Asymmetric Hex - conversion from an X8

[min0nim]

Hi guys,

I've been noticing my X8 is a bit unsteady in decent winds. I've adjusted gains, changed to smaller props and a few other tweaks. All have helped, but it still wobbles around a fair bit in any sort of stiff breeze.

I'm thinking about adding two arms and going to a hex frame. My smaller hex seems much more capable in wind - I'm thinking it's simply a limitation of the 'quad' layout - more arms in a hex or oct give a finer grain for the lever arms and therefore ability of the FC to adjust?

My frame is a folding Vulcan. The arms swing out into a pure X (each arm 90deg from the other). If I add two arms, these will be fixed and extend 90deg each side from the forward direction. The folding arms would then be 45deg forward and backwards from the fixed arms.

This is a bit different to the even 60deg spread of arms in a classic xHex layout. Will any FC's handle non-standard layouts? Has anyone had any experience with something like this?

And then also, am I really just barking up the wrong tree and should I be trying something else to get the X8 steady?!

Thanks all.

 

[PeteDee]

What FCU, stability has a lot to do with the FCU setup

 

[min0nim]

Yup, this is a good point. At the moment I'm running a Naza V2 - which I know according to DJI is not suitable for larger than a 600 or so frame. Mines a 900 dia.

Part of the thinking with a change in frame type is to go a different FC as a first step. I just can't get any reliable info on whether the Wookong or A2 are inherently more stable than the Naza (despite DJI saying so - they say lots of things!).

I do have some Hoverfly boards, but have been unable to get them working with the KDE motors and ESC's as of yet. On my smaller 550 though, the difference in flight characteristics between the two seems marginal - different styles, sure, but I wouldn't say one was more stable than the other.

I know Kloner's keen on the xAircraft FC's, but I'm not sure about their compatibility with the KDE's either.

 

[R_Lefebvre]

I think what you're talking about has a lot less to do with the actual arrangement of the motors and arms, and a lot more to do with larger machines with large props being less responsive than small machines with small props.

There was always a theoretical point where the inertia of the props would become a problem for stabilization with fixed pitch multi-rotors. I think we're seeing that.

For example, I've seen 26-29" machines teased over a year ago (ie: Big Hammer X8), but have yet to see anybody using them in actual service. Has anybody?

 

[gtranquilla]

I think what you're talking about has a lot less to do with the actual arrangement of the motors and arms, and a lot more to do with larger machines with large props being less responsive than small machines with small props.

There was always a theoretical point where the inertia of the props would become a problem for stabilization with fixed pitch multi-rotors. I think we're seeing that.

For example, I've seen 26-29" machines teased over a year ago (ie: Big Hammer X8), but have yet to see anybody using them in actual service. Has anybody?

Would not higher torque motors overcome the prop inertia more quickly so as to allow the FC output control to remain effective?
Or would rotational momentum of the motor arms then become then become the root cause of the problems?

 

[gtranquilla]

Would not higher torque motors overcome the prop inertia more quickly so as to allow the FC output control to remain effective?
Or would rotational momentum of the longer/ heavier motor arms then become then become the root cause of the problems?

I think what you're talking about has a lot less to do with the actual arrangement of the motors and arms, and a lot more to do with larger machines with large props being less responsive than small machines with small props.

There was always a theoretical point where the inertia of the props would become a problem for stabilization with fixed pitch multi-rotors. I think we're seeing that.

For example, I've seen 26-29" machines teased over a year ago (ie: Big Hammer X8), but have yet to see anybody using them in actual service. Has anybody?

 

[R_Lefebvre]

Would not higher torque motors overcome the prop inertia more quickly so as to allow the FC output control to remain effective?
Or would rotational momentum of the motor arms then become then become the root cause of the problems?

No, I don't think the numbers work out that way. I believe motor torque goes up as the square of motor size, but propeller inertia goes up with the 4th power of length.

 

[Old Man]

No, I don't think the numbers work out that way. I believe motor torque goes up as the square of motor size, but propeller inertia goes up with the 4th power of length.

That's been my experience with 28" props using 170kV motors as well. 1260mm Vulcan quad folder with X8 capability.

Hey, at least Vulcan sets his plates up where you have the option of adding more arms. Few others do that. For the record, the original Naza worked fine in up to medium winds on a 800mm+ flat 8 using 15" props. It may be the v2 is less capable. Wookong M works very well in your size class when dialed in.

 

[R_Lefebvre]

So what happened, you couldn't get it stable?

I was flying my 30 minute quad with 15" props with no payload yesterday and... it sure isn't sporty. Lots of power, but the rate control is just sloppy. Wind really knocks it around.

 

[min0nim]

Hey R_Lefebvre, not sure if you're asking Old Man or me, but sloppy is the way to describe what I'm seeing. It kind of wallows around in the wind. Plenty of power too - maybe over powered (hover on about 35% manual). This is another reason why I've been dropping the prop size.

I was running triple 15" 's with 400kv motors. Now on 1455's, have a set of 13"'s on the way. I'll see how that goes. Maybe the next step is to try a Wookong!

 

[R_Lefebvre]

With the DJI controller it's not a great test case, however, since they appear to be artificially limiting the frame size that you can use with the system by constraining your gain settings. Unless you are free to crank up the gains to whatever you need, then it's not an indication that you are having aerodynamic/physical control problems.

I'm using Arducopter with no such limitations.