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Sig Kadet LT-40 ARF EG Set Up

Old 10-11-2020, 05:09 AM
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Foxbat711
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Default Sig Kadet LT-40 ARF EG Set Up

I am getting back into the hobby and decided to go electric.....
I bought a Sig Kadet LT-40 ARF EG...
The plane "weighs" 6#......
SIG recommends a 500-800 watt Motor.... 50-75 amp ESC.... 3S-4S 4000-5000 Lipos...... and a 11x7 or 11x5 E Prop..... a Kv Range of 800-1000
I would like to use a Great Planes RimFire .46 42-60-800 Outrunner Brushless Motor.....
The specs for motor are....
Max. Constant Current: 60A
Max. Surge Current: 100A
Max. Constant Watts: 1110W
Burst Watts: 1850W
No Load Current: 4.6A
Input Voltage: 18.5-22.2V (5-6S LiPo)
RPM/V (kV Rating): 800
Weight: 9.5oz (268g)
Suggested Propeller Size: 10 x 5E-11 x 5E

Would this motor be suitable? Using 6S Lipos? What size ESC? Can you oversize the ESC?
Thanks for any support, Jim
Old 10-11-2020, 09:30 AM
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ron_van_sommeren
 
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Originally Posted by Foxbat711 View Post
...What size ESC? Can you oversize the ESC? ...
Nope, you can't oversize an ESC, there would only be a weight and size penalty.
An ESC does not force feed motor with current. At WOT, wide open throttle, the ESC has very little effect on current, it only effects by way of timing ('pre-ignition', because it takes time to build/collapse a magnetic field).
The ESC passes on the current asked/demanded by the system (pitch, #blades, voltage², Kv³, diameter⁴).

It is a good practice to have some headroom built in.
About derating motors, controllers, batteries, electronics in general:
Prettig weekend en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 10-11-2020 at 09:43 AM.
Old 10-11-2020, 10:51 AM
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Ron, have you been submitted for a Nobel prize in Physics? In over 50 years in electronics I have never seen anyone explain so beautifully how to create so much power out of nothing. Wow. Take a 500 Watt motor running on 3S, Put it on 4S and you now have a 1200 Watt motor. Amazing.

Last edited by rgburrill; 10-11-2020 at 12:20 PM.
Old 10-13-2020, 03:26 PM
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If only ...

And only if the motor can handle the extra power drawn, by a factor (4/3)³=2.3, and only if it can handle the extra current drawn, by factor (4/3)²=1.7.

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 10-14-2020 at 09:07 AM. Reason: B
Old 10-18-2020, 12:07 PM
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Originally Posted by Foxbat711 View Post
The specs for motor are....
Max. Constant Current: 60A ... ...
Max. Constant Power: 1110W ... ...
Kv : 800rpm/volt ... ...
Below an excellent quote about motor selection.
From
brushless motors Kv?.
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

Last edited by ron_van_sommeren; 10-18-2020 at 12:14 PM.
Old 10-18-2020, 12:17 PM
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Note that Kv says very little about max.power or max. current a motor can handle, or about max. torque, or propsize. A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same Kv = 2000pm/volt.

Kv is the velocity Konstant, a physical property (like mass, length and volume), expressed in the unit rpm/volt (like kg, meter and m³).

Kv is not a rating, not a figure of merit, not something the motor delivers like power and torque(explanation).
A Kv=2000rpm/volt motor on 10volt and a Kv=4000rpm/volt on 5volt will give same (no_load)speed of 10,000rpm, provided the motors can handle the current and power.


E-motor versus ic-engine behaviour
They have a fundamentaly different behaviour.
An engine tries to keep a constant torque: as load goes up, rpm goes down.
An e-motor tries to keep a constant rpm, no matter prop-pitch and -diameter: as load goes up, torque must go up, and therefore current has to go up.


It's all about what the motor wants to do versus what the motor can do.
Kv matches desired rpm and battery voltage, there's nothing more to it.
  1. Want, try: Kv
    Kv and voltage determine how fast motor wants/tries to run
    (rpm_noload = voltage × Kv, or, in other words, Kv = rpm_noload / voltage.)
  2. Current and torque
    RPM and prop determine torque needed, which in turn determines current drawn
    (current = torque × Kv, in SI units!, or proportional to Kv³ ).
  3. Can: max.current and max.power
    Max.current and max.power determine whether that battery/motor/rpm/prop combo can run without going up in smoke.
Old Yesterday, 02:54 AM
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Give it a rest.....
Old Yesterday, 02:21 PM
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Originally Posted by Foxbat711 View Post
I am getting back into the hobby and decided to go electric ...
...Thanks for any support, Jim
................

Last edited by ron_van_sommeren; Yesterday at 02:23 PM.

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