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MSFS20 How many coordinate systems does an MSFS aircraft need?

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If I have counted correctly, there are at least three involved in aircraft creation and debugging:

Firstly, the 3D model. The SDK says In the Z axis the center should be approximately where the water level would be. Meaning that positive Z is upwards. Which matches what most people would call the Z axis, I guess. And positive X is to the left, positive Y is towards the back of the aircraft. A right-hand coordinate system. OK.

Then, when talking about the aircraft once inside the sim, the SDK says longitudinal (z), where to the front is positive and to the back negative, lateral (x), where to the right is positive and to the left negative, vertical (y), where vertically up is positive and to the down is negative. This now suddenly then a left-hand coordinate system, and all axes different from those in the 3D model. Fun (not). But I guess there are some legacy reasons for this. And I guess the idea is that different people work on the 3D model and the flight model anyway. (Which surely is true at companies like Asobo or Aerosoft, but is not the case for individual add-on developers.)

But that is not all. If you look in the “Debug Aircraft Weight” window from the Aircraft Editor, there suddenly is this text, applying to part of (or all?) of the values: Negative is forwards! Offset backwards from Datum. Ouch.
 
I kinda have to agree with you, the coordinates are negative in the long(z) axis. And it does seem strange. But the values seem to be correct. There are three section ay that point.
Offset backwards from Datum
Offsets backwards from Plane front (propeller)
Offsets backwards from wing leading edge

Makes for difficult debugging your cfg files.
 
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Um, you guys know that during model creation in Max or Blender that the orientation of the aircraft is precisely backwards from what we use(d) for FS9, FSX and Prepar3D models, right? Seems to me to be a bit bass-akwards to me, but there it is...
 
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n4gix said:
Um, you guys know that during model creation in Max or Blender that the orientation of the aircraft is precisely backwards from what we use(d) for FS9, FSX and Prepar3D models, right? Seems to me to be a bit bass-akwards to me, but there it is...
Click to expand...
Actually I didn't know that. (I have never created aircraft (or anything else) for simulators before/other than MSFS, even if I have used at least FS2000, FSX, Fly! back in the days, and more recently X-Plane.)

In fact, that is one thing that is slightly (just a bit) annoying when following some tutorials etc that they keep pointing out how some things differ from FSX. Well, that is of no interest to me personally. But sure, I understand that there are a lot of people for whom that is important.
 
The classic body-fixed aerodynamic coordinate system used in real airplanes has X positive forwards, Y positive right and Z positive down. This is known as the right-hand rule. The airplane rolls about the x-axis, pitches about the y axis and yaws about the Z axis
Body-fixed is used to simplify the equations used and these results are transformed to Earth-fixed coordinates to update the airplane position and orientation for display.
The original Microsoft simulator versions and P3D use a different system for body-fixed with X positive right, Y positive up and Z positive forwards. The airplane rolls about the z-axis, pitches about the x axis and yaws about the y axis. This is known as the left-hand rule and is what MSFS uses. It is the same as the system used with computer graphics, so it makes sense.

Generally the centre of the coordinate systems is the airplane Center of Gravity, or at least a nominal one because loads make that a non-static parameter and you have offsets. Practically, CG positions tend to be given as distance aft of some feature on the fuselage so that weighing the airplane and calculating the results is simplified. This also makes sense moving the CG forwards makes the nose go down, just like in a boat.

Then you get to things like models and texture which have nothing to do with aerodynamics and they like to have things transposed left-right and up-down. I have ignored these since I gave up modeling.

So the one you pay attention to depends on what you are doing. Flight dynamics uses the left hand rule as in computer graphics. Modeling uses something else designed to confuse (which is why I gave up).
Roy
 
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