Greetings!

This is my first post. I've browsed around for quite a while and really enjoyed the content here. I am currently considering a design of a larger scale quadcopter. The main reason for a large scale is to carry payload. I have been searching around and I am not really finding the kind of information I'm looking for.

Current specs:

I am looking to build a quad of with about 25 lbs of thrust. I have no idea how much surplus thrust is 'ideal' to keep the quad stable. My weight is about 5 lbs for engine's/esc's, 6 or 7 lbs for frame and 3 lbs for batteries (4s2p 4000MaH's). This should leave considerable thrust for camera payloads and the like.

My frame in mind is 3/4" aluminum tubing, not sure what thickness yet, drilled out to reduce weight with machined platings for the body. The plates will be cnc'd to directly mount my hardware and support the arms with the excess cut out to save weight.

Problems:

Are there any obvious "Gotcha's" i am missing with this plan? Do I need to keep my frame within a certain size? Is there ideal sizes for arms? Is longer arms better in most cases? I have not really found any information on design of quads, only how to build specific kits.

Am I better off going with 6+ engines rather than 4? I imagine that would add stability if that is a concern at this size.

Any resources of this type would be greatly appreciated!

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Due to a lack of large props, I'd scale it as a octocopter with 14" apc's.   The extra motors will give you some failsafe in case one fails and are obtainable.  When I looked at making a large quad, I didn't really have a purpose, other than curiosity.  What stopped me was the cost of a bunch of large 6s lipos.

Messing around with the Static thrust calculator, http://personal.osi.hu/fuzesisz/strc_eng/index.htm it seems as though 8 motors spinning 14x4.7 apc's at 5500rpm using 158 watts, using 100amps at 12.6v 3s, would get 25 pounds GVW off the ground.  If you had (4) 5000mah 3s batteries, you should get almost 10 mins of flight time.  You would have to add up all of the weight of the vehicle and batteries and subtract that from 25 pounds to get your payload.  You just have to mix and match.

If you are serious, it would be best to look at the motor's performance chart to find the most efficient prop to use for your intended weight.

Just look at available motor/prop combos, and plug them it the calculator to get an idea of what may work for you.

Can a 6 engine copter handle the failure of 1 engine? or only 8? 8 seems a bit overkill.

Thanks for your input as well, but it doesn't really answer any frame questions which are my biggest ones. I understand the thrust to weight stuff, battery configurations, motor/prop rpm's, etc.etc.. but I really don't understand the details of frame design itself. How long should the arms be? should they always follow a certain shape?

Yes a hex can tolerate one motor failure.  Basically the opposite motor powers down also, making it a big quad copter.

I've never made a hex or octo as I never fly an expensive payload like a real movie camera.  A radial hex appears to be the more efficient than the coaxial Y arrangement, but it is larger due to the necessary prop clearance.  As far as arm length goes, I believe that shorter is more maneuverable and longer is more stable.  I usually just give and inch of clearance between the prop and center hub components.  I've heard that more clearance provides better efficiency, but I don't know if the added efficiency overcomes the added weight of longer arms.

I would choose my frame type based on the payload.  If you want a stable camera platform, I'd copy what others do and go with a radial and long arms.  If you want smaller size and maneuverability, I'd go with a short armed coaxial hex.

Additionally, most of the hardware store 3/4" tubing I've seen is 1/16th walled.  That is overkill.  I like using the Home Depot aluminum bulk square bathroom towel bar rods.  They are really strong for their weight.  Another advantage of using square rods is easy of assembly.  You just make (2) square plates for the hub and attach the arms.  No need to worry about rotation, or special clamping mechanisms.

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