MAAA Wingspan

20 I was a little disappointed with the performance of an electric model I recently purchased second hand and didn’t really know where to start tomake somemeaningful modifications. Right–o-then, so after a little research, (and I don’t claim to know everything about electric r/c modelling) I have learnt some good basic knowledge to get me sorted with themod’s with better power and knowledge to purchase the right parts. So, here’s what I learnt.… There are a couple of items you will need to purchase to put into your tool kit: 1. Rev / min counter (optical tacho) youmay already have one 2. Watt-Voltage-current meter 3. Calculator (there is a bit of maths needed to work it out) What does KV on a motor mean? Ok, in electrical terms KV = kilo volts, however inmotor terms K = RPM thus meaning RPMV = RPM per VOLT. So, this means that if you have a 1400kv motor and you intend to use a 3B cell li-poly battery, you then take the source power voltage andmultiply it by themotor KV. • 2 cell li-poly = 7.4 volts • 3 cell li-poly = 11.1 volts • 4 cell li-poly = 14.8 volts • 5 cell li-poly = 18.5 volts • 6 cell li-poly = 22.2 volts For example: Formula is RPMV where RPM = 1400 and V = 11.1 Therefore RPM x V = 1400 x 11.1 = 15,540 rpm Let’s say you have a 3600 kv motor and use a 3 cell li-poly battery, what would be the rpm? RPM x VOLTS = 3600 x 11.1 = 39,960 rpm What if I don’t know the KV of the motor? Ok, so using your optical tacho and the volt-watt-amp meter, connect a li-poly battery, run themotor up to full power and get the tacho and voltage reading. The test motor I used gave a reading of 26,400 rpmand I use a 3 cell li-poly. For example: Using RPM / V=KV , rpm = 13,400 and V = 11.1 Therefore RPM / V = 13400 / 11.1 =12,070 KV Thus meaning themotor is probably around the 1200 KV area. Why is KV important? The KV is important when I’m choosing the right motor for the airframe I want to power. For example a largemodel, such as with an 80-inch scale warbird, I’m going to want a low KV motor so it will swing a large propeller (22” x 10”). If you were to choose a high KV motor chances are the propeller would not protrude out past the cowl. • Low KV = high torque and lower rpm • High KV = low torque and high rpm Low KV motors are commonly used in larger slower aircraft and high KV motors are used in smaller quicker aircraft. What Is Power? Ok, this is where OHM’s law comes into play. OHM’s law states that V = I x R, where: • V = volts • I = current (amps) and • R = resistance (ohm) Therefore, using OHM’s law, we also know that power is measured in watts thus giving W = V x A where: • W = Watts • V = Volts • I = Current (amps) JOE ‘WISE’ WANGID: GETTING STARTED WITH AN ELECTRIC R/C MODEL AIRCRAFT