Fiala Tire Outputs/Requests - Instantanious friction coefficient (Mu vs Time Step)

Hi all,

Is there an easy way to extract the instantaneous longitudinal ground friction coefficient (Mu vs Time Step) from my Fiala Tire model?

I'm aware that Longitudinal Slip is already an available output, and thus, I could do some post-processing to derive Mu from the Mu_Slip_curve. However, this could lead to user errors if the post-processing Mu_Slip_curve is not the same as the one used in the simulation.

For context, this is for a Landing Gear drop test simulation.

Thank you!

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Accepted answers

Luca Cavagna

Yes usually from drop tests that is the only way to estimate mu. It is what I usually do indeed.

You have to enable the VARSUB flag in the tire panel then you can simply create an output as user defined function and access to your tire as {ts_1.sys_varsub.sv_cp_force_z.VARVAL} for Fz and similary for Fx. Make sure to use the correct name for the tire when you call the function. Another workaround I usually use is loading my plt or abf with compose and do all the magics there :D

All comments

GTT Adam

Hi @tadhnim,

What is the purpose of extracting the mu_slip curve?

Is it just to confirm that the values from the table in TIR file are being read correctly?

Hope this helps!

Adam Reid

tadhnim

Hi,

That is exact. Furthermore, it is a lot easier for us to compare friction cofficient with test data instead of slip.

Thanks.

Luca Cavagna

can you calculate Fx/Fz with a function and call it MU? In a drop test these are your outputs from the scale.

tadhnim

Hi,

True, that is indeed what we do to approximate Mu with test bench data. This could be a work around for now, of course, taking into acount the numerical edge case of Fz=0.

Any idea how I could implement contact patch Fx/Fz in an Output? I've tried using User Defined Expressions calling the mbdtire.dll "VARSUB", but haven't had any luck.

Thank you!

Luca Cavagna

Yes usually from drop tests that is the only way to estimate mu. It is what I usually do indeed.

tadhnim

It works!

It wasn't super intuitive finding my Fx and Fz VARVAL as the tire doesn't show up in the expression builder and I'm using Tire Pairs lol.
Note for anyone reading this, get the variable "path" by following the *BeginContext( ____ ) tree in the .mdl file. Ex: I had to write {ts_0.sys_tire_left.sys_varsub.sv_cp_force_x.VARVAL}.

As a comparison, the calculated Mu = Fx/Fz is not as numerically stable, but is nonetheless quite good. Shown below is the delta (Fx/Fz - Mu from slip) with ref. Mu max at 0.60 and ground contact from 0 to 0.3 seconds.

Kind of a shame that the Mu used in the tire calculation is not available. Consider my question solved, thanks :)

Luca Cavagna

I think they way you do actually makes more sense because MU is not available from ground tests. So now you compare apples with apples. glad you solved!