stunthanger.com
Electric Stunt => Gettin all AMP'ed up! => Topic started by: john vlna on June 27, 2010, 07:58:37 PM
-
I am new to Electric CL. I have converted 4 glow planes to electric and they all have worked, but I am trying to figure out what the difference is between the current draw I measure on the ground (static) vs. in-flight current draw. I purchased Motocalc to help me get started, and frequently it shows a big difference in static and in-flight. The motor results from Motocalc are in the attached example for an Alpha 480. Motocalc predicts that Static is 21A and in-flight only 12.6A. My static current was 17.8 with a MA 9X7 prop, measured with an Astro wattmeter. Motocalc's diffference between static and in-flight seems very large, and the RC guys tell me that the real difference shouldn’t be that great. Without a flight recorder I can’t say for sure. Anybody ever looked at this issue?
-
I usually figure that the static current draw is about 20% higher than flying. Your mileage may vary. The calc programs might be using the motor at full throttle while the C/L stunt group usually try to keep the motor at about 75 to 80%.
-
Hello John,
None of these calculators takes into account the action of the ESC's RPM governor. For "normal" RC-type wide open throttle running, the staic current will be anywhere from just above the same to 10% or 20% higher under static conditions than in a verical climb at flying speed ... but the governor completely changes the picture.
The governor changes the "throttle" in order to maintain PRM, so that the current on the ground is often 130% or 140% as much as in level flight at the same RPM.
When you use the calculator, do you assume the full battery voltage? I'll bet you do, but the ESC/governor is probably only applying 75% to 85% of the battery voltage. There's your big difference.
Hope that helps,
Dean P.
-
And then you have the aerodynamics. A thin wing has considerable less drag than a thicker one.
Bottom line fly the plane see how much current you need and size the battery appropriately.
-
Some interesting comments. I was testing a Brodak Clown with a turnigy 35-30 1400kv motor today. Motocalc shows more current and less rpm than I measured. I think sometimes this has to do with the motor constantsbeing not exactly as spec'd. Motocalc also shows a big difference in-flight from static, 22 to15.6 A, Static RPM was 11.1K I measured static at 15A and 16.2K RPM, it is like I have a different motor.
-
Dean
By the way when I make measurements I use a fresh battery and Astro Wattmeter. It does show, I believe, the actual voltage, for example in my previous post the simulator showed 10.7v and that is pretty close to what the wattmeter was reading. In fact on everything I have measured the one parameter that always is close is voltage. It loooks to me that most motors cause about the same drop from the battery. I am not quite sure why.
john