Richard Sutherland's Do-24T
This Do-24T is scratch-build by Richard Sutherland from Australia.
The wingspan is 1.90 meter and the aircraft is equipped with three
Astro 05C geared electric engines. The props are 10x7 and there
are twenty-one 900 SCR battery's on board. The maximum flight
time of this radiocontrolled airplane is 6 minutes.
The story in Richards own words coming from his own homepage:
A few years ago I decided to build a multi motor electric and eventually settled on the Dornier 24 as the subject. I already had an old well used ASTRO 05 cobalt so I ordered another two of them.
I wired the motors in series but the older ASTRO behaved differently to the new ones (less RPM, different accel/deccel, noisier etc. due to smaller magnets/pos different winding and being generally clapped out) so I put this motor in the center position. The model flew really well - short takeoffs - plenty of power - trainer like handling and I was really pleased with it.
A long holiday and other projects meant that the Dornier sat in the shed for a while until an upcoming seaplane day provided the incentive to bring it out of storage. Finances now allowed the purchase of a new ASTRO 05 to replace the clapped out one, I checked this on seven cells and it performed identical to the other two so I swapped the motor, installed the radio gear, charged the batteries and headed out to a secluded dam with my girlfriend for a test flight the day prior to the seaplane day.
I placed the Dornier in the water and attempted takeoff - acceleration was poor, but the batteries were charged a couple of days before so I assumed they had probably lost some of their charge. Peaked batteries and tried again - acceleration still poor - maybe the water is too flat to allow the Dornier to get on the step - should be OK if I can get it airborne - eventually manage to horse it off - flying really bad - elevator is hypersensitive (like the CG is on the trailing edge). About a 100 feet high now - low airspeed - too much up - stall - caught it - stall - mush -shut the throttle and down it comes - pretty heavily - SPLASH!!
Its still floating but will only taxi in tight circles about a 100 meters from shore. So I strip off to my jocks and swim out - friggin hell that water is cold!!, talk about "fffreezing the bbballs off a bbbrass monkey!!" I get to the Dornier and begin to tow it back (and start wondering if I might cramp up and drown!), I finally reach land and after about an hour have thawed out.
Damage is minimal - servo rails loose (hence rudder locked over),a couple of prop strikes on the canopy/hatch (strange, must have happened on impact) and BATTERY HOLDER BROKEN - Aha must have broken loose on takeoff allowing the battery to move back and result in a rearward CG. That night repairs are completed and the battery is firmly secured (so it cannot move back). I also added a couple of cells to the back (23 up from 21) because the battery is fairly old and it did seem down a bit on power.
Too late now for a test flight so its out to the lake on the seaplane day. Assemble model, charge battery, a quick blip of power to check the throttle and my flying buddy Mark Furgusen calmly states "your middle motor is running backwards". Bullshit I thought, another blip of power - hes right THE MIDDLE MOTOR IS RUNNING BACKWARDS!!
I cannot believe that the model was able to fly at all in this condition much less take off from water. I guess its like flying an 8lb model on one 05 motor; those ASTRO 05s sure have some power!. My girlfriend videod the flight.(She was suitably warned about videoing me on the retrieval swim). On watching the video it is pretty obvious something is seriously wrong. Also the propstrikes on the canopy were caused by reverse airflow lifting the flimsy canopy/hatch.
Its amazing how easy it is to overlook the obvious, but because I had tested the motor independently and then wired it up red wire to positive brush, black wire to negative brush I was absolutely sure it was right (I actually did not even consider the possibility of an error). The problem was basically due to wiring colour, stupidity and haste:
As can be seen the wiring colour to the center motor is arse about, I realised this when I built the model but forgot later on. This episode actually happened a while ago and my embarassment has now subsided to the point where I can talk about it.