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I have a 4s Nazgul5 drone. It is a 5'' drone and uses 2207 2750kv brushless motors. What is the most aggressive prop I can use on the drone without causing it to overheat or cause the battery to sag?

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    Battery sag is a fact of life for drones. We draw large currents from these batteries during high-throttle moves, and nearly all batteries will sag to some extent. – ifconfig Apr 19 at 21:07
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    Except that higher cell count batteries will sag less, as less current is drawn from them for the same power. This requires a drone explicitly built for the higher cell counts with lower Kv motors, though. – FlashCactus Apr 19 at 21:26
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    @FlashCactus This isn't necessarily true. There are reasons higher cell counts sag less, but it's not necessarily only due to lower current draw. In fact, many 6S builds draw equal or greater current at the upper ends of the throttle than their lower specced brethren as they are using a higher relative Kv, which overcomes any current reduction gained from Ohm's law. Spreading the current across multiple cells seems to play a larger role, because even at equivalent current draw higher cell count produces less sag. – QuadMcFly Apr 20 at 12:24
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    @QuadMcFly The current doesn't actually "get spread", though, as the cells are connected in series, thus each is experiencing the full current flowing through the pack. So it's more complicated than that as well. Also, I think the reason for higher relative Kv on high cell counts is precisely that you can get away with it, as the same relative Kv will sag less. – FlashCactus Apr 20 at 14:09
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    It's still being delivered by more cells, though you're correct that it's not spread in the sense that they're series not parallel. It changes things electrochemically though, and is still the core of the benefit. As I said, even at equivalent current the sag is less, hence people taking advantage of it by pushing to higher relative Kv. – QuadMcFly Apr 20 at 14:24
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Things like this are something that is usually only determined empirically, i.e. by trying and seeing what happens. That said, it is unlikely that you will cause damage to your motors by putting too high-pitched of a prop on your drone; racers have been known to put ridiculously high-pitch props on ridiculously high-KV motors and push them to the limit.

Your battery will sag somewhat in almost any case, though. 2750 is already a high KV for 4s; the consensus is that the "sweet spot", middle-of-the-road KV for 4s lies in the 2400-2500 range, and 2750 is towards the "ludicrous speed" side. Putting aggressive props on will add even more to that effect, drawing very high current at high throttle, but also providing very high top speed. So I'd probably advise starting from the lower pitch range, 4.0 and less, and increasing it if the drone flies okay and you feel like you want more.

The question you should be asking yourself, though, is "which prop will feel the best for me on this quad?". Order a bunch of different-pitch props (preferably from the same manufacturer), put up a current readout on your OSD and fly them all in order to see which ones you like best and which pitches strain your system too much. After that you can try out similar-pitch props from different manufacturers to see if they make any difference (i.e. some propellers may be more efficient than others, reducing current consumption).

In the end, the best prop for your drone will depend on a lot of factors besides just the technical specification of your drone, a very big part of which is what you're doing with it (racing, freestyle, long range, fun cruising, etc.), how you do it, e.g. your piloting style, and how you like your drone to feel.

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This table provides guidance to 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)

From this, your propellor size should be 5” for 2207/2750 KV motors, I wouldn't recommend going much smaller/bigger than that in order to not cause damage.

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  • How do you arrive at your conclusion of 4"? 2207 falls under 6", 2750kv falls under either 4" or 5" and his 5" drone would fit 5" props. Could you claify a bit more? – ifconfig Apr 19 at 21:08
  • @ifconfig sorry misread the numbers, adjusted – Daniil Apr 19 at 21:41

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