How to stop planes "nodding"

[p1t1o]

I can build planes, good ones even, fast ones, high altitude ones, SSTOs...

But every single plane I have ever built "nods" in flight whilst using any kind of automatic control system (SAS for example, or mechjeb's S.M.A.R.T.A.S.S). For example, when flying a long distance, or climbing to high altitude, you want your plane to hold a set attitude, you don't want to keep manually tapping the keys (which causes "nod" all on its own) so you either put SAS on, or an alternative. The control logic keeps you pointing in the right direction, but there is always "nodding".

Nodding, nodding, nodding. Has the word lost all meaning yet? Nodding.

The FAR stability augmentations are supposed to help, but if they do, they don't seem to eliminate the problem.

Is there a way to tweak the control SAS/FAR/mechjeb feedback settings? And if there is, in what way would I tweak them?

*Occasionally* an aircraft would fly through a certain speed/height regime where the control/response feedback loop settles down and I get a period of smooth flight, but its the exception to the rule, and any control input usually sets up a nice nod.

Could it be something inherent in the way I build my planes? Perhaps something related to CoL/CoM? I normally set them up with the CoL juuuuuuust behind the CoM (like, within a few blue-CoL-ball radii), with fuel positioned to affect CoM as little as possible.

Thanks!

[KOCOUR]

I put COL always exactly centerd with COM in XandZ axis and on top of COM in Y axis. Also i try to have COMs of my fuel tanks aligned with plane COM. Thats possible with smaller planes. In bigger planes i have two half empty "counterweight" fuel tanks not connected to any other tank or engine so theyr fuel is not used up and i transfer fuel between them when the plane starts to pitch down or up on itself. Dont know if it works with Mechjeb tho, i stopped using it after game introduces its own advanced SAS.

[Sharpy]

Try Pilot Assistant. It's a SAS/autopilot that is intended for airplanes in KSP (and specifically, don't try it in vacuum, as it tries to use control surfaces for maneuvering which doesn't work too well without air).

It has two modes: "SSAS" which works with more raw values, providing stability assist - keeping roll, yaw and pitch constant (and whatever climb, speed or turn results from it depends on other factors like throttle or plane geometry) and Pilot Assistant, which works on values like heading, climb speed, target alltitude, target speed.

I believe if you set given altitude, in Pilot Assist the plane will keep nodding as well, correcting all overcorrections. But in SSAS if you pick given pitch, it will be kept steady.

[Red Iron Crown]

Have you tried trimming the aircraft with Alt+WASD? I have found that trimming the plane to be neutral at the desired attitude reduces SAS oscillation greatly.

[Nich]

Cant you adjust the values on mechjebs PID filter? Make it less reactive to offset and more reactive to rate and intergul.

[monstah]

Have you tried trimming the aircraft with Alt+WASD? I have found that trimming the plane to be neutral at the desired attitude reduces SAS oscillation greatly.

But I thought trimming and SAS didn't work together? I'm usually with SAS on, and it took me a long time to realize that's why trimming wasn't working for me.

[p1t1o]

Cant you adjust the values on mechjebs PID filter? Make it less reactive to offset and more reactive to rate and intergul.

Does Mechjeb have that ability? Does it affect stock-SAS as well, or just the mechjeb autopilot modes?

I'm not sure how to modify those sorts of variables to get the result I want, I may have to do some trial-and-error tinkering.

[Red Iron Crown]

But I thought trimming and SAS didn't work together? I'm usually with SAS on, and it took me a long time to realize that's why trimming wasn't working for me.

I certainly could be mistaken, perhaps having it trimmed before engaging SAS means it has less to correct initially and that is what I am seeing.

[Tex_NL]

Have you tried trimming the aircraft with Alt+WASD? I have found that trimming the plane to be neutral at the desired attitude reduces SAS oscillation greatly.

But I thought trimming and SAS didn't work together? I'm usually with SAS on, and it took me a long time to realize that's why trimming wasn't working for me.

