Coaxial Inefficiency

[Jumpy07]

Interesting video...

Motor Configuration Efficiency Testing - RCTESTFLIGHT -

[HowDoIfly]

Interesting.
Love the flying lampshade 

[dockman]

Good video, be interesting to see if distance of props from each other in coax would make any difference.

As with most things multirotor though its all about trade offs.  Smaller footprint and better stability in wind, or slightly larger and longer flight times. 

[John A]

Interesting......
I have the following setup:-
Y6 configuration consisting of

Sunnysky 2212 980Kv motors, co-axial mount, prop separation of 95mm
10/4.7 SF props
Turnigy Plush 30A standard (non-SimonK)
Naza V2

AUW = 2050g including 2 X 3200mAh Lipos

I get around 12-13 minutes flight time for my 6400mAh.

Anyone using similar components and AUW in a standard Hex config, and what is your flight duration?

[MOFO64]

Interesting......
I have the following setup:-
Y6 configuration consisting of

Sunnysky 2212 980Kv motors, co-axial mount, prop separation of 95mm
10/4.7 SF props
Turnigy Plush 30A standard (non-SimonK)
Naza V2

AUW = 2050g including 2 X 3200mAh Lipos

I get around 12-13 minutes flight time for my 6400mAh.

Anyone using similar components and AUW in a standard Hex config, and what is your flight duration?

Longer than yours at a guess, lol.

Coaxial design looks cool but is inefficient.

[guest325]

Another thing that may improve coaxial efficiency is to run different pitch props - less pitch at the top than the bottom so that the lower prop with a greater pitch accelerates the output of the upper like multi stage turbines....just a thought!!!

[John A]

Another thing that may improve coaxial efficiency is to run different pitch props - less pitch at the top than the bottom so that the lower prop with a greater pitch accelerates the output of the upper like multi stage turbines....just a thought!!!

Yes, and with that in mind I began to research the theory, and with research comes conflicting viewpoints 

One school of thought is that reducing the lower prop diameter and increasing the pitch improves the efficiency. The explanation is that the diameter of the column of fast moving air from the top prop reduces the further away it gets from the prop, so the bottom prop diameter should ideally be the diameter of the column when they meet. The higher pitch then accelerates the column faster. In this scenario the prop spacing would also be important because of the shape of the column of fast moving air.

On the flip side, making the bottom prop diameter larger and greater pitch. In this theory the efficiency is increased because the tips of the bottom prop are running in "clean air", outside the column from the top prop. Again, the higher pitch accelerates the air faster.

I believe it is assumed that both of the props are running at the same number of revolutions to maintain the optimum airflow through the coaxial pair, probably by some form of gearbox or sharing a common driveshaft.

So, let';s get a bucketful of props and experiment......but wait.....
The yaw of the aircraft is controlled by balancing the torque reactions between the prop sets. If the load on each motor is different because  the prop sizes are different, won';t the torque reactions go out of balance? Our FC will do its level best to cancel out any unwanted yaw, and the only way it can do that is to change the torque reaction of each motor by speeding it up or slowing it down. Having balanced the torque reactions I would expect there to be a difference in the RPM between the top and bottom props which would affect the optimum airflow through the coaxial pair and also implies that one or other of the motors would be working harder? (thought: if the props are the same size, does this mean the FC is working one motor harder to balance the torque reactions?)

I like my Y6, I think it looks quite cool in flight, especially flying it backwards (one arm forward) which is how I ';hoon'; around but to save the brain-ache on the next build I think I will try a ';standard'; Hex 

....Unless you know better.....

[e.chipev]

Running larger lower props with greater pitch will not affect yaw.
Losing efficiency is part of the trade.. but in real world it';s not a problem.
The problem is in your head with this number mania thing.
If you are looking for something universal don';t look further just buy more juice/batteries.
I fly my y6 with nex5 stress free even in challenging weather(only problem is the landing).
I use 5 different sets of props for different weather starting from 11x5 up to 14x5.5
Auw-5kg including 2x5000mah 4s lipos
10min active flight with wooden props 13x4 top and 14x5 lower is draining ~65% of the batteries... not bad I think?
Will make tests with 3 or even 4 batteries... soon.
Regards