Hello,
The 60mm COG from leading edge is recommended by the designer. I don’t know if I should change it, or how changing it would impact the flight?
The motors didn’t prematurely shut down. The motors stayed running (q_assist) as best they could to try and keep the plane in the air and still transition to forward flight. The best flight contoller in the world can’t fly an airplane that is out of balance. I would verify that C of G is correct, and that you have the plane balanced at that point. The VTOL motor outputs suggest it isn’t in balance, as you’ve pointed out yourself. If the design is good the balance in VTOL and fixedwing should be the same.
I think they did.. At around 11m/s GPS speed, you can see the motor’s PWM signal clamping down. This is much below the AIRSPEED_STALL set as 14m/s, and AIRSPEED_MIN set as 17m/s.
Yes, that is true. I’m just following the recommended COG of 60mm from the leading edge by the designer. If you want, you can look at the “Flightory Super Stingray VTOL” guide. I have checked, and my build is balanced at that point.
I think that’s not necessary. I might be wrong. Let me clarify my statement.
For the WING mode, the COG point is what is recommended by the manufacturer.
For VTOL mode, since the battery is placed inside the fuselage on the front and not at the centre of the VTOL booms, the drone is front-heavy in VTOL mode, and the front motors have to work harder than the rear motors to maintain level in Q modes.
Full disclosure… I’m no expert so please take this as pure conjecture but based on personal experience of building VTOLs.
IMHO you’ve introduced a “defect” right off the bat. Your quadmotors are working more or less than needed since you’re making it compensate for the imbalance during the transition. Personally, I’ve always set my CG slightly aft the recommended point so that there’s less trim necessary for the elevators to maintain a positive angle of attack during FW cruise.
Consider this, before electric motors entered the R/C world, planes had their fuel tanks usually behind the firewall so as fuel was burnt, the CG went aft. Obviously you don’t have to consider this in today’s electric aircraft.
Most if not all of my builds, especially wing design have longer front booms to make sure CG would be the same in MC or FW mode.
my two cents worth…. Good luck.
Hello @rollys,
Thank you for your response.
I agree that in a perfect world, the centre of the VTOL motor booms should lie at the wing COG line, so that the VTOL’s axis of rotation lies exactly at the wing COG, and both front and rear VTOL motors are balanced.
In my case, if I tried to do that, I would have to increase the length of the VTOL motor booms towards the nose. It would make the wing COG move towards the nose (front-heavy for wing mode), and to push it back, I would have to add dummy weight to the tail.
It would be inefficient since the extra weight would be there for the entire flight. Since this is intended to fly primarily as a wing, I gave preference to wing efficiency.
According to me, the VTOL motors are powerful enough to handle the front heaviness without any issue.
Also, I feel that the current issue is left roll, not pitch down on transition, so the VTOL motors’ role in that might be minimal, considering they are already spooled down at transition attempt.
Thanks,
Divyanshu
You definitively DID NOT have a stall, as was suggested. Out of control situation really happened much later, when the speed was higher. The speed was growing steadily, from 11 to 16 m/s, and the out of control situation happened afterwards.
Second very important point about the boom length and CG. It is very false to assume that you can get away with short front arms just because the motors can handle it… the motors can, but the overall stability can not. You create a very disbalanced copter, and when you introduce small yaw upsets, these can quickly make the copter unfliable. You may have to tune very differently PIDs for such a setup. I once tried to load a test Vtol bed off center with a load (on purpose) where the CG was at 33%..66% arm length. The thing crashed in a few seconds. I am not saying that this was your case here, but I definitively would avoid moving the center of motor force and CG.
Hi @Michail_Belov,
Thanks for your reply.
Yes, you are right, that in a perfect world, the centre of the VTOL motor booms should lie at the wing COG line.
Currently, in my setup, that’s not possible. With the front-heavy quad, from the logs, I see that the front motors require around 110us more PWM pulse than the rear motors on average to maintain level flight.
That’s not ideal, I agree. But I feel that once the transition is done, the VTOL motors go out of the picture, so this might be manageable.
Regarding stall and other details, I have done some research. Read my next comment where I explain my hypothesis.
Hi @Rolf,
In the past days, I did some research on what can cause this, and came up with a theory that sounds reasonable to me.
PS: This might be a long message, but it will be worth it.
So, firstly, from the logs, here’s the time at which the VTOL motors’ spool-down starts during transition:
As you can see in both flights, the motors started spooling-down around 5s into transition, when the synthetic airspeed was nowhere near the AIRSPEED_MIN (17m/s). Due to this, I strongly believe that the wing stalled and rolled.
Earlier, I thought that the Q_TRANSITION_MS (also set around 5000) mistriggered, and spooled down the motos after 5s into transition. To verify this, I checked the “QTUN.Trn” in the logs, which is 0 (Meaning the state is “Airspeed Wait”) throughout the transition. So this theory rules out.
Next, I had a look at the attitude, altitude, and VTOL throttle during transition.
I found something interesting. This is my theory:
- At around 9-10m/s GPS speed, the pitch starts to move up (reason unknown), without any pilot input.
- With this pitch up, the altitude increases with the forward movement.
- As per the Ardupilot wiki link, it states that: “The VTOL motors will behave similar to that in QHOVER and will try to maintain present altitude throughout the transition”.
This is a very important aspect. - Due to an increase in altitude (around 0.5m to 1m), the altitude error increases, and the VTOL motors spool down to maintain altitude. In logs, the “QTUN.ThO” literally goes 0 during transition. Due to this, the VTOL motors spool down, and since the synthetic is already very low, the wing stalls.
Here’s what I believe happened:
Transition start → Drone starts moving forward → Around 9m/s pitch starts increasing → Altitude increases → Altitude error increases → Altitude controller cuts throttle to VTOL motors to maintain altitude (QTUN.ThO = 0) → The airspeed is still 11-12m/s and the wing stalls and rolls left → Pilot moves to QLOITER to save the aircraft.
I have uploaded an Excel sheet where all these details are entered at different times of the transition in each row. You can see in the sheet that the “QTUN.ThO” goes to 0 with an increase in altitude error, causing a stall, and the roll to increase.
What are your views on this? Does my theory sound right?
@harkhka Of course, the initial pitch instability (before the roll deviation) could also be due to the COG and the centre of the thrusters being far apart. The linked LUA applet lets you adjust the imbalance. I haven’t tried this myself yet, but it’s definitely worth a try.
x-quad-cg-allovatio LUA script: ardupilot/libraries/AP_Scripting/applets/x-quad-cg-allocation.lua at master · ArduPilot/ardupilot · GitHub
Rolf
Hi @Rolf,
Thanks for your reply.
I have some doubts.
- I don’t know how the pitch increase on forward movement can be caused by the COG and centre of thrusters being far apart.
- Even if I use the script, still the physical difference between the COG and VTOL thrusters centre remains the same. How will it affect that? Just curious.
hello!
Did you get the PID of the VTOL from flightory.com and the speed configuration? Is there where did you bought the VTOL 3d model right?
They only sell designs, not PID values.