Landing gear misaligns my plane's CoT from its CoM.

[NimbleJack3]

I'm currently building an air-breathing plane entirely out of stock parts, intended for atmospheric use. Thanks to keptin's Basic Aircraft Design tutorial I'm well on the way to a functional iteration, but aligning the plane's center of mass and center of thrust is very difficult.

It's easy enough up until the point I add the landing gear. At 3 x 0.5 units of mass it is heavy enough to pull the CoM down and out of alignment with the CoT, causing terrible pitch control problems in-flight. The only workable solution I've found is to add a 1.5 unit counterbalance to the top of the fuselage, but this is a poor fix and one that increases the overall mass of my plane. Not desirable.

What solutions are available?

Fig 1: The plane in question, seen from three-quarter view.

Fig 2: The same plane's CoM, CoL and CoT in profile. The plane is pointing to the left. The slightly raised CoL is intentional as it increases plane stability.

Edited November 1, 2013 by NimbleJack3

[Hodo]

Simple fix.

Add a counter weight to the top of the craft.

The funny thing is, I have never had this problem and I build planes all the time in FAR.

[tavert]

In the stock game, the landing gear are currently massless. Ignore the indicators in the SPH after you put the landing gear on.

[numerobis]

Landing gear has no such effect in flight. Build your plane, align everything, then slap on landing gear. As soon as you launch, suddenly the gear has no mass (and thus no drag).

[Scarecrow88]

I think you are trying to be too analytical. Put the thing on the runway, hit the go button and fly the thing. If it handles like a bucket of rusty nails, then maybe have a look at making a few tweaks, but until you know what it handles like in it's current form, you may be stressing over nothing.

[NimbleJack3]

In the stock game, the landing gear are currently massless. Ignore the indicators in the SPH after you put the landing gear on.

Landing gear has no such effect in flight. Build your plane, align everything, then slap on landing gear. As soon as you launch, suddenly the gear has no mass (and thus no drag).

Ah, I see. So my handling problems come from the design at large. Damn, back to the drawing board. Thanks for all your help.

[lipatden]

Assuming the gear does have mass (I cannot confirm), move the engines up or down to bring CoT in line with CoM.

[Epthelyn]

Oops, double posted for some reason :S

Edited November 1, 2013 by Epthelyn
double posted

[Epthelyn]

Have you done a test flight? Even with a plane that small the difference between the two markers is small enough such that it shouldn't cause any problems.

Oh, and, from SmallGearBay part.cfg

// --- standard part parameters ---

mass = 0.5

dragModelType = default

maximum_drag = 0.3

minimum_drag = 0.2

angularDrag = 1

crashTolerance = 7

maxTemp = 3600

crashTolerance = 50

breakingForce = 50

breakingTorque = 50

[Hodo]

Have you done a test flight? Even with a plane that small the difference between the two markers is small enough such that it shouldn't cause any problems.

Oh, and, from SmallGearBay part.cfg

// --- standard part parameters ---

mass = 0.5

dragModelType = default

maximum_drag = 0.3

minimum_drag = 0.2

angularDrag = 1

crashTolerance = 7

maxTemp = 3600

crashTolerance = 50

breakingForce = 50

breakingTorque = 50

They show as having mass but I think that is only when deployed.

[tavert]

Test it. Make a small rocket with TWR just barely over 1. Note its mass in the info panel on the map screen on the launch pad. Go back to the VAB, add a whole bunch of landing gear, and try again. Same mass, same TWR, everything flies more or less the same.

[vetrox]

Although I dont build small planes such as yours but your col looks to be quite a bit behind your com. Considering that your com is already quite close to the rear of your plane this could be the ptoblem

[KvickFlygarn87]

In the stock game, the landing gear are currently massless. Ignore the indicators in the SPH after you put the landing gear on.

...No they're not? They weigh 0.5t each!

http://wiki.kerbalspaceprogram.com/wiki/Small_Gear_Bay

[tavert]

...No they're not? They weigh 0.5t each!

http://wiki.kerbalspaceprogram.com/wiki/Small_Gear_Bay

Yes they are. From that same page,

Unlike landing legs, landing gear mass is not counted by the physics engine as of 0.19.1. This is similar to that behavior of struts and fuel lines.

And as I said, test it. The numbers in the cfg files and VAB/SPH tooltips are not always interpreted the same way by the game's physics engine.

[m4v]

Landing gear has no mass or drag during flight. You can quote the part.cfg settings and whatever, but the part gets its physical significance set to none when the flight starts. Same happens with ladders, but since their physical significance isn't hacked in the code (but defined in the part.cfg) it doesn't mess things up with the editor markers.

Edited November 1, 2013 by m4v

[Tex_NL]

Why bother with discussing whether landing gear has mass or nor? IT DOESN'T MATTER ! Even IF mass was calculated for landing gear it wouldn't have any significant effect. If a plane can't fly with such a tiny vertical imbalance it wasn't going to fly anyway.

[numerobis]

The imbalance is presumably much, much worse than presented in the OP: the center of lift and of thrust won't change, but the center of mass is going to move a lot given that there's 1.5t of false mass.

[Vanamonde]

An imbalance of that sort is not an unrealistic complication for aircraft design, but there are several things you can do about it. Move the engines slightly down so that they apply a bit of nose-up torque, or angle the tailplanes slightly (5 degrees at a time if you hold shift while hitting the WASDQE keys) to push the tail down and therefore the nose up. But mostly, just keep tinkering with it until it works.

[NimbleJack3]

I have indeed performed a test flight (quite a few test flights, at least once every iteration) and my current problem is a tendency to slowly pitch down and lose altitude. There seems to be enough lift to keep the plane aloft so I assumed that my problem was an off-center CoT giving the plane angular momentum.

EDIT: I also can't move the engines down - they're aligned with the same part of the fuselage that the landing gear is attached to, and KSP doesn't support translating just the engines.

[Vanamonde]

I'm not clear on the part arrangement you're describing, but if you want to see how the engines can be placed below fuselage centerline, take a look at these planes: http://forum.kerbalspaceprogram.com/threads/25413-Trainer-and-Example-Aircraft-for-Newbie-Pilots-(updated-for-21-1) I haven't tested their flight characteristics in 22, but you can download them and disassemble them for the purposes of reverse engineering.

[Javster]

Have you done a test flight? Even with a plane that small the difference between the two markers is small enough such that it shouldn't cause any problems.

Oh, and, from SmallGearBay part.cfg

// --- standard part parameters ---

mass = 0.5

dragModelType = default

maximum_drag = 0.3

minimum_drag = 0.2

angularDrag = 1

crashTolerance = 7

maxTemp = 3600

crashTolerance = 50

breakingForce = 50

breakingTorque = 50

But it has physicsSignificance = 0

[RexKramer]

Here's a way to check the effect of landing gear mounted above or below your COT. Mount an identical set of landing gear above the craft, exactly above the location of the lower gear assemblies. Fly and check results.

I'm guessing the location of the gear is not the root of your control issues. I'd start by investigating the forward-aft relationship between COM and COL, and how that changes as fuel is burned.

Next, are you turning on SAS? I find that extremely helpful. Short vessels with COM and COL very close together can be difficult to control, they rotate very readily and are easy to pitch rapidly past your target.

To add pitch stability, try moving your COL a little further aft, and increasing the arm of your pitch control surface- with a traditional elevator move the elevator location further aft. (With a canard style pitch actuator, move it further forward. In a real aircraft, increasing pitch stability this way would come with a small drag increase, but I'm not sure if KSP would give a significant amount of extra drag.

Good luck!

[Claw]

Here's a way to check the effect of landing gear mounted above or below your COT. Mount an identical set of landing gear above the craft, exactly above the location of the lower gear assemblies. Fly and check results.

I'm guessing the location of the gear is not the root of your control issues. I'd start by investigating the forward-aft relationship between COM and COL, and how that changes as fuel is burned.

Next, are you turning on SAS? I find that extremely helpful. Short vessels with COM and COL very close together can be difficult to control, they rotate very readily and are easy to pitch rapidly past your target.

To add pitch stability, try moving your COL a little further aft, and increasing the arm of your pitch control surface- with a traditional elevator move the elevator location further aft. (With a canard style pitch actuator, move it further forward. In a real aircraft, increasing pitch stability this way would come with a small drag increase, but I'm not sure if KSP would give a significant amount of extra drag.

Good luck!

(BTW, This thread dates back to November.) All of what you say is valid, but KSP has a few quirks. Such as the fact that landing gear actually have no mass or drag.

So the landing gear isn't going to cause pitch stability issues, except that it will cause the CoM in the SPH to lie, since the CoM marker takes landing gear mass into account. In flight, landing gear have no mass or drag effects so if you plan very little margin between CoM and CoL (likely for a small plane), KSP will bite you because the in-flight CoM usually ends up slightly aft of the SPH position.

Edit: Here's an example of what I'm talking about. This thing supposedly weighs 35 tons.

[MisterFister]

Why bother with discussing whether landing gear has mass or nor? IT DOESN'T MATTER ! Even IF mass was calculated for landing gear it wouldn't have any significant effect. If a plane can't fly with such a tiny vertical imbalance it wasn't going to fly anyway.

Sorry to reply so late in the thread's lifecycle, but this reply above didn't receive enough attention. Frankly, it's shortsighted in terms of scope -- if the OP were discussing a CoL / CoM imbalance of the size we're discussing, and the plane massed hundreds of tons, dozens of meters wide, five or six meters tall, etc., then this Co[X] discrepancy as a function of overall fuselage length, wing chord length, and the length of distribution of pitch authority surfaces fore and aft... then yes, this would be an insignificant imbalance.

But look at the photo in the OP, d00d. He's designing a piloted mosquito for crying out loud -- the margins are razor thin, and the imbalance we're discussing here is ginormous as a proportion to the overall vehicle size.

Be nicer next time.