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.



May 6

What Is An RC Plane Setup About?

All Present & Correct

My last post took us through the various installations needed to complete the radio and power train. Now that all of the essential components are installed, (I have to say it looks pretty busy in there)  I need to go through a full setup procedure to make the model ready for its first trial flights.

So what is an rc plane setup all about? Well really its just a matter of checking and making sure all of the active functions are correctly connected and operating as they should. Let us go through some of these checks together.

Arranging Aileron Differential

I have decided to use this arrangement as in the past I have found it beneficial in certain circumstance. If you visit my previous post entitled Differential on Ailerons for Radio Control Aeroplanes you will learn why this feature makes life easier when flying models that suffer from ‘Adverse Yaw’.Servo Offset Aileron Diff

Now, I’m not suggesting that the Hawklett is one such model but I have found that a little aileron differential helps the roll function of most planes.

Here in this diagram the differential is arranged by setting the servo output arm a few degrees forward of the vertical. As the arm moves in a circular arc, the forward linear movement is less than the rearward movement.

When this is transferred to the control surface horn the difference in linear movement causes the upward deflection to be greater than the downward deflection (As > Bs therefore Ah > Bh). Hence we have ‘Aileron Differential’.

Now I know that I could have arranged this with the two wing mounted aileron servos connected to two receiver sockets (aileron & auxiliary) along with a mixing facility but I prefer to keep the spare sixth channel for another function if needed. Consequently I find this mechanical option equally as effective.

Single Servo Rudder & Steering Control

Rudder & Steering Servo

As you can see from this diagram, there are two ways to arrange the pushrods to achieve control of both Rudder and Steering from a single servo.

In the first instance the steering horn on the nose leg is connected on the same side as the servo output arm connection. To have the rudder turn in the same sense as the wheel the closed loop cables have to cross over in front of the fin inside the fuselage.

In case two the steering pushrod crosses over from one side to the other so that the wheel turns in the opposite direction. but the closed loop cables do not cross over. Alternatively the ball link connector to the wheel could be moved over to the other side of the servo output arm whilst leaving the steering leg linkage as in the first diagram.

Separate Elevator Control Arrangement

I thought that it was worth spending a little time on an explanation of this arrangement. I personally have never seen this done before on any commercially available model kits or plans. Having said this, it does work very well and could be applied to other projects where rear fuselage space is at a premium.

Elevator Pushrod & Drive Yoke assembly

The basis of this arrangement is the clever little yoke that connects the two halves of the elevators together. It comprises a wire ‘U’ with a ‘Z’ bend soldered to the bottom of the ‘U’. This fits into a standard clevis pin hole in the horizontal leg of the bell crank. The upright leg of the bell crank is connected to the pushrod that runs the full length of the rear fuselage to where the servos are located under the cockpit.

On each end of the Yoke arms are ball link sockets that mate with the balls attached to ply extensions on the trailing edge of the elevators.

In use this arrangement gives very positive and easily adjusted elevator movement.  As there are at least three points at which the movement ratios can be adjusted (servo & bell crank x 2) the range of movement is adjustable to the Nth degree. The all important consideration is that all linkages are a good fit into the drive components to eliminate unwanted slop.

The final ball link connections on the elevators give fine adjustment to ensure the elevator halves are exactly in alignment on each side.

Setting Up The Retracts

The important thing to ensure when installing and operating mechanical retracts is:

a) That the travel of the pushrod(s) do not exceed the operating range of the servo(s).

b) That the travel is sufficient to locate the pushrod and actuator in the locked leg position.

Retract leg up

Retract leg down

In the two diagrams above you can see that the pushrod link/activator moves horizontally sliding a bar in the activation slot from one end to the other. This in turn pivots the leg mounting block, shown mainly in dashed lines, so that the leg travels in a 90 degree arc from the retracted position to the down position.

I have shown the pin that sits in the activation slot short of the maximum travel. This prevents the servo from being put under continuous load. Having said that, I have checked that the amount of travel is sufficient to ensure that the position of this pin at full travel locks the leg in the required position.

The main adjustment is made via selection of the correct pushrod connecting hole in the servo output arm. With some radio gear that does not have a ‘travel adjust’ programme, this is the only method available. Most modern digital gear has this facility so final adjustment can be made by reducing or increasing the travel slightly at each end. I have set mine at 95% at each extremity.

The drawings here show the nose leg where the mounting plate is hidden inside the fuselage. On the wing units the leg extends from the opposite side of the block so that the mounting plate sits flat against the wing surface.

Final Testing

At the beginning of this post I asked the question; “what is an rc plane setup about”? I hope that this has been answered for you in the above explanations.Digital Wattmeter

Now that all of the active controls are setup the next task is to do some motor run tests using different propellers and monitoring the current draw and logging the static power readings. This is where that invaluable piece of kit, the Power Meter or Watt Meter comes into its own.

I very strongly suggest that if you intend to go down the electric flight route, you should invest in one of these meters. It will save you not only a lot of time but also the money you could lose in burnt out motors, ESCs and blown Lipos. It will also help you select the best propeller for your plane.

I’ll catch you next week with some test results. Don’t forget to mention my website if you know anyone who’s just getting started. Also, if you want to follow the full series of my Hawklett build posts, the first is here.