# About the Inertia of the model

• Hello everyone,

Reading the manual I understood that Inertia is a parameter that is already as a default to AF, but I would like to understand

how I can input data related with the inertia of the model. I understand that as in all model data eveything is related with 3D axis,

In the example model PT40 we have this data:

<[float64][Mass][1.600000]>

<[tmvector3d][InertiaLength][1.150000 0.100000 0.150000]>

<[tmmatrix3d][Inertia][0.004333 0.000000 0.000000 0.000000 0.179333 0.000000 0.000000 0.000000 0.177667]>

<[tmvector3d][R0][-0.034669 0.000000 0.000000]>

<[tmmatrix3d][B0][1.000000 0.000000 0.000000 0.000000 1.000000 0.000000 0.000000 0.000000 1.000000]>

<[tmvector3d][C0][0.000000 0.000000 0.000000]>

I understand [Mass] that is the total weight of that specific part of the model.

What exactly mean [InertiaLength]?

Why [Inertia] have 9 fields of data, if we are working with 3 axis?

What are the Labels

[R0] = ?

[B0] = ?

[C0] = Initial Inertia, so obviously it should be 0 in all axis

I believe that this is not so important for the overall model's performance but I would like to know and I can not get it from the manual that I translated from German.

Edited once, last by Aboim2 ().

• Official Post

If you remove the line Inertia for the 3d inertia tensor you make your life much easier.

InertiaLength is then used instead to calculate the Inertia 3x3 matrix. At least it worked like that for a long time, it should still be like that as far as I know.

When you save the aircraft again I think it also saves the Inertia values for you.

InertiaLength is much easier to explain than the individual Inertia values.

The x, y and z values represent the dimensions in meters of an imaginary box. x is the box length, y is the box width and z is the box height.

You can take the actual fuselage dimensions for this as a good first educated guess. If your fuselage is 1.15 meters long, 0.1 meters wide and 0.15 meters high you get the inertia values that you posted: [1.15 0.1 0.15]

Increasing the x value of the InertiaLength will make the fuselage accelerate slower when you try to pitch up or when you try to yaw the aircraft. If you decrease the values the aircraft will be more agile for pitching up and down or for yawing.

R0 is the center of mass

B0 is the attitude (3x3 orientation matrix) - for wings we usually create a rotation around the longitudinal axis to account for the dihedral angle. And you can rotate it around the vertical axis as well to account for the wing's sweeping angle. I programmed a tool to generate these matrices: https://www.aerofly-sim.de/dow…rotation-matrix-generator

C0 line can be removed. This is to offset the broadcasted center position. If you do this you have to use the "Shift" property in your graphics objects. It can cause sound to be played from a different location as well. This is for advanced users and has no impact on the aircraft flight behavior at all.

Regards,

Jan

• Hi Jan,

Thank you for your reply.

I'll follow on from there.

EDIT-

It didn't run on Windows 10 but as a beginner I'll take the default values from my model guide.

Thank you guys for the massive help anyway!!

Re-Edit:

So sorry but I thing I am very excited with the all process and I didn't give the time to my antivirus run throught the file!

It is working alright!

best regards,

Carlos

Edited 2 times, last by Aboim2 ().

• Hi again,

And I am sorry for so much post but I am really new to model building!

How do you calculate the wings angle for example, in AC3D if you know

or if you know other tip or tecnique to find it in the 3D space!

Thanks

Carlos