Hi All,
I finally have some data from my electric experience from last year until now. (I hope that I can get these attachments to work on the forum) I started testing with one of my own old stunters and then went on to design my Electron as a purpose-built e-stunter from scratch. As some of you know, I burnt out a couple of the MVVS 4,6/840 in/out runners plus some Jeti Spin esc's, so I needed to do some more research and fiddling with all of this stuff. I now have an Eagletree data recorder with an additional temperature probe, so I thought that it's time to share my findings.
In case someone needs to know why I chose to start out with MVVS motors and not AXI's, or any other makes, this is quite simply because I'm the South African agent for MVVS, and since they ventured into the electric game, they sent me a few samples to play with. I was most impressed by the quality of workmanship, and they very idea of an out-runner inside a casing seemd to me like a good idea. Just the concept of not worrying about a rotor touching anything in the nose, is a good idea in my book. After burning my motor out plus one of Igor's in the world champs, I was totally puzzled as to the cause. This was a puzzle because it had worked so well for some time before the champs. The only change on the days when the motor's popped, was that it was a few degrees hotter. After each flight I checked the temperature and it was never so hot that you couldn't touch the motor. The MVVS factory said that the temperature must have gone over 130 degrees, but there is no way that it was ever over more than 70 degrees. They did state however that this 4,6/840 is rated as a 4,6 cc (.28 cu.inch) glow motor and that my 1920 gram (68 oz)stunter was too much for it. Maybe they do in fact, know what they are talking about?? Igor kindly lent me his AXI 2826/10 for my final rounds in France, and it certainly felt to me (this is very subjective) that it was not as powerful as the MVVS, but maybe the AXI is also not really made for my overweight model. Just by the way, the target weight was 1700 grams (60 oz.), but I found that making the Electron fully take-apart, this target was difficult to achieve.
I now believe that it was most likely the high current spikes that took out the windings and not that it was too hot. The other problem with this in/out runner, is that I used the Sport version because it has a built-in fan. I thought that this is also a good idea, but the problem is that the fan is in front so the air is sucked through the rear. The Electron's nose is made with cheek cowls to feed air in the area behind the motor, but in hindsight, this does not work well.
I then bought the bigger MVVS 6,5/910 which is their .40 size glow equivalent. It also has an integral fan, but you mount the motor either way around. My first attempt had the motor with the fan in front, because it was easier to mount. It had huge power and felt good, but a little hot to the touch, so I swung it around, and this is when the Eagletree recorder arrived. I put the tiny temperature probe right inside against the windings, and you can see the results in the chart. I do not have any fancy thermal compound, but I don't believe that it will be that much hotter anyway. I found it interesting to see just how much the temperature rises after switch off. I was told by someone that I must always let the motor cool down properly before flying again. This did not seem reasonable to me, so I did two flights with just a short stop to change batteries. The chart shows the 3 flights that I did, and flight#1 used more current and became hotter, due to the battery pack losing voltage prematurely. I run in governor mode, and of course this means that the current will go up to try and keep the rpm constant if the voltage drops. Flight#2 shows that the temperature simply goes back down to normal operating temperature, and starts all over again, so in fact, this is no problem at all. Flight#3 was much later, but for this flight I blocked up 3 small (6mm) holes on top of the nose just behind the spinner. The holes were drilled to see if I could get some more air into the mouth of the motor, and I really thought that were not doing much, so I blocked them. As you can see, the temperature was higher, so like the man says "Heat is your enemy......!!"
After quite a few more flights with the Eagletree, the high temperature after switch-off is around 58 degrees C, and this is with a summer's day of around 30 degrees C. I think that the Eagletree data recorder is a most wonderful toy to have when playing with this electric stuff. I've done so much "surgery" to my poor Electron now that it's a mess, but the testing is certainly worthwhile for me. Once I am reasonably happy with the installation, I intend to re-build the nose section once again.
In the meantime, here are the main details of my set-up with the MVVS 6,5/910:
Line length: 20,750 metres (68 ft) from the centre of the model to the centre of the handle. The flying line is PAW Staystrate steel line of about .014” (0.355 mm)
Prop: APC 12 x 6 E set at 9000 rpm with governor mode on Jeti Spin 44 esc.
Lap times: 5.1 to 5.2 secs/lap
Data: Eagletree V3 data recorder with temperature probe inside motor windings
Altitude: 5500 ft. (1676.4 Metres) ASL Air temperature 30 to 32 degrees C.
Weight 1850 grams (65.26 ounces)
I hope that some of you will find these results interesting.
Keith Renecle