Choosing an RC Brushless Motor
Weight and Dimensions
Apart from Power there are two important things to keep in mind when choosing an rc brushless motor. These are a) its Weight and b) its Dimensions. Having to add extra weight at the front of a model to achieve the correct centre of gravity (CofG) is far from desirable. If we can incorporate at least some of this extra weight by using a heavier and more powerful motor, this is preferable to having a smaller less powerful motor and a lump of lead at the front of a model.
The trainer on the right would fly with a smaller motor but this selection fits well, suits the installation, and provides a surplus of power for emergency situations whilst helping bring the CofG to its desired position.
Occasionally there is no choice but to use additional weight to achieve the correct CofG, but don’t forget about its relationship to the weight of your motor. The dimensions of a motor are obviously important, as it needs to fit within the space provided.
So now we come on to the amount of Power (Watts) required for the model to perform as it should. 3D models need thrust to weight ratios greater than 1:1, whilst scale WW1 biplanes need considerably less. The table below provides recommendations of performance in Watts per pound for the styles of model commonly flown. It is important to understand that if you run your motor above its maximum rated efficiency the Watts per pound rule won’t be accurate. The motor will tend to overheat as a result of a higher percentage of the Watts going into the motor producing heat instead of power. This is something that should be avoided at all cost.
- 70-90 watts/lb. Trainers 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.
These figures are generally optimum requirements but many model planes will actually fly quite successfully with a lower power to weight ratio. It is not uncommon for lightly loaded vintage style model to fly on 50 watts/lb.
RC Brushless Motor Formats
Brushless Motors are available in two formats. Inrunners and Outrunners. Let us take a look at each type in turn so that we can decide which is best for our application.
The electromagnetic windings of a brushless motor are made stationary (the stator) and the permanent magnets made to revolve (the rotor). The wiring connections are permanent and fixed.
Unlike brushed motors there is no need to make any electrical connection to the revolving parts and two configurations are possible.
Inrunner Brushless Motor
In this type the stator is in the form of a cylinder around the outside of the motor and the rotor spins inside it. The resulting motor is known as an ‘Inrunner’. They look more like conventional motors, with a fixed cylindrical casing. With a small rotor, they turn at a high rpm but do not produce as much torque as outrunner types.
If the stator with its windings is in the centre, the result is an ‘Outrunner’. Now the outer casing becomes the rotor, with the permanent magnets arranged around the inside of its rim. The casing is fixed to the motor shaft at one end whilst the other end runs in a bearing mounted in the non-moving part from which the coil lead out wires protrude. They turn more slowly and because the magnetic interaction occurs at a greater distance from the motor axis, they develop more torque.
Brushless Motor Benefits
There are major benefits to be had using brushless motors as an alternative to the brushed types.
- They are more efficient – Brushless motors are much more efficient. This efficiency has been measured to be between 85% to 95% better than brushed motors.
- Less electrical energy is wasted as heat – This means that more is used to do useful work.
- Reduced Noise – Brushless motors have fewer mechanical parts than brushed motors, so they emit less sound.
- Longer Lifetime – The only parts that are in mechanical contact in brushless motors is the shaft bearings, compared to both bearings and brushes in brushed motors. This results in a considerable reduction in wear and tear.
- Reduced Electro Magnetic Interference – Brushless motors emit less energy as electromagnetic (EM) waves than brushed motors do. As a result they are more efficient, and create less radio interference.
- Torque, RPM and Voltage are Linearly Related – This means that the amount of Torque or RPM produced by the motor divided by the Voltage supplied is a constant value. This makes it easy to predict how much power your motor is going to produce.
Selecting The Best RC Brushless Motor For your Plane
The choice of motor type, either Inrunner or Outrunner, will depend on the requirement of your particular model. Inrunners are more efficient and powerful. Generally they produce higher revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like pylon racers and ducted fans, inrunners are a popular choice.
It is possible to use inrunners with gearboxes to turn larger propellers but this is a more expensive option that adds extra weight and is less efficient due to mechanical losses within the gearing.
For most normal applications the outrunner is a better option. Its ability to turn larger propellers in direct drive mode reduces both mechanical losses and cost and provides greater efficiency.
In the earlier stages of your model flying experience it is unlikely that you will be getting involved with high performance aeroplanes. For this reason I propose to concentrate on outrunner motors suitable for sport and basic aerobatic flying performance.
If we refer to the chart above it is very convenient that a figure of around 100 watts/lb. of model weight is appropriate. I like round figures, they make calculations very simple!
