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egrave1 06-08-2014 02:37 PM

Motor type
I am still a little confutes on what motor to buy for a cretin airplane If I have a airplane that weights 5LB and I go with 150 watts per Lb do I buy a motor by wattage or KV ????

fhhuber 06-08-2014 03:07 PM

Watts more than kv... but it also depends n the style of airplane. If you don't have enough watts then you won't fly at all. If you have the wrong kv it won't fly well.

High kv for high speed style models and lower kv for slow flyers and 3D aerobatics. The high kv motors swing short props very fast which is great for going fast but bad for "static thrust"

egrave1 06-08-2014 03:24 PM

thanks that helps alot

max2112 06-08-2014 05:13 PM


Originally Posted by egrave1 (Post 950090)
I am still a little confutes on what motor to buy for a cretin airplane If I have a airplane that weights 5LB and I go with 150 watts per Lb do I buy a motor by wattage or KV ????

A lot of motor specification sheets spell out the intended performance range for that model#. That's what I usually go by.

As an example, the following spec.s were pulled from an E-Flite Power 32 web page:

Product Description\Key Features
  • Equivalent to a 30- to 36-size glow engine for sport and scale airplanes weighing 3.5–6 lb (1.6–2.7 kg)
  • Ideal for 25- to 36-size 3D airplanes up to 4.5 lb (2 kg)
So this motor could perform aggressive aerobatics on a 5 pound model. But not true 3D performance.

That is just one example, A lot of manufacturers include info like that.

It usually entails a lot of reading, but a lot of times you can find on the forums (here, RCG, etc) where someone has built that same model and talks about their setups.

hayofstacks 06-08-2014 06:00 PM

Kv is a determination of prop size and cell count.

a 600 watt motor with three cells might be 3000kv and have a 4.1x4.1 prop with would barely move a 6lb airframe. a 350 watt motor around 400kv running on three cells could have 180oz of thrust on three cells, but would require a 16" prop. there are quite a few variables, and they all matter.

you need to properly match all needs for the plane.

id reccomend using headsuprc at the very least as a guideline for what motors to look at. anything from a .25-.60 size motor may be appropreate for your plane. compare the cell counts of the motors to what you have and the size prop that would best fit the plane.

you moght want a 10" prop for ground clerance, or a 14" for lower watts and same thrust on the same motor, again depending on cell count.

i learned far more from reading on headsup site than almost anything else. another perk, send an email or call jeff at heads up, he will give you a recommendation, and you will not.be disapointed.with what you will end up with.

ron_van_sommeren 06-10-2014 05:56 PM


Originally Posted by egrave1 (Post 950090)
... do I buy a motor by wattage or Kv

By max.current, it is a better indicator than max. power. AND by Kv.

The Kv constant (motors have just one Kv, not 180Kv), is not a rating, not a figure of merit. More windings will give lower Kv, less windings will give higher Kv, that's all there is to it, no big deal, anyone can do that.
A motors Kv constant says nothing about a motors max.power, max.current and rpm.
Motorcurrent wants to go up with voltage squared and with Kv cubed.

About choosing Kv

Originally Posted by scirocco (Post 27290157)
Kv, while an absolutely critical part of the system, is actually the item one should choose last.

Decide your peak power requirement based on the weight of the model and how you want to fly it
Pick a preferred cell count (voltage) and pack capacity for how to deliver the power
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
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.

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. OTOH, 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, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

brushless motors Kv? - RC Groups

kyleservicetech 06-10-2014 05:59 PM


Originally Posted by egrave1 (Post 950090)
I am still a little confutes on what motor to buy for a cretin airplane If I have a airplane that weights 5LB and I go with 150 watts per Lb do I buy a motor by wattage or KV ????

Here is a whole bunch of information on this sort of thing.

Check Posting #5 in this thread. This is the size model you're looking at:

These models are slightly larger than 5 pound, but you might find something useful.

Thread on 70 size glow engine conversion to electric

Hacker 6S2P A123 powered Models

Hangar 9 Kantana Model

Hanger 9 Twist 40 Model (This model weighs exactly 5 pounds)

AEAJR's Site on Electric Power

dahawk 06-10-2014 07:53 PM

What's a cretin? Not familiar with the airplane. Wing Span? High wing? Low wing? LG?
I have a few 5# + models . Using a 32 on the Escapade but a 46 on the HK Beaver and Phoenix EMB 312 Tucano and GP Piper Cherokee.

+1 on Headsup RC as a quick reference to Motors, lipo voltage, Watts generated, amp draw by prop size, etc.

Sometimes getting the perfect combo takes a little experimentation.

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