Rudder "feel" during takeoff/landing

[gospadin]

So, after doing 15-20 takeoffs & landings with 1.5.3.52018.61 this morning, I think I've finally put my finger on why ground control is so difficult with the M-2000C and causing so many of us problems.

 

There seems to be a moment or inertia to the rudder position either in the flight control system or in the physics calculations, that doesn't appear on the animated exterior model.

 

When I'm rolling down the runway at about 100 kts, and give it some rudder input, the nose moves left or right as expected. However, if I closely watch the horizon, I notice that as soon as I release the rudder and let the rudder center itself, there's a yaw inertia to the plane that shouldn't be there at all. As soon as the rudder centers itself, the nose of the plane should fix in whichever direction it is pointing.

 

When I look at the external model, the rudder is clearly centering quickly. However, I can see the plane continuing to turn for 2-3 more seconds after the rudder has been centered.

 

I believe this behavior is likely incorrect. As soon as the forces on the rudder are centered, the yaw moment should immediately be centered too.

 

Does anyone else "feel" this too?

 

EDIT: In case anyone asks, I have nose wheel steering disabled. I am purely working on keeping the plane centered with the rudder during takeoff and landing.

Edited April 9, 2016 by gospadin

[FishBike]

That sounds kind of right to me, actually. The plane should have a moment of inertia in the yaw axis, so once you get any kind of yaw rate happening, it should want to keep on happening until some moment in the opposite direction stops it.

 

In the air, if you get some yaw going with the rudder and then release it, you have quite a bit of sideslip, which results in an aerodynamic force on the vertical stabilizer that tends to point the nose back into the relative wind.

 

On the ground, with the main gear on the runway, you don't get nearly as much sideslip happening and so there's not much of a side load on the vertical stabilizer when you let off the rudder again. So unless the flight control system deliberately puts in some opposite rudder momentarily to stop the yaw rate, I think it would continue for a little bit after you center the rudder.

[gospadin]

Even with the friction of the tires on the ground? With the huge tail and a centered rudder, I'd have expected the yaw moment to go to zero as soon as the rudder is centered.

 

A yaw moment on the ground either means I'm skidding or my rudder is still deflected, no? Additionally, my max turn rate was perhaps 1-2 degrees per second. That shouldn't be enough momentum to require significant countering.

Edited April 10, 2016 by gospadin

[FishBike]

Even with the friction of the tires on the ground? With the huge tail and a centered rudder, I'd have expected the yaw moment to go to zero as soon as the rudder is centered.

 

A yaw moment on the ground either means I'm skidding or my rudder is still deflected, no? Additionally, my max turn rate was perhaps 1-2 degrees per second. That shouldn't be enough momentum to require significant countering.

 

What I'm saying is the yaw moment DOES go to zero when the rudder is centered. And because you have a 1-2 degree per second yaw rate, that rate will continue until a yaw moment in the opposite direction makes it stop.

 

In the air that yaw moment happens due to sideslip, but on the ground there is hardly any sideslip. The main gear would contribute some yaw moment to reduce yaw rate, but they are very close to the center of gravity, so may not do that much.