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Motor Help

#1
I am new to the hobby. We bought a cheap trainer with a togglable gyro. I am looking into venture into bugger airframes, so I built the FT Simple Storch. I bought the electronics from my local hobby store, assembled it, and subsequently crashed it. I have done some looking into the motor I bought and realized that is where my error was. The Storch has an 810kV minimum requirement, my E-flite park 400 has a rating of 740kV. It is the ELFM1300 instead of the ELFM1305. I have the recommended 10x5 prop, the recommended 3 cell 2200mAh lipo, and the recommended 30 amp ESC. Is there some change I can do to be able to use this motor or do I need to just buy a new one?
 

quorneng

Well-known member
#5
GWgameing
The Park 400 740 kV is a bit less powerful than the 810 kV version, 7A continuous against 10 A which could be significant however even the Park 400 810kv at 10 A (just over 100W power) is a rather meagre for a plane that weighs a bit over over 2 lbs.

I note the FT Simple Storch speed build web site specifies a bigger heavier Park 425 of 1200kv or equivalent. This is 220 W (20 A) motor on a 3s battery. This would give the Simple Storch a much more reasonable power to weight ratio of 100 W/lb.

I may be wrong but I believe you will need a physically bigger motor or use the Park 400 motor in a smaller lighter air frame.

Be aware that the kV of a motor does not indicate the power it can produce only the maximum rpm it can achieve for a given battery voltage.
Power is given by the maximum Watts the motor can draw from a given battery. Most motor manufacturers give this figure.
 

Merv

Well-known member
#6
I agree with @quorneng, Kv is a measure of RMPs per voltage, not power. The difference between 740kv & 810kv is not that significant. Wattage is the measure of power.

If you want to fly slowly use a low Kv motor, 1,000 or lower. If you want to fly fast use a high Kv motor, 1,800 or higher. If you want a compromise use something in between.

Think of Kv somewhat like a transmission, low Kv is like low gear, it will deliver the motors power at low air speed. High Kv is like high gear, delivering power at high air speed.
 

FDS

Well-known member
#7
If you have a difference of 100kv more throttle will equalise the rpm difference, you can also use a slightly higher battery voltage if you want more RPM.
 
#9
Thanks for all the suggestions and comments, they came quicker than expected. I decided to order a 1300 kV 210W motor. I will likely assemble a mini-series plane and use the park 400 on that.
Just a quick question, what was the cause of the crash? Did it feel underpowered or was it just pilot error?
I believe the motor to have been underpowered. I had to add some weight to the nose to get it within their CG spec. When that weight was removed (only about 100 ounces) the plane flew until the tail heaviness kicked in. Some of this extra weight was due to the too-small motor. Hopefully, this new one will correct the issue, if it doesn't then it is likely operator error.
 

BATTLEAXE

Well-known member
#10
A Park 400 740Kv should pull that plane around just fine, plenty of torque. Which means that you can use a larger higher pitch prop. An 11x6 or 11x7 could work fine on 3s. You could step up to 4s with your 10x5, there is where your power would be. Mind you if =f you are new to the hobby, no offense, but it could more then likely be a lack of experienced pilot error. It could be anything from a tip stall, to much up elevator , not enough air speed, reversed controls, wind or a gust of gravity. So many variables to count. That Park 400 power combo you have would be awesome in the Scout