Center of mass calculated wrong

[Skeltek]

I just noticed trying to construct an airplane.

I got the center of mass pretty much in the middle of the craft.

Attaching 2 heavy engines in the far back of the craft actually moves the shown center of mass forward instead of backward

It is really a pain to balance out all the craft and center of mass, lift and drag, just to see the attachment of engines at the far end of the plane to actually move the center of mass forward to the cockpit.

Anyone else can give me a hint on what that is all about?

Apparently it seems the mass is added at the location, where the frontmost part connects to the mid section.

Dont know how to upload pictures here and my dropbox doesnt give me a direct link to the content, so heres the dropbox links:

https://www.dropbox.com/s/lers4y0046nox9y/WithoutEngines.jpg?dl=0

https://www.dropbox.com/s/20001fi163jjuu2/WithEngines.jpg?dl=0

As you can see, the center of mass with the engines attached is significantly futher front instead of the back.

The game just seems to add the engine mass to the fuel containers attachment point and forgets all about the z-axis difference.

Also the center of drag and center of lift behave strange for me it seems(nosecone-effect-like, but with the exactly opposite of expected result), but Im still trying to figure out the exact reason.

[Red Iron Crown]

Jet engines have their centers of mass ahead of the part itself. Try placing a jet engine only in the VAB/SPH and turning on the CoM indicator and you'll see.

I believe this was done to make balancing more conventional aircraft easier (i.e. cockpit in front, engines at rear).

[Skeltek]

"Belief" is a squeeky little devil.

After some futher testing and excluding a glitch with the nodebased linkage system of vehicle parts, I was convinced they just got the sign of the z-axis wrong for the engine.

It is as hard as it is already to get the center of mass towards the back of the craft. I dont think it would had been a wise decision to implement it like you describe.

With larger crafts a shift of weight towards the front by just a few inch doesnt help much at all. And with smaller craft it is just nearly impossible to balance.

With most real aircrafts, most the fuel is located inside the wings, so center of variable fuel mass is automatically pretty close(slightly in front) to the center of lift.

Inside the game for small craft it is especially difficult to have something behind the fuel tanks to balance out the cockpit mass in front of the craft.

Basically you have the cockpit in front, a fuel tank in the middle and the engines in the back, so the aproximate center of mass if about where the fuel tanks are.

With how it is now, you have to place some placeholder or something else behind the fuel tanks to make the engines able to shift the center of mass backwards; effectively making your "small aircraft" too long to be really able to lift off safely with the small landing gear keeping minimum distance from ground...

[Red Iron Crown]

Why would you want the CoM closer to the rear of the craft? That makes it harder to make stable.

[Skeltek]

You dont get it... I want it in the middle of the craft and not at the very front.

[KerikBalm]

CoM on jet enjines is offset forward.

This makes things much easier in the majority of cases.

In your case, it would still be beneficial, as your CoM won't change as much when the fuel is consumed.

All the other engines behave as you would expect.

Think of it has the engine being forward, inside the structure it is attached to, and all you see is the nozzle.

RL engines don't look like the engines we have.

"Realistically" (Yes, I know it is a game), the engines should have to be directly behind a precooler/engine nacelle - which do look like actual turbine engines - and then you just place the exhaust nozzle to determine what kind of jet it is...

[Chaos_Klaus]

The problem with the jet engines has been that the part only depicts the visible nozzle. An actual jet engine would extend far into the fuselage. That's why the center of mass is where it would be if you imagined the engine with all its components.

With out that, jet engines shifted CoM too far back. Even for your vessel, having CoM further foward will prabably greatly benefit the flight characteristics.

It's not a bug, it's a feature!

[DocMoriarty]

I disagree with the COM of engines being moved forward completely. It doesn't make things easier and leads to retarded looking aircraft actually. And who can't get his alignments right with the old and better COM won't get them right with the new one either! The most superflous change of all time!

Example aircraft: I even reversed order of elevators and main wings on this craft but the main wings are still at middle of the craft thanks to the new totally misaligned COM. On a real craft the main wings would be way back at the rear end of this plane with this configuration. And with this new COM way outside the jet engine it's actually impossible now to build well aligned and balanced VTOL's anymore. Also the argumentation that a real jet engine also has a body is wrong either because you need to attach stuff to the engine that makes it look like a real jet engine, tanks or intakes, too which have their own weight and move COM forward anyway.

