1. Decide your power requirement based on the weight of the plane and how you intend to fly it.
As a rule of thumb, the input power for a sports plane (no EDF), should be about 110 W/kg (50 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.
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, 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. You may check out our Database
Note this is a conservative choice assuming motors with about 75% 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 75% to 80% efficiency gives an increase in power handling of 25%: Factor 1.25 = (1 - 0.75) / (1 - 0.80)
5. Find a motor in that power range that has the Kv to achieve the power desired with the chosen props
- This calculator gets you in the "ballpark" by trial and error.
The Kv was chosen last because prop choices have limits: the max diameter that will physically fit the
plane and the minimum size that can absorb the power you want. Whereas the combinations of voltage
and Kv are much less constrained before the components have been purchased.
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.
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 if
it's a pusher, or if you already have a bunch of 3S packs and don't want to buy more, and so on.
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.
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 one hour before it gets
totally discharged (flat). However, you should not use more than 80% of the battery's capacity.
Discharge Rate refers to battery's max discharge current without getting damaged.
For example, a 500mAh 30C battery is capable of deliver 15000mA (15A) without getting damaged.
However, you should not use more than 80% of battery's discharge rate.
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.
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.
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 stallspeed.
Prop's Output Power = Thrust x Pitch Speed
Thus, with a given power, the more thrust you have, the less top speed you get.
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 5000 RPM.
To produce the same thrust, a 6x4 APC E prop needs about 156 W at 18630 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.