Correct. SAS/SMARTASS completely ignores all trim settings. A few years, and a few versions ago SAS and trim did works together perfectly but sadly not any more.

However RIC's suggestion is a valid one. A properly designed plane does not need SAS/SMARTASS. I often prefer to fly planes without it and totally rely on trim to keep them stable.

[Nich]

I would think anything with canards is unstable and can not be trimmed into stable flight

[DancesWithSquirrels]

SAS works with trim, but makes it rather irrelevant - the PID controller (Proportional Integral Derivative) used in SAS will build up an equivalent 'integral' offset that acts like trim. And if you add in trim that drives the plane away from the SAS vector the integral will cancel it out.

I use trim on my aircraft when flying without SAS. If my plane is positively stable (CoG slightly ahead of the CoL) that works just fine, and I can make long flights at 4X speed only slowing time accel. and correcting occasionally.

A canard should provide less lift than the wing, with the wing lift vector (without canards) rather behind the CoG - the plane will be statically stable if the canard lift * distance from CoG is smaller than the wing lift * distance from CoG, with the wing having greater leverage. That should be the case if the CoL is behind the CoG, except when the canard authority is greater than the rest of the pitch authority - consider dropping back from a AV-R8 winglet to a less authoritative Delta-deluxe.

Edited August 21, 2015 by DancesWithSquirrels

[Nich]

Actually even if we assume the wing and canard have the same lift profile you need the delta Lift(Alpha) because at a stable state canard lift * distance from CoM = wing lift * distance from CoM However alpha is not the same for the wing and canard. Because the center of lift is behind the COM alpha will be greater for the canard. In most flight profiles (not stalling or flying very slow) with a low alpha lift is approximately linear thus;

Canard 10m from COM with 1t lift = 10mt

Wing -1m from COM with 10t lift = -10mt

is stable

Change alpha on the canard by 1 degree

Canard 10m from COM with 1xt lift = 10x mt

Wing -1m from COM with 10t lift = -10 mt

Plane pulls up 1 degree

Canard 10m from COM with 2xt lift = 20x mt

Wing -1m from COM with 10xt lift = -10x mt

You now have a ton torque and the plane flips out until front canard starts to stall out but at this point the wing pretty well stalled out as well

Edited August 21, 2015 by Nich

[Mikki]

I certainly could be mistaken, perhaps having it trimmed before engaging SAS means it has less to correct initially and that is what I am seeing.

Agreed, and flying without SAS is also doable, needs a balanced, fine trimmable construction. Try to make a fuselage with wings and/ or engines (and cockpit...) angled pointing downwards some degrees, expect very interesting flight behaviours... really worth trying out, you won`t regret the time spent.

[panzer1b]

Wobbling is 90% of the time caused by too many control surfaces or reaction wheels.

For whatever reason all my supermaneuverable craft will do this at higher speeds as the sas overcompensates with too much control authority. Usually using the precision controls and disabling sas entirely is what i do when the wobbling gets very bad. Anyways, for anything that is super fast, you dont want as many control surfaces/wings as its extra drag and causes the stock sas to go nuts. Either use a mod sas (the MJ one seems pretty good), or just disable control surfaces in flight when u get up to those speeds (and disable gimbals too as those can also overcompensate if the engine is very powerful and has good vectoring range).

[SmashBrown]

I usually put canards on the nose.

[DancesWithSquirrels]

Nich - Yes, differing angles of attack between wings and canards do make matters more complicated. A quick test for this is to rotate the ship in the SPH in pitch, and see what happens to the CoL. If it moves ahead of the CoG at certain angles the plane won't be stable there.

[Tex_NL]

I usually put canards on the nose.

Canards are ALWAYS placed on or near the nose of a plane. No matter how the part is called in the game if you do not place it on or near the nose of a plane it is NOT a canard.

[A_name]

I usually put canards on the nose.

Like a unicorn?