Dual-motor tailsitters

I see. Yep, pitch is oscillate a lot. Not sure why. After autotune, it still happens. I suspect that it’s something to do with hardware. Specifically, both flap hinges have certain clearance of moving.

I never try cutting throttle in flight. But I think the plane will not glide so well as it has low aspect ratio wing.

My advise is to have a close look at your logs. Check the log when flying straight with and without throttle and compare RCout ( ppm out for elevons), RCin (ppm in for ELEV) to the trim value you have entered for SERVO_ (elevons). These trim values should give your airfoil a little reflex but not too much. RCout and SERVO_trim should be not very different whatever the throttle. If not, it means you have a CG issue or motor pitch issue.
The CG MUST BE somewhere between 17 to 20% of the mean cord. If not the pixhawk have to struggle a lot to keep the wing flying. This could be a reason of oscillation…
To find the right relative angle between motor and neutral cord of the airfoil, I look at the airfoil polar and choose the angle for which Cz is 0.5. In my case this is 3°, so both motor point downward by 3° relatively to the airfoil.
Pierre

While I was reading this
http://ardupilot.org/plane/docs/roll-pitch-controller-tuning.html
I found this tip that can be of interest for your pitch question

•WARNING : If PTCH2SRV_IMAX is set too high, then there is a danger that in FBW-A, if the model has been levelled so that zero pitch is too nose-up to glide at a safe speed, that the integrator will continue to keep increasing the elevator to maintain the demanded pitch angle until the model stalls. PTCH2SRV_IMAX should be set to a value that is big enough to allow from trim changes, but small enough so that it cannot stall the plane.

This seems to be more likely the cause of your problem. My understanding is that the pitch integrator is set to high and your pixhawk is not level (or maybe your radio calibration is bad ?). So you can make accelerometer and radio calibration again or test AHRS_TRIM_Y.
Pierre

Hi Pierre,

I think you got it! Because I use a sym. airfoil, I set it up to be level on pointing 5deg or so upwards, otherwise you can not fly in FBWA without pulling the stick all the time. ( at least I think so, pls tell if anyone got solution) But then when you cut thr, it should lower its nose the stock 3 deg. Then its still pointing up 2 deg and the explanation with the integrator gain makes sense.
Maybe i should just raise this 3 deg to some more like 10 deg or so…

I did a walkaround and just set “FBW min thr” to a value that this wont happen, cuz there is always enough airflow over the control surface.

BTW thanks for the link, they updated quite a lot! The graphs how the different axis controller work is nice to have.

To the CG topic @losawing : CG is critical on a tailsitter. There is a range of two actual CG locations which makes sense. The most forward is the one you would use when this plane is just a manually controlled flying wing. The most backward position you should use is the neutral point, which varies depending on your airfoil. I use FLZ Vortex to determine where this point is.
But hte is a good and bad side on evey solution.

If CG set at the normal position, the plane will fly great in forward flight but if will not fly good in high AOA settings, because of the induced pitch momentum.

If CG is set on the planes neutral point, you will have a awesome flying at any high AOA (cuz no pitch momentum is induced) and you can fly any angle without having much rudder deflection. You also wont run into the angle limit of your servo/ flap. BUT as your plane then is completly instable, it will be VERY sensitive to any deflection of control surfaces and you MUST set the PIDs perfect for forward flight.

