Akom's Tech Ruminations

Various tech outbursts - code and solutions to practical problems

Sapac Pitts S2A electricals and motor replacement

Posted by Akom • Monday, October 6. 2008 • Category: Toys

I killed the motor. Apparently ramming into pavement at full throttle (don't ask. From now on, when something goes wrong on takeoff, I abort) is not such a good thing for a long thin shaft and a firmly bolted prop. A few flights later, the motor died and started singing instead of spinning. One of the windings seemed to be busted (based on measuring voltage with a drill).

Measuring Current

Anyhow, I did some measurements when it was all good, and here are some specs: With the stock Sapac 2830 motor and the 3-blade 9x8 prop, the battery put out as much as 12.5A at full load (my measurement, massive analog meter). The battery is 10C, 1200mAh, meaning that 12.5A is about as much as you could ever expect it to put out, in other words - running dangerously high. I also got a 10C 1800mAh battery and the plane had noticeably better thrust (haven't measured the current), so it actually did want more than 12A!

I did some research on motors and decided to replace it with the Scorpion 2212-26 Brushless Motor($35) and a GWS 1060 prop (10x6). A lot of reading various forums and opinions. People seem to like the Scorpions (in a wow, I didn't expect this level of quality from a discount chinese part kind of way), so I spent an extra $5 or so. The motor is nice, bolted on the same way, but then I had to figure out how to use a wobbly mount (prop saver) because the motor shaft is not long enough and not threaded to mount the prop the way it was. In retrospect, it is much safer for the motor.

Prop Saver instead of a nose cone


The plane performs overall about the same as it did with stock hardware. There may be a slight improvement (this was not my goal anyway), not sure - I think it recovers from turns and loops a little better. What did change though, was the battery life - it seems that I got about 20% more flight time out of the batteries. But then again, it could just be in my head. I measured the new motor current as before, and it was exactly the same - 12.5A momentary, then full throttle levels out at about 11.5A. The same happens with both the 1200 and the 1800 battery, which suggests that this is all it wants, or the ESC simply cannot provide anything more. (Plane claims to have an 18A ESC)

Do Not Mount This Way!


This is what happens

I first mounted the motor exactly as the stock unit, but the screws were not long enough to use washers. As a result, by the second flight, the screws ripped through the plastic of the engine mount, causing the motor to fall in (mid flight, mind you), and twist the wires around the shaft, stripping some insulation and miraculously not frying the ESC. The prop was slightly injured from rubbing on the nose shroud. (I did land reasonably well though, I'm getting better).

I headed to a hardware store, picked up some longer screws (the tiny screws are hard to find! The scorpion uses 3mmx0.50 screws) and some washers, and then finally used the cross as a giant washer. Seems pretty sturdy now.

Properly Mounted Motor

Update: it is sturdy.. but not sturdy enough to withstand a 15 foot drop onto the motor: Rebuilding The Front End

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