After my Talon build part 1 & 2 (here), you'll find herefater my first experiments with the installation of Pixhawk and other components in a fixed wing UAV. What's great with this Xuav Talon model is its huge internal bay. So I started to draw on paper the location of all of the components of my setup, as shown in top picture.
I imposed to myself some rules/principles:
-to keep different antennas and receiver as far apart as possible.
-not to use of the wings to install any components. This is a compromise to keep transport/assembly/unassembly easy (wings can be removed with two screws).
-contrarily to certain builds, keep the gps with a clear sky view (eventhough it would probably still work buried in foam)
-have backup power on pixhawk: 3DR power module + SBEC on RC rail (+ required to power servos)
-to plan wiring before installing components; to draw a wiring scheme as shown above. This to avoid bad/wrong connections and ground loops. On the latter principle, Pixhawk has a weak point : if you power , as a backup, via a SBEC the output rail, not only do you have to mess with the installation of a 5.6V zener diode to protect the board, but you are forced to connect the ground wire too (creating a ground loop with the ground coming from the power module). This should be adressed in a next card's version : integrate the zener diode in the PCB and make it right like APM was: a common shared ground for all circuits of the FC.
Let's see what was done,
1. Customized electronics platform to host Pixhawk:
I don't know what these foamy airplane manufacturers think about when they design their model : they always forget to plan for a leveled horizontal surface, inside the bay, to host the flight controller board (maybe they are old school and never use a FC ?).
As a consequence comes a first question: where to place the pixhawk board so that it is firmly held in place (with room to use an anti-vibe platfrom underneath) , at the center of gravity and perfectly horizontal when the plane is horizontal ?
My answer is to add a diy custom plywood plate held horizontally in between the two existing plywood pieces that are used to reinforce the wing attachment points on the fuselage:
Side view of the added black painted plate:
As can be seen on this last picture, Pixhawk sits on the custom platform. It leaves a big gap underneath which is useful to pass cables and/or position other components on the bottom below the platform. It gives also the advantage to move pixhawk far away enough from possible electromagnetic interferences of cables/electronic components that would be positionned underneath, on the bottom of the bay.
2. RC receiver and antenna's positionning:
I decided to start with a FrSky 2.4Ghz rc control (a long range opensource Hawkeye DTF UHF alternative is ready and tested but will wait for now).
I use a X8R FrSky receiver with the PCB antennas. It is supposed to give a 20% range increase versus the simple coax wire antenna's. To be efficient they have to be placed perpendicular to each other (to cover horizontal and vertical polarisation. Especially useful when the plane banks, in order to not loose the signal).
Question : where to place the receiver, pcb antennas and how to have them perpendicular ?
Answer : eat a popsicle; then insert and glue the popsicle stick in the fuselage on the back of the bay. This location is far away from the motor/ESC, far from the telemetry antenna and FPV antenna. One pcb antenna is then velcro'ed vertically on it.
Using a solder iron, I melted a hole through the fuselage to pass the antenna wires. The receiver is velcro'ed on the side of the fuselage as seen on picture.
The second antenna is place perpendicular to the first one on the fuselage body.
3. 3DR GPS and compass module installation:
I wanted an external GPS installation. I did not want it on the wing, nor drill a hole in the top foam cover.
Furthermore it needed to be close enough to Pixhawk because of these damned DF13 cables that are delivered way too short in the 3DR kit... (=> request for 3DR, in your next FC version, replace these cables by longer ones and use connectors that DIYers can modify easily)
The GPS + compass module is installed on the back of the bay where the air inlet (cooling the motor) is located. It provides a small horizontal surface that is perfect for the GPS to sit on. However a "cable pass-through" hole has to be melted through the foam, to reach to Pixhawk (again a fined tip solder iron makes the job).
4. Telemetry module installation
Current 3DR telemetry modules are delivered in a plastic casing that makes it easy to velcro to a supporting surface.
I decided to locate the module in the front of the bay where this telemetry casing fits perfectly on the internal side of the fuselage.
An iron solder was again needed to melt an antenna hole.
5. Airspeed sensor (Pitot tube) installation:
The airspeed sensor consists in a pitot tube that ideally should be installed on the front (nose) of the plane. However that will be reserved for the installation of the FPV equipment. Also, the sensor's silicon tubes and I2C wires are too short to go all the way from the nose to pixhawk.
So i chose an alternative installation on the side of the fuselage,
I found an easy way to attach the pitot tube to the fuselage so it is maintained fixed and horizontally about 1,5 inch away from the fuselage. A zip tie is used around the metallic tube. The "plastic tail" of the zip tie is glued with CA on a CF rod. Hot glue is used to reinforce and fix the rod perpendicular to the tube.
The cf rod is pushed through the foam on the side and hot glue on both sides seal the rod firmly.
When travelling, the metallic tube of the sensor can be slided out of the zip tie.
6. Side note on servo control horn and control rod installation:
It is said by xuav talon experts on rcgroups forum that the maximum deflection of control surfaces should not exceed 20 degrees. Indeed only very small surface movements are required to fly the Talon (and people tend to move their sticks too much).
So I decided to mechanically limit the surface's deflections by positionning the control rod attachment point outward on the control horn. That way surface's movements are limited to about 20 degrees.
The consequence of this is I do not get perfect 90 degrees angles between horns and the control rod. I hope this will not create more adverse effects than not limiting the amplitude of the surface's deflection...
This ends part 3 of the XUAV Talon build.
Soon to follow : FPV components installation.
Cheers,
Hugues
Comments
This is what I did on my Talon.......
And airspeed/groundspeed correlates nicely :-)
@Dave, did you follow the airspeed sensor calibration procedure? That might correct your readings?
@John, yes I am installing the wing extensions, especially to reduce the stall speed and allow me to land at lower speed.
Great article. Have you considered the wing extensions for your Talon project? They add some good gains on performance.
Thx Dave.
Gary, where on the wing would you advise to put the pitot? I was wondering if it would be good on the wing because the airflow is accelerated on the wing, thus wouldn't that give false speed readings?
No I don't do auto landings because I sometimes operate in very tight spots where it just would not work. I also don't do auto take offs because I prefer to feel that the aircraft is flying properly before committing to auto missions. It works for me its good enough. The pitot should probably be a little further out on the wing for you but if it works it works. I think Hugues pitot will likely be affected by the fuse and wing, but there will be somebody with better knowledge than me along in a minute I am sure ;-)
Put the pitot out on the wing it will not be accurate there. Don't fly with all your FPV gear until you have at least 10 hours on the airframe and know what it flies like. I don't bother with pitots any more the wind model is good enough.
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