(Dis)assembly

There's been some good news and some bad news: the bad is that I have not had and will continue not to have enough time to work alone on the oktokopter project, and the good is that a team of robotics students and engineers at Arizona State University have volunteered to continue the work. We are shipping the 'kopter overseas, disassembled so as to pack well for the trip. To help the ASU team (and anyone in future working on this project!) here's an overview of how to put the 'kopter back together.

Remove the propellers: use an 8mm hexagonal spanner to undo the self-locking nuts. There is a black washer between each nut and propeller.

Note that the propellers are on in alternating clockwise and clockwise configurations: the red (port) spar  is shown below with the correct propeller so you can put them back on correctly. If you get them all wrong, the copter won't take off, but push itself into the ground!

After removing the clear plastic hood and putting the cottle pins somewhere safe, undo all of the black bolts with an allen key or ball screw, holding on to the 3.5mm nut at the bottom each time with either your fingers or a small spanner. (These bolts hold the strut saddles in place around the frame.) Put the black bolts and nuts together in once place.

Undo the silver bolts: these not only hold half of a strut saddle each, but also hold the platform onto the landing gear frame. As such they are slightly longer than the black bolts.

This is a view of the connection between the platform at the landing gear, before disassembly. Note that the platform touches the clear plastic frame directly, and then the load is distributed with a carbon fibre insert. This falls out when the silver bolts are removed.

After the platform is removed, the struts can be rotated around so that the starboard and port struts join the  front struts, and the remaining struts fold back with the rear struts.

All the cables connecting the landing gear to the platform have to be disconnected so here is a photo record of the connections:

We have since extended the camera mount lead since the
one that came with the 'kopter was not long enough.

Heading and Altitude Test

Plan
The plan today was to test a series of waypoints with different headings. I hoped to program the 'copter to fly between two points, pointing in a different direction each time. This would help with aerial photography, as I could then choose where I want to point the camera. It would also give me a feel for how accurate the compass is, if I pick cardinal points with clear landmarks.

Flight Log

I carried out two separate flights, both attempting to execute the above plan. Weather was perfectly still, 100% humidity, and about 30C.

Flight start: 11.46am
I accidentally had 'home' engaged during take-off. I flipped back to 'manual', and then to 'home' again, hoping to execute the waypoints. The 'copter flew to its first waypoint but hovered there without moving. Later, in the video, I found that it was pointing at the POI, rather than North, as I expected. I had programmed in a heading of '00' but apparently this causes it to use the POI rather than point at zero degrees - useful to know!

I set the 'copter back to PH/AH and flew it back to the point at which I had taken off. While moving it back, I found that Carefree mode made it actually more difficult to control. Perhaps I had moved from my starting point, or because it had moved to a 90degree angle to my starting point, the controls were not quite as I expected. In any case, I returned it to full manual mode, brought it back over the landing point and landed it very safely and gently,
Flight end: 11.48am

Flight start: 11.53am
The next flight, I was determined to execute the waypoint pattern and perhaps was not as thorough with my pre-flight check as I should have been. This time, I correctly set the switches on the controller ('manual', 'carefree on'), but I forgot to check the tightness of the nuts on the propellors. I also did not review the video of the last attempt, assuming that the reason it did not execute further waypoints was my mistake in starting with waypoint mode already on, but I should have realised that in that case it would have returned to the landing point, as that was the last waypoint on the list.

I calibrated, started the engines, and smoothly took off, then engaged waypoint mode again. The 'copter flew to its first waypoint but again hovered for a good twenty seconds without moving on to the next waypoint. I realised that this might be something to do with the altitude settings - that it would like to be at 50m, but that it could not get there, because the climb rate was too slow. (I had seen complaints of this on various forums). At this stage, I probably should have landed it and reprogrammed it with lower-altitude waypoints, but I was wary of being too close to the trees. Instead, I increased throttle to move it to 50m. It then moved to the next waypoint, but again hovered for ~ten seconds or so without moving, rather than the expected five. I assumed I was again too low, and increased altitude, but the 'copter did not execute the next waypoint. I realised in retrospect that I had probably gone too high, and again it was waiting to get to 50m before it could execute the next waypoint.

I decided at this point to abort the flight and return it to the secondary landing spot on the west end of the courtyard. I engaged PH/AH, reduced throttle, yawed around clockwise 360 to check responsivity, and began to bring the 'copter back toward the courtyard. At this point, one of the propellors flew loose, and the 'copter began to fly erratically. With PH/AH engaged, the 'copter was attempting to engage the motors in a way that it thought would keep it at constant altitude, but missing a motor, it began to descend, and also slew. For thirty seconds or so I fought this behaviour, until I realised what the problem was, and switched it to full manual mode. I then yawed it so that its back end was facing me again, and brought it back to the courtyard, and landed it again. However I was rather anxious at this point (I had nearly lost it!) and the landing was not perfect, so one of the stressed carbon struts finally broke loose.
Flight end: 11.58am

Lessons

• I didn't manage to thoroughly test the functionality I wanted to test - i.e. pointing in different cardinal directions while moving - because the very first heading I used, 00, caused the kopter to point at POI. I will try 360 next time, and perhaps 01.
• Altitude control is tough, and if the 'copter needs to be within a certain altitude in order to execute the next waypoint, then it's crucial to either
• get better at using the MX-20 transmitter display so I can read and adjust the altitude quickly
• use a lower altitude so I can more easily see whether it is too high or too low - at previously tested altitudes of 10--30m I had none of these problems
• figure out how to get the 'copter to completely control its own altitude - in the firmware documentation there is a good discussion of this
• Slewing clockwise may be a particularly bad idea and cause propellers to come loose - always slew anticlockwise.
• A pre-flight-check needs to be designed and more carefully attended to.
• Bring lots of spares - and investigate the issue with the struts.

