Coaxial Question

[teecee]

I just built my first coaxial. It is a big SkyJib X4XL and I am doing my first flights with it. I find it flies a little differently then any of my other UAVs and I wanted to see what other people were finding with their rigs. I find it yaws quite slowly for one and when I continue rotating a couple of times around it gets a little unstable. What is that all about? Is there anything else different about coaxial that I should know / do? Any tips are greatly appreciated.

Thanks!

 

[Old Man]

I fly a smaller x-8 than yours but I will say it's not as "sprightly" as a quad or tricopter and yaw rotation is normally slower unless I dial up the yaw rate a lot. The lack of stability you mention is a little troubling because i find my 8 is considerably more stable than a quad, even if in a constant yaw.

 

[teecee]

I wonder if the instability during prolonged yaw is due to so much air moving under the rig. I find that all my bigger birds get a bit unstable when I don't move around horizontally and drop vertically or sometimes even when I stay in one place. I have gotten in the habit of always moving horizontally while descending for this reason. How out there knows more about the real physics going on here??

 

[Bartman]

How hard over on the sticks are you getting to increase yaw? If the stick is full over then the motors are using most of what they have to make the yaw happen and will have little left to effectively control the helicopter.

Most all of the coaxial quads I've flown have had all of the top props spinning clockwise and all of the bottoms spinning counter-clockwise with one or two inches of pitch higher on the bottoms. Control was always excellent and descending flight was actually very stable if you weren't coming down too fast.

the top prop imparts a spin to the downward flowing air and that spin makes the air hit the lower props at an angle that essentially reduces the pitch of the lower props making them less effective. by using a higher pitch prop on the bottom you're restoring that pitch and making the prop perform more closely to the top one without having to play with motor speeds. i don't think you'll get full efficiency of a flat oktokopter but it will fly better.

with the same pitch top and bottom mine always felt similar to what it feels like when driving on ice, you can do it but if you're too quick with the throttle the response sort of slips. using higher pitch on the bottom got rid of that.

what diameter props are you using? I've got a box full of 14" XOAR PJN and PJN-P, new in the wrapper that I'd sell cheap if anyone wants to try it.

 

[teecee]

My top props spin counter clockwise and bottom clockwise. This is the ONLY quad coaxial configuration I see in the A2 assistant software. What controller are you using to do the reverse?

All my props are are 18" long with 6.1 pitch. These props were what was what was recommended by both the motor supplier (KDE) and the frame supplier (Aeronavics). I had heard of using different sized props top and bottom but not different pitch.

The instability I noticed was with full rudder and repeated rotations. Not something you would normally do but these are my first few flights with this machine and I am experimenting. Until I figure this out I will not be performing multiple rotations again.

 

[Bartman]

full rotation is telling the flight controller to use all available YAW authority to YAW which leaves nothing for other control functions so of course it will be unstable. full YAW isn't really a useful flight regime for any media product unless you're trying to make viewers puke!

I typically use the flat X-Okto mixer (no arm pointing straight forward) and then just put the clockwise front-right motor on top and the CCW front right motor on the bottom, then go around the points and do the same on the other arms. so long as the motors are turning the correct way the FC won't know the difference.

 

[teecee]

full rotation is telling the flight controller to use all available YAW authority to YAW which leaves nothing for other control functions so of course it will be unstable. full YAW isn't really a useful flight regime for any media product unless you're trying to make viewers puke!

I typically use the flat X-Okto mixer (no arm pointing straight forward) and then just put the clockwise front-right motor on top and the CCW front right motor on the bottom, then go around the points and do the same on the other arms. so long as the motors are turning the correct way the FC won't know the difference.

Yes, I don't tent to YAW forever on my shoots either. I do like to test all functions of a new machine prior to strapping thousands of dollars worth of cameras and gimbal to it though. I have performed the same test on octos and hex machines and never had this dramatic of a result.

Is there a benefit to clockwise props on the top VS on the bottom?

 

[Bartman]

@teecee

think about it this way, you're hovering over one spot and the motors are all equally supporting the helicopter, yaw is stable, it's a nice day for a flight.

now you go full right yaw on the radio, the flight controller tries to comply by running half of the motors to full throttle and the other half to a much reduced level to provide max yaw response......but with half the motors at full and the other half at somewhere much less the helicopter still wants to maintain altitude so it's got to balance making hover lift (equals helicopter weight) using all eight motors but with the rudder commanded full right....and it still wants to maintain pitch neutral and "wings" level.....do you see the challenge it has to do all of this simultaneously?

I put the clockwise ones on top because they're harder to come by and usually a bit more expensive...the CCW lowers are easier to get so i put them on the bottom where they are more likely to get dinged in an unexpected encounter with the ground.

not bad to test for unusual outcomes but some instability in full yaw isn't completely unreasonable.

 

[teecee]

Got it. Thanks for all the great info!

 

[Bartman]

I had heard of using different sized props top and bottom but not different pitch.

the problem with this is that the weight of the props isn't equal so the spinning masses make yaw harder to control. i've tried everything, i've even flown with two-blade props on top and three-blade props on the bottom. it might have helped but the weights of the props were off by too much.

yaw is a matter of action-and-reaction, the motors spin one way, the helicopter yaws the other way so the mass is important as a higher mass prop induces a greater reaction.