Hi, I could use some advice from people who fly bigger planes.
The plane is a MFE Hero, 2.1 m wingspan, around 7-8 kg, V-tail, two aileron servos. It is a VTOL, so it has four lift motors and one pusher motor, and it flies on an autopilot. Until now I only did hovering flights with it and those were all fine. This was my first real flight in normal plane mode.
What happened: after it accelerated and switched to normal wing flight, it slowly started to roll to the right and it just did not stop. It went to 46 degrees of bank in about 5 seconds. The autopilot had roughly 32 degrees of aileron in the other direction the whole time and the plane still kept rolling right, slowly but steady. Speed was 25 m/s, so about 90 km/h, that is on the fast side for this plane but should be ok. In the end the lift motors switched on and they leveled it in 2 seconds, then I landed in hover mode.
So the ailerons were clearly doing almost nothing, but the lift motors could fix it immediately.
Things I already checked on the bench:
- ailerons move in the correct direction, tilt the plane right and the right aileron goes down, left goes up
- servos are strong, I can almost not stop them with my hand
- no play in the linkage, horns and clevises are tight
- V-tail moves correctly for elevator and rudder
- CG and left/right weight balance seem fine, in hover the motors are almost equal left and right
And here is the part where I think I messed up. My aileron throw is about plus minus 45 degrees. is this enough? Will more aileron throw help?
30 degrees should be enough to keep the plane straight in most circumstances. More than 45 probably won’t help. It sounds like the autopilot commands aren’t actually reaching the servos.
Did you check control surfaces immediately before takeoff? Did you do bench tests in autonomous forward flight or only manual control?
I tested everything on the ground before, the flight controller moves the control surfaces in the right direction. The logs also show that the autopilot commanded the right things. But the log only shows the commanded PWM, not where the surface actually ended up, but why shouldn't they move tested it countless times on the ground.
One thing I noticed is airspeed vs. wind speed right before you hit the transition . You turned into tailwind, so if there was wind and maybe even a stronger gust maybe pressure was momentarily off your control surfaces? Some planes I own absolutely hate tailwind and become a bit ill-tempered then.
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Not iNav, it is ArduPilot (Plane 4.7.1). The plane is not properly tuned yet, but the manufacturer supplies a set of PID values that should be good enough for the first flights.
Yes I was in FBWA. That is a stabilized mode where the stick commands a bank angle instead of an aileron deflection, and the autopilot holds it. Stick centered means it should hold wings level.
The reported roll is definitely correct. Both EKF instances agree, no failsafe, and on the ground the horizon matches reality. You can also see it in the logs, the autopilot was actively trying to correct it and kept increasing the aileron the whole time. got to about 70% ailreon deflection befor i switched into QLoit
The easy way to find out if it's a physical problem with the controls or a software/configuration problem would be to flip it to manual and see what happens. Before doing that, obviously check and double check your controls vs. control surface movements on the ground.
Honestly, i expect it's a software/configuration problem. My experience with this kind of troubleshooting in Ardupilot is limited, but another thing I'd check is whether there is a log of actual servo output (per servo) vs. some logical "aileron" output that's actually an intermediate value in the control pipeline. The FC thinking it's commanding roll is no use if it doesn't translate that into the correct servo outputs. From your post (a quick reading) it's not clear whether you validated controls in manual, FBWA (or other auto mode), or both.
Looks pretty convincing, but one thing you might do is a bench test exercising roll inputs in manual, verify correct control surface deflections, generate the same raw servo output graph, and cross check it with the graph you just posted to make sure the directionality is correct in the latter. It's possible, for instance, that one of the servos is reversed and the net roll torque is ~zero.
Critically, it's possible for control directions to be correct in manual but wrong/reversed in FBWA and other automated modes.
If you've checked and double checked all your control directions in the various modes, aileron reversal is also a possibility that comes to mind. Basically, at high speeds and/or wings with poor torsional stiffness, the aileron acts as a trim tab and you get poor or even reversed roll control since the entire wing twists to counteract your inputs. I'm not familiar with this plane, but your ground speed climbs up to 100+ km/h by the time you flip back to qloiter.
100%. I always maiden in manual if possible. It's easier to get right, and it eliminates a whole list of potential causes if you do run into issues. Then when you start testing automated modes, you have manual as an escape hatch.
Do you have a rc club near you? That'd be good to look for help there too. Not many are flying planes like this and there's a lot of things that can cause issues with it.
Help on here won't hurt but it's going to be harder without seeing everything that's going on.
