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It’s drawing Watt?!

Posted by Tom Benedict on 18/04/2013

As part of the whole spate of orders I placed when jumping into the RC airplane hobby, I picked up a wattmeter. The one I got is pretty cool. It’ll tell you (semi) instantaneous current draw, power draw, and supply voltage, as well as the peak current, peak power, and minimum voltage. It’s got two inputs, one for high power systems (think battery/ESC/motor) and one for low power systems (think battery/Rx/servos). For me, this is perfect. I can use it to characterize the servos in my planes, find out the stall torques on each of them, how much I’m likely to draw when pulling out of a dive, etc. And I can also use it to characterize the servos in my KAP rigs, estimate run time, see from current draw how balanced my rig is, etc. And I can use it to measure current draw on my video transmitter, receiver, and monitor. In essence I can finally find out how big a battery I actually need for each application, and get the run time I want out of each one.

My KAP gear works well as-is, so I started with the planes. I hooked it up to my Bixler and fired things up. The Turnigy TG50090M servos I put in the flaps draw 0.6A stalled. Put two together, and a full on airbrake landing can potentially draw up to 1.2A. Not too bad. Then I fired up the motor and ran it up to 100% throttle: 13A. At 12V (give or take), that’s 156 watts of power to run the motor at full throttle. I have a 25A ESC on the plane, so that’s not a problem. The BEC on the ESC can supply 2.5A to the servo system, so as long as I don’t hammer it too hard, I’m safe. Excellent!

Then I put it on the Raptor. I have six of those TG50090M servos in that plane, so there’s the potential for over-drawing the BEC if everything is working at once. Since this is largely a thermal glider, I don’t expect to stall all six servos simultaneously. But it’s something to be aware of.

Then I spun up the motor… CRIPES! The current meter went up to 45A before I shut it down. It’s got a 30A ESC in it that can handle 40A peak current draw. Already I was exceeding what the equipment could handle. That’s when I did the math… 45A @ 12V = 540W. It’s drawing Watt?!

I honestly didn’t expect this plane to draw half a kilowatt of power. The thrust was tremendous! But after I shut everything down as quickly as I could, I felt the ESC (warmish) and the motor (HOT!). I knew I couldn’t run this way. I played around with the endpoints on the throttle, and dialed down the upper end to about 70%. That kept the constant draw under 30A, but the motor was still running incredibly hot. Hot enough that the prop and spinner were getting warm.

That raised red flags in my head. The shaft on this motor is steel, and the collet style prop adapter is aluminum. What this means is that if the temperature goes up high enough, the prop adapter will become loose on the shaft just from differential thermal expansion. Clearly something had to be done.

I asked on the RCGroups forum, and was advised to cut cooling ducts into the plane: two in the fuselage next to the motor, and one more in the canopy to cool the ESC and battery. Exit vents underneath the wings finish the job, and should serve to remove at least some of the heat generated by the motor and ESC. A few weeks ago I put together a CAD drawing of a NACA submerged duct, based on the original NACA report. What timing! It’s easy enough to scale the drawing to the right size, trace the ducts onto the fuselage and canopy, and cut them out using a Foredom and some needle files.

So the maiden flight of the Raptor has to wait another couple of weeks while I ventilate the fuselage and wait for the new ESC to arrive: 40A constant, 55A peak, and a 3A BEC to boot.

But here’s the real funny: The Bixler 2, with its 13A draw on a 2200mAh battery, has a total run time of ten minutes at full throttle. The Raptor with its 1300mAh battery? 2.5 minutes. My hope is that with that much thrust and with the plane’s ability to catch thermals, though, their actual in-the-air flight times will be comparable. I’ve flown my Bixler 2 for over an hour on a single charge. I hope to see the Raptor perform as well or better.

– Tom


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