Octo with overlaping propeller arcs

[eskil23]

Altough a large heavy lift MR is still far out of my budget, I can't help planning it. One thing that struck me about octos like the S1000 is that despite large span the propeller size is limited (the S1000 max prop size is 15", just like the smaller hex S800). A coaxial configuration like the Cyclone X8 allows larger space between motors, which means space for larger props. The drawback is that the lower props have to work in turbulent air lowering their efficiency.

A compromise would be an eight-armed octo with four of the arms mounted higher than the others (like two quads on top och each other with one rotated 45 degrees). This would allow for large props just like a coaxial, but the lower props would only partially operate in turbulent air.
Have anyone tried this?

 

[Pumpkinguy]

I've thought about this many times. It stands to reason since there would only be a slight overlap it's gotta be more efficient than a coax setup.

 

[Carapau]

Not sure how much more efficient it will be given that most lift is generated by the tips I believe.

 

[eskil23]

The tips will also be only partially overlaped.

 

[stevemaller]

I have thought about this as well. I think the only way to test it is to actually build it and test it. You'd need a suitably well-instrumented copter with good data logging and telemetry. Also being able to experiment with different mixer configs would be important, as well as somehow quantifying things like pitch, roll, and yaw responsiveness. I could see advantages, especially for efficiency and flight duration, as well as payload capacity.

I wish I had some time to devote to this myself.

 

[tstrike]

A couple of guys did that here with their CS8. It let them run way bigger props, they said they benefitted with more lift having more prop surface area.
I thought it was a clever solution for running bigger props vs. using longer arms.

They've got some pictures of their rig on their instagram page- http://instagram.com/overridefilms/

Last time I saw it was last summer, looks like they've gone to X8 config. now.

 

[Dylan M]

Interesting idea, still much larger footprint than true X8.

You also still have all the extra weight having 8 single booms...

 

[eskil23]

You also still have all the extra weight having 8 single booms...

Yes. On the other hand, each boom only have to carry 1/8 of the weight, so they can be thiner and lighter than on a coaxial configuration.

 

[Dylan M]

Yes. On the other hand, each boom only have to carry 1/8 of the weight, so they can be thiner and lighter than on a coaxial configuration.

Also remember that all 8 of them need to be longer to be able to run the same size props.

 

[eskil23]

Also remember that all 8 of them need to be longer to be able to run the same size props.

No, they don't. That's the whole point with this configuration.

 

[SleepyC]

I had an aerospace engineer look into this for me a while back... Using this:
View attachment 23354

His reply:
My intuition says that the top rotor will still induce down wash on the tip section of the lower rotor and thus mess up the span wise lift distribution with a slight loss in efficiency. But probably not as much as a straight coaxial set up. My concern would be manuverability.
The overlap might cancel out some of the flight controls commands if left in an onto configuration. I guess it would be an expensive experiment for you if you try it and it doesn't work but I'd say go for it.

I never went for it, but I have thought about it a lot!

 

[Dylan M]

It is an interesting idea, the thrust differential with the overlap would possibly be confusing to the motor mix but it would be cool to see it done.

 

[eskil23]

His reply:
My intuition says that the top rotor will still induce down wash on the tip section of the lower rotor and thus mess up the span wise lift distribution with a slight loss in efficiency. But probably not as much as a straight coaxial set up. My concern would be manuverability.
The overlap might cancel out some of the flight controls commands if left in an onto configuration. I guess it would be an expensive experiment for you if you try it and it doesn't work but I'd say go for it.

I never went for it, but I have thought about it a lot!

I also like the idea. What I'm worried about is vibrations. A propeller that works a quarter of an turn in downwash, next quarter in clean air, next quarter in downwash etc would probably meet varying resistance. But still, some full size helicopters have overlaping and even intermeshing rotors.

 

[SleepyC]

It can't be any worse than coaxial!
If I could source some materials I would be willing to give this a try!

 

[Dylan M]

It would look pretty cool as well as being slightly smaller footprint... If I have any time I might mock something up as well, I'm curious...

 

[JoeBob]

Take a Hex frame. Mount 3 up and 3 down.View attachment 23405
Fewer prop tip overlaps than an Octo.
Control inputs might be more responsive.
24% cheaper to experiment on!

 

[Remoteworks]

This guy did some testing about an year a go

 

[eskil23]

This guy did some testing about an year a go

More efficient than coaxial but less efficient than a conventional setup with same size propellers. Well, thats no surprise. A comparison with a conventional multirotor with same footprint would be more intresting. But nevertheles, the test shows that the weight saved by having shoter booms/arms is lost.

He screwed up the ducted fan test, though. The profile of the duct is crucial to efficiency. It should not be a plain tube. The ducted fans I've seen used for real is wider at the ends and narrowest at the propeller.

 

[SleepyC]

More efficient than coaxial but less efficient than a conventional setup with same size propellers. Well, thats no surprise. A comparison with a conventional multirotor with same footprint would be more intresting. But nevertheles, the test shows that the weight saved by having shoter booms/arms is lost.

He screwed up the ducted fan test, though. The profile of the duct is crucial to efficiency. It should not be a plain tube. The ducted fans I've seen used for real is wider at the ends and narrowest at the propeller.

Right a proper duct is like a turbo charger, it pulls more air in, and compresses it and then pushes it out on the other side. Also to be effective the prop to shroud clearance has to be VERY tight.

 

[Dylan M]

If you could tune it then it might fly pretty well because the thrust inputs would be closer together than the conventional. I wonder if there is anyway to see if the prop wash from overlapping slightly is different than plain coaxial. It is definitely an interesting idea.

Get 2 quad frames and offset the arms half way in between each other. If nothing else it would look badass...