[FAR] Plane rolling problems

[Itsdavyjones]

I've made a plane that wouldn't roll at all (great for turns) without input or having FAR keep the wings level, that flew like a beauty, yet the same craft now rolls and keeps rolling while in flight. I have tried to solve it my self (plane has been modified to not look like it used to), and have looked on the forums, but didn't find anything. Is rolling any time, even with the plane designed not to roll, for I have tested with another craft with high wings and dihedral, normal?

http://imgur.com/cQ6C9Ns

Edited January 13, 2015 by Itsdavyjones

[Alshain]

1. Define "roll". You mean actual roll or is it flipping end over end? (I ask because I've run into that terminology brick wall before)

2. A picture of the plane in the air isn't all that useful, get us one in the SPH with CoM, CoL, and CoT enabled.

3. What altitude does it roll? What speed?

4. Rolling while hands off or are you doing a non-roll maneuver like pitch?

My intitial thought is that landing gear has a flat edge on it, the way you have it mounted it's creating an air block which could induce roll.

Edited January 12, 2015 by Alshain

[Van Disaster]

Seems to be completely lacking any means of aerodynamic roll control ( the cockpit torque will have some effect ), but maybe the ailerons are hidden away somewhere I can't see.

[LethalDose]

Seems to be completely lacking any means of aerodynamic roll control ( the cockpit torque will have some effect ), but maybe the ailerons are hidden away somewhere I can't see.

The canards are winglets that can induce roll, unless they've been tweaked to not act on roll inputs. Which brings me to my question: Have they been tweaked to not act on roll input? Also, as Van points out, there appears to be a general lack of control surfaces on the aircraft in the pic.

[Itsdavyjones]

First, more images as asked.

http://imgur.com/a/XWxZ5

1. Define "roll". You mean actual roll or is it flipping end over end? (I ask because I've run into that terminology brick wall before)

2. A picture of the plane in the air isn't all that useful, get us one in the SPH with CoM, CoL, and CoT enabled.

3. What altitude does it roll? What speed?

4. Rolling while hands off or are you doing a non-roll maneuver like pitch?

My intitial thought is that landing gear has a flat edge on it, the way you have it mounted it's creating an air block which could induce roll.

1) Roll, means that it wants to roll around the longitudinal axis.

2) In images

3) Any altitude above the runway, any speed.

4) Hands off, which I did for turns, with minor roll corrections for large turns, and with pitching now.

Also tested the landing gear being on the main body, but still rolled.

Seems to be completely lacking any means of aerodynamic roll control ( the cockpit torque will have some effect ), but maybe the ailerons are hidden away somewhere I can't see.

The image didn't show the elevons, but they are there.

The canards are winglets that can induce roll, unless they've been tweaked to not act on roll inputs. Which brings me to my question: Have they been tweaked to not act on roll input? Also, as Van points out, there appears to be a general lack of control surfaces on the aircraft in the pic.

Gave it a shot with the canards with no roll, but it still rolls.

And with the last image, FAR seems confused what level means.

[Alshain]

Ultimately I think your vertical stabilizer is too small and that is likely the problem. The whole idea of a vertical stabilizer is to split the air and put equal pressure on both sides, holding it.. well stable... and preventing undesired roll and yaw.

Other tips:

I'm a little concerned about the wings, there is considerable space between the segments. Because FAR uses the craft shape to determine aerodynamics, the sudden "dip" in the wing surface may be causing issues. Some like the "black bar" appearance but the grey area of the wing should never be visible when two wings meet because that's where it starts change shape, those should overlap. I'm not certain how that will react to be honest.

Ideally your roll control would be stronger at the outside of the wing, that's where it has to do the least work to roll the plane. In fact I would put it only on the outside elevons. This however is just a suggestion for improvement, if your not actively rolling it shouldn't be causing your problem.

What are the front elevons intended to do? Spoilers? You should know that FAR does not simulate the stall effect from spoilers. You can right click the wing and see it's stalled 0% when the spoilers are deployed. It only simulates the drag, so they are really only good as either air brakes or on the runway as braking spoilers. You generally don't want to create drag ahead of the center of mass. It's not your roll issue if they aren't deployed but it struck me as odd so I thought I'd mention it.

Edited January 13, 2015 by Alshain

[Itsdavyjones]

I placed those spoilers in the front since it was the place where the stability of the craft wouldn't be affected when deployed, full or empty, and as for the black bars, this was recreated after trying to solve the problems earlier, but its still stable in that regard. Also, both elevons induce roll, but the inner ones move farther than the outer ones to increase force, as for the speed which the elevons do roll when applied, its a perfect speed to avoid sudden forces.

[Alshain]

Well I guess if it works for you. But for your problem, swap out that vertical stabilizer for a larger wing segment and throw an elevon on the back for yaw control.

