Servers by jDrones

Dual-motor tailsitters

(letpi) #1483

Thank you for the dashware OSD, I am not familiar with this tool and I hope to be able to use it.
Your tests pushed me to think about wing design that allow the CG to be at the right place whatever the flight mode. As a copter, I assume that only the wing section behind the propellers are “flying” and the CG is set at the neutral point in order to give “free rotating” behaviour. As a fixed wing the CG position gives a static margin of 3% and that should be OK with the jwl airfoil. The weather should be OK next week, so we will see soon…
Page 4 of this link, a very good article about a flying plank wing Wihok 60 that helped me a lot
An other subject, When draining high current I have a motor synchronisation issue. After a lot of test I found this problem is due to ESC signal wire too close from ESC power wires (all wires are trapped in the foam core wing and the esc is close to the motor). I think DSHOT protocol could be the solution but the wiki said Dshot is not supported is the stable release. Is that true and is the Chibios built mandatory ?

(Mallikarjun SE) #1484

Hello Guys!
I’m trying Dual-motor tailsitter aircraft without tilt/vector. Hover flight looks good, transition is ok, fixed wing mode is very bad. It oscillates in pitch axis a lot. I’ll attach video and log below. Any help would be appreciated.
Thank you.

(David Kaden) #1485

Actually you attached just a photo, no video.

What you describe is a known behavior, move your CG 1-2 cm more to the nose and try again. You will find the hover getting worse but the forward flight getting better.

What airfoil did you use?
Where is your cg relativ to the neutral point?

(Mallikarjun SE) #1486

I’m actually uploading the video to Google drive! Will share the link soon! Currently the CG is at quarterchord! I’m using NACA 2412.

(David Kaden) #1487

So you use a chambered airfoil with no reflex? Why?
How you counter the induced pitching down moment?

(letpi) #1488

For sure a negative Cm airfoil is not a straightforward choice. Lets explain a little.
Negative Cm means your airfoil, considered as slick airfoil, want to pitch down. The stability condition is CG in front of the neutral point which is again a negative pitching moment. The condition of level flight is the sum of pitching moment =0. In your case this will be achieved with a lot of upward flaps deflection. So yes, your wing can fly level and stable. But there are two big BUT.
The first one is about the lift coefficient you can expect with a such flap deflection, fore sure a very bad one and your wing will probably glide like an iron. But this is not critical because even an iron can fly with a powerful motor.
The second one is more critical and is related to stability because the pitching coefficient Cm is not constant and is dependent of Cl. The more it is dependent the more your airfoil risk to become unstable. This is why modern airfoils like PW51 ot TP100 for plank flying wing are designed in order to have a Cm almost constant over a wide range of Cl. This in turn allows to set the cg close to the neutral point because the static margin can be reduced.
If you want a better explanation you can read the link of my post 1461

(Mallikarjun SE) #1489

Hey Guys! Here’s the video! Can someone check why there’s oscillation in forward flight? I have uploaded log in the previous reply!
Please suggest me things after watching the video!
Thank you

(letpi) #1491

I cant read the video but it does not matter.
The log confirm your wing has a lot of oscillations in hover and in fixed wing flight. I am not good at analyzing log so I cant say much. But it seems to me that EKF works properly so all calibrations should be OK and that’s a very good point.
About the fixed wing flight I cant say if they come from a bad CG or from bad tuning, probably both. As it was already said, move your CG to the front, at 25% unstable behavior is normal. 15 to 22% of mean cord is the range. The more you move CG to the front, the more hover will be difficult.
The parameter responsible for oscillation is ptch2srv-P. You set this parameter =1 which is probably too much considering your aircraft has large flaps. Less than 0.2 is probably more appropriate. If too low, the risk is to get a sluggish behavior.
Last point is something I dont understand It seems you tried a transition by switching to autotune. To tune your aircraft with autotune is a very good idea but I dont know if it works as a transition and I think it is better to perform autotune only when your aircraft fly almost correct.
Concerning hover tuning, please read above posts, there are plenty of information.

(Mallikarjun SE) #1492

Hover flight feels really stable in QHOVER. Transition is good too. In forward flight, there rapid pitch up and down. I’ll try by moving the CG front.
And I’m not able to attach video link. Any suggestion?
Thank you!

(David Kaden) #1493

Put any signs like $$$ in front of the link and it does not get recognized

(Mallikarjun SE) #1494


(letpi) #1495

Thanks for the video. Yes, hover is already good. To my opinion, the way your wing fly is typical of a bad CG and probably the airfoil does not help. The pixhawk really make a good job to stabilize the wing, that’s amazing.

(Mallikarjun SE) #1496

Thanks! I’ll move the CG front and check!
Did you try forward flight on your aircraft?

(letpi) #1497

this is monsoon time this WE but I hope to have 5 minutes without rain to give my wing a try :wink:

(Zevi) #1498

Has anyone tried using these foxtech Nimbus motor tilts for a belly sitter? They look decently beefy.

Could the servo be setup to til 90 degrees the other way as well?

(lorbass) #1499
  1. Poor technical information (Weight, Tilt angle, Voltage)
  2. For large Wings only (>150 cm)
  3. You need about 180 degrees.
  4. Expensive
  5. I would use this
    Then you are free to select the correct motor by yourself.

(letpi) #1500

This is the second tentative of fixed wing flight. I am happy of the result.
During the first flight I have got long period oscillations of ±10° so I moved the CG about 3mm forward and the result is very good. An interesting fact is ±10° oscillations correspond to a range of 40 PWM of servo throw. You can imagine how precise must be the linkages. This is probably due to large surface of flaps together with the CG at the limit.
Also the average power needed to fly FBWA is about 55 W. Who said dual motor tailsitter is power hungry ?

(lorbass) #1501

Congratulation, the verry best Tailsitter Flight in all Modes so far.

We can see, it makes fun!

(Mallikarjun SE) #1502

That’s amazing! Really stable flight! :airplane:

(David Kaden) #1503

Very good!
What static margin do you use now?
Can you post a log?

So we see again how sensible this configuration is to the CG. I doubt it is a good configuration for transporting varying stuff :frowning: