Documentation

# Simple Variable Mass 6DOF (Euler Angles)

Implement Euler angle representation of six-degrees-of-freedom equations of motion of simple variable mass

## Library

Equations of Motion/6DOF

## Description

The Simple Variable Mass 6DOF (Euler Angles) block considers the rotation of a body-fixed coordinate frame (Xb, Yb, Zb) about a flat Earth reference frame (Xe, Ye, Ze). The origin of the body-fixed coordinate frame is the center of gravity of the body, and the body is assumed to be rigid, an assumption that eliminates the need to consider the forces acting between individual elements of mass. The flat Earth reference frame is considered inertial, an excellent approximation that allows the forces due to the Earth's motion relative to the fixed stars to be neglected.

The translational motion of the body-fixed coordinate frame is given below, where the applied forces [Fx Fy Fz]T are in the body-fixed frame. Vreb is the relative velocity in the body axes at which the mass flow ($\stackrel{˙}{m}$) is ejected or added to the body in body axes.

`$\begin{array}{l}{\overline{F}}_{b}=\left[\begin{array}{l}{F}_{x}\\ {F}_{y}\\ {F}_{z}\end{array}\right]=m\left({\stackrel{˙}{\overline{V}}}_{b}+\overline{\omega }×{\overline{V}}_{b}\right)+\stackrel{˙}{m}\overline{V}r{e}_{b}\\ {A}_{be}=\frac{{\overline{F}}_{b}-\stackrel{˙}{m}{\overline{V}}_{re}}{m}\\ {A}_{bb}=\left[\begin{array}{c}{\stackrel{˙}{u}}_{b}\\ {\stackrel{˙}{v}}_{b}\\ {\stackrel{˙}{\omega }}_{b}\end{array}\right]=\frac{{\overline{F}}_{b}-\stackrel{˙}{m}{\overline{V}}_{re}}{m}-\overline{\omega }×{\overline{V}}_{b}\\ {\overline{V}}_{b}=\left[\begin{array}{l}{u}_{b}\\ {v}_{b}\\ {w}_{b}\end{array}\right],\overline{\omega }=\left[\begin{array}{l}p\\ q\\ r\end{array}\right]\end{array}$`

The rotational dynamics of the body-fixed frame are given below, where the applied moments are [L M N]T, and the inertia tensor I is with respect to the origin O.

`$\begin{array}{l}{\overline{M}}_{B}=\left[\begin{array}{l}L\\ M\\ N\end{array}\right]=I\stackrel{˙}{\overline{\omega }}+\overline{\omega }×\left(I\overline{\omega }\right)+\stackrel{˙}{I}\overline{\omega }\\ I=\left[\begin{array}{lll}{I}_{xx}\hfill & -{I}_{xy}\hfill & -{I}_{xz}\hfill \\ -{I}_{yx}\hfill & {I}_{yy}\hfill & -{I}_{yz}\hfill \\ -{I}_{zx}\hfill & -{I}_{zy}\hfill & {I}_{zz}\hfill \end{array}\right]\end{array}$`

The inertia tensor is determined using a table lookup which linearly interpolates between Ifull and Iempty based on mass (m). While the rate of change of the inertia tensor is estimated by the following equation.

`$\stackrel{˙}{I}=\frac{{I}_{full}-{I}_{empty}}{{m}_{full}-{m}_{empty}}\stackrel{˙}{m}$`

The relationship between the body-fixed angular velocity vector, [p q r]T, and the rate of change of the Euler angles, [$\stackrel{˙}{\varphi }\stackrel{˙}{\theta }\stackrel{˙}{\psi }$]T, can be determined by resolving the Euler rates into the body-fixed coordinate frame.

`$\left[\begin{array}{l}p\\ q\\ r\end{array}\right]=\left[\begin{array}{l}\stackrel{˙}{\varphi }\\ 0\\ 0\end{array}\right]+\left[\begin{array}{lll}1\hfill & 0\hfill & 0\hfill \\ 0\hfill & \mathrm{cos}\varphi \hfill & \mathrm{sin}\varphi \hfill \\ 0\hfill & -\mathrm{sin}\varphi \hfill & \mathrm{cos}\varphi \hfill \end{array}\right]\left[\begin{array}{l}0\\ \stackrel{˙}{\theta }\\ 0\end{array}\right]+\left[\begin{array}{lll}1\hfill & 0\hfill & 0\hfill \\ 0\hfill & \mathrm{cos}\varphi \hfill & \mathrm{sin}\varphi \hfill \\ 0\hfill & -\mathrm{sin}\varphi \hfill & \mathrm{cos}\varphi \hfill \end{array}\right]\left[\begin{array}{lll}\mathrm{cos}\theta \hfill & 0\hfill & -\mathrm{sin}\theta \hfill \\ 0\hfill & 1\hfill & 0\hfill \\ \mathrm{sin}\theta \hfill & 0\hfill & \mathrm{cos}\theta \hfill \end{array}\right]\left[\begin{array}{l}0\\ 0\\ \stackrel{˙}{\psi }\end{array}\right]\equiv {J}^{-1}\left[\begin{array}{l}\stackrel{˙}{\varphi }\\ \stackrel{˙}{\theta }\\ \stackrel{˙}{\psi }\end{array}\right]$`

Inverting J then gives the required relationship to determine the Euler rate vector.

