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In quadcopter model creation is needed to choose components in order to have enough thrust ratio to grant the hovering plus some extra for movement performances, taking in account other constraints as minimum flight time needed and max battery size that could be used for the selected build.

Smaller motors don't mean small thrust ratio considering that parameter depends on the model weight (that would be proportionated with smaller battery, smaller frame etc.), but obviously, smaller motors mean smaller thrust absolute value and way more impact on flight specifications if any accessory or other passive component adds weight to the model (e.g. if you add 200g on a 900g drone its flight time will be crushed, add the same weight on a heavy drone and the impact on flight time of same 200g load is negligible).

Now considering you have to build a drone model that should have enough extra load to carry a specific accessory component: how to calculate what is the minimum motor size needed to make sure that extra load will be manageable without crushing the model flight specs?

e.g. Considering to have these requirements:

  • minimum thrust ratio 3:1
  • extra carry load of 400g
  • impact on flight time of the extra load max 20%

How to calculate the minimum motor dimensioning?

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  • $\begingroup$ The only bulletproof way to size motors is to look at vendor-provided thrust curves for specific propeller geometries. If none exist, you'd likely need to benchmark different options yourself. Aerodynamics and CFD are difficult problems to simulate and there is no substitute for empirical testing. $\endgroup$
    – ifconfig ♦
    Commented Oct 21, 2022 at 18:23
  • $\begingroup$ @ifconfig Checking the curves may be bulletproof but It should be possible to have a primary estimation since average spec at given BLDC motor volume/wattage are comparable and e.g. you know for sure that if you want to build a 3Kg drone 2850 motors (or comparable volume with different form factor) are too small without need to check curves. $\endgroup$
    – AndreaF
    Commented Oct 21, 2022 at 21:13

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The thrust depends on your propeller and how fast it's turning. It's easy enough to build a thrust stand to measure thrust and power for a range of RPM. Here there's a black optical tachometer, a blue power meter and a silver digital scale for measuring thrust.

DIY thrust stand

Then you would need to find a motor that can reach the desired RPM, and handle the power required.

The motor's Kv multiplied by the battery voltage gives you it's no-load speed. As you load it, the speed drops, with maximum power at about half speed. However, it's rare to run at that speed as the motor probably can't handle that power for long and it's not at its most efficient. There are theoretical curves for speed and power of permanent magnet motors, but you can aim for 70-80% of the no-load speed.

The maximum power for a motor is usually specified, although it's not precise. It depends on how hot you're willing to let the motor get, how long it's run for and how much heat it can lose to the air. Motors have a lot of thermal mass and can handle large bursts for short periods.

Many popular propellers and motors have been tested, and the data combined into on-line calculators. RCGroups has a compilation of e-flight calculators, motor- and prop-databases. These usually allow you to pick a motor, prop and voltage, and give you the expected static thrust and power.

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  • $\begingroup$ Thanks for the reply but I know this and I'm interested in an analytical way to calculate dimensioning starting from extra load and thrust ratio requirement (where with extra load I mean the weight of a passive component to add in addition to base drone components). $\endgroup$
    – AndreaF
    Commented Oct 28, 2022 at 21:31
  • $\begingroup$ There's nothing special about 'extra' weight, it's just weight. If you want to add 200g to a 900g drone, just work out what you'd need for a 1100g drone. If that means you need larger, heavier motors, add on that weight and do the calculation again. It should only take a couple of iterations to find a solution, you don't need any fancy maths. $\endgroup$
    – Robin Bennett
    Commented Oct 29, 2022 at 22:20
  • $\begingroup$ Not exactly. Extra weight impact changes a lot according dimensioning and this relation should be modeled in order to avoid random trial and errors among thousands of configurations that will lead to a lot of time wasted and likely an unoptimal combination. $\endgroup$
    – AndreaF
    Commented Oct 31, 2022 at 3:15

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