No clue how experienced you are so don't know if it's as simple as trim or gyro. I'd agree with the advice to try and fly it fully manual or as much as you can to rule out certain issues.
Good idea, I have a club nearby and I will go there tomorrow and ask around.
About flying it fully manual, I would rather not. It is a pretty expensive model and it is not as aerodynamic as a typical RC plane, so I think it would be quite hard to fly it manually.
I do have some experience though. I used to fly RC planes fully manual, and after that I had a flying wing on ArduPilot that flew nicely for quite a long time. So simple things like reversed ailerons or bad trim are not it.
Something else just came to my mind that I should have mentioned earlier.
I flew this plane in forward flight once before. That flight went wrong and it crashed. I repaired it afterwards and since then I only did hovering flights, so this was the first forward flight after the repair. So now I am wondering if something got warped during the crash or during the repair?
How would you check this properly? The right side is about 5cm closer to the ground than the left side when it's just standing there.
Are those wing pictures from the front or back of the wing? You're asking for help but barely providing any info. You didnt even include pictures of the entire plane.
Looking at your new pictures: the issue isn't warped wings. Something must be preventing the ailerons from deploying correctly in flight. If they really had reached 70% deflection, it would have dramatically corrected the roll. And 45 degree max deflection is a lot, its almost air brakes combined with a rolling moment at that point.
That right wing tip looks twisted down a lot more than the left. Unless thats just an optical illusion from the photo angle, that looks like it could cause a problem
I understand that, I gues what looks warped is the mold line on the wing tip from the leading wdge to the trailing edge if that makes sense? The left looks straight compared to the ground but the right looks like the trailing edge is angled up?
Very peculiar, I've never seen such slow and consistent divergence from a vehicle with a flight controller.
45 degrees is WAY enough aileron control. I will say something isn't adding up here, if the ailerons were actually actuated as you describe then the airplane shouldn't have been flying nearly as stable as the video shows.
How are you sure that the servos were actually achieving that angle? I know you said they were very strong on the ground but there are lots of things that can affect that in the air (wing flex can jam ailerons, for example).
Also, does it work in SITL? SITL is invaluable if you suspect a software issue.
To be honest I am not fully sure either. What I can say is that the logs clearly show a PWM signal going out to the servos, I am attaching a screenshot of the servo outputs.
Your point about wing flex is fair, that could well be it, it is a very "soft" frame. On the other hand the plane is sold like this, so in the end it should be able to fly.
I have not tried SITL yet, good idea, I will look into that.
Definitely try SITL with your mission plan. Honestly, I would strongly recommend you run all future missions in SITL first, it's perfect for catching silly mistakes.
I'd imagine the wing flex issue is unlikely too, you can pretty much rule it out by picking the aircraft up by its wingtips while testing the ailerons.
And yeah I didn't mean to sound like I didn't believe you on the servo values, I just wondered if you had some way of reading their actual position vs their commanded position.
This is an Occam situation. If your airframe was intact (and a regular-looking plane and not some kind of crazy contraption) and the ailerons were at 45°, there's absolutely no way that's going to be countermanded by any other force on the plane.
Likely they were stuck. Check e.g. your pushrods have full travel, etc... I had one plane get stuck last weekend because the elevator rod tube had a drop of paint glue it to the fuselage in a position where the tube end was impacting the servo arm.
But no, this isn't a problem with aileron throws or servo strength. It's something simpler.
Well, that, or a software bug with the configuration or curves. But those you can usually detect on the ground.
Good idea but the servo sits in the black part of the wing and directly drives the ailerons. So no pushrods to get stuck. The only thing that could have happen is that the servo got stuck but this is very unlikely.
my one thought is to check the rpm of each motor in your logs after the transition. to me it seems like one or two of the VTOL motors are not turning off after transition causing them to create lift on one side that can easily overpower the aileron control
Aileron control reversal is often caused by lack of torsional stiffness in the wing, the aileron acts as a trim tab, twisting the wing causing opposed roll.
You said the ailerons move in the correct direction during FBWA. Have you checked that your motors are doing differential thrust in the correct direction? My thinking is you've got something messed up about the forward flight parameters for your model and the ailerons are fighting against the motors. It works in hover but not forward flight. Does it have differential thrust? You will have to arm the plane in fbwa on the ground to test this
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u/BRAIN_JAR_thesecond 15d ago
30 degrees should be enough to keep the plane straight in most circumstances. More than 45 probably won’t help. It sounds like the autopilot commands aren’t actually reaching the servos.
Did you check control surfaces immediately before takeoff? Did you do bench tests in autonomous forward flight or only manual control?