May 13

What Is Brushless Motors Best Performance Criteria

Last week I ran through the various linkage and servo setupemp_n4250_950kv procedures, ensuring that appropriate movement and travel adjustments were correct. This post will look at the way I determine what is brushless motors best performance for my model and how is brushless motors best performance obtained.

With any model I build the performance I need from my motor very much depends on what is an rc plane designed for. Trainers, WW1 Biplanes and Vintage types require much less power and performance than Aerobatic, Warplane and 3D types. So I have to decide the appropriate power output needed for this model.

What Power Do I Need?

The Hawklett is a sport aerobatic design so it needs a good margin of surplus power to fly through vertical manoeuvres. There are several charts available online that suggest suitable power levels  for different types of plane. The one I favour is here:

70-90 watts/lb. Trainer and slow flying aerobatic models.

90-110 watts/lb. Sport aerobatic and fast flying scale models.

110-130 watts/lb. advanced aerobatic and high speed models.

130-150 watts/lb. Lightly loaded 3D models and ducted fans.

150-200+ watts/lb. Unlimited performance 3D models.

 From this chart it is fairly obvious that my power requirement falls somewhere in the range of 90 to 130 watts per pound of fully loaded airplane. I have weighed my model with everything on board and it comes out at 5.2lbs (for our European friends that amounts to 2.36Kg). Taking a mid range value of 110 watts per pound (242 watts/Kg) I calculate that I need:

5.2 lbs x 110 watts = 572 Watts (2.36Kg x 242 = 572 Watts)

If you remember back to some of my earlier posts and if you’ve checked out my website,, you will know that power in Watts = Volts x Amps. For a motor of the size and kv rating I am using with a plane of this weight, the best Lipo options are either 3 cell (11.1 Volts nominal) or 4 cell (14.8 Volts nominal). If I were to use a 3 cell Lipo then I would need to pull more amps to attain the 572 watts than if I use a 4 cell Lipo.

Amps = watts/volts:    572/11.1 = 51.5 Amps whereas 572/14.8 = 38.65 Amps

The motor I am using has a continuous current rating of 50 amps so if I used a 3 cell Lipo I would risk the motor overheating and burning the coils out. On the other hand a 4 cell Lipo will provide the power without ever pushing the current to this maximum rating.

RC Motor KV Rating

The next important consideration is the kv rating of my motor. What is motor kv rating going to tell me? It is going to tell me a great deal. Rc motor kv rating influences the parameters of the propeller I will use.

My motor is an EMP 4250 – 950kv which, for its size, is a fairly fast spinning motor. The 950kv figure tells me that at full throttle it will be rotating at around 13,500 rpm ( 950 x 14 =.13,300).  This means that I will have to be careful not to over prop it so that I take the risk of pulling too much current and overheating the motor itself, ESC or Lipo or possibly all three!

You will notice that I have dropped the running voltage at full throttle to 14 volts. This is because the greater the load on the Lipo, the lower the actual voltage available will drop to. You will see this in my test results for the three propellers I tried.

Propeller Size v Power Output

Digital WattmeterWhilst testing the various propellers I needed to record values for Voltage from the Lipo, Current drawn in Amps and the resulting Power in Watts. The best way to do this in one test session was by using probably the most important tool in my electric flyers kit, my Watt Meter.

If you already fly electric or intend to do so, I strongly suggest you invest in a Watt Meter. It will save you so much time and expense avoiding damage to your motors, ESCs and Lipos.

Here are the results of the tests I ran with three sizes of propeller:-

Propeller Size & Make              Voltage              Current         Watts 

      APC 9 x 6                                13.9                       38A                  572 

       EMP 9 x 6                               13.8                        36A                  538 

         EMP 10 x 6                             13.6                        48A                  671   

These just happen to be the three sizes I had available to make a start with. It just so happens that the third one, the EMP 10 x 6, gives me what I need, plus some, without pushing the motor into the danger area. 671 watts/5.2lbs = 129 watts per pound (283.8 watts per kg). Most of my flying will be at around 1/2 to 2/3 throttle settings so the full current draw will not be called for regularly.

 the APC 9 x 6 propeller would be totally adequate but the overall performance would not be quite so good. So long as I don’t push the power train into the danger area, I prefer to have the extra power available. I can always throttle back if I don’t want to fly so fast.

In due course I propose to try a 9 x 7 or possibly a 10 x 5 just to see what difference there is between their performances but for the moment I will be test flying my plane with the 10 x 6.

As a matter of interest, this motor is capable of producing 1100 watts using up to a 7 cell Lipo so I am not expecting it to feel to much strain from my usage.

The Finished Product

I’m sure you would like to see a photograph of the model in all her glory so here she is. I have to say I’m quite pleased with the end result. Will she fly as well as she looks? That remains to be seen.

Finished Hawklett

Hopefully by this time next week I will have committed her to the wide blue yonder and be able to report that all went well. We should know what is brushless motors performance compared to the previous version I built with an old HP40 two stroke glow engine up front some years ago.

Watch this space, see you next time.



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Posted May 13, 2016 by Colin Bedson in category "Modelling Skills

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