Plane pulling

[locustgate]

So I'm trying to circumnavigate Kerbin, and collect sciency stuff from every biome on the way. I've run into a minor problem with my current design, the plane likes to drift between 83-89 degrees and likes to tilt diagonally /. Any idea how to fix both, I'm thinking adding more vertical lift.

[Anquietas314]

We're going to need screenshots of the plane showing center of mass, center of lift and center of thrust to accurately diagnose the problem. Also, are you using stock aero, FAR or NEAR (or anything else)? I doubt adding lift will fix your problem though; it sounds like either asymmetric thrust or dodgy(/missing) rudder placement.

From your screenshot, the design seems to be mostly fine. What altitude does the issue occur at? If it's above say 15km then it's probably the early stages of asymmetric flame-out. You can use the intake placement trick (there's a thread in the tutorials forum), or the intake build aid mod (far less tedious) to fix that.

I should add: if your plane is yawing left or right, it will generally tend to bank (lean left/right) as well.

Edited January 9, 2015 by armagheddonsgw

[Wanderfound]

Move the rudders as far back as you can. The distance between a control surface and CoM provides leverage.

[MarvinKitFox]

Move your rudders back, for more positive control.

Move your Air intakes back, as they are actually mostly in front of your CoM

This places your Center of Drag in front of your CoM.

This is a bad, bad, bad idea.

Moving the high-drag intakes back will fix this.

(that cockpit will help offset this,but still....)

P.S.

As for the rolling action...

I see you are using a single set of control surfaces for both roll and pitch control?

This ...can... work, but it is much better to have separate controls for each function of pitch, jaw, roll control.

You want your pitch controls far back(or front), and as close to the centerline as practical.

You want your roll controls a far out from the centerline as possible, and pretty close to the CoM in both the forward-aft-line and the belly-roof orientation. (i.e. aligned with CoM viewed from the side, but way left-right of it)

You want yaw control at the rear, in line with the CoM, and a bit *above* the CoM. This makes a Yaw initiate the correct bank to go with the resulting turn, and keeps your belly pointing towards gravity.

Edited January 9, 2015 by MarvinKitFox

[locustgate]

We're going to need screenshots of the plane showing center of mass, center of lift and center of thrust to accurately diagnose the problem. Also, are you using stock aero, FAR or NEAR (or anything else)? I doubt adding lift will fix your problem though; it sounds like either asymmetric thrust or dodgy(/missing) rudder placement.

From your screenshot, the design seems to be mostly fine. What altitude does the issue occur at? If it's above say 15km then it's probably the early stages of asymmetric flame-out. You can use the intake placement trick (there's a thread in the tutorials forum), or the intake build aid mod (far less tedious) to fix that.

I should add: if your plane is yawing left or right, it will generally tend to bank (lean left/right) as well.

It likes to pull to the left on the runway, and stay at 89 until I hit 10km, then it pulls to 83 for about 15km. No mods other than scansat, final frontiers, and chaterer.

[GoSlash27]

Judging by the pic, your rudders are too close to the center of mass. Try moving them back to engines 2&5.

Also, they should not be actively controlled surfaces, but rather plain old fins.

Best,

-Slashy

[Anquietas314]

It likes to pull to the left on the runway, and stay at 89 until I hit 10km, then it pulls to 83 for about 15km. No mods other than scansat, final frontiers, and chaterer.

Don't worry too much about the runway - almost any design doesn't quite behave on the runway; yay physics glitches. As long as it's only a slight tendency to turn left or right it doesn't matter very much; if you enable turning on the front wheels it should be easy to correct it (be careful!).

Above 10-15km intake air distribution is probably at least part of your issue; try the solutions suggested. You may want to make the intake types a little more evenly balanced to the number and type of engines you have as well; as they are, it's going to be difficult to get them balanced properly since the shock cones have much more intake area than the radial ones. Even if you can't get them balanced perfectly, making sure the "assignment" is symmetrical should be good enough to keep it stable. Technically it should fly fairly well as it is if you can make thrust stay symmetrical, but it's not going to be the most abuse-tolerant design ever .

The intakes are "close enough" to the center of mass (especially given most of the radials are just behind it) to not be a serious concern given the cockpit etc, but it would certainly help with stability to move the radial ones as far back as possible. As others have said you could try moving the rudders further back too, although realistically it'll just give the plane more yaw authority to overcome minor tendencies to yaw left or right.

A couple things I didn't address before:

1) Can it lift off the runway cleanly before flying off the end of it? If not, add more wing parts. If so, never mind.

2) Later in the flight (particularly when everything's nearly empty) your plane is likely to have issues due to fuel arrangement; the center of mass will move backwards until it's behind center of lift. For designs with a lot of control authority that's not necessarily a major problem, but in your case it'll make the plane want to pull up, possibly beyond your control.

Sliding the side tanks forward so the center of mass is nearer the middle of the plane, as well as emptying the tanks nearest the nose and tail of the plane should help with that. If all of those tanks are full, you have way more fuel than you need on that thing to circumnavigate Kerbin at least twice in a single flight .

I don't see any rocket engines on the design either, so be sure to drain the oxidizer from the bicouplers unless it helps with balancing; it's just excess weight. Same goes for the monoprop in the cockpit .

[locustgate]

A couple things I didn't address before:

1) Can it lift off the runway cleanly before flying off the end of it? If not, add more wing parts. If so, never mind.

2) Later in the flight (particularly when everything's nearly empty) your plane is likely to have issues due to fuel arrangement; the center of mass will move backwards until it's behind center of lift. For designs with a lot of control authority that's not necessarily a major problem, but in your case it'll make the plane want to pull up, possibly beyond your control.

Sliding the side tanks forward so the center of mass is nearer the middle of the plane, as well as emptying the tanks nearest the nose and tail of the plane should help with that. If all of those tanks are full, you have way more fuel than you need on that thing to circumnavigate Kerbin at least twice in a single flight .

I don't see any rocket engines on the design either, so be sure to drain the oxidizer from the bicouplers unless it helps with balancing; it's just excess weight. Same goes for the monoprop in the cockpit .

Yes, when it hits 140, about halfway.

Haven't gotten farther than across the pond. need to move some parts forward it is rear heavy.