Quad vs Hexa vs Octo -copter. (Radial and coaxial) Advantages-Disadvantages?

Hi everyone :)

Althought it seems to me it's the first question that comes in mind when building a multirotor, there's not really much interest in determining which is the best design (deppending on the use) on the net.

I only could find some tri/quad comparation, but nothing comparing the CPU-balanced ones.

Could you please tell me the main (relative) differences between them (including their possible layouts like raddial/coaxial). Just smack here what you now  :P


 Quadcopter

 Skylens Aerial Photography





 Hexacopter Radial

 Holger Buss' MK Hexacopter




 Octocopter (Radial)

 Mikrokoopter
 



 Octocoper (Coaxial)

 www.robfrench.co.uk



 Octocopter (linear?)
 
 AscTec Falcon 8


Photo credicts go to their mentioned owners

Thanks in advance :)
Andy

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Let me gove a try at this...

Y or Tri copters are NOT unstable at all, in fact they are far superior in some flight modes, especially when it comes to turning, or what some people more specifically call "Swooshing", which can be advantageous when making video.

There are yet more advanced versions using tilt rotors in order to 'Pan' directly sideways, rather than the craft turning and tilting.

More motors/rotors are generally liked for power, but there is a trade off that actually takes away from that claimed power - more motors means more weight of the craft itself, means more weight from extra batteries to power the craft for the same flight time, which paradoixically is slightly self-defeating if you get what I am saying here.

The main factors: Craft weight, battery wieght, potential payload weight, and flight time.

If you just add motors, say 6 instead of 4, then your potential payload and flight time suffer, so you have to add more battery to maintain the same flight time, etc.

Unless you are going for building a heavy lifter, or are building something with lots of bells and whistles, thus more on-board equipment and craft weight, more motors may not give you a whole lot of advantage.

Aside from that, one of the main benefits of more motors means that you can potentially lose one or more props or motors without the craft going down, so if there were a collision, you may still be able to either bring the craft back or keep using it if there is some important reason to.

Octo's fly great, but again, unless you NEED serious lift, they are not necessary, but no reason to not build one if you have the parts spare, or want to burn money.

For racers, small quads (250 class) are almost universally prefered.

Videographers use almost every other kind of multirotor depending on thier needs.

Personally, I don't think much about 'spider' or raddial/coaxial types - the Octo's definitely have the power potential for heavy lifters, and people going for records tend to prefer them, but they are not a design that is best for lifting object unless they are  hung from underneath, by a strat or hoist or similar, they also waste craft frame weight, because every motor has its own arm, when you don't need 8 arms, other frames are better for that, such as the "H" style, like the last picture, "
 AscTec Falcon 8".

I personally prefer Y Copters with 3 or 6 motors - but not with the motors doubled-up, in pairs, with one motor facign up and the oither under it facing down, as it has been shown to be inefficient. I otherwise also prefer various designs using 6 motors. They seem to have the best balance of motors, weight, etc.

I am working on a wind-resistant copter with 15" props and 2/3 kgs payload.

I am leaning towards an Y-6 configuration with coaxial motors. It minimizes the surface area for susceptibility to gusts of wind and interference with the visual field of the 360cam. I hear that if the motors are not too close together on the Z-axis, coaxial motors can be quite efficient. @JSmith do you have any data showing how much less efficient this is?

I am also considering ducted fans although I am wondering how much that will affect the susceptibility to winds.

If you do more than hover around then Y's have a distinct disadvantage because of dis-symmetry of lift. 

Ralph Zoontjens said:

I am working on a wind-resistant copter with 15" props and 2/3 kgs payload.

I am leaning towards an Y-6 configuration with coaxial motors. It minimizes the surface area for susceptibility to gusts of wind and interference with the visual field of the 360cam. I hear that if the motors are not too close together on the Z-axis, coaxial motors can be quite efficient. @JSmith do you have any data showing how much less efficient this is?

I am also considering ducted fans although I am wondering how much that will affect the susceptibility to winds.

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