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Pumpkin drop event

Motor application

#1
I recently asked a question on the comments section of the FT Spear build but unfortunately no-one took time to reply so I wondered if anyone out there could help out? I have built 3 or 4 FT models so far and I would love to try the Spear. I've never flown a wing before. My question is that I have a new EMax 2215/12 905kv motor which I purchased by mistake, I thought it was the 1180kv version? So can anyone tell me if this will be suitable for use on this wing? also what would be the choice of prop to go with it please. If not what would it fly? Hope someone can help? Thanks
 
#2
Well I don't fly wings but I can answer your question by comparing the motor you list versus the Spears specs. The Spear calls for a 425 sized motor 1200 Kv minimum and 9 x 4.5 CW (reverse) prop. The EMaxx motor is a 905 Kv that needs a 10x4 or 11x4 prop. so it would not meet the the requirements. The 905 Kv is a slow speed and that makes it a big prop spinner. The limitation of the Spear is prop diameter as it rests between the elevons, so to get by the need for smaller diameter prop you need to spin it faster so 1200Kv with a 9x4.5 prop works to get the needed thrust. I don't know your level of motor understanding but the basic kv formula works like this 1 volt x kv equals prop RPM. So a 905 kv x 12v battery would be around 10860 RPM and on a 1200 kv x 12v= 14400 not a huge number difference until you factor in that the theoretical distance a prop moves forward in one revolution is its 2nd number so a 9x4.5 prop moves 4.5 inches forward in one turn multiplied by its max RPM of 14400 is 64800 inches or 5400 feet and the 10x4 @ 10860 is 43440 inches or 3620 feet. Big difference. To complicate it even more is easy to do, adding in diameter, which I won't do! So lower kv lower speed but more power (torque) high kv high speed less power (torque) Anyway hope this helps your understanding of motors somewhat, I am no expert on all this, it is just what I have learned in the last 3 years.
 
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#3
Thanks Vipertech for taking the time to reply. I found your info very helpful, I'm a retired motor engineer so I did have an idea about torque/revs ratio as with low revving high torque diesel engines and low torque high revving petrol engines. I just didn't have a clue on what kind of speeds/prop matching it takes to take off so thanks again for the info at least I have an idea what type of plane I can use the motor on now.
 
#4
You are welcome! Great comparison diesel vs. petrol (gas for us USA guys). That's exactly the way I would compare it as I am an auto tech in real life and like you I have a good understanding of engines and the way they develop torque.