Attitude instability.

[Confutus]

I've noticed in a 5% power test of my Apis 1 aircraft (shown here ) that it goes yawing back and forth as it goes down the runway at a blazing 5 m/s. I know this isn't a landing gear problem, since other vehicles with identical gear configurations go perfectly straight. It's aerodynamic.  The rudder tail fin can be seen swinging back and forth as SAS enters a cycle of overcorrecting first to one direction, then the other.  I've already encountered a similar problem with pitch, so I expect it can occur with roll as well.  Whenever and however it occurs, it can make control of the aircraft difficult. Pilot input at just the wrong direction at just the wrong time can deliver compelete control to the laws of physics.  I have the idea, based on what I remember of the physics of vibration and resonance, that I can simulate damping by reducing the control authority of the various control surfaces.  To save time in testing, is this an effective approach?

 

[OHara]

Maybe you don't want to save time testing; the idea of KSP is that testing is fun.

In general, reducing the feedback in a servo system reduces oscillation, so reducing the control-surface amplitudes should work.  

There is a FAQ entry about wobbling on the runway, giving the standard KSP solutions. My favorite is to trim up to take weight off the nose-wheel

[Confutus]

3 hours ago, OHara said:

In general, reducing the feedback in a servo system reduces oscillation, so reducing the control-surface amplitudes should work.  

There is a FAQ entry about wobbling on the runway, giving the standard KSP solutions. My favorite is to trim up to take weight off the nose-wheel

 I see information about veering off the runway, but not wobbling while still on it, not in the FAQ or either tutorial.

[RizzoTheRat]

Landing gear and wheels can be a bit flakey.  Looking at your picture it looks like the main gear is fair way behind the CoM (although hard to be sure at the angle of the pic).  If so then moving the main gear forward a bit might well help by taking load off the front wheel, the same as OHaras trim suggestion.  The theory being that as you start to get some lift you have more weight and grip on the front than the back so it gets a bit oversteery.

Another thing that can help with aircraft (again hard to see in the pic so you may already be doing this) is to design it so that it sits a bit nose up on the runway, meaning you start off already pitched up and don't need any control input to takeoff.

[Confutus]

3 hours ago, RizzoTheRat said:

Landing gear and wheels can be a bit flakey.  ...

Another thing that can help with aircraft (again hard to see in the pic so you may already be doing this) is to design it so that it sits a bit nose up on the runway, meaning you start off already pitched up and don't need any control input to takeoff.

I've done enough tests with different models of vehicle that I don't think its a landing gear problem. You can't see it in the picture, but the rear landing gear is spread just wide enough to give me a free degree or two on the AoA while still on the runway. The CoM is close to and ahead of the CoL, and both are close to and ahead of the main (rear) landing gear. I'm also flying at about a 10% fuel load in engine tanks and empty main tanks because I don't need more for runway tests and short hops around the KSC. One model has gotten into full flight at 40% engine thrust. I expect others to follow soon. A couple are lifting the nose wheel already at about 38 m/sec, before crossing in front of the SPH building, and without touching the pitch control, so I think I'm good with with these issues.

What concerns me is the oscillation in attitude before I even leave the ground. I have pitch on the one hand, yaw on the other, and why not roll, too?  This appears to be highly sensitive to details of vehicle configuration and speed. It appears in some models but not others, and it could be due to interaction with the SAS; one more thing to test.  It may either go away or get worse with increasing speed.  If it persists in flight, it's an invitation to a crash. 

I don't think this is a typical beginner problem, although it could be one of the many factors contributing to the high crash rate among beginners. Most beginners  don't take the cautious, finicky approach to flight testing that I'm doing. I didn't, when I started, and I've been told it's not The Kerbal Way. But that's precisely why I'm doing it.

Edited January 16, 2020 by Confutus
fixed grammar errors

[Fierce Wolf]

If the plane is the one in the link, one set of wheels goes forward, and the other backwards.

And that wheel model require some twitching to get them right, manually setting the spring and dampers. Auto setting cause them to start bouncing like a muscle car

Edited January 16, 2020 by Fierce Wolf
typo

[Confutus]

2 hours ago, Fierce Wolf said:

If the plane is the one in the link, one set of wheels goes forward, and the other backwards.

And that wheel model require some twitching to get them right, manually setting the spring and dampers. Auto setting cause them to start bouncing like a muscle car

 It's the one in report #4. I don't seem to be able to get both sets of wheels pointing the same way.  On the 4 wheeled ground vehicle, the editor insisted on flipping the front pair back around when I rotated them 180 degrees, so I gave up fighting it and let it have its way. The nose wheel was another problem.  That's really finicky to set going straight manually and the snap setting didn't want to do it in the forward direction.  (I've think I've managed it now, but it took some fiddling.  As long as it's going straight down the runway... )

And yes, they are rather bouncy. I see that I can set the friction on the wheels,  but does this first tier of wheel parts even have spring and damper settings? I couldn't find them.

Edited January 16, 2020 by Confutus

[Fierce Wolf]

My advice:

 

1) Use only 1 pair of wheels near center of mass, the pointy thingy going backwards

2) Use 1 steering wheel in the back of the plane.

3) To access spring and damper you need to able the Advanced Tweakeables (in the main menu options, before starting your campaing). Play around with the wheels stats on the runway and once you find a good configuration for them, write down the numbers to tweak in the VAB and save the craft.

4) Test that MOFO!