I've been trying to design and build, my experimental Ultralight using electric propulsion system, My plane designed base on Slaler and Gensis ultralight , following is the primary calculation

Empty weight 120KG

Wingspan 9.1

Length 2.8

cruise speed 120 Km/h

propulsion 20kw

I'm trying to select a pair of Brush-Less Direct Current motors (BLDC) for my airplane, but I have no experience using BLDC's before, whereas there is a lot of BLDC ambiguous options (mostly in Aliexpress) which claim to be a different KW.

My design goal is to find the best possible options for propulsion to have longest possible flight time duration.

Following is the 3D image of my design (located in my GrabCAD account) any advice really appreciated enter image description here

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    $\begingroup$ I can't help you with the motors, but what kind of calculations is the design based upon? I'm asking because the tail assembly seems to be way too small. $\endgroup$
    – Jpe61
    Feb 9, 2020 at 10:59
  • $\begingroup$ The tail boom also looks beyond flimsy. Both of the designs you are taking inspiration from had much more substantial tail structures AND required less torque from the HTP due to the CG being very close to the wing. Your design moves the CG forward with that long (2 crew?) cockpit while weakening the tail. $\endgroup$ Feb 9, 2020 at 13:53
  • $\begingroup$ Thanks for your comment but I've tried to move CP to CG as close as possible to reduce the amount of tail volume and reduce wight and negative lift by tail $\endgroup$
    – Hamid s k
    Feb 10, 2020 at 2:26

2 Answers 2


FLÜGEL World Directory Of Light Aviation 2019/2020 - English PDF could be a good starting point. On pages 278 - 281, a total of 71 lines of electric motors from 42 manufacturers are featured. Disclaimer 1: some data fields are incomplete for many of the entries. There are German, English French and Chinese versions in paper and pdf, starting at €8. Here's the paper version in German: Welt-Index Ultraleicht und Flugzeug 2019/20 Wings of the World

In any version, the aircraft listings themselves may also be of interest, as aircraft models which are available with electric motors as the primary powerplant are highlighted with a green e icon. In browsing those, you may glean some emperical inspiration in terms of electric motor vs aircraft size. Disclaimer 2: the power-plant details for the aircraft listings are not always complete.

The free annual e-flight Journal by the same publisher includes motor manufacturer adverts which may be of some use.

Lastly, at the end of the online Kitplanes 2020 Engine Buyer’s Guide are four electric motors from one manufacturer.

Final Disclaimer: I have no vested interest in these magazines, publisher or any products listed or advertised.


The rated power of an electric motor can vary wildly; it's not a fixed quantity like an IC engine. If you fit a larger prop, or run it at a higher voltage, it will draw (and hopefully produce) more power. However it'll also produce more waste heat, and may well be less efficient.

You can run it way beyond the rated power for a few seconds, or overheat a motor way below its rated power if there isn't enough cooling airflow.

Ideally you want a chart showing the efficiency for a range of speed and power. Then you need a chart for your proposed propeller, showing the thrust produced at a range of rpm, and the power required to reach that speed.

Then you can set out some design requirements, perhaps static thrust for take-off, climb and cruise performance. Look those up in the charts, and see if the motor will be operating efficiently, or if you're pushing it beyond its limits.

In the model world, you can usually use someone-else's project as a starting point, but some bench-testing and experimentation is required to find the best combination.


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