Last updated: November, 22 - 2023
1. Decide your power requirement based on the weight of the plane and how you intend to fly it.
The heavier the plane or higher the wing loading, the more power it needs to fly properly.
As a rule of thumb, the input power for a trainer plane (no EDF) should be about 120 W/kg (55 W/lb).
Gliders and parkflyers may need much less power; 65 W/kg (30 W/lb), while scale and aerobatics may
need much more power, e.g. > 200 W/kg (90 W/lb). This is assuming a motor efficiency of about 75%.
For a given input power, the higher the motor efficiency, the more power it delivers at its shaft. For instance, a motor
with 70% efficiency delivers 350 W with 500 W input power, whereas a motor with 85% efficiency delivers 425 W with
the same input power.
As a rule of thumb, the larger the wingspan, the higher the # of cells in series (voltage).
For instance: 30-40in wingspan: 2-3s LiPo, 40-55in wingspan: 3-4s LiPo, 55-68in wingspan: 4-6s LiPo,
68-84in wingspan: 6-8s LiPo, 84-96in wingspan: 8-10s LiPo, 96-105in wingspan: 10-12s LiPo.
3. Choose a prop that fits the plane and will fly it the way you want - often as big diameter as it fits
the plane is a good choice (since larger diameter prop is more efficient), but if high speed is the goal,
a smaller diameter prop with higher pitch may be more appropriate.
4. Find a motor size that can handle the power: 3 W per gram motor weight is a reasonable choice.
For example, a 300 W motor should weight about 100 grams.
Note this is a conservative choice assuming motors with about 70% efficiency, since the motor's efficiency significantly
affects its ability to handle power. For the same weight, the motor with higher efficiency is able to handle more power.
For instance, going from 70% to 80% efficiency increases the power handling by 50%: Factor 1.5 = (1 - 0.70) / (1 - 0.80)
Thus, high efficient motors can handle 4 to 5W per gram motor weight or more depending on the cooling.
Manufacturers of motors intended for multirotors, usually claim even higher Watts per gram thanks to
the excellent motor cooling.You may check out our Database
5. Find a motor in that power range with the Kv suitable for the chosen prop & voltage to achieve the desired
flying speed, which should be significantly higher than the plane’s stall speed.
- This calculator gets you in the "ballpark" by trial and error.
It refers to the motor's RPM per volt without load. Kv = RPM / (Vin - Vloss).
Vin is the supplied voltage and Vloss is the voltage drop due to the coil's resistance (Rm) times the
no-load current (Io): Vloss = Io x Rm.
Note that the motor Kv is not a figure of performance, it's just a motor parameter that you may use to
make your power system do what you want, within the limitations you have, e.g. limited prop diameter.
So, which Kv is the best?
It depends on the sort of plane, the type of flying, as well as on the supply voltage (# of cells in series) and
the prop type & size.
For instance, assuming the same power:
A lower Kv allows the use of a larger diameter prop, giving higher thrust at the expense of top speed,
whereas a higher Kv requires a smaller prop spinning at higher RPM resulting in higher top speed but
gives less thrust (lower acceleration capability).
So, if you intend to hover, have fast climb and acceleration, have got enough ground clearance to use a
larger diameter prop and the top speed is not of concern, then a low Kv is preferable.
For a chosen power & prop, you may need higher Kv if using 2S or 3S cell pack compared to 4S or more.
Or for a chosen power & cell count, you may need higher Kv if using a small diameter high pitch prop
compared to a large diameter prop.
If your motor constants are unknown or if you don't trust those provided by the manufacturer, you might
wish to check out Homebuilt Electric Motors, which provides detailed info on 3 important motor constants:
Velocity Constant (Kv), No-load Current (Io) and Winding Resistance (Rm), including various methods
for measuring them.
It stands for Electronic Speed Controller, and is a common device used to control the electric motor's speed.
The ESC should be able to handle the max. expected current drawn and also get proper cooling.
It's recommended that the ESC's current rating is at least 20% higher than the max. expected current drawn
in case of the device's inefficient cooling.
Timing: Many ESC's intended for brushless motors allow the user to set the Electronic Advance Timing.
Timing refers to when the ESC energizes the stator's windings relative to the position of the rotor's poles.
