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What do motor size and kv translate to in flight performance?
#1
I'm looking at getting the LDPower FR2305 motors but they come in 3 different variations.  They have a 2300kv, 2450kv and 2600kv.  
They are also abnormal in that they are 2305 when we usually see 2204, 2205, 2206, etc.

Which got me wondering, what do those numbers translate to in terms of flight performance?
Will 2600kv motors have more acceleration than 2300kv?  More top end?  Eat more battery?
If I wanted a smooth and agile quad and don't care about top speed, would a lower KV be better or worse?
I've also seen some people build "smooth" quads with 2000kv motors.  What about that kv makes it "smooth"?
#2
KVs equal how fast your motors spin per volt. So as an example a 1500 kv motor spins 1500 times a minute per volt. Therefore if you are using a 10 volt battery, the motors will spin at 15,000 RPM. (This is without props on) I kinda relate kv to torque. A lower kv motor, say 700kv, has a lot of torque so you're able to use a much bigger prop. Whereas a 2300kv motor has less torque so the prop needs to be much smaller. So yes, a lower kv motor with larger props is going to be smoother, but eat more power.
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#3
So a 2600kv motor on 5" props is going to use less power than a 2000kv motor on 6" props.
That is the opposite of what I thought it would be.
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#4
There's lots of variables like prop pitch and motor amp draw. I wouldn't know where to start to do the math, I typically just dump my info into ecalc and see what results I get. But basically, if you had a 2000kv and 2600kv with the same size props, it would take more volts for the 2000kv to spin at the same RPMs as the 2600kv.
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#5
(01-03-2017, 07:44 PM)RENOV8R Wrote: There's lots of variables like prop pitch and motor amp draw.  I wouldn't know where to start to do the math, I typically just dump my info into ecalc and see what results I get.  But basically, if you had a 2000kv and  2600kv with the same size props, it would take more volts for the 2000kv to spin at the same RPMs as the 2600kv.

But in that direct comparison, when using the same prop same battery, amp draw might get much higher for the 2600kV motor, while the 2000kV motor might get away with a lower rated esc, for example.

Test results from miniquadtestbench show that a 2305 does not gain much over a 2205, aside from increased current draw under load. A higher stator (à la 2206) makes a bigger difference in delivering torque (what you need to drive the heavier, 3-blade etc, props).
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#6
Theoretically, a bigger prop is normally more efficient, and a 6 inch producing the same amount of thrust as a 5 inch will use less power due to less drag because of the lower spinning speed of the 6 inch prop. The disk area is also bigger which also will provide more thrust
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#7
So let's say I like long flight times, smooth acrobatic flying (with occasional punches) and have 20A escs. What would be an ideal motor stator and kv rating for that?

What about for flying more oriented toward racing?

What about if I only had an 18A esc? Would that change much?
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#8
My 2 cents: I don't think one (or even two) factors alone can answer any of your questions.

My thoughts (and these are generalities):
If you want long flight times, then you want higher efficiency (thrust delivered per milliamps consumed).
If you want more agility, then you want less mass to throw around the sky (smaller quads tend to be more agile).
If you want speed, then you want higher pitch props and/or higher KV.
IMHO, if you want smoothness, then practice stick control, and tune your quad to the 9's.

It's a very difficult question to answer what's best for a 20A esc. I've got 20A esc's on a 130 quad with 3" props and 4s battery that is very agile, but is a real beast. Maybe someday I'll fly it smoothly - but it won't be soon... Big Grin
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