Edited January 13, 2015 by Alshain

[Hodo]

Davy sounds like you have an issue with roll authority. Let me see if I am reading the problem correctly? The craft is rolling randomly or lacking of roll stability during flight?

I have to ask what control surfaces do you have on the craft other than the canards I see in the pictures? Do you have any horizontal rudder control, ailerons or elevators other than the canards at the front of the craft?

If you are relying on the canards for all of your roll and pitch controls you are asking them to do to much this will cause the craft to roll unexpectedly when attempting to climb or dive and make it unstable in roll situations. It is generally best to set canards to pitch control only and then set a set of ailerons out towards the edges of the wing trailing edge to act as your roll controls. This with an actual rudder not some horizontal stabilizers and no control surface on them you will have a pretty stable aircraft there.

[Wanderfound]

"Not rolling at all" is not a beneficial design feature for planes; they need to be able to roll in order to make banked turns. "Doesn't roll unless I tell it to, but then rolls quickly and stably holds a desired bank angle" is what you're after.

The image provided is a bit too unclear to be useful; much better would be to post FAR stability analysis screens at the speeds and altitudes in which the problem occurs, and images from directly above and to the side in the SPH with CoM/CoL/CoT indicators turned on.

We also need to know if any of your control surfaces are set as ailerons or elevons (i.e. tweakables set to influence roll). It'd also be good to know if you're using a PID tuner (you should; Kerbal Pilot Assistant is a good mod that includes a PID tuner).

[Itsdavyjones]

Well I guess if it works for you. But for your problem, swap out that vertical stabilizer for a larger wing segment and throw an elevon on the back for yaw control.

I'll give that a shot when I can upgrade the SPH, at part cap as it is, unless i start skimping on fuel. This craft has a 2000 km range, so a trip to the poles and back seems like it can be done (was doing this when this occurred)

Davy sounds like you have an issue with roll authority. Let me see if I am reading the problem correctly? The craft is rolling randomly or lacking of roll stability during flight?

I have to ask what control surfaces do you have on the craft other than the canards I see in the pictures? Do you have any horizontal rudder control, ailerons or elevators other than the canards at the front of the craft?

If you are relying on the canards for all of your roll and pitch controls you are asking them to do to much this will cause the craft to roll unexpectedly when attempting to climb or dive and make it unstable in roll situations. It is generally best to set canards to pitch control only and then set a set of ailerons out towards the edges of the wing trailing edge to act as your roll controls. This with an actual rudder not some horizontal stabilizers and no control surface on them you will have a pretty stable aircraft there.

There are two sets of elevons on the back of the wings, the first picture isn't very good at showing it, but the pictures farther down show them. Also, I tested without roll enabled on the canards, but it made no difference, and the rudder can be upgraded when the SPH is upgraded, unless I can get it stable without the wingtip vertical stabilizers.

"Not rolling at all" is not a beneficial design feature for planes; they need to be able to roll in order to make banked turns. "Doesn't roll unless I tell it to, but then rolls quickly and stably holds a desired bank angle" is what you're after.

The image provided is a bit too unclear to be useful; much better would be to post FAR stability analysis screens at the speeds and altitudes in which the problem occurs, and images from directly above and to the side in the SPH with CoM/CoL/CoT indicators turned on.

We also need to know if any of your control surfaces are set as ailerons or elevons (i.e. tweakables set to influence roll). It'd also be good to know if you're using a PID tuner (you should; Kerbal Pilot Assistant is a good mod that includes a PID tuner).

I guess "Doesn't roll unless I tell it to, but then rolls quickly and stably holds a desired bank angle" would better describe my plane. Will also provide stability stats before i try adding a different rudder setup, in case that fails. As for the PID, never heard one till now, and will give it a shot, maybe it will help.

Edit: Preliminary testing with the PID allows it to maintain level flight with maintain heading, something FAR couldn't handle. As for the rolling, its still there, but now under control. Still no idea what is causing it, so will follow up with some more stats, for potential solving.

Edit 2: http://imgur.com/a/J9su8

Edited January 13, 2015 by Itsdavyjones

[Wanderfound]

Looks like a well-built plane to me; once you get the PID tuning right it should fly cleanly.

Exactly how to set up the PID tuner [1] varies a bit based on ship design and personal preference [1], but I usually get good results from cutting all kp values to 1/3rd of default and scalar values to 1/2 default.

[1] For those that don't know already, this is a thingie that allows you to adjust the sensitivity of your SAS.

[2] Basically, more control authority = needs lower kp values.

