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I’m looking to use some spare motors I have to build a 7” rig.

The problem is that they are 2450Kv, and I don’t want to burn out my other components. As I understand it, if I set a motor output limit of 70%, this should give me similar performance to a 1700Kv motor. Is this the case, and are there any disadvantages to doing this?

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7" props have a disk area that is almost twice as large as 5" props, so the motors will be heavily overloaded. (7 squared is 49, 5 squared is 25, the other terms in the equation for the area of a circle cancel out)

Assuming you use 5x4.5 3-blade props on a 5" quad, you could reduce the load 33% by switching to 2-blade props. You can reduce the load further with a lower pitch prop. 7x3 is about as low as you can find, and will give you another 33% reduction, so now the motors are only running at 133%.

I think that's worth a try, but if the motors get hot you may need to step down to 6" props. 6x4 props may work better than 7x3, giving you more speed.

Reducing throttle to 70% will reduce the maximum voltage, similar to switching to 3s batteries. However torque and heating are related to current. Reducing voltage will limit the maximum power and thus the maximum current, but won't make any difference at part throttle.

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    $\begingroup$ Thank you for your answer! $\endgroup$
    – Drones and Whatnot
    Commented Jun 3, 2020 at 13:54
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    $\begingroup$ @DronesandWhatnot maybe you should accept one of the two answers and/or upvote them if they helped you. That would help future visitors and it serves as an incentive to answer questions $\endgroup$
    – Bastian Springer
    Commented Jun 4, 2020 at 8:39
  • $\begingroup$ Or explain what else you'd like to know, so we can expand the existing answers. $\endgroup$
    – Robin Bennett
    Commented Jun 4, 2020 at 9:56
  • $\begingroup$ @BastianSpringer there’s still some discussion going on in the comments of the other answer. I’d like to see what comes of that before accepting an answer. $\endgroup$
    – Drones and Whatnot
    Commented Jun 5, 2020 at 23:13
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    $\begingroup$ This is the correct answer. A 2207 will have plenty of torque to push a 7" prop and limiting the motor output in the flight controller should be a perfectly acceptable solution, I would also recommend reducing rampup power (or startup power in BLHeli_S) in the ESC to limit the current spikes caused by higher torque constant of the higher Kv motor when pushing the higher torque load. Once you've done that you should be in good shape, though you'll probably see a slightly lower overall efficiency than a correctly specced motor. Should be manageable though. $\endgroup$
    – QuadMcFly
    Commented Jun 5, 2020 at 23:52

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