Tim Mackey Posted May 12, 2009 Share Posted May 12, 2009 Anyone dabbling with electric flight stuff probably knows by now that the Kv of a motor is the RPM per volt applied, so theoretically, a motor with 1000Kv rating "fed" by a battery of 12V should spin at 12000 RPM. Rather than slow down dramatically as bigger and bigger props are fitted, ( like an IC engine woud ) the electric motor tries to maintain this RPM by simply drawing more and more current from the battery. Of course, in reality, the prop does slow the motor down a bit, and this has to reckoned with when choosing a powertrain and trying to assess the pitch speed etc. I have always worked on the basis that this RPM drop would be roughly 5% to 10% of specification, depending on just how much extra load we are putting on the motor, I did check this once against a couple of motors and prop combos some years back, but have not done so recently, or indeed with an EDF unit. In final prep for a maiden flight later this week ( I hope ) of my flyfly hawk, I just did some more eagle tree data logging, and found the following. motor is 1600 Kv, and under load pack volts are 21.33. Theory says that this should be spinning at 34128 RPM. The logged data shows peak RPM reached 32608, approx 5% lower, so..... not far off then This will change in the air of course, as the motor will unload a little ( EDFs not as much as props ) and also will vary with different motors and props etc. Still, its a fair guide I reckon? Edited By Timbo - Moderator on 12/05/2009 08:11:47 Quote Link to comment Share on other sites More sharing options...
Garry Pollard Posted May 12, 2009 Share Posted May 12, 2009 Thanks for that Timbo It has answered a couple of questions I was going to post Thanks again Garry Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 12, 2009 Author Share Posted May 12, 2009 Quote Link to comment Share on other sites More sharing options...
Tim Hooper Posted May 12, 2009 Share Posted May 12, 2009 Cheers Guv'nor! One of those little things that always made me take the theoretical predictions of Motorcalc with a pinch of the proverbial........ tim Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted May 12, 2009 Share Posted May 12, 2009 Intersting Timbo saves trying to count the revs,one of the flyers at the club checks his revs with a guage every flight Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 12, 2009 Share Posted May 12, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 12, 2009 Share Posted May 12, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 12, 2009 Share Posted May 12, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 12, 2009 Author Share Posted May 12, 2009 Ha ha I never got on too well with motor calc TBH - preferring instead to splash out a bit on a variety of props and use the wattmeter ( prior to eagle tree days ) to see for myself. The way I look at it is that props will almost always end up useful for something - even if its only stirring paint ! Stephen - trying to measure RPM on an enclosed EDF unit with 6 baldes etc is not feasible with a regualar opto meter - the eagle tree unit uses a pin probe inserted into one or two of the motor connections from the ESC - which can be easily inserted /removed....or simply left in place and connected with a small jst style plug for future use. i also have a range of other expanders for things like servo current draw, altitude, and throttle position sensing etc. All this can be logged during flight, and then simply downloaded to a PC at either the field or back home. Edited By Timbo - Moderator on 12/05/2009 23:09:43 Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted May 13, 2009 Share Posted May 13, 2009 Few, Timbo which part of your hobby do you enjoy the most it seems you take technically very studiously deeply ,then with all this information arrive to slope soar intent on destroying anything in your flight path,whilst downing copious sticky buns with the other hand or am I misreading your dialogue? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 13, 2009 Author Share Posted May 13, 2009 Nope.... spot on Stephen I think I enjoy final finishing /covering the least, and radio /powerplant installation ( and testing of course ) the most. Quote Link to comment Share on other sites More sharing options...
Stephen Grigg Posted May 13, 2009 Share Posted May 13, 2009 I do enjoy when a box of parts come alive Quote Link to comment Share on other sites More sharing options...
Gemma Jane Posted May 13, 2009 Share Posted May 13, 2009 The rest of the world has motocalc, we have Tim, cheaper, more reliable and capable of burning 50 billion calories worth of sticky buns in a day... works for me Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 13, 2009 Share Posted May 13, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 13, 2009 Author Share Posted May 13, 2009 How true Phil.... and fancy Gemma suggesting I was cheap - huh, I am gonna sulk now Quote Link to comment Share on other sites More sharing options...
Danny Fenton Posted May 16, 2009 Share Posted May 16, 2009 Hi Timbo, great information as usual. I have to agree with you, airbourne numbers via Eagle tree is the only way. Motorcalc etc never has worked for me either. Talking of sticky buns, are there going to be any at weston this year? Cheers Danny Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted May 16, 2009 Author Share Posted May 16, 2009 Hi Danny.... ( long time no "see" ) - dont think I will be doing Weston this year - my allowance for model shows has been used up with Rufforth, The Lakes slope in, Roodee, and WMAC....plus hopefully, one of the Greenacres days. Quote Link to comment Share on other sites More sharing options...
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