Hi,

I would like to build an octocopter, with an X8 config. Would need some help with choosing the relevant (1) motors and (2) props (3) ESCs and also the (4) batteries.  I would be using an APM2.5 board.

However, I do have some restrains / requirements.

  1. The X8 octocopter AUW will be approximately 35 lbs (16kg).
  2. The prop can only have a maximum diameter of 9 inches.

I am worried that with such specs, the quad may not be stable or even maybe take off.

I was wondering if having 4 or 6 blades to each prop instead of 2 would solve the problem (if the basic 2 props can’t do the job). But having 4 or 6 blades, would the motors be able to take the stress?

Anyone have any recommendations on how many kv motor to use and prop size?

Please help.

 

Thank you.

Nicholas

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  • The smallest I could get to even get close to working in this size, was a 9*5" 5 blade prop, which running on Alle KDE500XF-1450 motors, can give enough thrust to hover at about 76% power. However you are then talking about 80amp per motor at hover (4s), and over 1KW/motor. With 8 motors, total current 640amps. These would be noisy though, since the prop tips will be going at over 500mph at hover. This kills efficiency, and will be _loud_. You need to get the tips below perhaps 350mph, before sound will be even remotely acceptable. You need to use larger props, to produce anything even remotely useable. You are still going to have big problems with something around 8KW needed to hover....

  • My setup is 3.5KG heavy and already runs 13 inch 8 pitch 3-blade props. Yes, I have more capacity (around 18kg total trust) but flight time and system overall use is important how efficent your copter will fly. If your ESC gets burning hot due to constant near max load, your flight time will be short, your fail rate will be higher... I would go back to the drawing board and think again what you want to achive.

    cheers

    J

  • I think that what you're asking for is unrealistic.  I did poke around at http://www.ecalc.ch/xcoptercalc.htm a bit, but I think these conditions would be out of the range of what can be reliably predicted on that site.

    Your first problem is props.  I did look around briefly for 3+ bladed props at 9", and didn't find anything right away.  So, playing around at http://adamone.rchomepage.com/calc_thrust.htm, to hover (2kg thrust) you would need to turn an APC 9x4.7 SF at over 13k RPM.  APC suggests a max RPM for 9" SF props around 7200.  Switch to APC E 9x6 and you're still in the 13k RPM ballpark.  The safe limit for those is around 16k RPM.  That's not a lot of room to maneuver, but they come in both CW and CCW, so for now let's go with it.

    Now you need a motor.  You need something big that can sustain at least 500W.  And with high enough kv to spin at 13k RPM, under load, at probably no more than 66% throttle.  That probably means something like a 1000 kv motor with a 6s LiPo, at the low end.  Most of the big outrunners that I've seen have too low kv.  But something like http://www.hobbyking.com/hobbyking/store/__17698__XK4082_1500KV_Bru... might get you in the ballpark.

    Going back to ecalc with that, and dropping to a 5S LiPo, it looks like you might be able to get it flying.  With 4x 6000 mAh packs and 120A ESCs, you'd be looking at less than 5 minutes of flight time.  And you're looking at something like 8-9 kg for motors, ESCs, and batteries.  But I really have no idea if these numbers would hold up to experimental values.

    In any case, I think you will have a hard time reaching your requirements.  One thing you might look at, though is an electric ducted fan.  I don't know if something like that would work any better for you or not.

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