It's similar to the timing of a combustion engine when the sparkplug can be set to ignite earlier or later
relative to the piston's position. Timing may be set from zero up to 30 plus degrees.
High advance timing increases the motor's RPM and gives higher output power but at expense of efficiency,
which may result in overheating of the motor, ESC & battery.
Setting the right timing is important for a smooth running motor, since a stuttering or screaming motor may
harm the ESC. High advance timing is mostly suitable for high pole count motors (above 6 poles).
A moderate advancing timing may make it easier for a brushless motor and ESC to sync.
This sync issue tends to occur mostly with outrunners.
Low or zero advance timing (soft timing) is suitable for low pole count motors, since it gives higher efficiency
with some loss of output power, gives more torque at lower RPM, so it's recommended when long run-time
cooler motor, ESC & battery are the goals.
To find the most suitable setting is some times a matter of trial and error.
And since the "ideal" timing depends on several factors, such as the motor load, RPM, number of poles, etc.,
some manufacturers use software to enable the ESC to handle timing automatically.
Capacity C refers to stored electrical energy expressed either in amps-hour Ah or milliamps-hour mAh.
For example, a battery with a capacity of 500mAh should deliver 500mA during an hour before it gets
totally discharged (flat).
However, to prevent reducing the battery's lifespan, one should not often use more than 80% of its capacity.
The Discharge Rate, a.k.a. C-Rate refers to the battery's max discharge current without getting damaged.
For example, a 500mAh 30C battery is capable of deliver 15000mA (15A) without getting damaged.
However, one should not use more than 60% of battery's discharge rate continuously, as it would shorten
the battery's lifespan.
For the same capacity, the battery with the higher recommended max discharge rate, has the lower
internal resistance and the greater ability to deliver power, which significantly affects the calculator results.
A battery with a larger capacity or a higher C-Rate, will result in higher current draw for each prop used.
Cold temperature negatively affects the battery's performance. One may expect degraded performance at temperatures below 15oC (59oF).
You should always test your power system with a watt meter whenever a prop is used, to ensure that
you are not exceeding the recommended rating of the motor and ESC.
Further, make sure that the temperature of the electrical components such as ESC, Battery and Motor
does not exceed 60oC (140oF).
Without having a thermometer one may check whether they are comfortable to touch for about 5 sec.
In order to get reasonable climb and acceleration capabilities, the Static Thrust should be at least about
1/3 of the planes' weight.
In order to takeoff of the ground, the static thrust should be greater than 1/2 of the planes' weight.
However, thrust alone is not enough to guarantee the plane to fly, since other factors, such as Pitch Speed
must also to be taken into account.
Unless it's a glider, the adequate Static Pitch Speed should be greater than 2.5 times the plane's Stall Speed.
So, in order to get the right power system, it's also important to know the plane's stall speed.
Prop's Output Power = Thrust x Pitch Speed
Thus, with a given output power, the more thrust you have, the less top speed you get and vice versa.
For example, assuming the same power:
Larger diameter & less pitch = more thrust, less top speed. (like the low gear of a car)
Smaller diameter & more pitch = less thrust, more top speed. (like the high gear of a car)
A smaller prop requires more power to produce the same thrust compared to a larger one.
For instance, a 12x8 APC E prop takes about 86 W to produce 27 oz of thrust at 5,000 RPM.
To produce the same thrust, a 6x4 APC E prop needs about 156 W at 18,630 RPM.
You may estimate the power needed if you know the static pitch speed and the thrust you need.
The recommended prop P/D (Pitch/Diameter) ratio for sport models is 1/2 to 1/1.
Again, assuming the same power:
With a too large pitch, the prop becomes inefficient at low forward speed and high RPM, as when during the
take-off and/or climbing (you may need to hand-launch the plane).
Whereas a propeller designed for greatest efficiency at take-off and climb (with small pitch and large diameter)
will accelerate the plane very quickly from standstill but will give less top speed.
The graph below shows Thrust & Drag vs Speed for 3 props with different P/D ratios.
The plane reaches max level flight speed when the Thrust becomes equal to Drag.
The prop with P/D ratio of 1/2 yields higher thrust at low airspeed, but gives lower top airspeed.
Whereas the prop with P/D ratio of 1/1 yields lower thrust at low airspeed, but gives higher top speed.
Parts of the above information are excerpts from the following RCGroups' Blog:
How to design a custom electric power system