New Octo and Motors Question

[rwilabee]

I'm getting ready to put together a scratch built Octo. I have 8 Avroto 2814 motors from another project I am going to use. Any thoughts on 12 x 4.5 vs 12 x 3.8 props and 3S or 4S battery

 

[Electro 2]

Generally multirotor props work best in the 1:2 range for pitch-to-diameter ratios. Your 1247s should be fine for 2814s. I fly 1247s on my 2810s and it works well. As for 3S vs. 4S batteries, you don't give the Kv rating for the motors, or the AUW for the aircraft, so it's impossible to tell. If they're 750 or higher, should be OK with 3 cells and 1247s for a sub-5 pound machine. it they are lower than this, 4 cells would be a better choice. If we had curves for these motors we could look at the results and pick the optimum efficiency point, (in G/W) and know that props can be one inch up or down from this point for fine tuning.

 

[rwilabee]

The Avroto's are 770KV just a bit higher than 750 so it sounds like I could you either 3S or 4S. I guess I'll have to try both. I don't know the weight yet as I haven't started building the Octo.

 

[Electro 2]

Most manufacturers give weights for components on their Web sites, just add 'em up, add the batteries weight, and you've got a rough idea of AUW. The difference in Kv is trivial, you'd have to go plus or minus 200 or so, to see effects. If you had said they were 1000 or 490, that would have made the choice for you.

 

[rwilabee]

APC 11x4.7 @ 11.1V The distributor just sent me some test data. He said that the 12 inch props at 4S tend to run hot a full throttle.
Max Watts- 190 W
Max Thrust- 41.3 oz

APC 12x3.8 @ 11.1V
Max Watts- 226 W
Max Thrust- 47.2 oz

APC 11x4.7 @ 14.8V
Max Watts- 390 W
Max Thrust- 64.2 oz

APC 12x3.8 @ 14.8V
Max Watts- 469 W HOT
Max Thrust- 73.4 oz

 

[holco]

 

[Electro 2]

Well, there you go, some data. What isn't detailed here is efficiency, i.e. G/W. So you'll kind have to sort this out for yourself.