Aerospace Blockset / Incorrect Variable Mass Equation

[Arda]

Hi,
I have discovered an incorrect usage of "variable mass equation" in Simulink.
Under Aerospace Blockset / Equations of motion, there are 3Dof and 6Dof equations related with variable mass. After looking under these blocks i have seen that the equation used for mass change is simply this;
F = m*vDot + mDot*v
This can be accepted true considering F=d(m*v)/dt which leads the above equation with a simple differentiation. However things are a little bit different for mass variable systems.
True equation should have been this;
F = m*vDot + mDot(exhaust)*v(relative)
Derivation of the above equation can be seen in Military Handbook MIL-HDBK-1211 at page 77. Please see;
http://www.everyspec.com/MIL-HDBK/MIL-HDBK+(1000+-+1299)/MIL_HDBK_1211_2041/

Two equations given above are clearly different from each other. I have created a toy system in 1Dof frictionless environment to compare them. I also included "3Dof Simple Variable Mass Blockset" from Simulink Library. Not that in order to get a 1Dof system from a 3Dof Blockset, i gave zero to the second force (Fz), moment and gravity. It appeared that "3Dof Simple Variable Mass Blockset" results are same with the incorrect equation.

I intend to write a report for variable mass equation used in Aerospace Blocksets. Whom and/or where should i apply to warn Mathworks?

Thanks in advance.

[Arda]

[Arda]

I have posted the simulink file in which three blocks are compared, but it has not been approved yet. Once it appears under file exchange catagory, i will give the link here.

[Arda]

Here is the simulink file;
http://www.mathworks.com/matlabcentral/fileexchange/23256

Any help would be much appreciated.

[James Tursa]

"Arda " <arda...@hotmail.com> wrote in message <gp8d5d$t1v$1...@fred.mathworks.com>...

>
> Derivation of the above equation can be seen in Military Handbook MIL-HDBK-1211 at page 77. Please see;
> http://www.everyspec.com/MIL-HDBK/MIL-HDBK+(1000+-+1299)/MIL_HDBK_1211_2041/

I don't see page 77 ... could you look at this link again & point me to the right page?

James Tursa

[Arda]

> > Derivation of the above equation can be seen in Military Handbook MIL-HDBK-1211 at page 77. Please see;
> > http://www.everyspec.com/MIL-HDBK/MIL-HDBK+(1000+-+1299)/MIL_HDBK_1211_2041/
>
> I don't see page 77 ... could you look at this link again & point me to the right page?
>
> James Tursa

Actually, there is PDF file at that link. I should have said page 77 of PDF file. Anyway, here is the direct link to the PDF;

http://www.everyspec.com/MIL-HDBK/MIL-HDBK+(1000+-+1299)/download.php?spec=MIL_HDBK_1211.2041.pdf

[Alan Weiss]

I have informed The MathWorks of this issue.

Alan Weiss
MATLAB mathematical toolbox documentation

[Stacey Gage]

Hi Arda,

This is just a matter of how you look at a problem for an example of how to use the variable mass equations of motion, please see the example aeroblk_vmm.mdl.

As you can see the relative velocity term is handled by the use of exhaust velocity as part of the Forces input.

All the best,
Stacey

"Arda " <arda...@hotmail.com> wrote in message <gp8d5d$t1v$1...@fred.mathworks.com>...

[samil]

"Stacey Gage" <sgage....@mathworks.com> wrote in message <gpb4iv$c65$1...@fred.mathworks.com>...

> Hi Arda,
>
> This is just a matter of how you look at a problem for an example of how to use the variable mass equations of motion, please see the example aeroblk_vmm.mdl.
>
> As you can see the relative velocity term is handled by the use of exhaust velocity as part of the Forces input.
>
> All the best,
> Stacey

Hi Stacey, for your implementation to be correct you have to delete the mdot*Vb term inside the equation of motion block.