Servers by jDrones

Dual-motor tailsitters


(David Kaden) #1313

Here guys, may be useful:

PID Test Sheet.pdf (81.3 KB)


(lorbass) #1314

Good Idea is really usefull

Otto


(lorbass) #1315

Yes, in the new version of MissionPlanner they are now shown in the graph.


(letpi) #1316

Among new features there is one concerning VTOL: improved VTOL flight code, with improved transition support and loiter
It should be applicable to tailsitter, however I don’t really understand what improvement has to be expected.


(David Kaden) #1317

(Peter Hall) #1318

I have added Q_assist support to my latest pull request, here is a quick demo with then without it enabled.


( .) #1319

Mark, your ‘Dart’ quad-tailsitter has inspired me very much. So far I built single copter and carried out few succesful flights. I can see that tailsitter software is giving more possibilities than single copter one. I am looking for proper assignment of SERVO#_FUNCTION. I can see that for dual motors tailsitters elevon feature is quite good. But I would like to use single motor and four fins below in the prop wash. My idea is to use elevons for two fins and rudder for third one. Still do not know what to do with the fourth one? Just duplicate rudder or there is another assignment for this type of frame. It would be appreciate to get some guidance. Kind Regards.


(David Kaden) #1320

First flight after some time.

AUW is now 1400g from the before ~800g.
6s 5200, 10x6, 580kv.

Again calculated PIDs for Q_roll with Ziegler/Nichols method but they were not usable because of oscilations. Had to manually reduce them. For hover pitch and yaw I used the same PIDs as I used in my earlier videos and It seems to work. Did not try to retune them.

In Horizontal flight you see the yaw drift while doing the transition.
Then you can see enormous oscilations in both roll and pitch. They increase with speed. Log said that it had a maximum of ±11g (!!!) where I put full throttle and went to about 28 m/s.
I dont think that I will be able to make this more stable in horizontal flight and still use the symmetrical airfoil (NACA 0015).


(David Kaden) #1321

FYI BLHeli passthrough currently does not work with tailsitter https://github.com/ArduPilot/ardupilot/issues/9340


(letpi) #1322

You have got probably the best stability in hover for a non vectored ardupilot tailsitter.
Did you try to move forward the CG ? It will stabilize your wing in fixed wing mode. The more you move forward the CG the more the elevons neutral position will be upward giving some positive moment. It could be at the expense of the lift coefficient though.
How do you look at the acceleration ?


(David Kaden) #1323

When I move the CG forward it won’t hover that great…

I think I first need to have another airfoil and then try to find a CG which works acceptable in hover and forward flight.

For Hover the CG optimum is where the wing has its pressure point, bus this means tha the wing is completly instable in FF.
For FF the CG must be a bit more in front of the pressure point, depending on required stability, but this means that the wing won’t hover at every angle of attack with zero flap deflection because it will always have to counter the aerodynamic moment of the wing.


(David Kaden) #1324

Was flying again this week. Tried to tune the pitch axis. No luck yet. I also destroyed the craft after the last flight and will go on building a completely new one with adjustable CG location and other fuselage + other airfoils. Should then solve all problems. :slight_smile:


(lorbass) #1325

Great, thats staying power.
Good luck

But for me perfect tuning in the first flight until autotuning.
Perhaps to much trust in “Autotuning”
Or not enoughe autotuning cycles.
PlaneWiki:
So you need at least 20 full stick movements to learn a reasonable tuning value.

Otto


(.) #1326

Hi palm369
nice project! I have three questions

  1. Where is your COG? Wouln’t it be better if you had a wing with more wing sweep that you could put your propellers nearer to the COG? I read that tractor configuration on wings had a destabilising effect.
  2. Can you post a pic of your motor mount? It looks strong and easy.
  3. Do you have any down thrust because of your tractor configuration?

(David Kaden) #1327

Autotune does not work when the plane is aerodynamically instable. Actually the only thing that can make it fly is a super fast and perfectly tuned control loop. Or building it aerodyn. stable, so with the CG infront of the wings pressure point and not right ontop of it.

Thx.

  1. CG is at 1/3. What difference you think it makes if props are in CG location?
  2. Actually I destroyed plane after last flight. sorry
  3. Don’t unterstand the question

(.) #1328
  1. I thought it could make it more stable if the pulling propellers are more near the CG or pusher configuration. Is your airplane aerodynamically instable? I think not, CoG at 1/3 seems to be normal, maybe tail-heavy for a wing.
  2. Bad news! Hopefully easy to fix.
  3. In german it is called “Motorsturz”. Normally tractor configurations have a little negative and pushers a little positive. Do you have that?

(David Kaden) #1329
  1. According to FLZ Vortex its instable. I did it intentionally so I have good hover performance on any AOA without saturation of flap angles. Take the Tailsitter of ETH Zürich as reference. I also explained a lot of my design rules in this thread already, including this.
  2. I threw away the plane because I want to built a new one with “S-Schlag Profil” like MH45 or so.
  3. No I don’t. “motorsturz” is done so that the thrust vector passes through the aerodynamic center. eg with high-wingers and no pitch up moment is induced by gas. Everything is intentionally symmetrical with mine.

(.) #1330
  1. When it is instable, can you fly in manual mode (horizontal)? I though flying an instable airplane is impossible without fly-by-wire.
  2. Can you tell me how you attached the motors to the x-shaped wood? I would like to do it like you.
  3. Okay so I won’t use it.

(lorbass) #1331

Thanks for the interesting CG Video. Many things clear now.

So if in your case the CG can’t be moved due to Hover Stability, you think about moving
the neutral point backwards by modifying the wing shape?


(David Kaden) #1332

and it is like that

I does not matter if I move the CG or NP. What matters is the distance from each other.

  1. CG infront of NP (common case): Stable, but there is a moment needed for the plane not pitching down. See video posted before. Tailless aircraft use reflex airfoils for that.
    So plane auto pitch up with speed and pitch down when speed is to low.
    Problem for Tailsitter is that you need to have elevator pulled when flying at high AOA. Than flap angles can saturate and you not able to get in the vertical hover position cuz you cannot pull more elevator.
  2. CG right on NP: Instable layout. Advantage for Tailsitter is that you can fly on any AOA without need for rudder deflaction (!!!). See paper of ETH Zürich Tailsitter for that. Already posted that before.
  3. CG after NP: I am not sure if this is flyable in any way. Normally immediate crash. Like throwing an dart arrow with tail first.

So you all see that Wingtra solved that. I think they did it by having a combination of a very good tune and a CG just a little bit infront of the NP, so you get aerodyn. stability but don’t have the problem with flap saturation. This is what I want to try next, but I need a new plane with reflex airfoil for that.

So If you wonder why my plane flies so good in hover -> because my CG is where the NP is. This is also the explaination for why it flies so crappy in forward flight. :laughing:

You want to know where your NP is? Do one of this: (I did both)

  1. Built a FLZ Vortex model of your craft and it will tell you http://flz-vortex.de/flz_vortex.html
  2. More easy -> Change your CG like that: When you fly in hover at about 45° AOA and your elevator goes up, then put CG bit more in the rear. When flaps point down, put CG more to the nose. Repeat this until you find the CG spot where flaps stay neutral even at high AOA. That is where your NP is.