[What Determines a Motor Power]

On 04/10/2023 at 18:11, Nigel Heather said:

Say I have two motors, one a 3648-900 and the other a 4050-900.  
 

The 4050 is quoted as being more powerful, more watts but if I attach the same prop and connect the same battery, the prop will rotate at the same speed because they motors have the same kV.

 

So what cause the 4050 to be more powerful than the 3648.  Is it because it can swing a bigger, pitchier prop.  But if you swing the same prop is it a waste, are they same power motor in that case?

I'm a bit late to the party, but you may compare different motors by entering your data into this online calculator. https://rcplanes.online/calc_motor.htm
You may open separate windows for different motors and put them side by side to compare the results.

Then click "Print Screen" on keyboard to copy the results into "Word" or "Paint" if you want to save and/or print them.

[ESM Focke Wulf center of gravity?]

If you know or measure the plane's wingspan and then measure it on the drawing you've posted, you may find how much percent smaller the drawing is compared to the actual model and then calculate how much 140mm corresponds on the drawing.

 

Hope it made sense.

 

 

[Calculating Thrust]

This site might also be of help:

https://rcplanes.online/setup_guidelines.htm

[What Size Electric Motor/ Prop??]

1 hour ago, Peter Wedlake said:

In 1990 I built a 70" Dornier Do 335 which flew with 2 brushed motors and 35 nicad cells complete with air retracts, we used to attend quite a few electric fly-ins with it. Over 30 years later it has been re-motored with 2 brushless motors and Lipos, performance has been somewhat transformed.

 

The 1990 version

 

The 2022 version

Transformed for the better or for the worse?
 

Could you please provide more details, such as the plane's weight, props size, how many Lipos (voltage), motors' type, power, and KV?

Thanks.

 

 

[What Size Electric Motor/ Prop??]

As already mentioned, the KV means how fast the motor will try to spin the prop at a given supply voltage.

Lower KV means the motor turns slower for the same voltage.

For the same motor size, prop and cell count (voltage), the motor with the lower KV would draw less current than a motor with higher KV.

The power and current drawn are proportional to Kv³.

[What Size Electric Motor/ Prop??]

59 minutes ago, OLD FLIER said:

Hi all, I too am in the same position as Tosh, after 30 odd years of ic.  Been reading quite a lot about the subject and I am reasonably happy on choosing a power train now.  The only part that gets me is the motor kv.  What is a ball park number for our normal size sports models ? Thank you.

Here’s my two cents:

The motor Kv is chosen depending on certain factors, such as the number of cells in series you’ve access to or are willing to use and the max prop diameter that fits your plane to produce the power needed to enable the plane flying the way you want.

For a chosen power & prop, you may need higher Kv if only using 2S or 3S cell pack compared to using 4S or more.
Or for a chosen power & cell count, you may need higher Kv if using a small diameter high pitch prop compared to a large diameter small pitch prop.

You may find this link useful:
https://rcplanes.online/setup_guidelines.htm

 

[What Size Electric Motor/ Prop??]

Here's another motor prop combo calculator (estimator) free to use online:

https://rcplanes.online/calc_motor.htm

[c of g help for all]

The Giggler: since your plane has a relatively long tail moment, the CG would be more aft than conventional. So, maybe the calculated CG is not far off.