Today we’re going to fly test our smaller plane, a c-1 chaser, which has the same exact configuration as the bigger x8! Will post photos later on!
Here’s Cyril and his C-1 Chaser! We’re gonna fly it today!
A bit about Cyril - He’s made this while me (Teo) and Johanna we’re working on setting up the X8 for forward flight. I’ve seen him put in a lot of time doing many iterations to finally achieve his goal. He’s truly a genius guy! I am obviously biased as he’s not only an awesome engineer but also an awesome friend.
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Hey!
We had a successful flight followed by a crash which we cannot explain! The pixhawk used in the chaser doesn’t seem to want to store any data on the dataflash logs…
Below is a video of the crash! Prior to the crash we had changed the LIM_ROLL_CD from 6000 to 7000. Everything else stayed the same. Is there any reason for the pixhawk to behave differently after reboots? I feel like this has happened to us before where we are left dumbstruck and short of answers…
Any questions or help is welcome!
Something is not happening correctly when you transition and not sure why it flew so fast right away.
Pixhawk will bot behave differently after reboot as long as parameters are the same.
What a pitty your crash 
Its easier to have help if you share your crash log, its looks that fails at transition too.
ok I can see that the transition of front two motors was like a switched flipped instantaneously. It’s not supposed to be like that. You are supposed to give them an angle, not 90 degree flip. Thats why your plane went into a roll.
"## Tilt Angle
The Q_TILT_MAX parameter controls the tilt angle during transitions for continuous tilt vehicles. It is the angle in degrees that the rotors will move to while waiting for the transition airspeed to be reached.
The right value for Q_TILT_MAX depends on how much tilt you need to achieve sufficient airspeed for the wings to provide most of the lift. For most tilt-rotors the default of 45 degrees is good.
Tilt Rate
A critical parameter for tilt rotors is how quickly they move the tilt servos when transitioning between hover and forward flight.
The two parameters that control tilt rate are:
- Q_TILT_RATE_UP is the tilt rate upwards in degrees per second
- Q_TILT_RATE_DN is the tilt rate downwards in degrees per second
If Q_TILT_RATE_DN is zero then Q_TILT_RATE_UP is used for both directions.
How fast you should move the tilt servos depends on a number of factors, particularly on how well tuned your vehicle is for multi-rotor flight. In general it is recommended to err on the side of slow transitions for initial testing, then slowly speed it up as needed.
A typical value would be 15 degrees per second for both up and down."
Source: https://ardupilot.org/plane/docs/guide-tilt-rotor.html
We did have a successful flight just before that where it flew almost perfectly. Between the flights we changed one parameter: LIM_ROLL_CD from 6000 -> 7000. We made sure to reboot the plane before the flight to make sure everything is fine. We checked the elevon deflection and it seemed ok initially but once in the air we could barely roll at all.
We had set Q_TILT_RATE_UP/DN to 200 and 50 respectively. Q_TILT_MAX is set at 60 degrees.
I see that our tilt up and down are way larger than what you recommend. What would this imply to the stability of the plane?
Oh yeah, heres the successful flight 
Why are you changing this parameter value?
The default is LIM_ROLL_CD = 4500
You are flying VTOL, this parameter is good to play with fixed wing planes.
FBWA Mode (FLY BY WIRE_A)
“This is the most popular mode for assisted flying in Plane, and is the best mode for inexperienced flyers. In this mode Plane will hold the roll and pitch specified by the control sticks. So if you hold the aileron stick hard right then the plane will hold its pitch level and will bank right by the angle specified in the LIM_ROLL_CD option (in centidegrees). It is not possible to roll the plane past the roll limit specified in LIM_ROLL_CD , and it is not possible to pitch the plane beyond the LIM_PITCH_MAX / LIM_PITCH_MIN settings.”
The pilot wanted more roll. I don’t think the roll is the issue here.
What was your reasoning to change these values from default?
I think your transition rate is too high. Bring it down its not a fighter jet 
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Hahah yeah true it is not! 
We got the values from the Nimbus 1800 which has a similar configuration. We figured it’s better to transition fast and spend less time in the “transition” phase.
We’ll lower it and see if it behaves better
Way too high. I ran your video in slow motion and your servo literally snap to 90 degrees. No good…
Are you all located in Germany?
We will for sure try a lower setting. No but close, we’re from Stockholm, Sweden.
What do you think about the sudden yaw when entering FBWA? (Around the 8s mark on the crash video). To me that almost says esc issues.
Do you guys check log files after each test flight?
That would pretty much will tell you and you can analyze every aspect of the flight. Give it a try and also upload the tlog and bin files.
Also, the rpm of your motors in transition is way too much. Your First goal is to reach Cruising speed and then go full throttle.
The formula for the lift coefficient, Cl, is:
Cl = 2L ÷ (r × V2 × A), where L is the lift, r is the density, V is the velocity and A is the wing area.
Coefficient Of lift is published ratios based on airfoil type at versions speeds. You can lookup the Clark-y table on line.
Density of air as per google.
You will have to use newton 2nd law to estimate velocity based on motor/ prop thirst and than u can calculate lift.
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Do you have counter rotating props on the front motors?
If you look at how the motors are tilted when you fly in multicopter mode, you will notice one motor is tilted towards the front, the other to the back. When you initiate the transition, the motor tilted forward will reach the Q_tilt_max angle faster than the one tilted back, hence it will create a yawing moment. There is nothing you can do about this, as far as I know, except having counter rotating props. It will fly with same props, but this effect is accentuated.
Also if you are not pointed straight into the wind, again you have more tilt difference in the front motors!
Hello,
Nice project!
You have to trust your plane’s behaviour in order to try it over the water 
We also used a flying wing (already available in our company) and tried to turn it into a VTOL.
We first tried to use only two front tilt motors but it would be too sensitive to the wind.
Recently we modified it to a tri-copter configuration (using the same front tilting motors) and all the pixhawk parametrization was based on the information available in the Aticof’s project:
We ran into several difficulties during this modification but now succeeded (at least with no crash) in a controllable vertical flight (no adjustments to PIDs were still performed):
We now have to try to correct the vibrations caused by the back motor as you can see/hear in the video (we still don’t know if it can be corrected by reducing the pitch PIDs or if we have to increase the rigidity of the rear part of the foam structure).
We also want to use AUTOTUNE to adjust the PIDs but we will not have enough battery to fly for a long period of time (unfortunately it has a 5800mAh 4S 60C battery and only flies for about 3min with wind 
).
Does anyone knows if we can “pause” the autotune, land the aircraft, change the battery, take-off again and continue the autotune?
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Hi! Cool project!
No as far as I am aware you have to complete the autotune for it to save its values. We ditched autotuning and went for manual tune as we found more success with it. We still have a oscillation in yaw but I’m confident it will be solved as soon as the plane is rebuilt and I get a chance to fly it a bit!
/Teo
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Yessir, counter rotating. Interesting note about the tilt servo speed!
Hello David, I am glad that my project has been useful to you.
To autotune you can do it for each axis if you do not have a battery for all 3 axes.
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