`$\left[\begin{array}{l}\stackrel{˙}{\varphi }\\ \stackrel{˙}{\theta }\\ \stackrel{˙}{\psi }\end{array}\right]=J\left[\begin{array}{l}p\\ q\\ r\end{array}\right]=\left[\begin{array}{lll}1\hfill & \left(\mathrm{sin}\varphi \mathrm{tan}\theta \right)\hfill & \left(\mathrm{cos}\varphi \mathrm{tan}\theta \right)\hfill \\ 0\hfill & \mathrm{cos}\varphi \hfill & -\mathrm{sin}\varphi \hfill \\ 0\hfill & \frac{\mathrm{sin}\varphi }{\mathrm{cos}\theta }\hfill & \frac{\mathrm{cos}\varphi }{\mathrm{cos}\theta }\hfill \end{array}\right]\left[\begin{array}{l}p\\ q\\ r\end{array}\right]$`

## Parameters

### Main

Units

Specifies the input and output units.

UnitsForcesMomentAccelerationVelocityPositionMassInertia
`Metric (MKS)`NewtonNewton meterMeters per second squaredMeters per secondMetersKilogramKilogram meter squared
`English (Velocity in ft/s)`PoundFoot poundFeet per second squaredFeet per secondFeetSlugSlug foot squared
`English (Velocity in kts)`PoundFoot poundFeet per second squaredKnotsFeetSlugSlug foot squared
Mass Type

Select the type of mass to use.

 `Fixed` Mass is constant throughout the simulation. `Simple Variable` Mass and inertia vary linearly as a function of mass rate. `Custom Variable` Mass and inertia variations are customizable.

The `Simple Variable` selection conforms to the previously described equations of motion.

Representation

Select the representation to use.

 `Euler Angles` Use Euler angles within equations of motion. `Quaternion` Use quaternions within equations of motion.

The `Euler Angles` selection conforms to the previously described equations of motion.

Initial position in inertial axes

The three-element vector for the initial location of the body in the flat Earth reference frame.

Initial velocity in body axes

The three-element vector for the initial velocity in the body-fixed coordinate frame.

Initial Euler rotation

The three-element vector for the initial Euler rotation angles [roll, pitch, yaw], in radians.

Initial body rotation rates

The three-element vector for the initial body-fixed angular rates, in radians per second.

Initial mass

The initial mass of the rigid body.

Empty mass

A scalar value for the empty mass of the body.

Full mass

A scalar value for the full mass of the body.

Empty inertia matrix

A 3-by-3 inertia tensor matrix for the empty inertia of the body.

Full inertia matrix

A 3-by-3 inertia tensor matrix for the full inertia of the body.

Include mass flow relative velocity

Select this check box to add a mass flow relative velocity port. This is the relative velocity at which the mass is accreted or ablated.

Include inertial acceleration

Select this check box to enable an additional output port for the accelerations in body-fixed axes with respect to the inertial frame. You typically connect this signal to the accelerometer.

### State Attributes

Assign unique name to each state. You can use state names instead of block paths during linearization.

• To assign a name to a single state, enter a unique name between quotes, for example, `'velocity'`.

• To assign names to multiple states, enter a comma-delimited list surrounded by braces, for example, `{'a', 'b', 'c'}`. Each name must be unique.

• If a parameter is empty (`' '`), no name assignment occurs.

• The state names apply only to the selected block with the name parameter.

• The number of states must divide evenly among the number of state names.

• You can specify fewer names than states, but you cannot specify more names than states.

For example, you can specify two names in a system with four states. The first name applies to the first two states and the second name to the last two states.

• To assign state names with a variable in the MATLAB® workspace, enter the variable without quotes. A variable can be a character vector, cell array, or structure.

Position: e.g., {'Xe', 'Ye', 'Ze'}

Specify position state names.

Default value is `''`.

Velocity: e.g., {'U', 'v', 'w'}

Specify velocity state names.

Default value is `''`.

Euler rotation angles: e.g., {'phi', 'theta', 'psi'}

Specify Euler rotation angle state names. This parameter appears if the Representation parameter is set to `Euler Angles`.

Default value is `''`.

Body rotation rates: e.g., {'p', 'q', 'r'}

Specify body rotation rate state names.

Default value is `''`.

Mass: e.g., 'mass'

Specify mass state name.

Default value is `''`.

## Inputs and Outputs

InputDimension TypeDescription

First

VectorContains the three applied forces.

Second

VectorContains the three applied moments.

Third

ScalarContains one or more rates of change of mass.

Fourth (Optional)

Three-element vectorContains one or more relative velocities at which the mass is accreted to or ablated from the body in body-fixed axes.
OutputDimension TypeDescription

First

Three-element vectorContains the velocity in the flat Earth reference frame.

Second

Three-element vectorContains the position in the flat Earth reference frame.

Third

Three-element vectorContains the Euler rotation angles [roll, pitch, yaw], within ±pi, in radians.

Fourth

3-by-3 matrixApplies to the coordinate transformation from flat Earth axes to body-fixed axes.

Fifth

Three-element vectorContains the velocity in the body-fixed frame.

Sixth

Three-element vectorContains the angular rates in body-fixed axes, in radians per second.

Seventh

Three-element vectorContains the angular accelerations in body-fixed axes, in radians per second squared.

Eight

Three-element vectorContains the accelerations in body-fixed axes with respect to body frame.

Ninth

Scalar elementContains a flag for fuel tank status:
• 1 indicates that the tank is full.

• 0 indicates that the integral is neither full nor empty.

• -1 indicates that the tank is empty.

Tenth (Optional)

Three-element vectorContains the accelerations in body-fixed axes with respect to inertial frame (flat Earth). You typically connect this signal to the accelerometer.

## Assumptions and Limitations

The block assumes that the applied forces are acting at the center of gravity of the body.

## Reference

Stevens, Brian, and Frank Lewis, Aircraft Control and Simulation. Second Edition. Hoboken, NJ: John Wiley & Sons, 2003.

Zipfel, Peter H., Modeling and Simulation of Aerospace Vehicle Dynamics. Second Edition. Reston, VA: AIAA Education Series, 2007.

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