Monday, January 17, 2005

Andre Kilian on a DIY outrunner motor

Herewith some results of my tests for using the AC VL DIY out runner brushless motor kit in a Zoom/Shogun micro heli (motor supplied with kit isa Speed 400).
I started off by drilling out the standard 2.3mm pinion to3mm, and used a shaft from a CD-ROM motor in order to be able to set the spacing between the motor and the pinion at 5.5mm.
As the motor pinion is placed between the main gear and the tail drive gear, there is not much room for changing the number of teeth on the pinion. As the pinion is also longer than normal, chances of sourcing alternative pinions are also very slim.
Another option would be to use two GWS pinions on the same shaft, but I tried this before I realized that the distance between the motor and the pinion is important.
Needless to say, my pinions were installed too close to the motor and I did not have any more 2mm pinions.
As I was on leave and had some spare time available, I took up the challenge to optimize the windings to suit the standard pinion which would also make it possible for others to replicate my results.
In order to get some indication of the performance of the motor I placed a 50mm wide and 250mm long ply plank on the skids, and weighted it down on my workbench. I set up the throttle curve to give about 85% throttle at zero pitch, and full throttle at full pitch (7 degrees as recommended).
My aim was to obtain about 70W from the VL motor at full pitch and full throttle, with a head speed of about 2000RPM.
My experience is that the standard Speed 400 motor gives about 80W on a fresh battery pack, and then drops to about 60W as the motor starts heating up. As can be seen from the results below the current draw on 9winds was way too low, so I tried 7 winds.
Satisfied with the results of the bench tests, it was off to the local cricket field for flying tests.
Performance was much better than on the standard brushed motor, as there was no power sag after flying for a minute or two), and aerobatics was possible using a 3 cell Kokam 1500 battery.
Flying time was more than adequate -after flying for more than 15 minutes it took less than 1100mAh to fully recharge the battery.
As I was aiming for about 10-15 minute flying time and it seemed that the current draw in the air was significantly lower than the static draw, it seemed that it would be worthwhile to rewind the motor with 6 winds.
This proved to be the ideal setup, with more than adequate power for full 3D aerobatics, and the motor only slightly warm.
Unfortunately we did not set the timer on the radio, but flying time was in excess of 10 minutes!

Winds Pitch Volts Amps Watts Blade RPM

9 0 11.6 2.4 27.8 1740

9 +7 11.3 3.6 40.7 1590

7 0 11.5 3.1 35.7 2250

7 +7 10.7 5.6 59.9 2070

6 0 10.6 6.0 66.8 2130

6 +7 9.3 10.4 96.7 1740

Regards, Andre

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