The main thing is to count on certain software tools:

This is a freeware program which calculates the efficiency of a motor based on voltage, current, and prop for a predetermined rpms.

Select a prop and input rpms, amperage and voltage – then it calculates thrust and speed, with graphs.

A very useful program with a data base of motors, ESCs, batteries and airplanes; free for 30 days.

Program similar to Motocalc but free – very useful.

The type of plane drives the power required and the size of the propeller as well as the speed and necessary power for good performance. The total weight of the model with all installed equipment is another determinant.

This method consists of 4 steps:

Guidelines may be provided by the manufacturer, but sometimes these are not to your liking, or the plane is your own design, or these are not included in the model’s instructions. We can help using this approach.

We look for, calculate or deduce the final weight of the model. Also we determine the wing’s surface area in square inches. We load this in the Motocalc program in the airframe section. In order to register the model, we include a motor “X” and battery “X” that agrees with the weight. We then calculate and it gives us data in where the main one is the Stall Speed – the minimum flying speed. We multiply this by a factor which determines the desired terminal velocity:

• Factor of 2: For Trainers, Gliders, 3D and for very light airplanes with a great helix by far push.
• Factor of 2.5: For airplanes sport, pattern, and scale.
• Factor of 3: For pylon airplanes or any where speed is required.

Thus we obtain the palne’s speed. And with this we determine the RPM for different props in order to find the static thrust that will provide the power needed.

Let us consider this example:

• Mini XT Presses, ARF, Wing Span: 42.5 in (1080mm)
• Overall Length: 37.5 in (875mm)
• Wing Area: 330 sq in (21.3 sq dm)
• Flying Weight: 25-27 ounces (795-965g)
• Radio: 4 channels Servo: 4 sub-micros
• Prop Size: 10×8 Electric, 25A Pro Brushless ESC
• Recommended Battery: 2100 mAh 3 cell 11.1 volt Lipo

It seems to be a sport airplane or an aerobatic trainer. Using Motocalc we find:

For a weight of 27 ounces, the stall speed will be of 17 mph. We multiply this by a factor of 2.5 to obtain its WOT speed – that equals 42.5 mph.

Note: For 3D airplanes we skipped this step and just considered static thrust, so we multiplied its weight by 1.5 and that will be the required static thrust.

Using Badcock.net/MotorXL with drivecalc.de in the Tools section, we can calculate the prop required. We calculate with different props the rpms needed to produce the desired speed. With this we will obtain the prop’s Pitch Speed.

Considering the speed of the model, it could be obtained using a radar gun. We will use the speed produced by the prop for its rpms. The speed of the model depends on its drag due the wing’s shape.

• APC E 10 x 7 = 6415 rpm, 42.5 mph, 87.6 watts out, 22 ounces
• APC E 11 x 5.5 = 8350 rpm, 202 watts out, 48 ounces
• GWS HD 10 x6 = 7660 rpm, 105 watts out, 26 ounces

An airplane weighing 27 ounces would fly acceptably in sport mode with a static thrust of 22-26 ounces, with 48 ounces being way outside our performance envelope. Therefore we will choose either the 10×7 at 6415 rpm and the 10×6 at 7660 rpm – we will use the GWS HD 10×6 prop which gives us 7660 rpm, 105 watts out, 26 ounces thrust.

Obtaining the required rpms will depend on the applied Voltage (Generally they will be 2s Lipo @ 7.4 volts and 3s Lipo @ 11.1 volts.) multiplied by the Amps – this will be the Power Output. For this calculation we will use the values of 10.5 volts for 3s and 7 volts for 2s, since these values are typical when applying a load to these batteries.

As in all the electrical motors, we have losses and the motor’s efficiency cannot be 100%. Rather it varies between 50% and 80%, with a good motor giving its maximum power at an efficiency of 70% to 80%. Now if 105 watts out is only at 70% efficiency, the motor’s Watts Input (ie, 100%) will be 150 watts – that will be our Power In with 70% motor efficiency. Now if 105 Watts out is at 80% motor efficiency, the Watts Input (100%) will be 131.3 Watts – that will be our power in at 80% motor efficiency.

With 2s: 7 Volts * 21.42 Amps = 150 Watts @ 70% efficiency
With 2s: 7 Volts * 18.75 Amps = 131.3 Watts @ 80% efficiency
With 3s: 10.5V * 14.3 Amps = 150 Watts @ 70% efficiency
With 3s: 10.5V * 12.5 Amps = 131.3 Watts @ 80% efficiency

An advantage is clearly seen when using a motor of better efficiency and also with a greater voltage – better to use 3s rather than 2s.

So we have determined the following:

GWS HD 10×6 = 7660 rpm, 105 watts out, 26 ounces thrust, 10.5 volts, 14.28 amps, 150 watts in @ 70% efficiency
GWS HD 10×6 = 7660 rpm, 105 watts out, 26 ounces thrust, 10.5 volts, 12.5 amps, 131.3 watts in @ 80% efficiency

A calculation that I do without being “scientifically correct” is the one to find the Kv for the motor: Multiply RPM * 100 at a given efficiency and voltage. In the above example at 7660 rpm @ 70% efficiency, I multiply ((RPM * 100) / 70%)) / 10.5 volts to give 1042 Kv. At 80% of efficiency, I multiply ((RPM *100) / 80%) / 10.5 volts to give 911 Kv.

Therefore we need a motor with a Kv between these that can handle a prop of 10 inches to 131 – 150 watts. One rule of thumb says that a motor weighs about 1 ounce for each 100 watts, but it’s better to consider a maximum of 75 watts per ounce. Putting it all together, we need a motor that weighs over 2 ounces with 1042 Kv @ 70% efficiency, and 911 Kv for one @ 80% of efficiency.

A motor with 80% efficiency will have to support a minimum of 131 watts; a motor with 70% of efficiency will have to support a minimum of 150 Watts; a motor with 60% of efficiency will have to support a minimum of 175 Watts.

The disadvantage is clearly seen in using a relatively inefficient motor. WHen selecting the motor within the parameters determined, the appropriate one will be the one with the smallest weight and greatest efficiency.

I understand that this method is not totally correct, but it is very simple and easy to use. In addition, it tracks the real world closely.

A starting point for any RC flyer is to estimate the power load on the motor of the prop currently on it. The load can be estimated by using the following formula:

Solving this formula yields a pure number (no dimensions) – for a 10×8 prop, it equals 8000 (10 x 10 x 10 x 8).

To find a prop that will load the motor the same as the 10×8, it must be the same prop type (eg, slow flyer). By substituting terms, you can then find a prop that is perhaps a larger diameter, different pitch, etc. The table shown below shows some examples:

If you want to use a larger prop for more speed than what you are using currently, using the formula will give you an idea of how much more power the new prop will take. For example, substituting an 11×8 for a 10×8 will take 33% more power. Users must determine if the increased load due to a prop that demands more power is feasible with their current setup.

If your objective is to find a three or four blade prop to substitute for a two blade prop, the PLF formula to use:

With two blades, the square root is 1 (ie, 2-1), so it drops out. For a three blade prop, the square root term becomes the square root of 2, or 1.414:

To use this formula, calculate the PLF for a two blade prop; then use this value as the PLF in the formula above, pick either a diameter or pitch, substitute terms and solve. The result will give you the three blade equivalent to the two blade prop you want to replace.

Many thanks to Lucien Miller of Scorpion Products for sharing this formula with the RC Community.