And to top it off, it's totally unintuitive and not the first post about that topic! Change it back already!

Edited September 12, 2015 by DocMoriarty

[NathanKell]

Jet nozzles don't tend to mass over a ton. That usually involves an engine.

[GoSlash27]

Jet nozzles don't tend to mass over a ton. That usually involves an engine.

^This.

The mass offset for engines is fine as it is.

Best,

-Slashy

[sal_vager]

If the CoM was on the end of this...

It'd be rather inaccurate, there's a lot of metal in front of the nozzle, and the offset CoM simulates where the KSP jet engine CoM should be.

Of course, if you want to make unrealistically stubby turbojets you're free to do so

Also, punted to S&D.

[cantab]

In my view this jet engine CoM offset is among the most worst decisions Squad has made. The centre of mass of an object is inside that object, or to be more precise inside the convex hull of that object. For it to be otherwise is unphysical nonsense. It's completely unexpected by the player and as this thread shows results in behaviour that looks like a bug. It's exploitable for further unphysical results, like rovers with a centre of mass underneath the ground they are driving on.

If the size, shape, and appearance of the jet engine is unrealistic then the *right* solution is to get a new model which is more realistic. Not to do a hack that breaks basic laws of physics. Now 1.1 is getting new jet engines from Porkjet, and I really hope they have a centre of mass that isn't three feet outside them, but from what I've seen so far I'm expecting they still will.

(And if one wants, the node can be put halfway down the engine, allowing its guts to clip inside whatever part the player attaches it to so designs can be largely unchanged but this nonsense is done away with.)

[pincushionman]

What Squad has done is a step in the right direction. However, I can't disagree that it's klunky, unintuitive if you don't know what it represents, and subject to abuse if you really do.

What we need are dedicated "engine core" parts that contain the bulk of the engine mass and represent the power-generating portion of the engine system. They could do this by throwing a flag that is necessary for the nozzles to work, or by generating some kind of engine-power virtual resource that the output devices (the nozzles…or maybe propellers…or even wheels…) use to generate thrust (or traction…or electric charge…see where this is going? It's flexible). It's not as intuitive as stacking fuel tanks on an engine, but it's clear that the CoM offset is causing confusion.

[DocMoriarty]

I disagree. I could build lots of aircraft without that change and had no problems whatsoever. BTW, its not only a nozzle but also something else inbetween, like a tank or nacelle.

Edit: If the COM in this engine + nacelle + nozzle would be in the center of the nacelle it would be right. The current COM though doesn't make no sense whatsoever and is unintuitive for that matter, nobody would expect it there. This looks like the nozzle weighs 5 tons.

All jet engines should be modified so that the COM is right in the center of the nacelle when using the above assembly.

Edited September 13, 2015 by DocMoriarty

[Stoney3K]

What Squad has done is a step in the right direction. However, I can't disagree that it's klunky, unintuitive if you don't know what it represents, and subject to abuse if you really do.

What we need are dedicated "engine core" parts that contain the bulk of the engine mass and represent the power-generating portion of the engine system. They could do this by throwing a flag that is necessary for the nozzles to work, or by generating some kind of engine-power virtual resource that the output devices (the nozzles…or maybe propellers…or even wheels…) use to generate thrust (or traction…or electric charge…see where this is going? It's flexible). It's not as intuitive as stacking fuel tanks on an engine, but it's clear that the CoM offset is causing confusion.

^^^This. Already working on such a thing, having a set of interchangeable engine bodies and nozzles which just provide a "CompressedAir" resource, that is expelled through the nozzle you can attach wherever you want. This would be similar to real jet engines which can have air ducts and other plumbing to redirect the air flow if necessary.

It also allows you to close exhaust nozzles as well as intakes, so for low altitude flight, you could have a high-flow, low-pressure nozzle, and when you're supersonic, those exhausts are closed and the high-pressure exhaust opened, which is more efficient at high altitude. The nozzle only defines Isp, not mass flow.

Jet-powered RCS ("puffer ducts") and re-using the same engine core for both VTOL and horizontal flight is also a possibility then.