Coaxial configuration prop directions

[iconflyer]

Does anyone have concrete testing results of having props on one arm of any coaxial configuraiton(X8 or Y6) rotate in the same direction vs rotate in oposing direction(which is the more common configuration I see on many existing designs)?

 

[DucktileMedia]

I have both Y6 and X8 on DJI and Hoverfly. Both turn opp top/bottom. Not sure what controller does it otherwise but why would you want this?

 

[iconflyer]

I too have both these controllers and the default config is the standard counter rotating props. I've managed to create a custom motor mixer on my WKM to achieve same rotation prop configuration on the same arm for my X8. The reason I wanted this is to be able to use vectored thrust from slightly rotated arms to achieve yaw rather than RPM/torque changes only. Using vectored thrust(for example on my quad setups) achieves super smooth yaw(pan) and less attitude variations when doing so. The other problem I was having with my WKM X8 setup, which I guess is appliable to hot environments like the one I work in(45 degrees celcuis), is that due to the inherent loss of efficiency in coaxial setup, the bottom motors always accumulate more heat because they are trying to maitain yaw authority while also compensating for the thrust loss due to the fact that they are running in the prop wash of the top motors. The result of this overheating is very interesting: as their temperature increases, the apparent strenght of their magnets decreases very slightly(temperature difference between top and bottom motors upon landing is 10-15 degrees celcuis) causing them to drop their RPM very slightly and inadvedently causing a slow yaw in the direction opposite to their direction of rotation. In other words, because they work so hard and slightly overheat, they end up providing less thrust(and torque) and lose their ability to counter act the yaw torque generated by the top motors. My conclusion that this is infact what's happening came from numerous experiments where my WKM X8 would start out fine and after 5-6minutes of flight(after the motors get nice and hot) it would start yawing on its own(more so if I'm carying my camera/gimbal load)....landing for a couple of minutes resets this behavior. I never noticed this behavior in the winter. Without any modifications except for the prop redistribution, motor direction modification, applying the custom motor mixer solved this problem immediatly...plus it gave me the added advantage of being able to tilt my motor arms 3-4 degrees to use thrust rather than torque only to create the yaw forces. On the WKM, I could probably increase the yaw gain but I guess that would just wastes valvuable energy. Anyway the overall effect on my specific X8 with the custom motor mixer is better yaw authority, less attitude impact from yaw, and less thermal stress on the bottom motors. This is all good, but I wonder why no one else has this same setup, it is very likely that my logic is flawed in one way or another

If you have round arms on your X8(Y6 would be a bit more tricky to setup vectored thrust) and you're interested in testing this out, I can pass on my custom motor mixer. If it works for you, it may significantly improve the pan movements on your X8 especially if you have a fixed camera gimbal(i.e. gimbal with no independant 360 deg rotation).

Very interested to hear your thoughts on this.

Thanks!

 

[iconflyer]

Anyone else with any experience on coaxial setups?

 

[R_Lefebvre]

I have to think this would be pretty bad for your thrust efficiency.

 

[iconflyer]

you're saying having the props rotate in the same direction is very bad for thrust effeciency? Any idea what the physics behind that is?

 

[jes1111]

Interesting setup - you don' t really have an octo - it's a quad with doubled motors.

Rob is right IMO - much reduced thrust from the bottom engine because the angle of attack of the bottom blade will be approaching zero - the airflow from the top is not straight down - it's a spiral - the bottom prop won't be able to get good bite on the air because it's rotating in the same direction as the spiral, effectively free-wheeling. I'd say its not getting hot because its not doing much work. What prop sizes and pitches are we talking about here?

 

[iconflyer]

GWS 9x5x3 props...What do you mean by angle of attack of bottom blades approaching zero?

makes sense on the free-wheeling explanation...bottom line though is a I get better power performance(just finshed a few rounnds of testing) and defintely much better yaw authority. The top motors are hotter but not by much

 

[jes1111]

Imagine an aircraft wing - the leading edge has to be angled upwards for it to work well - that's angle of attack. A prop is just two (or three) wings attached to a hub - same/similar aerodynamics. Except on a prop the angle of attack is called the pitch - on a prop it varies from the root to the tip - the quoted figure is the average or nominal pitch. A propeller expects to have the air coming straight at it from the front - if it isn't, if it's coming from an angle, it either stalls the blade (too high an angle of attack) or the blade free-wheels (too low).

You're probably benefiting from the prop being a fairly high pitch ratio - that is the ratio of diameter to pitch - over 50% in your case. This gives the bottom blade a better chance of getting a bite on the spiral of air. But the bottom engine being cooler means its not doing the same amount of work as the top motor (where previously it was doing more work and therefore getting hotter). The ideal is for both motors to end up the same temperature (same amount of "work", i.e. Watts).

Can you change your mixer to increase the speed of the bottom motors? - that way you might be able to spread the work evenly between the two and end up at the same temperature. In which case you'd have discovered an interesting way to beat the "efficiency loss" of a coax setup (which is what this is all about).

 

[iconflyer]

Thanks for the explanation...makes sense to me. I can change the motor mixer to that the top motors contribute less to the force they applied to any given attitude change including vertical movement. I'll try that and see if I can even things out. Thanks for advice!

 

[makarthur]

Anyone else with any experience on coaxial setups?

Hi Iconflyer,

New to X8 configuration and would like to experiment with the same rotation config.

Grateful if you could share the custom mixer.

Actually I am using A2 and N3.

Thanks