The three most significant figures published by brushless motor manufacturers are:-
- Kv – This refers to the RPM (Revolutions Per Minute) constant of a motor. It approximates to the number of revolutions per minute that the motor will rotate at when 1V (one volt) is applied to the motor. This figure is called ‘revs per volt’ designated the abbreviation Kv. (Be very careful not to confuse this with kV which is a totally different abbreviation for ‘kilo-volts’ or multiples of 1000 volts). Please understand that you will never obtain the revs calculated from this constant. As soon as you attach a propeller to the motor the load will reduce the RPMs your motor can attain.
- Continuous Current Rating – This is the maximum value of current that the motor is capable of handling continuously during operation without incurring damage through overheating and potential bur- out of the wire coils. They will also quote a figure of Maximum Current the motor is capable of withstanding for a very limited duration. this figure should never be used for the purpose of calculating the power figures you will need for your plane.
- Cell Count – This will tell you the range of voltages appropriate to the motor’s operation. these figure will often include both NiMH cell counts and Lipo cell counts. Not many people use NiMH batteries nowadays so the significant information for us is the Lipo cell count. Most Lipo cells give a typical average voltage under load of 3.3 volts.
Power (Watts) = Volts (Cell Count x 3.3V) x Continuous Current Rating (Amps)
Using this equation it is possible to calculate the working power figure for any motor. With this figure as a maximum beyond which it is undesirable to operate, the only other variable capable of increasing or decreasing the power consumption of the motor is the size of propeller fitted.
The amount of power required of the motor to turn a given propeller is affected by both pitch and diameter. Below is the formula for calculating power requirements for a given size of propeller:-
power = k x rpm3 x diameter4 x pitch
The factor k is a constant depending on the units used to define power, pitch and diameter as well as the airfoil, thickness and shape of the propeller. Because RPM is controlled by the Kv of the motor, the only variables we can control are diameter and pitch.
You can see that changing the pitch of the propeller has only a small effect on power but a change in diameter has a major effect. Because pitch has such a small effect on power, it can be used very effectively to improve model performance at the expense of very little change in current. If we are looking for an increase in top end speed increasing the propeller pitch by a factor of one (e.g. from a 10 x 6 to a 10 x 7) will only increase the power required by 14% hence current will increase by 14%. So if the motor was drawing 25A with propeller one then propeller two will require 29A, a small penalty for the improved performance.
If, however, we increase the diameter of the propeller from a 10 inch to an 11 inch of the same pitch, it would require 1.46 times the power to maintain the same rpm (11/10 to the fourth power i.e. 1.1 x 1.1 x 1.1 x 1.1 = 1.46). If we continue to use the same battery the voltage remains the same so the current would have to increase by a factor of 1.46, i.e. 25A x 1.46 = 37A. An increase of this magnitude could push the current beyond the safe operating parameters of the motor.
This leads us to consider that maybe we can get the same amount of performance from our motor by reducing the propeller diameter but increase the pitch. Lets use a 9″ diameter propeller instead of the 10″. This will reduce the current consumption by a factor of 0.9 x 0.9 x 0.9 x 0.9 = 0.66. So if we take the 25A and multiply it be 0.66 = 16.5A. Now lets increase the pitch from 6″ to 8″ we increase the current by 8/6 = 1.33, i.e. 16.5 x 1.33 = 22A.
Clearly just by reducing the diameter by 1″ and increasing the pitch by 2″ we have been able to reduce the current consumption by 3A. This means that our flight time can be longer for a minimal reduction in overall power consumption. Performance will suffer very little as it is not normal to operate a plane at maximum power continuously. Besides which it is unlikely that the actual power loss will be noticeable for sport flying applications.
It is always a good thing to experiment with different propellers
to find the one that gives the best performance for a particular model. It is normally best to start with the size recommended by the motor manufacturer then make small variations either side of this size.
It goes without saying that you are going to need our faithful servant the Wattmeter to check how the changes are affecting current and power figures. You can buy yours through these links:
Summarising Our Choice Of RC Brushless Motor
Although there are a considerable number of factors to be considered when deciding on which motor to install in your plane, the principle one is the Power to Weight Ratio. By referring to the chart at the beginning of this post you can decide the sort of performance you need and select your motor based on this consideration.
It is always good to err on the high side of the power capability of the motor. You can always throttle back for normal flight leaving a reserve of power for those occasions when it is needed. A motor that is having to perform at its maximum capacity continuously will not last very long and will not provide the reserve you may need.
The performance you get from your rc brushless motor results from a combination of motor, propeller and lipo capacity and rating, given that your ESC has at least a 30% margin above the anticipated maximum current handling capacity.
Don’t forget to visit our main website at www.rookiercflyer.com for all the help you will need if you are just getting started. If I can help you pleas don’t hesitate to ask your questions via the comments facility below.