Rear elevons don't respond to roll input

[Corona688]

Why do the two back elevons not respond to roll control input, and how do I get them to do so?  They will do so if I rotate them 'straight' but there's no reason they shouldn't work as is.

[bzimac]

The only thing I can think of is to check in the hangar if roll input is allowed for them.

As for rotating, I'm not sure what you mean exactly. They should be effective the way they are on the picture.

If nothing does it, try adding another pair and have separate elevators and ailerons. Hope some of this works.

[Plusck]

I'm pretty certain it's because of their axis with respect to the CoM of the ship.

KSP decides which control surfaces it will use for each axis of rotation depending on the control axis with reference to the CoM.
If you look at the control axis of your elevons compared to a line drawn from the CoM of the ship, you'll see the angle is nearly 90°.* Therefore, as far as KSP is concerned, those control surfaces cannot roll the ship. Each has a much greater change on the ship's pitch, and the fact that their combined input will cause a roll (as a human can easily see) is not taken into account by the game.
 

* edit: I realise this isn't a great description, but I'm struggling to find a more logical way of putting it. Clearly KSP is deciding that the torque applied by the control surface is not in the right axis, but I don't have any idea as to how it does that, and I'm at a loss as to how you could describe it properly. Maybe not every axis is treated in exactly the same way? :shrug:

Edited September 15, 2016 by Plusck

[Corona688]

Roll input is allowed for them, it just doesn't work.

If I add another pair, they also do not function.

As you say, the game is clearly wrong in this decision.

How can it be overridden?

Edited September 15, 2016 by Corona688

[Plusck]

19 minutes ago, Corona688 said:

How can it be overridden?

I don't think it can.

Since roll authority never needs to be huge, I'd just add a pair of the very smallest elevons to the far end of the wing, and rotate them so that they are facing mostly (or wholly) forwards. Aesthetically displeasing, perhaps, but better than rotating the whole of the large elevons.

Also, if your CoM moves forward as fuel tanks are emptied, the sufficiently-rotated elevons should ensure that the game doesn't decide to stop using them for roll input (which would be a danger if you just rotated the large elevons far enough to work when fully laden).

Actually, you could probably clip them wholly into the outside centre of the wing. That won't eliminate them for drag purposes, but might be less uglifying.

Edited September 15, 2016 by Plusck

[Corona688]

So, bug?

[Plusck]

2 minutes ago, Corona688 said:

So, bug?

For that, you'd need to work out why a rudder will be deemed "usable" for roll, but your elevons aren't. As I said, I'm at a loss to explain exactly why...

[Corona688]

So, bug.  OK.

[Corona688]

For future struggling googlers, the "swept elevons" don't appear to suffer this problem.  Hooray.

I take that back.  They move only slightly.

[Plusck]

6 minutes ago, Corona688 said:

For future struggling googlers, the "swept elevons" don't appear to suffer this problem.  Hooray.

Good to know.

 

I made a test:

Those are all Elevon 4s. The left side and vertical wing have the innermost elevons flush with the surface and the outer ones rotated 10° forwards. The right side have the opposite (inner elevons rotated, outer ones flush with the angled wing edge).

On max roll trim, the inner flush ones on the left and centre are all activated the wrong way; the rotated ones are all activated the right way; the flush ones on the right hand side are activated the right way. And the inner ones are all deflected less than the outer ones.

So there you go.

[Corona688]

I just had to change the angle of the wing to work around the bug.

[Boris-Barboris]

1 hour ago, Corona688 said:

So, bug?

Yes, pretty old one. AtmosphereAutopilot's control surface module can help with that, if you don't mind reconfiguring toggles on your old crafts after install. If you do, rotating those surfaces is the solution as you already noticed.