[Itsdavyjones]

I did as suggested, and it works, mostly. The plane constantly pitches up and down in a rapid movement, not rapid enough to cause URD, but enough to prevent it to get stable, along with me not really knowing how to operate the PID best, any suggestions to stable that out? .craft file provided too, only mod needed, other than far, is the adjustable landing gears.

https://www.dropbox.com/s/47v4f07k8pwzrmb/KRAP.craft?dl=0

The name stands for Kerbal Reconnaissance AirPlane.

[Wanderfound]

If you're getting pitch oscillation, you probably need to lower the pitch kp value in the PID tuner. The SAS is overcorrecting in pitch, causing the oscillation; reducing kp will make it less prone to doing that.

I haven't been giving specific numbers as KPA uses different PID scales depending on whether you're using it in "stock SAS" mode or "surface SAS" mode. Stock mode is purely a tuner for the standard SAS; surface mode adds some extra KPA trickery.

I tend to use it in stock mode, with the settings as described above: pitch and yaw kp cut to 5000, roll kp to 3000. Scalar values for all three set to 500. Other values left at default.

The numbers are different in Surface mode, but the principle is the same: if you're getting oscillation, cut kp on that axis. If doing so causes the plane to gradually drift off course, lower the scalar.

Edited January 13, 2015 by Wanderfound

[Janos1986]

Have you tried to lower the wing roots and pull up the tips a bit? They seem a bit too straight to me.

I'm not using FAR, but it helps greatly with stability in my personal experience.

[Hodo]

If you're getting pitch oscillation, you probably need to lower the pitch kp value in the PID tuner. The SAS is overcorrecting in pitch, causing the oscillation; reducing kp will make it less prone to doing that.

I haven't been giving specific numbers as KPA uses different PID scales depending on whether you're using it in "stock SAS" mode or "surface SAS" mode. Stock mode is purely a tuner for the standard SAS; surface mode adds some extra KPA trickery.

I tend to use it in stock mode, with the settings as described above: pitch and yaw kp cut to 5000, roll kp to 3000. Scalar values for all three set to 500. Other values left at default.

The numbers are different in Surface mode, but the principle is the same: if you're getting oscillation, cut kp on that axis. If doing so causes the plane to gradually drift off course, lower the scalar.

This is my catch all planes with FAR, I set all the pitch roll and yaw kd to half of the original values. This usually kills the oscilation with the pitch issue with SAS on. Oh and if you are using the Pilot Assistant tool PID, then use stock SAS settings it will help with the tuning effort.

Otherwise you have an otherwise amazingly stable aircraft, it should do fine for long flights around the globe.

[Van Disaster]

Turn off the dynamic control range thing and tune oscillations out using the other dampers, or lower the dynamic control adjustment a lot - I suspect it *is* trying to compensate for roll but doesn't have the authority anymore because the dynamic adjustment has clamped too hard. I would grab Pilot Assistant though, does the same thing with a bit more functionality.

[Itsdavyjones]

Ok, will try tuning it better when I get home today, thanks all.

[Hodo]

Have you tried to lower the wing roots and pull up the tips a bit? They seem a bit too straight to me.

I'm not using FAR, but it helps greatly with stability in my personal experience.

This unfortunately does not always work with real aircraft or in FAR. If you are looking for a long distance slow flying stable aircraft then you want a high wing design. Nothing overly fancy, basically a Cessna or Piper Cub.

[Itsdavyjones]

Looks more like a Hercules than a Cessna, but that does seem like a good idea to try. As for range, my current plane can go appx 2000 km, so a round trip to the poles is available, if just, and if I got the time to wait out the hour to go one way, and the hour to go the other, I don't like phys warping my planes.

[Hodo]

Looks more like a Hercules than a Cessna, but that does seem like a good idea to try. As for range, my current plane can go appx 2000 km, so a round trip to the poles is available, if just, and if I got the time to wait out the hour to go one way, and the hour to go the other, I don't like phys warping my planes.

There is the one I was looking for.

And the previous one was actually modeled after a common passenger aircraft used by many companies for short flights.

[Itsdavyjones]

Thats a nifty fuel efficient plane, will give one a shot later.

[Hodo]

Thats a nifty fuel efficient plane, will give one a shot later.

Oh its not fast, but it will go a good distance. I built it in my RO install last year. Could go something like 800km on a single fuel load, and never climb about 7km on Earth.

[Van Disaster]

Works for jets too:

Range is more than I care to test, I use it for dropping RT relay rovers so it has to be able to fly tight circles until they land - however also cruises around at 600kts at altitude so going places doesn't take all day. I suspect something similar could be built from stock Mk3 parts.

That, not so much but that's airliner-fast too.

[Hodo]

Works for jets too:

https://farm8.staticflickr.com/7571/16271108852_cb49d71bdb_b.jpg

That, not so much but that's airliner-fast too.

Thats kind of like my FB-1 attack craft.