Aircraft Lawn dart despite CoL in front of CoM.

[Frozen_Heart]

I'm trying to build something out of Mk-3 parts vaguely resembling a passenger airliner.

Trouble is my aircraft is nosediving into the ground as though the CoM is too far forward.

Except its not. The CoM is behind the CoL and the aircraft is still nosediving. Does anyone have a clue about how to solve this issue as nothing I do prevents this.

I've tried: Adding counterweights to the tail, tilting the wings to give them more lift, moving the wings forward until it looked stupid. None of this worked, no matter how far I moved the CoL and CoM.

Edited March 10, 2015 by Frozen_Heart

[Docsprock]

CoL should be slightly BEHIND the CoM.

[Frozen_Heart]

CoL should be slightly BEHIND the CoM.

That makes it more stable. This is already so stable it will nose down and head straight for the ground.

[mudkest]

what happend with com when the fuel tanks get drained?

move wings a bit forward? what about adding a canard?

maybe your wings are rotated too much? theye have a pretty big aoa, move your nose up a bit and it gets even worse, might be theye are stalling

Edited March 7, 2015 by mudkest

[Frozen_Heart]

what happend with com when the fuel tanks get drained?

move wings a bit forward? what about adding a canard?

maybe your wings are rotated too much? theye have a pretty big aoa, move your nose up a bit and it gets even worse, might be theye are stalling

I haven't tested with tanks drained yet. I've yet to add wheels and am just hyperediting it into the air.

I've tried moving the wings forward. They pretty much have to be hanging off the nose before this behaviour stops. :/

And I tried the wings level first. That didn't work so I rotated them a bit to try to help which didn't work either.

I've been building aircraft in KSP for 2 1/2 years and have never encountered this behaviour before. Previously moving the CoM and CoL around solves the issue.

[kahlzun]

hyperedit can muck up the physics, so fair warning.

[eddiew]

That's why your aircraft is lawn darting. Rather than centre of lift, imagine it as the centre of air pressure. If the CoL is in front of the CoM, then the plane is more stable when flying backwards.

Also, check your control surfaces and add canards if necessary. Pitch forces should come from the nose or tail, the wings won't have any significant pitch leverage

*edit* I've probably advised you as if you have FAR running, which you may not have. If not, and you want to make planes, then really you're just punishing yourself by not using it

[Red Iron Crown]

I would move the wings aft until the CoL is just behind the CoM. Then I would angle the horizontal tail surfaces down a little bit, this will have the effect of adding some inherent nose up "trim".

[Frozen_Heart]

I would move the wings aft until the CoL is just behind the CoM. Then I would angle the horizontal tail surfaces down a little bit, this will have the effect of adding some inherent nose up "trim".

This has seemed to work. It only flys at a certain speed before pitching up or down but at least it flys at all.

Had to empty most of the fuel as well to get it to lift off from the runway.

[LethalDose]

Why does no ever bring up pitch authority when discussing lawn darting?

OP, do you have susbstantial pitch control surfaces on your tail/empanage? If you don't have a sufficient lever arm to torque the nose up, that combined with the high AoA wings will probably lead to a ver stable nose down flight profile.

EDIT: EddieW did. Good.

[Northstar1989]

Assuming you're using FAR (as was already suggested- you're punishing yourself without it), your problem appears to be that you lack pitch control.

Just looking at your plane, I can visually tell this is the problem. All your elevators are located on the lagging surface of the wings- very close to the Center of Mass. You want your control surfaces to ideally be as far from the Center of Mass as possible in order to maximize control-authority.

I wouldn't suggest removing those control surfaces- they're actually well-positioned to make great flaps or spoilers (they're close to the Center of Mass and Center of Lift, so they won't significantly destabilize your pitch-moment while being used to increase or decrease lift).

But if you want to actually control your plane's Angle of Attack, you need to place control-surfaces oriented to control pitch on either the very front or the very rear of your aircraft. I suggest the front (canards) because they will be less likely to cause the tail to rip off at excessively high dynamic pressures there, and will shift your Center of Mass slightly forward (the canards have mass, after all). But if you want an airplane that looks more like a commercial airliner, place the elevators on the tail instead...

Regards,

Northstar

[Sampa]

...just hyperediting it into the air.

Add wheels and take off from the runway like a normal plane.

[Lhathron the Elf]

If I may, where are the engines? And how many do you even have?

[kujuman]

I have this problem too with the Mk3 parts. I think part of it is that the Mk3 parts are so much more massive the pitch surfaces just don't scale up large enough. On smaller planes, the cockpit torque can compensate for bad aerodynamics: mk3 cockpit just doesn't have the same amount or relative torque.

My solution has been to do what real airliners do and have an all moving tail for trim (attach the horizontal stabilizaer to the empennage with a IR rototron or the like) and then I can use the rototron to trim for speed/alt and use the elevator for fine control.

[dryer_lint]

Canards usually seem to fix control issues... unfortunately, they look ugly. Especially on an airliner.

[Frozen_Heart]

All your elevators are located on the lagging surface of the wings

That's the ailerons. The pitch control is on the tail of the aircraft on mine.

I suggest the front (canards)

Canards usually seem to fix control issues

I'm trying to make it look like a conventional jet liner. Canards usually help but just don't fit the look.

If I may, where are the engines? And how many do you even have?

4 engines under the wings. The screenshot is a glide test.

Add wheels and take off from the runway like a normal plane.

If its not even capable flying, how am I expected to take off? Hyperedit allows me to sort out aerodynamics first.

Anyway I found what was wrong. The huge weight of the Mk-3 part just meant it was much too heavy. Draining most of the fuel meant it was about a quarter of the weight and could still make it halfway round the world.

Undercarriage looks ugly but next updates comes with a better selection.

[cantab]

The stability derivatives page in the FAR analysis lets you put in a height and a Mach number, and one of the readouts is angle of attack. That's what you need for level flight, and if it says 0 that's code for the plane CAN'T fly level at that speed and altitude.

[Red Iron Crown]

Judging by the screenshots the OP is not using FAR, so those readouts are not available.

[Frozen_Heart]

Judging by the screenshots the OP is not using FAR, so those readouts are not available.

No I've always used the stock aero. I share craft and do turn based multiplayer with other members so FAR would prevent that.

And why are all the Mk3 cargo bays without textures? They've been like that the whole time.

[LordFjord]

Could it be a similar issue as I had here?

Try to grab your root part and rotate it, check if the CoL moves in an odd way (what it did in my case).

To the white cargo bays:

There have been several support tickets for that. here is one.

I had that once as well, a steam "validate game files" thing fixed it for me.