What size motor for my plane??
10-07-2012, 04:43 PM
#1
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What size motor for my plane??
Hey guys, I made a cardboard jet ( yes i know foam is better but i just had cardboard around ) and I am using an exceed rc rocket 1820 kv, bluelipo20c 7.4v 1500 mah battery, and a dynam rc 70a esc, ( yes i know it's an overkill ) i Tried it with a 6x4 prop and it didn't have enough thrust to get it going, and then the 7x4 had just a bit more thrust.
Here's the question:
I am not familiar with kv ratings and what exactly they mean, i was looking at going for a 3000kv exceed rocket or a 2850 exceed rocket, i just don't know what will be stronger? mayber lower kv... IDK!! haha, I also want quite a bit of speed.
Here's the question:
I am not familiar with kv ratings and what exactly they mean, i was looking at going for a 3000kv exceed rocket or a 2850 exceed rocket, i just don't know what will be stronger? mayber lower kv... IDK!! haha, I also want quite a bit of speed.
10-08-2012, 04:32 AM
#2
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RE: What size motor for my plane??
You're skipping an awful lot of knowledge that you need for this project in going straight to motor recommendations. I'll offer the simple version so you can figure this out for yourself.
1: Watts per pound- weight your plane with all the components and add in the battery and motor you're considering. A solid minimum is 100 watts per pound, but if you're wanting this thing to haul you should be around 150-175 watts per pound. That tells you what wattage of motor you should be considering.
2: Kv- This is a measure of how many rpm a motor makes per volt of battery it gets. There is always some loss so numbers will be a touch lower in practice, but theoretically speaking a 1000Kv motor on 12v would turn 12k rpm. It doesn't matter what prop is on the motor for this number because the motor will draw as many amps at it needs to turn that rpm. The issue you'll see is when you over prop the system your batteries will either not be able to keep their voltage up under load which will cause an rpm drop, or you'll burn up your motor or ESC because you are exceeding the wattage limit. Therefore, prop selection is a big variable in setting up an electric power system.
3: Airframe design- There are no hard formulas here, but rather just practical experience. A draggy airframe needs a big diameter prop to pull it through the air, so with a given wattage requirement you'll choose a big prop and a low Kv motor to drive it. So you could compare two systems set up for 400 watts of power, one using a 10x7 prop and another using an 8x4 at a much higher RPM (higher Kv motor). Both pull the same watts from the same battery and speed controller, but the flight characteristics will be really different. The high speed/small prop system will have a lot of top end speed for a slippery airframe, while the other system would haul around a draggy biplane or Cub with authority. If what you've built is something you came up with yourself, all you can really do is look at the manufacturers' recommendations for power systems of similar planes to estimate what would work in yours.
1: Watts per pound- weight your plane with all the components and add in the battery and motor you're considering. A solid minimum is 100 watts per pound, but if you're wanting this thing to haul you should be around 150-175 watts per pound. That tells you what wattage of motor you should be considering.
2: Kv- This is a measure of how many rpm a motor makes per volt of battery it gets. There is always some loss so numbers will be a touch lower in practice, but theoretically speaking a 1000Kv motor on 12v would turn 12k rpm. It doesn't matter what prop is on the motor for this number because the motor will draw as many amps at it needs to turn that rpm. The issue you'll see is when you over prop the system your batteries will either not be able to keep their voltage up under load which will cause an rpm drop, or you'll burn up your motor or ESC because you are exceeding the wattage limit. Therefore, prop selection is a big variable in setting up an electric power system.
3: Airframe design- There are no hard formulas here, but rather just practical experience. A draggy airframe needs a big diameter prop to pull it through the air, so with a given wattage requirement you'll choose a big prop and a low Kv motor to drive it. So you could compare two systems set up for 400 watts of power, one using a 10x7 prop and another using an 8x4 at a much higher RPM (higher Kv motor). Both pull the same watts from the same battery and speed controller, but the flight characteristics will be really different. The high speed/small prop system will have a lot of top end speed for a slippery airframe, while the other system would haul around a draggy biplane or Cub with authority. If what you've built is something you came up with yourself, all you can really do is look at the manufacturers' recommendations for power systems of similar planes to estimate what would work in yours.
