising the electronics and cables inside ?, etc
As there is no step by step guide out there I decided based on intuitive logic in which order to proceed.
I first followed the mounting PDF guide that is the generic mounting guide coming with the XUAV Talon. I followed the guide up to the "gluing the wood pieces" stage. I then left the two halfs of the fuse unglued at this stage.
For gluing I decided to use UHU POR. It is said that CA is too rigid and breaks apart, that hot glue when too hot melt the foam, that the EPO material of the Talon has built-in an unmolding substance that makes it hard to adhere to.
When I received the fpvmodel box, it had been slightly crushed and had one of the two EPO dome pieces broken.
I managed to repair the dome piece and glue everything together nicely with UHU POR
The next step that was clear to me was to understand, before gluing anything definitively, how to mount servos, control rods, control horns.
How to mount servos, control rods and control horns ?
This is the most difficult part as I had read in this thread, even from senior builders like chanyote and blueprint, that they had lost control horns in flight (bad fixing)!
I tried manually to move a servo , just holding by the fingers a control horn : indeed there is a huge force playing on them, especially at the end positions of the servo: it was immediately obvious that just gluing a control horm would not work (I would not trust it in flight).
My approach to this issue : reinforcements of the control horn fixing on the moving surface,
To do so I gathered some fiber glass fabric that I had left from my multicopters builds, Titebond wood glue (the very best of wood glues) and a paintbrush
The idea is to apply on both sides of the control surfaces, around the spot where the control horm has to be fixed, a piece of fiber glass fabric glued with wood glue (applied with paintbrush to smooth and even the surface). This results in a quite hard composite that adheres extremely well to EPO foam.
It remains also very smooth, flat and thin (would not have been the case with UHU POR for ex). As a result, the two sides of the control surfaces, that will receive the control horns, are armed with fiber glass. Not only armed EPO offers more resistance to ripping forces, it also reinforces the foam against the squeazing pressure of the screwed control horns.
I decided to use Titebond wood glue versus foam CA because of a small test I did before : I coated a test piece of fiberglass with foam CA ->apparently CA destroys the fiber glass resistance somehow and the pieces could easily be broken apart. Then I coated another test piece with of fiber glass with wood glue->excellent mechanical property and more rigidity.
I do not have the screwed type control horms yet, they are being delivered. I expect them next week.
Another reinforcement decision was about the control rods. I had also read on this thread that the rods that came with the kit are flexing too much. So what I did was to double the main wing rods (the longer ones; the rods of the V tail are short enough to be left alone as they came in the kit). I cut in piano wire the same length of wire. I then used heat shrink tube pieces to fix the two rods together, as show in picture 8.
Then came the question of the servos :
What servos to use ?
I had read in this thread that the servos for the ailerons need to be stronger than the servos for the v-tail. There is a mention somewhere that the V tail servos should be 9g and the servos for the ailerons should be 17g.
I got mistaken from a kit on fpvmodel that provides four 12g servos for the XUAV Talon. So I got 4 of them, model EMAX ES08MD (digital, metal gear, 1.6/2.0 Kg.cm).
Unfortunately when I tried them on the plane, they only are a good fit for the V tail, and they are way too small for the ailerons.
So I got 2 Hitec HS-82MG (Digital, metal gear, 19g, 2.8/3.4 Kg.cm) for the ailerons.
One of the hardest thing for a beginner like me in fixed wings, is how to mount the servo arm, in which position, etc.
After a bit of reading, I had to place the servo in neutral position and than mount it with the servo horn perpendicular to the control rod when the control surface is in neutral position too. It seems easy to sayd (read), but it is much harder to implement.
Especially I am still waiting for the transmitter/receiver (a Hawkeye DTF UHF 433Mhz) that will be used, so I had no way to define the neutral position. I got thus a servo tester for this. It is very cheap and practical; it allowed me to test all servos before installation. To test not only that servo worked ok, but also to test the extreme positions of the control surfaces.
I still have an issue with the movements of the surfaces because they move too much : I have +- 45 degrees. According to what I read in this thread, I should limit the movement to +-20 degrees.
How can I achieve that movement reduction mechanically ? (or is it ok to use servo limits in the transmitter without loosing servo resolution ?)
That's where I am at for the moment. If there is some interest I will continue posting progress on this build.
Finalized servo and servo horns on the control surfaces
I have finally found some time to finalize the servo and servo horns attachments on the surfaces. I found this step to be the most difficult of the build so far as the stock control horns are too weak and are not of the screw type (so in practice this would mean a high risk to loose a control horn during flight if it is just glued...not good).
What has been done:
-the default control horns were replaced by screw types control horns
-but in order to fix screws on foam, you first need to reinforce the zone where the screws will put pressure on the foam
-it was also necessary to fill in the factory precut holes for the stock control horns. They were filled flush to the control surface with balsa and foam CA.
-the new control horns were positioned on the same spots where the factory precut holes were. Then the screw holes were marked and drilled.
-the new control horns were then both glued and screwed on the control surfaces:
To attach servos:
-The servos were positionned at neutral with the servo tester
-The servo horns were screwed on to form a 90 degree angle (or as close as possible to it) with the surface of the wings. Due to the finite number of possible positions on the servo gear, it was sometimes an angle of 80 degrees instead of 90. I hope this will not have any negative effects on flight control.
-the control rods of the ailerons were doubled to reinforce them (otherwise they will flex in flight under the air pressure , they are too thin). The double control rods were fixed to each other with heatshrink:
-Little bit of blue loctite and hot glue was used on all screws so that vibrations will not loosen them during flight
Fixing system for the XUAV Talon motor :
The XUAV Talon comes standard in the kit with a plywood piece on which you are supposed to screw the motor. However the defaut manual is so badly done that you risk to glue it in the plane fuselage, as prescribedn and then realize it is too late to fix your motor : because you will not have any access to this plywood piece to screw bolts and/or screws of your motor attachment system.
So BEFORE gluing anything, mark the position of the screw holes of your motor fixing plate (it is usually an aluminium cross shaped piece with pre drilled holes for both motor screws and fixation screws on the frame) and drill them.
Then, saw another piece of about 5mm thick plywood a bit smaller than the original kit piece like so:
-Screw holes are drilled
-Blind nuts are hammered and glued into these drilled holes. (i used 3mm diameter blind nuts for standard 3mm screws we usually use in our UAVs)
-Finally an opening is carved out to pass the motor wires through:
This will allow me to screw my RC tiger motors on it, even after the two parts of the fuselage will be glued on. It makes it possible to change your motors easily afterwards (which is absolutely necessary to try out different motor/prop combinations).
More updates to come as the build progresses,