I agree with your summary about CG. I am currently tuning my second vectored tail sitter. I think I got a good compromise between FW et heli mode, hope to be able to post a video soon.
If you want to use a symmetrical airfoil, some amount of twist at wing tip will help to get the wing self stable and maybe elevons can be set at 0°. But the question of CG remain the same.
Pierre

a video of my vectored tail sitter

It is a 1 m span, 33dm² balsa wing. Weight is 1.6 kg, CG at 17% of mean cord. Elevon surface is around 30% of wing surface. Elevon throw is ±45°. To design the wing I made a little benchmark with the wingtra. My goal is to improve wind resistance and to be able to fly more than 30km at 50Km/h. We will see…
Airfoil clarkys, 12.7 % thick, Cm0=+0.043, Czmax=1.2, flat bottom and thick trailing edge make it easy to built.
sunnysky 2212 KV1400, graupner eprop 10*5: powerful combo, good bearing and vibration free, hover at 25A.
lipo 3s 10Ah
4 JX digital servo 4409MG: fast, strong and only small amout of gear play, they are able to 180° throw provided they are supplied with 500 to 2500 µs pulses. After about 3 hours of flight those which support motor mount got a little more play. not bad…for 10€. To supply servo, I use an 8A 5V external BEC to power the servo rail, it is also a backup for the power module.
Pixhawk is mounted with pad of sticky tape directly on the balsa so it cannot move. I squeeze some hear plug between the pixhawk and the balsa. The result in term of measured vibration is almost perfect.
Tuning is not yet done, there are still a lot to do.
Pierre

6 Likes

looks great! where did you find the 8A BEC?

Hi Letpi, great project.

I thought the pixhawk is only ablye to generate pulses between 900 and 2100 µs.
500 to 2500 PWM realy works without firmware modification ?

Rolf

Rolf,
when you go to the full parameter list, the indicated pwm range is 800 to 2200. And yes it work beyond those limits.
But there is a small bug, lets try to explain. when you enter a value higher than 2200 it will not work at first, you have to modify the trim value (which must stay within the 800-2200 range). Once you have entered the new trim value, the new limit is active.
I have 750 to 2240 at tilt motor left and 900 to 2400 at tilt motor right. I get +90 to -60 °

Tilt,
The 8A UBEC is a turnigy.
https://hobbyking.com/fr_fr/turnigy-8-15a-ubec-for-lipoly.html
but obviously it is a hobbywing brand.

Verry good stability.
Especially in Hover forward and back. I got always a pumping effect.
Tuning not yet done? I’m surprised, nearly not to belive with my experience.
Can you poste a Log and/or Params?
Regards, Otto

Lorbass,
First of all, thank you for the tilt motor mount you designed, mine is not exactly the same but it was the design I started from.
Yes I will post log and parameter as soon as possible. About tuning I was speaking mainly of fixed wing.
What do you mean by pumping, do you mean elevons flutter ? This is a very complex problem with so much interacting parameters. Some ideas to think about:
Hardware part: make every thing as stiff as possible, including the wing, servo link, flight controller mount… I had a very bad experience on a quad copter with the kind of flight controller mount you have (with damping balls) as the flight controller was bouncing. Also I would make your servo link short and straight and relocate them to the middle of the elevon.
Software: nothing to say, this is a total black box for me.
Parameters: apart from the PID that must be tuned, I think that PID scaling is a key. As you know, Q_M_THST_HOVER is used to scale the PID, this is why we can increase the throttle without flutter and it works great but what happen when throttle is bellow Q_M_THST_HOVER ? Are the PID increased ? I think yes, because when flying fast forward or fast backward, the wing begin to get some lift and I can hear motors spinning slower. And yes in that case elevons begin to flutter. This is the same at the beginning of the back transition when motors spin slowly. Tridge would be of great help for that.

Link for the log, I am not an expert, I hope it will work.


I think first 6 minutes correspond to the video.

link for parameters


Since the log above I have performed autotune.
Some interesting parameters I have tried
KFF_RDDRMIX =0.4 and RUDD_DT_GAIN = 20% together give interesting maneuverability. To be used carefully because at low speed if you put full rudder and increase throttle at the same time you can make nice Uturn.
Q_AZ_P is higher than standard
Q_VELZ_MAX is 1m/s, I tried 2 but back transition was ugly
Q_TAILSIT_VFGAIN is 0.2. I really want to push further this parameter as it seems to be very powerful against stall and could be very funny in acro mode.

