I'm writing a proposal for a $2000 summer stipend from my university, for development on an APM UAV.

For the project, I'm looking at the X8 wing. However, I foresee a couple issues:

Most layouts I see have the (3 or 4s) batteries mounted in front, with a long cable going to the ESC in the back. This seems to be a problem because making a foot and a half cable run to carry 60+ amps DC from the battery to the motor is going to lose a lot of efficiency, and the cables would have to run very close to the ardupilot's magnetometer (compass), which is definitely going to cause problems.

So I'm looking for suggestions... I don't see an obvious way around mounting the batteries in the front, because I don't have anything else heavy to put there for balance that won't be deadweight. Does anyone else see a way around this?

I can use a 6s or higher system to get more energy transfer efficiency. My goals, however, are for efficiency and endurance, not speed and power. So, can anyone suggest a battery/esc/motor/prop combo that will provide max endurance on a 6s or higher system?

My final problem is no landing gear. I fly off a runway, so ideally I would add retracts. Is this doable?

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I've seen that thread. It doesn't help with my specific issues, to the best of my knowledge.

I would suggest using shielded cable to run the wire extension.  That would help with the interference issue a bit.  You can also try and run the wire through a CF tube.  I'll get back to you on the 6S motor/prop setup.  Retracts should not be a problem to put on the X8.  Here are some good ones that can support up to 10 lbs.  I really hate how no one helps out the newbies or general questioners on DIYDrones..  

OMG that is exactly what my first dream design looked like only better!  That my diydrones peeps is a sexy aircraft if ever there was one.  I just woke my wife up to show her!

She's all kinds of happy.

Now I have to decide between that or a six foot glider for a platform.  OMFG.

Shielded wire will not help the magnetometer interference problems.  You'd have to use MuMetal shielding or something like that.  Now, if you use the smallest gauge wire you can get away with, and run them tightly bound together, better yet twisted, and mount the magnetometer as far out on a wing as you can, it should work out OK.

I also wouldn't worry about the voltage drop along 1.5 feet of wire.  12 Ga wire is 0.0015 ohm/foot, so at 60A, you're looking at a voltage drop of 0.12V. From 16V, it's not a big problem.  Less than 1% power loss.

Sorry, I should have been more specific when I said shielded wire.  Obviously the shielded wire used in a home won't work, but magnetic shielding cable like this should attenuate the interference some.  You can twist the wires tight, use a foil braid, and try a CF tube to help with the attenuation.  I do not have the hardware to test this out but I would love to actually see an interference test with 1.5 ft wire with 60A using these methods.  He's right about the power drop.  It's pretty nominal.  The 6S setup that Alishamao uses, is a custom motor from BEVRC.com and a 13x7 prop.  

A good 6S motor setup would be a Hacker A40-10L-14pHacker X-70 SB Pro ESC, and a 13x7 Graupner folding prop.  This setup has great static thrust and a 90.3% efficiency, 45A, 929W.  With a 6S 8000mAh battery, you can get approx. 11 min at full throttle.  At 30% throttle (~7A), you can get approx. 68 minutes of flight time.

I'm not sure what your goal is after setting up the UAV but seriously keep it simple. There are enough things to go wrong, go with what works in the simplest possible way or you'll spend all summer debugging extra stuff. Bixler or Skywalker, no landing gear just tape the bottom and land on the pavement. Forget efficiency, 3 or 4S is fine just don't mount the APM directly on the battery cable. A big battery in front can always balance the plane, mount it with velcro. Good luck!

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