One for the physicists -

 

It would seem obvious that the lower rotor in a Y6 (or any vertical rotor set-up) is working in a whole different way to any rotor in a hexa set-up.  The fact that it's pushing down "pre-pushed" air suggests, intuitively, that it's not working as hard as it's upper neighbor and therefore not generating as much lift.  

 

So is a Y6 weaker than a hexacopter, with every other spec identical?

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Y6 configs are widely considered to be 15-30% less efficient than a non-coaxial configuration. I think most of the efficiency is lost through the "slap" when the top and bottom blades pass.

It is the engineers, not the physicists, whom you want, I believe.

 

14% is mostly theoretical/optimistic for full scale, variable pitch, but larger than 33%, I think, in this case, and in typical coaxial arrangements vs similarly arranged single and tandem rotor systems. 

 

See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_19... for a good read on a survey of the topic, across major research performed by several countries, which I believe you will find applicable.  

Very nice link Mr P!

Having researched this a great deal, and having tested (under admittedly sub-optimal, less-than-laboratory conditions ) a number of different rotor configurations, might I suggest a more modern paper might worth perusing.  I know many aeronautical engineers believe that coaxial configurations offer promise of increased efficiencies, but I am unconvinced.  Intuitively, having four or more drag-inducing blades working the same column of air just doesn't seem like it would work well (for much the same basic reasons we don't see biplanes much any more).  Dr. Leishman's analysis of the available data, including review of the free vortex wake models he produced, are summarized nicely by the chart on page 9 of the attached paper.  If anything, it should illustrate the fact that even acknowledged experts often can't agree on something seemingly so basic.

http://terpconnect.umd.edu/~leishman/Aero/AHS2002_Griffiths.pdf

excellent, good material. And these variations, in both documents, are why I have suggested it is more in the domain of engineers than physicists - there might be mathematical models detailed enough to account for all the issues, but I'd rather trust the experiments than the models in this case. I suspect it takes some fairly complex computer simulations to get as meaningful results, and in our case, it is better/more accurate/less costly to just build and test

Great material.  I did not expect such a thorough, fascinating set of responses.  Raises questions as to optimal lateral rotor-layout, also.  Lots to experiment with over the coming decades!  Thanks, gents.  Still absorbing the seeming fact that a Y6 is more elegant regarding aesthetic design and camera-practicalities than it is in aeronautic function.  There had to be some trade-off in reducing the canopy of threshing blades, of course.  

 

I was actually surprised to note that the reduced efficiency reported in the NASA paper is only by 5% and that the coaxial configuration gives better stability when hovering. Its also noted that the efficiency is improved if the top propeller size is increased by 8%. I will do some testing of these configurations on my test rig and let you know the results.

Two thoughts, sitting in the same section of the (Nasa) document. Emphasis added.

 

'He also found that there was a "trade off between increase in induced power on the upper rotor with the reduction in upper rotor radius and the enhanced performance of the lower rotor as proportionately more disc is exposed to clean air. Most promising results were obtained for a 8% reduction in upper rotor radius."

Harrington demonstrated that scale effect plays an important role in coaxial rotor testing (as in any rotor testing). Throughout this report, large differences in Reynolds numbers have been reported, probably resulting in different testing conditions.  '

My line of thinking is that a full Octo mutil prop copter compared to an X8 quad coaxial copter configuration  having 4x less arms and motor mounts,  hence less this amount of payload, might just be the difference to make the flight time and efficiency nearly equal. It may even surprise us and be better. This is the line I am following at this stage and will put some resources in doing some testing on this. Another gut feeling why I think this might also be the case is when I see a company like Draganflyer who build multi wing copter in X8 quad configuration for $30K, some how I feel they would have done their home work to get a reasonable efficient aircraft.

I don't see that in the math, it seems to me that in those cases, they are trading off inefficiencies for other benefits.

 

Everything in multicopters is compromise towards a purpose. However, the purpose varies between projects. It all begins with that second blade, then that second prop. We usually quickly jump to a quad because of the simplification found in yaw stability. 

 

Even examples of well engineered multicopters are not, in themselves, evidence that a design is more efficient. 

 

That being said, this general thrust of investigation is a worthwhile effort. Every minute spent investigating these topics will provide a return for you in your projects. 

What I am unsure of at this stage and having difficulty in finding is more info on does the APM support Octo configuration and X8 quad coaxial configuration? Its doesn't seem to be on the GUI software

There are a number of folks running different octo configurations, including X8, V8, and Vtail8 setups. As multicopters go, none of these are the most popular, so full support in the UI has not yet been developed. 

 

For those so motivated, it is possible to get these configurations running on ArduCopter without too much effort. Because of the increased cost of an octo, and because they are not generally a "starter" build, it makes sense that this is one area that is not top priority. For example, I believe that the more popular Y frame is suffering and could use more attention, testing, and refining, and configuration support aside, there are dozens of areas like obstacle avoidance, banked turns, enhanced launch and landing support, enhanced maneuvers and so on that benefit everyone. 

 

The more people we have contributing, testing, and supporting these projects, the better it will get. But the wonderful things about this project is that even if main line support does not exist, there is probably someone doing it, and often it is not all that difficult to get to because of how it's built.

 

I'll find some resources for you. Then it'd be a good idea to contact some of the authors of the posts for more help.

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