Servers by jDrones

Dual-motor tailsitters


(letpi) #1544

I have loaded and tested the prquadts fw for some days now for a total of 9 flights and dozens of transitions. I have tested all kind of attitude and speed to enter back transition, I also flyed acro and manual as I did before and this time all transitions were perfect. So impossible to reproduce the bad back transition, very good job…


Maybe the only thing to be aware is the back transition need some speed. Because I set Q_tailsit_vfgain=0.1 (almost 0) and because there is no differential thrust from top and bottom motors until the transition angle, the pitch rate rely on flaps efficiency and wing equilibrium.

Now I am going to make an other fast wing non vectored with 2208 2600KV motors on wings and 1808 2600KV top and bottom motors. On 4s and 5*7.5 props it will be around 1500W and a little more than 400Km/h pitch speed. The TP100 airfoil is already printed…first tests within 2 months


(Mark Whitehorn) #1545

That’s good news; there must have been a bugfix applied to master in the meantime. Thanks for the log, I’ll take a look at the transitions to see if I can think of any ways to improve them.


(James Near) #1546

Has anyone come up with a good writeup of steps to follow before the first flight after everything is built. Want to make sure I don’t forget anything.


(Pavel) #1547

Hello,

We are trying to perform a SITL simulation of a tailsitter model with an AUTO mission. We are not using any flight simulation SW, just bare model.

And we cannot launch the model - due to “Gyros inconsistent” error the arming is failed.
The mission is pretty simple - it consist of 4 WPs only, and we were able to fly it with at least qiadplane sim. model.

N-ytt-kuva-16


N-ytt-kuva-19

The SITL is freshly installed to a Ubuntu 18.04 VM.


(letpi) #1548

This is at least how I proceed. This is surely not complete but a start if someone wants to add his own experience.

CG position :
To fly as fixed wing the CG MUST be somewhere between let’s say 15 to 22 % of the wing mean aerodynamic cord. The link bellow is a tool to calculate the M.A.C. and find CG location as a function of the wing design.
http://tracfoil.free.fr/cm/indexE.html
Keep in mind that the small fuselage of a plank wing may change the CG location by 1 or 2%.
CG in the middle of the wing thickness with respect to the Z axis is better. Check both wings are equal weight.
I think the hover is more stable when the CG is at ¼ of the wing cord of the wing section behind the propellers. For most wing design it means you take advantage to set the CG as close as possible to the neutral point of the wing. But beware that bellow 3 % of static margin there are a lot of chance the wing will be totally not flyable as a fixed wing.
Flaps:
Large flaps and ±40 ° throw for a non-vectored tailsitter. Less surface and less throw (±30°) is acceptable for a vectored.
Tilt motor : -45 +80 °
Both motor equal thrust

Parametrers
AHRS_EKF_TYPE=3
EK2_ENABLE=0
EK3_ENABLE=1
MIXING GAIN=1
Q_A_ANGLE_BOOST=0 (for vectored I guess but I am not very sure)
ARMING_CHECK =1
Q_ANGLE_MAX=2000 for first tests then increase step by step.
Q_VELZ=100 for first tests then increase step by step. This low value gives safe back transition (to q_hover) whatever the throttle stick position. Of course it will take a while to land.
Q_transition _ms : I don’t remember what is the default value but 3000 seems to me a good start.
Q_TAILSIT_ANGLE = 40
All Q_a_rate default values

Hover flight
Wait for a calm day without wind.
If your wing is vectored and takeoff from belly, remove propellers and switch to q_hover to verify is you get tilt motor oscillations. If yes you can break propellers. I have no solution for that.
Takeoff with q_hover or Q_stabilise mode, control there is almost no flaps or tilt motor oscillations, observe flaps neutral position.
Pull and release the elevator stick and observe, try to increase q_a_rat_pitch_p as much as possible, look for overshoot and oscillations.
Yaw the wing for several 360 turn and observe the EKF compass gauge.
Roll the wing carefully because it can go fast in that direction.
When after takeoff the wing drift in one direction (without wind) and take time to stop you can try to increase the relevant Q_a_rat_xxx_I parameter. But try to understand first why the wing is out of trim.
PID tuning take a lot of time, to save time you can test qautotune available in the last built. There is a dedicated qautotune blog.
When you are happy with your PID, increase Q_max_angle step by step.
If you observe flaps oscillation at high lean angle try to decrease Q_tailsit_thscmx from the defaut value of 5 up to 1.
If you observe throttle pulses while flying q_hover and high lean angle you can try all Q_AZ parameters.

Log analysis
Plot Q_Tune - throut, look at the hover value and use it to set Q_M_THST_HOVER, Set Q_M_SPIN_MIN about 1/2 of hover value and not lower than 0.25. These 2 parameters are critical for transitions.
Plot XKF1 pitch, XKF6 pitch, AHR2 pitch, ATT pitch : curves should be superposed except ATT pitch
Plot XKF3 “IVN, IVE, IVD” “IPN, IPE, IPD” “IMX, IMY, IMZ” refer to the wiki for interpretation
Plot VIBE
Plot Power VCC and Vservo

Transition
From q_hover to FBWA and fbwa to Q_hover: should be OK with above recommendations
It is always better to have the throttle stick centered.
If the wing has not enough speed to complete a smooth back transition, try to increase Q_M_THST_HOVER and Q_M_SPIN_MIN.
If you want the back transition to proceed sooner with less altitude gain try to reduce Q_TAILSIT_ANGLE

FW flight
Wait again for a calm day and proceed autotune as soon as possible.
For the beginning, avoid any back transition from an auto mode flight and do not use Q_loiter.
Q_tailsit_vf_gain is not really useful: a low value of 0.1-0.2 is OK.
Rudd_dt_gain together with kff_rddrmix allow using differential thrust as rudder. That’s very nice but should be increased carefully if you don’t want to make U turn.
Make the same log analysis as above except for the throttle tuning I use Ctune
Look at your flaps position when flying level at different speed and compare them to the neutral position. If your flaps are below (or at) the neutral position it may indicate the CG is after, that’s really dangerous.
good luck