

Feb 19, 2002, 05:05 PM  

Jochen,
I too have an indoor fly coming up on Friday and a Micro IFO to finish and some changes to make to my Punkin bipe. So, doubtful I'll do any measuring constants between now and then. Here's your N20 post reposted here from the Gearbox thread http://www.rcgroups.com/forums/showt...n&pagenumber=1 =============== I've made some measurements and done some calculations. The N20 motor I've examinated has the following characteristics: c_n ~ 7000 rpm/V c_M ~ 1.36 mNm/A Ri ~ 2.5 Ohm I_0 ~ 0.17A (with gearbox, without prop) The following formulas give the basic data at every operation point: U: voltage [Volt] I: current [Ampere] n: rpm n = c_n * (U  Ri x I) [rpm] M = c_M x (I  I_0) [Nm] P_el = U x I [W] P_mech = (U  Ri x I) x (I  I_0) [W] eta = P_mech / P_el x 100% [%] These formulas show that the N20 will have about 50% of efficiency at about 5.2V and 0.7A. That is quite good for a motor of this size, the N20 is not an amphog, but a good performer *with proper gearing*. Always let him turn, don't overprop him! This combined with prop data gives some interesting insight. The Günther prop (125mm dia, 110mm pitch) has the following characteristics: static thrust produced: F ~ n_prop^2 / 0.84e6 [g] mechanical power consumed: P ~ n_prop^3 / 49e9 [W] So assumed that a single stage gearbox has an efficiency of about 90% including all bearings (don't be too optimistic here) you can expect >21g of thrust at 5.2V and 0.7A and a gearfactor of about 1:5.5. Now you know why I would never treat this motor with high current and low voltage. This motor is hotwound, let him turn! And you know why I built a DC booster to get 5.2V out of one single Lithium cell. It is quite interesting and instructive to type these formulas into an Excel sheet and to play around with them. Always take into account that the data differ from motor to motor, even of the same type. ============== and another post ============== you need two multimeters to measure amps and volts simultaneously, a tacho to measure rpm and (if you want to investigate props) a scale to measure prop thrust. Everything else can be done by calculations. I don't have a dynamometer. In fact with a bit of clever calculation you can use the motor that you investigate as dynamometer at the same time. The Günther prop will pressfit on 22.3mm shafts. I use a 2mm carbon shaft in my gearboxes. Gordon, my N20 is from allelectronics.com. I think that is the lowvoltage version. Resistance is about 2.5 Ohm. Regarding the flywheel: a bit of numerical math a la RungeKutta, and we will even deal with this. No black magic is involved (though the efforts would be a bit bigger). But I don't think that we need this. Or do you want to perform a short takeoff by revving up a variable pitch prop to the maximum and then giving more pitch suddenly to create a thrust burst? And besides that, the N20 does not have a second shaft end at the back side which would be needed to put the flywheel to the proper highrpm place. ========= and another ========= just to make it clear: the load does not need to be constant with the method I use. You *need* to have two different loads if you don't change voltage. The easiest way is to use two different props at the same voltage (if the props differ too less then the calculated values could be inaccurate). ========= and one from Gene Bond ========= I see what you are doing now. Makes sense. Didn't realize we were on the same page, where you change one variable, and calculate the constants! 
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