### Yaw Stability - derivative coupling between yaw and pitch

Hello,

I ran across this thread over at google code drones discussion and would like to bring attention to it, hoping 3DR chimes in and can clarify if indeed the rear blades need to be swapped (and documentation updated to reflect this). You will notice on your Y6 that if you yaw, it induces a pitch change.  Here is a portion of the discussion where a solution is found

Problem definitely solved : it was caused by the rear arm propellers arrangement (reversed compared to the front arms).

Here is how i did solve it :

- put a pusher prop on the rear arm upper position (instead of the normal one)

- put a normal prop on the rear arm lower position

- exchange ESC outputs for those two motors and reverse spinning directions

So now i have an Y6 with the 3 pusher props in upper location on the three arms.

No more Yaw / Pitch coupling at all.

This is confirming what i thought, a thrust asymmetry is induced if a propeller tandem is reversed on a arm.

The probable reason :

On a coaxial setup the two propellers of a same arm do not have the same efficiency. During yawing, we are accelerating three propellers, and slowing down the three other ones.

The problem is that the vertical thrust result is not the same if one arm has a different prop arrangement : rising the speed of the lower prop is more efficient than rising the speed of the upper prop.

So if two arms of the Y6 frame have pusher props in the up location, and the other arm have the pusher prop in the lower location, then when yawing one side, we produce a different vertical thrust on the rear arm compared to the front arms, because we are not speeding up the same arm propeller. This is causing a pitch derivative coupling.

The coupling is only derivative because the induced pitch unleveling is compensated by the APM accelerometers as soon as it is detected but with a small delay.

So the definitive solution is to put the three pusher props in the same location on the arms. That's it.

I think that reintroducing a top bottom ratio is not the solution, at least not without a more complex motor mixing to avoid axis coupling.

Olivier.

If a simple change can increase stability, I think it's really worth looking at and addressing.

#### Replies

• I know this is an old topic now, but I have a question in regards to prop size. Has anyone tested the Yb (frame10) with different size props vs the same on top and bottom? I ordered a 3dr Y6 (2013) a month ago but do to missing parts and now a bad motor, I haven't been able to fly. Ive decided to go with the Yb configuration since I have to rebuild anyway. But should I go with larger on the bottom and small props on top? I've read that having larger props on bottom provide "cleaner" air, from the prop wash of the top motors. This makes sense to me, but I'm no aerospace engineer. If no one has tested this I'm going to bench test it before I put everything together (of course I'll post the results here).
• Hi,

As for my opinion the answer to your question depends on your setup (frame, motor, prop, distance,...) and you should test that yourself. It is possible that different prop size (top, bottom) or different pitch (top, bottom) may be more efficient, but it really depends on your confirugartion and you should test that yourself.

If have tested a lot of motor/prop combinations (http://ax2210n.blogspot.be/) and for my setup I had the best efficiency with 9x47 prop on top and bottom. The only combination that give me the same result was a 9x47 top and a 9x60 bottom. When I mounted a larger prop on the bottom I had less efficiency.

But other poeple tell different stories, so I think this subject is really setup depended.

• I had all 10x45 props for a while before I recently switched to the standard 10x45/11x45 configuration. Honestly saying, I didn't notice any huge difference. Flight time may be a little bit better by a minute, not more. No difference in terms of stability, controllabilty etc

• Developer

Hi all,

Basicly 10's on the top and 11's on the bottom are ever so slightly more efficient. However, the lower motor and esc tends to get hot and the copter isn't as agile. Tests by others apparently showed that using different props top and bottom results in less control during motor out situations.

I personally recommend 10's top and bottom for performance, reliability and simplicity with very little compromise in efficiency.

• thank you for taking the time leonard, it is much appreciated.

• hi!

The truth is no one knows and I have yet to find a definite answer.. basically have to try different configurations and see what works best. Leonard (one of the devs) was supposed to be getting a comped Y6 & different prop configuration around christmas time to use his in-depth knowledge to find the best prop config.  I have no idea what happened with that - I hope he reports back!

• Leonard posted the results of his prop testing here.

An extract follows (talking about top / bottom props):

10x4.7/11x4.7 had the best efficiency, closely followed by 11x4.7/11x4.7 and 10x4.7/10x4.7.

11x4.6/11/7 had the best pwm match, closely followed by 10x4.7/10x7 but both took hits in efficiency and lift capacity.
Anything with 11 inch props on the bottom suffered from significant heating of the lower motor and esc
..and summary...
(10/11 and 10/10) configurations are very close but I will be putting 10/10 on the Y6 I am building (my own design). I think it is a safer combination that runs cooler and is a little more agile. However, I wouldn't fault 3dr for choosing either combination!
• Hi all,

Let me say hello to everybody: I'm a guy who makes my multis, for pro purposes.

I setup an Y6 on APM 2.6 FW 3.1

I saw Y6A and Y6B in the instructions on http://copter.ardupilot.com/ , and decided to wire Y6B config because maybe later older Y6A won't be supported..

But after in wizard, or in frame selection in APM, no way to specify Y6B...

Then the mapping was wrong. I saw in this post that I had to change manually FRAME=10 in full parameters: it works! THANKS!

(but there is a problem in APM interface.. no possibility to chose Y6A or B in wizard nor Frame selection panel)

Now it flies well, but I have a mixing appearing between Yaw and Throttle:

on big ressources, the Y6 turns slightly in YAW (cf video, at 1'30).

My motors are not exactly coaxial, but I guess this phenomenon will occur on all configs. Those how are in Y6B can maybe try that and tell me if they have same effect? It is because the top props have fresh air, and are much more effective for YAW than bottom ones.

Question: does anyone knows if there is a way in deep params to modify top and bottom props yaw gains? (MK allows this trim, DJII not was I told)

• Ok Tried doing the prop change thing, but ran into a few issues :

1. we changed all the esc-apm wires, (apart from 4 I think), according to this thread. When changing the frame to 10, during the motor test the motors would spin in a "all over the place" sequence. When I change the frame back to 1, only then did they spin Right top then bottom, Left top then bottom, Back Top then bottom.

Can someone confirm the motor numbering now, and the correct order they must spin in again for me please?

2. I am unsure about the A vs B firmware implementation. Do I load a new version of the firmware? If so which esc wires do I change, and does the motor sequence change too?

3. Once we changed the props, and took it for a flight, the y6 flew ok, even in strong wind, but the only thing that seemed weird is that the yaw stability has gone for a ball. The Y6 will sometimes spin around in windy conditions instead of holding its heading.  do you think this could be Yaw Pids that need adjusting, or related to the above. She flew rock solid and great yaw holding before the prop swop experiment.

TIA

Wessie

• Really Appreciate this Info!

I converted to Y6 recently and noticed that yaw was weird, dipping in the direction of the stick instead of staying on plane.

I changed the frame type to 10, re-wired my motors to turn clockwise on top, counter-clockwise on bottom, pushers on top, sf on bottom. I am using 10's on top and 11's on the bottom.

Now it is flying much more predictably and yaw rotates on axis like it should.

I have uploaded a log file. The logs are beyond my grasp and I don't understand how to use them. Perhaps someone will find them useful.

Also, it was VERY windy today but I could tell right away that things were "right". Still have some tuning to do but I will wait for less windy conditions.

Thanks to those who figured this out!

cheers,

plex.

2014-01-09 15-42.log