coaxial different motors top and bottom

[saidquad]

I have lots of satisfactory experience setting up coax octocopters with wookong. all of birds using same motors and props top and bottom. But now I have 4 spare dji-4114-11-320kv motos and 4 spare tmotors-4014-9-400kv motors and 8 15*5 props. Can I use different motors top and bottom (same prop)? all using 40 amp esc (two different brands). which motors should I mount on top and which one bottom? Will FC (WKM) handle this? thanks.

 

[jdennings]

That's an interesting question, and I hope others will chip in. I don't have experience with WKM on X8s (only on flat8) but quite a bit with Pixhawk, where using different motors or propellers is not an issue. I can't see why it would be on WKM, but sorry, no firm answer.

When flying X8s I have systematically observed that top motors always work harder than bottom ones (higher temperature, also confirmed in flight logs). This makes sense given the downwash from the top propellers , and is a well known fact. So in short more power consumed by top motors than bottom, meaning higher amperage.

If this is also the case with wkm, then given that your two motors appear to have the following specs: 320kv, max 360w, 147g for DJI, and 400kv, 150g, max 900w for t-motor, the t-motor should spin faster (higher kV, everything else equal) so would "work more", as such it would make the t-motors good candidates for bottom mounts.

My problem is with that "everything else equal". Not sure how WKM FC deals by default with mixing on X8s.
Messing with the mix manually could optimize performance no matter what.

 

[saidquad]

Thank you jdennings. Normally I have same motors and props top and bottom, but I manually customize the top motors to 95% and bottom ones to 100%. This resolves exactly what you explained. But this time with different motors top and bottoms I don't know if WKM will handle the situation at all. Mixing manually needs very much and precise calculation which is impossible for me. If there is someone who has tested such situation with WKM please help. My Boss (wife) is watching my wallet very carefully! no room to expend for new motors.

 

[saidquad]

any one? I don't dare to test it.

 

[Bartman]

i say go for it (but be careful)

the flight controller is a black box with little awareness of what is actually happening outside. so it you give it different motors and it can find a combination of motor speeds to allow it to control the heli then it'll just do it. it might not be pretty, or stable, or efficient but it'll try.

i'd probably put the higher KV motors on the bottom.

but be careful!

 

[maxwelltub]

Make sure you have the two stacked quad set up. So motor 1 top, 2 bottom, 3 top, four bottom and so on. Not sure if that was mentioned or assumed.

 

[saidquad]

Thanks to Bartman and maxwelltub. Anyways After 6 days of searching I decided not to try this setup. I guess it is too risky. It seems that the best scenario would be to have a unstable bird (if flying at all) which is not what I want. I need it for survey jobs, so the stability and reliability are most important factors. If anyone has done such setup in real life I'm still eager to hear about the experience.

 

[Old Man]

With the FC working off of gyros and accelerometers it should not matter if the motors are matched or not. The FC will increase or decrease current based on what the the aircraft is doing. I'm with Bart in thinking it would be worth trying on a test frame, tethered of course. It could turn out to be quite a breakthrough in efficiency? In a way, the motor kV difference makes sense.

 

[jinly]

View attachment 26156 I use a 1448 on the top prop and a 1555 on the bottom prop All using SS V3508-29 380kV Motors. Good handling and powerful yaw characteristics. The vertical take off is tremendous. Check this link on youtube about efficeincy..very interesting.