Now it looks better.


Hi Losawing,
Thanks for the Upload.
Will have a look to learn.
With pumping I meant here from 1min57 sec. (not shutter)

To see in the graph right, down

May be Not yet in this forum, :wink:
But with the wing already in front of the crew. :+1:
Can you share a more detailed Picture of the wing and the tilt units?
(To upload by the icon with the horizontal bar and flesh upwards in this edit field)
So you know that the damping balls are not nescessary or even bad?

Upload works, I can read your files.
Regards, Otto

Hi palm369, thanks for replying!.

The post you’re referring to is September 2017, right? I’ll try to prove it as soon as possible and tell you if it works or not.

Regarding the fuselage, it is made of cork cut with hot wire and lined with balsa wood. (I don’t know if it’s the right way to write it in English.)

Just a stab here:

Can the code handle 4 motors instead of 2?

hi Otto,
I have exactly the same pumping phenomena described in your video when flying fast forward in Q_hover and a little less when flying backward. I tried almost all Q parameters that are related to stabilization and not be able to get rid of the pumping.
I have analysed my log.
Looking at PIQP: P and D oscillate but not I. So the oscillation is at least not driven by I (This was my first though as this is a slow movement)
Looking at RCout 5 and 6 (esc for me): RCout begin to oscillate when PWM decrease to around 1550 (the wing hover at 1700). This is why I think it could be linked to PID scaling.
About parameters, the ones that mitigate best the pumping are related to throttle controller. So what I did
Q_AZ_P from 1.5 to 0.7
Q_AZ_D to 0.06
Q_AZ_IMAX to 1000
Q_AZ_I default

Before playing with the throttle controller I saw little improvement
lowering Q_A_RAT_PIT_D
increasing Q_A_RAT_PIT_I
Lowering Q_M_THST_HOVER

But maybe the most important, I found flying the wing in Q_Hover full speed forward and trying to make short turns that there is a contradiction between the motor tilt and demanded roll (body frame). When I apply full aileron right, the right motor tilt down and the wing turn left…Sometime the elevons are stronger than the motor and the wing turn right… This could be the cause of crazy things when flying auto and maybe related to the pumping. To be tested.

About your flight controller mount, I cant say if it is bad or not but my approach is to make things as simple as possible and just in case to remove something that could be a problem. Sometime, I feel that to understand all is a luxury :grinning:

Regards,
Pierre

Hi Pierre, I did several tests with different CG locations and found that the pumping only occurred (on my wing) when CG was in the front. When I took it a little more back, then it stopped.
Easy to try out. I just taped some nails on the trailing edge.
Cheers
David

Hi David,
Again a steam machine counterweight solution to compensate a firmware work :wink:
Regards, Otto

hi David,
let me tell you a funny story. I made the maiden flight of the blue tail sitter 1 month ago. Begin with Q_ hover and loiter, I was very happy, then FBWA, not bad. As I was more and more confident one time I switched from FBWA to Manual and in less than 2s things went very badly so I switched back to FBWA but the pixhawk was unable to take control and then CRASH in the lake. My first reaction was Merde !! and the second was to remove ma pant and to go for a bath in the 10°C water. My neighbor who was resting on the beach despite the cold whether was happy that the motor buz stopped and was amused by the scene. So what ?
The good thing: not much damage to the wing
The bad thing: water in the electronic. Very bad because servo, esc, power module are trapped into the wing and glued so I had to make windows and break a lot of the structure to remove them. Hopefully during the crash, batteries were ejected and nothing was burnt. I found the batteries some hours later with my paddle board and believe me they still work nicely.
The problem was the after CG, so definitively I don’t want to play with that and I can witness that a flying wing does not fly at all with a bad CG.
An other conclusion: I am amazed by the ability of the pixhawk to fly something so unstable. Many thanks to all developer…

Regards,
Pierre