GPS Logging

Having enabled the POI functionality, I wanted to properly log the GPS co-ordinates of the 'copter. I had another demonstration to perform to another group of NYSF students, so I figured I would try to get logging working and see if I could record anything. I didn't have time to do much before the demo at 11, so I just quickly popped it up in the air for them and then back down again:

The flight was between 11.08 and 11.09 am WST in the courtyard, and it was just a tad windier than yesterday. Unfortunately the POI was still too far to the south, so I was again not directly pointing at the group of kids!

After this I researched a bit more about how to log the GPS. I found that the MX-20 transmitter can perform logging, but more of the telemetry, like temperatures, control stick settings etc. It also produces very large files which have to be opened using a proprietary program from Graupner. I tested it out and it wasn't very user-friendly, mostly in German and with many dialogues and help boxes empty or containing filler text. As it can only log what it is sent by the 'copter, I realised it probably wouldn't help us with the main effort, to accurately measure the 'copter's position, any more than just recording directly. In the interests of completeness though, here is how you set up some auto-logging that begins when your flight starts:

• Go to 'control switches'
• Move the throttle joystick
• Set the threshold to just past off, e.g. -88%
• Go to 'timers'
• Set that control switch as starting the 'Flt' (flight) time

If there's a micro-SD card in the transmitter, this will start logging when you bring the throttle up after starting the engines. Unfortunately it currently won't stop until you turn the transmitter off.

To log on the 'copter, it's a lot easier - just put a micro-SD card in the Navi-Ctrl board. 2GB is more than enough space as it generates very small files. It creates both a GPX file and a KML file: The KML file has  simple entries like

+115.8894829,-32.0038644, 1.789

showing the longitude, latitude and altitude, and it logs these every second.

The GPX files show a lot more data: 34 different data objects like roll angle and heading, and more information on the 'copter status, like available motor power and the motor temperatures. These points are logged every 2.5 seconds.

Now that I had a way of logging the position of the 'copter, I wanted to try a flight path that resembled the a possible flight plan for the MWA tile measurement: a curved flight with varying altitude, so as to eliminate the effect of taking a straight cut through a spherical beam. I came up with a simple series of waypoints using a sin curve, since I didn't need to be exact, just test the functionality.

When we were in the field, I uploaded the POI as the first waypoint, as I wanted to fly the 'copter over to its exact start position and land it there. I left the POI in the file, a mistake!

Flight start 16:33

I started the 'copter in CF mode and took it to 10m, then engaged 'come home' to send it to the POI. It reached it, but then behaviour I had not accounted for kicked in. Hovering directly over the POI, the 'copter was straining to tilt the camera mount at the point directly beneath it, and was slewing around in a clockwise arc trying to get to an impossible angle. The landing gear was also disrupting the servos on the camera mount. I immediately put it in PH/AH and began to land it, but it was difficult as the 'copter continued to slew around. I had seen warnings about this on the forums, but I didn't realise that it would be so difficult to control. After a few clockwise turns it lost a propeller! I brought it down and we managed to find both the prop and the washer, but lost the nut holding the propeller on. I realised afterward that I should have turned off CF mode, as then the 'copter would not have been trying to point at the POI, since that functionality is then disabled.

Flight end 16:34

Flight start 16:53

Having lost one propeller, I still hoped to see if the curved waypoints would work, so we brought the 'copter back to around 10m east of WP2, and deleted WP1 from the log. I brought it up and started the waypoints, but the 'copter rose poorly and slewed around, spending a lot of time trying to get to each waypoint even though I had told it to spend zero seconds at each one. After a minute I brought it down again, as it was clear it was not going to be a good test of the functionality.

Flight end 16:34

I looked at the logs and they were difficult to understand at-a-glance. I tried putting them into Google Earth but it runs very slowly on my computer and GE's handling of altitude is terrible. Then I found this excellent little online app: http://www.gpsvisualizer.com/ which can produce a nice png of each flight, or transform it into files for use in other programs.

Clockwise spirals while hovering directly over POI.

Poorly-executed curved flight path - missing a propeller.

Lessons:

• Never hover directly over a POI; the 'copter becomes extremely erratic as it tries to get the camera pointing directly at it. If you do accidentally end up doing this, turn off CF mode, turn on PH/AH and land immediately, then reprogram.
• Flying manually is ok with seven propellers, and it can execute simple waypoints, but it can't make fine adjustments well enough to perform a densely-waypointed flight path.
• The propellers have a tendency to come loose when you yaw clockwise - probably due to the unreversed threads on half of the motors.