How should one chose the correct motor KV for a build?

Question

For a 5 inch, 4S build, the general advice is to go 2300-2750KV. For the same size but 6S, it should be ~1800KV. However the same motors could be used on a 4S 7 inch quad. And all of this scaling seems to go into the window when moving onto something like whoops or toothpicks, and up to X-class.

How should one choose the most appropriate KV for their intended build? Is there a formula to use, or is it just learned through experience?

Answer 1

I think it’s learned through experience and it can also be roughly figured out using this table based on frame and propellor size:

    ╔══════════════════╦═══════════════╦═════════════════╦══════════════════╗
    ║ Frame size       ║ Prop size     ║ Motor size      ║ KV               ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 150mm or smaller ║ 3" or smaller ║ 1306 or smaller ║ 3000KV or higher ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 180mm            ║ 4"            ║ 1806            ║ 2600KV           ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 210mm            ║ 5"            ║ 2204-2206       ║ 2300KV-2600KV    ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 250mm            ║ 6"            ║ 2204-2208       ║ 2000KV-2300KV    ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 350mm            ║ 7"            ║ 2208            ║ 1600KV           ║
    ╠══════════════════╬═══════════════╬═════════════════╬══════════════════╣
    ║ 450mm            ║ 8", 9", 10"   ║ 2212 or larger  ║ 1000KV or lower  ║
    ╚══════════════════╩═══════════════╩═════════════════╩══════════════════╝

(table source)

Answer 2

The basic formula by which you determine (at least the ballpark of) the desirable motor KV is:

KV * Battery Voltage/2 = characteristic RPM (for the prop size)

Basically, what you want is for the motors to "want" to spin at the RPM that is just enough to keep your drone in the air with the corresponding prop size.

In your example, a 6S battery has 1.5x more voltage than a 4S, so the motor has to have that much lower KV to spin at the same RPM. A 7 inch prop, however, needs to spin slower than a 5 inch prop, so also needs lower KV motors if you use same-voltage batteries.

Unfortunately, there are some caveats to this simplistic view.

• First, characteristic RPMs are dependent on a lot of factors other than prop size (which is why it's only a ballpark).
• Second, RPMs are generally an uncommon stat in hobbyist circles; it seems that no one really measures RPM other than super nerdy types with thrust stands (although that might change soon with the advent of bidirectional DSHOT), so you're unlikely to come by a list of those for different prop sizes.
• And finally, the relationship itself is more complicated. For example, it's now well known that higher cell counts tend to tolerate (and even "like") higher KV motors that simple scaling would suggest: 2450KV scaled down 1.5x is about 1600, but the actual sweet spot for 6S is higher, around 1800. Why? Well, there's a lot of speculation about the contribution of the battery's chemistry and how it reacts to higher currents, coil resistance and other factors, but the truth is, no one knows. It just works like that.

So in the end, the best way seems to be to just look at what others are doing if you want to get something that works, and experiment if you want to maybe get something that works better :)

Answer 3

The calculations are relatively straightforward for determining unloaded RPM with battery voltage and motor KV. In most practical terms, batteries will never be producing rated full charged voltage in these scenarios (quite often much closer to 3.7-3.9V in actual wide open throttle scenarios on fresh charges - and this is as the props unload allowing the battery to recover slightly), and the actual achieved RPM will correspondingly drop.

There are additional pragmatic considerations that need to be taken into the initial consideration, the most important of which are what propellers are to be used, and what stators are going to be paired with those.

The largest driving consideration is one that makes intuitive sense, but the primacy of it isn't necessarily intuitive: it is that higher KV motors enable (but do not require) larger wattage draw from the battery as an overall system. Higher KV motors will convert this power into thrust, and have much greater top end capability, but the cost is always battery amperage in each given case, and particularly where thrust grows as a relationship to RPM^2, this current draw growth (and eventually additional thermal resistance losses) are what becomes the limit. Battery chemistry and its interaction IS a known quantity, but the non-linear relationship and multivariate response to this makes it challenging to usefully estimate.

Practically, the reason not to run the lowest KV possible is that the flight dynamics benefit from having overhead for demanded thrust and additional ability to modulate the overall thrust available without reducing thrust from the other [N-1] motors to retain attitude control. Particularly because these edge case limitations occur when demanding maximum performance (or else the craft will crash), overhead is basically always desired, but the cost of this overhead is quite often the ability to push the overall powertrain to very high exertion points, which reduces flight time and part life.

In racing considerations where maximum speed is going to be tied to a practical limit (i.e. where additional speed is unlikely to be gained through having additional motor RPM possible at higher KV, but the props or battery will not sustain that), there are some quick guidelines.

General RPM targets are about 38k RPM for 5" quads, which grows towards 52k RPM in high speed 3" quads, and drops into the 26k regime While this isn't a hard & fast rule to be followed, and quite often optimally performing racing setups will exceed these values, but with very specific goals of maximizing responsiveness in instances where the motor RPM isn't at redline, but to maximize available thrust and responsiveness of the powertrain in mid-throttle high load situations common in racing courses.

The other key trends to consider are how stator dimensions and props interact. Lower pitch props obviously benefit from higher RPM, and can produce extremely precise three-axis control through maintaining higher RPM while gaining better thrust resolution at each corner, but the cost is typically small losses in efficiency. Higher pitch props, somewhat paradoxically, are often more efficient when paired with lower KV motors, though the result if typically slightly damped craft response in propwash environments. Stator dimensions and the propeller chord (thickness of blades) also plays a significant response - lower surface area props paired with taller stators respond very well with a smoothed linear response and make use of the taller stator tendency to work well at higher RPM, while thicker chord props paired with wider stators do a great job of managing the extra rotational inertia of the propeller and produce a remarkably smooth response across the most typically used thrust band.

Ultimately, being able to visualize a histogram of throttle inputs can help determine this significantly - something that spends the majority of its time in the 30-60% throttle may benefit from lower KV and/or lower pitch props, but to retain the grip selecting higher effective surface are props work extremely well. For something that spends lots of time above 60% throttle, higher KV despite the drawbacks makes sense, and pairing that with taller stators and thinner chord props can regain some of the responsiveness without making the current draw excessive.

Answer 4

You can try using the propeller speed calculator to simulate motor and propeller combo for a good approximation.

https://www.mrd-rc.com/tutorials-tools-and-testing/useful-tools/propeller-speed-calculator