Old dog, new tricks
After my crash near Harrismith, I threw a bit of a temper tantrum and took a break from everything drone. The crash really baffled me and killed the autopilot in the process. So I could not ever retrieve telemetry to see what went wrong. I did figure out the reason eventually (see details below).
Life went along and my job at ESRI South-Africa assigned me to do a talk on UAVs and their use in aerial photography in the upcoming summit. The topic is covered over a whole day and includes a demo and processing of the data.
So, I had to procrastinate later and get the drone up and flying again. Since the nose was a bit mashed after the last crash, I decided to implement and idea that I have toyed with for a long time – rebuild the nose completely. So I started removing what was left of the nose:
My experimentation with 75x75mm PVC gutter downpipe on other planes were quite successful and matched the width of the old body quite closely. I decided on a design that allows the nose to be detached. The “avionics package” (RX, autopilot, speed controller, BEC, etc.) will stay in the body of the plane, whilst the “payload” (cameras, batteries, etc.) will go into the detachable nose. This way I can make noses custom crafted to the mission.
The new nose has way, way more space than the old one. Gone are the 2-3cm thick foam walls, replaced by 2-3mm of PVC pipe wall. This is going to make packaging much easier in the future. The autopilot and camera equipment will also now be nestled in the middle of the plane body, far away from crashes. The canopy is made from old 5 litre purified water bottle heat formed over a wooden blank. So the canopy also has very thin wall thickness. I also repainted the plane white as I was tired of the ugly orange that did not really help much with visibility of the plane.
I also decided to enlarge the rudder fin. I always felt it is too small and have read that other people had good results enlarging this on this plane body. In the process of rebuilding it, I finally found out what caused the plane to crash. The rudder servo was dead. It explains the behaviour of the plane right before it crashed. The auto pilot was flying the let-down pattern and was on the downwind leg, and on trying to turn to the base leg, it would not turn. The plane banked, but would not turn and kept heading away. I took over from the Autopilot and tried to turn it manually, giving it more bank and pulling more on the elevator. It suddenly spinned and was on the ground in two rotations. My guess is that the servo died with the rudder stuck hard over (I could not see it after the crash as the rudder was ripped off) and it was banking in the opposite direction. The low speed, high angle of attack with lots of rudder in the steep bank was probably just too much and physics took over.
This is the second servo failure on this plane. I tried to use good quality metal gear digital servos. Bot failures so far was the expensive ones. The cheap replacements have never died in other planes nor this one. The first failure was an aileron failure and did not cause much trouble. I have now rebuilt the tail with a bigger fin, and a split elevator with a servo driving each half, giving me redundancy on the elevator (the possibility of a servo failure on the elevator has kept me awake for long enough now). Once I get my replacement autopilot, I will setup it back to it’s default of not using the rudder at all. It makes for less tidy flying, but since I don’t have redundancy on the rudder, I will leave it for manual flying and compensating for wind on landings only.
Here is a side view showing the new tail a bit better:
I hope to maiden the new setup this week-end and will post updates and more photos of the insides.