Overlapping Propeller Designs

[R_Lefebvre]

im encouraged to hear this, was just thinking last nite on how to build a Y6 with all the motors on top, and if overlapping prop disc's works, thats just fantastic

Yes, you could do that. If you mounted the motors such that the lower prop just clears the upper motor, you'd be near 40% overlap, and probably would have good efficiency. It's worth checking on a bench test though.

 

[R_Lefebvre]

What does change is the moments of inertia, in the same way as longer arms. So hanging a big, heavy gimbal with a big, heavy camera under the frame will slow the craft's roll/pitch responses. The craft will always rotate around its centre of gravity - the further that point is displaced from the centre of thrust (in any direction), the more work the motors have to do to effect a change of attitude. This can be visualised as holding a broom-stick with a weight on the end - the closer your hand is to the weight, the easier is to rotate the stick in space.

So it is just about MoI? I thought there might be more going on, like the longer lever arms made the motors produce less angular rotation for a given control input. It would make their response "softer". That's a good and a bad thing. Good in calm winds, bad in turbulence. However, if all that the designer wanted to do is drive up the MoI, it could be easily done by using 4 battery packs, and hang them out on the ends of the arms in a plus configuration.

The discussion of MoI is interesting, as sports car guys argue about this all the time. Mass centralization makes a sports car more responsive, but if you increase the MoI, it feels more stable. You'll notice some cars such as the old Porsche 944, and now the Corvette, they did this by putting the transmission in the back. The motor up front and transaxle in the back are connected with a long torque tube. Really drives up the MoI. Compare that to any Mid-rear sports car like a Lotus Elise where the powertrain is compact and centered. And then you have the worst of both worlds: The 911.

 

[flytofly]

I was hoping this topic didn't die off from the other thread

Just like the coax Y6 how about an overlapping X8. Two motors mounted on the same arm at different overlapped heights. With only four arms that would be a very compact powerhouse. The tricky part for me is figuring out a solid way to mount a set of 2826/12 on spacers. Or I could flip four motors upside down... Then there's the limited camera tilt range due to motor/prop hanging in front of the lens. Decisions decisions.

It was mentioned in another thread that a quad setup may be more stable in breezy weather than a hexa or octo due to the quad having less disc surface for wind to catch..thoughts?

 

[R_Lefebvre]

Sure, you could do that. I've qot sketches somewhere of a Quad-T-Copter. I think you could do something where... ok, make it an X-quad-Octo copter (as opposed to a plus-quad-Octo). Space the motors such that you have 40% overlap on each pair, but no overlap between the arms. Then, put the two motors directly in front of the camera "up". So it would leave a "tunnel" of sorts that the camera could look out of. I dunno. It's would have to be sketched out.

Also, it wouldn't be as compact as an overlapping Octo.

As for spacers, I'm planning on buying 40mm screws with 30mm aluminum standoffs. The spacers won't move.

Alternatively, I have though about cutting 30mm sections of heavy wall aluminum tube. then drilling it axially for each screw. No way that would move. The only issue here (besides weight obviously) is that the bolt pattern isn't round, it's more like a parallelogram. So the tube would have to be really thick, or a complicated elipsoid machining. It would be trivial to do on a CNC, But I don't have one.

 

[Bartman]

how do you plan to mount the arms and motors to make the overlapping possible? it would seem that in addition to the extra weight of the arms and hardware you're going to have to put more space between the frame plates which means your hardware/fixtures will weigh more.

if you're up for some lab work, it would be helpful to see how prop sizes affect efficiency, like having 13" props on the bottoms and 11" props on top. Pitch differences would also be interesting to investigate.

regards,
bart

 

[R_Lefebvre]

Are you referring to my Octo? I'm simply going to use some spacers. Won't weight much, and is easily made up for by the savings on arm length.

If you're talking about the quad-T-Octo, I have no idea. I didn't get much past the sketching phase on that one.

 

[Bartman]

it would appear that if the props are going to overlap then there has to be vertical clearance between the arms/motors/props. you're saying you're going to put the motors up on spacers to get every other one higher than the two adjacent to it? i wonder if that might not be quite as impact resistant as the motors mounted more closely to the arms.

just thinking out loud.

 

[R_Lefebvre]

Yep, that is true, less impact resistant. But if I have an impact like that, I figure the bent spacers is the least of my worries. I would like to come up with a more elegant way of doing this, but I haven't yet.

 

[Bartman]

you could try one up and one down alternating around the heli

 

[R_Lefebvre]

you could try one up and one down alternating around the heli

Yes, I know, that's what I did first. But it makes it very hard to keep motors out of the camera frame.

 

[Bartman]

Yes, I know, that's what I did first. But it makes it very hard to keep motors out of the camera frame.

i'm very familiar with that which is why i developed my XY-8 format.

 

[R_Lefebvre]

i'm very familiar with that which is why i developed my XY-8 format.

Yes, I almost build XH-8, but I couldn't find motor mounts I liked, and the efficiency is reduced, so I just decided to give this a shot.

 

[Bartman]

i'm beginning to suspect that larger diameter and higher pitch props on the bottoms don't suffer quite as much of an efficiency loss as you think.

 

[R_Lefebvre]

It's entirely possible. But I haven't seen any test results yet.

 

[flytofly]

What about wind stability for a filming bird with an X config, whether it be overlap or coax, compared to a standard hexa/octo layout? I've heard that the X config will be more stable. Does anyone use both configs regularly enough to see a difference in stability?

 

[R_Lefebvre]

I've also heard the statement that X-octos are more wind resistant than flat Octos.

I'm not sure if the reason is because an X-octo has half as many booms and thus less frame to get caught in the wind? Or is it because an X-octo has shorter booms, so they don't get caught in the wind? Or is it something to do with the an interaction between the column of air coming off the motors vs. the wind.

And then how will the overlapping-octo compare? No idea yet.

 

[hexacop]

I can definitely confirm that the coax setup is much stable in wind compared to a traditional multi rotor.
I have build many copters in the meantime and my CX8 is the preferred one in windy situations.
This is my CX8 test flight during heavy wind:

Sure it has some other downside, it is noisy and probably 10% less efficient in terms of flight time, but is is so nice and small compared to a real octo ;-)

 

[jes1111]

It's to do with the air columns (they effectively offer resistance to lateral wind) and the shorter arms (so response to disturbance is quicker). National Geographic Magazine uses a coax-8 and stated in a video (can't find the link right now) that it was much better for wind stability than their previous flat 8.

 

[R_Lefebvre]

I've seen that NatGeo video. I have an SRH that does even better in the wind.

 

[matwelli]

View attachment 6407

decided to try it out on my Y6 - bit rough but the mounts are all done, and esc wireing bodged...just need to add flight controller and test her out tomorrow