I was flying and testing modes today - stabilize, loiter, manual all work great. I tried to switch to FBWA and there just does not seem to be enough ‘control’ given by the APM with as compared to my inouts; I couldn’t even make even bank hard enough to turn around - forcing me into manual mode.
What parameters do I need to mess with to get decent control in FBWA? I’m afraid RTL won’t be able to come home if the behavior is similar…
APM 2.6 with external compass and GPS
3DR Power Module
parameters attached
[quote=“Graham Dyer”]Your Roll and Pitch P are quite low at 0.42, you should be able to go higher to almost double for better responsiveness.
What plane are you using?[/quote]
Thank you for the help Graham, very much appreciated.
I am flying a TBS caipirinha. It’s a very small flying wing. The P was 0.5 originally and I lowered it slightly because in stabilize, it would try to correct itself too fast and overshoot - am I tuning the wrong parameter? I was under the impression “P” is the rate of which it will try to ‘level’ the aircraft… higher values = more aggresive, lower = smoother but less responsive. Correct me if my understanding is wrong (please).
the issue i’m having is that in FWBA it won’t roll the plane very much at all. the throws for FBWA are 1/3 of what stabilize is…
One thing to check with a flying wing is when you do your radio calibration, put the roll and pitch sticks into the four corners, not just up and down/left and right so that you record the full range of the “elevon” movement.
You’re tuning the right parameter, you should be able to turn it up quite far before you get oscillations. Also add more I and D, One test is to check the [color=#cccccc] pitch and Nav_pitch[/color]roll and nav_roll, in a tlog they should follow each other very closely. Something like the attached pic.
[quote=“Graham Dyer”]One thing to check with a flying wing is when you do your radio calibration, put the roll and pitch sticks into the four corners, not just up and down/left and right so that you record the full movement of the “elevon” movement.
You’re tuning the right parameter, you should be able to turn it up quite far before you get oscillations. Also add more I and D, One test is to check the pitch and Nav_pitch in a tlog, they should follow each other very closely. Something like the attached pic.[/quote]
I did calibrate the radio properly but thank you for pointing that out. I’ll try increasing these terms and see how it flies, but I’m still not sure how this is going to increase elevon travel in FBWA ? I literally can’t make turns in this mode at all…
You can also increase you bank max to allow for a greater bank angle, but FBW-A is a rate limited mode so it’s not supposed to roll like stabilize.
Check if you have bank max set to 45 and you hold in full left roll that you are getting 45 deg of roll, if not then either your servo’s can’t get that far or your P is too low with the solutions previously mentioned.
Post a tlog here too so we can have a look.
(In my previous post it should read “roll and nav_roll” not pitch)
I do not have tlogs but attached are a couple logs from the APM. They have been truncated since log space ran out as I was flying, so i think the log starts mid way through a flight. also attached my PID’s - bank max seems to be set to 30, maybe that’s the issue?
awesome thank you, I will try to adjust the bank max value.
Just so I am clear…
-the bank values are only used in FBWA/cruise but not stabilize?
-The main tuning parameters parameters that effect stabilize are servo roll/pitch PID’s?
Bank max is only used in Navigation modes, FBW_A is a nav mode where you, not the APM provide the inputs. It’s uses angle limits that apply to all the nav modes. You can do a loop in stabilize but not in Auto or FBW_A modes.
The servo roll/pitch PID’s control all the modes, so if they’re high it’ll affect Stabilize, FBW_A, Loiter, etc, etc
Turn radius is a function of airspeed and bank angle. Specifically, it is r = v^2/(g*tan(bank)) - Your planes must be very slow.
OP: set your LIM_ROLL_CD to 60 degrees for 2g max airframe stress, or 70 degrees for 3g max airframe stress. I wouldn’t go any further.
LIM_PITCH_MIN and LIM_PITCH_MAX are your pitch limits, default 15 degrees up, 25 degrees down.
FBWA will adhere to these limits.[/quote]
Thank you for the info! My frame has a minimum speed of 20km/h, high end at 60km/h. Any recommendations on what these values should be for this frame?[/quote]
If you just want to be able to fly FBWA, it is all about what you like the feel of. I would start by setting your roll limit to 60 degrees and your pitch limits to +20/-35 degrees.
I agree, 60 is a good maximum bank angle. Beyond that a full sized airplane must be certified for aerobatics because the wing loading gets to the steep part of its logarithmic scale. At 60 degrees bank, holding level altitude, you will pull exactly 2 G’s your stall speed will increase by 40%. At 70 degrees your stall speed increases by 70%.
Had this same problem. Increased roll angle but that had no effect, FWBA was still extremely sluggish. Found out that new code has stall prevention feature that is active by default. It requires airspeed indicator to function correctly. By disabling and setting STALL_PREVENTION = 0 you will get the full specified roll angle and much more response from FBWA. Hope this helps.