Fiala tire and static runs

Luca Cavagna_21621
Luca Cavagna_21621 Altair Community Member
edited February 29 in Community Q&A

Hello, 

I made a MB model for my aircraft. It's a rigid body at CG which connects rigid nose and main gears with spring-dampers and tire elements. I run a static simulation and it runs smoothly in very few iterations.

image

The model is as sketched above. I apply at each axle 0.5 the vertical force on each wheel. At the CG I report the sum of all the side forces to have the overall balance in side direction.

The tire model does not work if i set MU to 0.5. I cant get a side force in the output. So I decided to apply the side force at each axle (and the transport moment from ground to axle). I recover the vertical force on each tire using its varsub variable (it provides the forces in W frame) and define a force and the transport moment at tire center point on the axle.

It works. The aircraft rolls and one side is compressed. However, if I check the total sum of the vertical forces I do not get the weight. The tires actually are not normal to the ground  and they roll as well. I think this is the source of the problem. I tried to attach the tire to the axle with a ball joint so that they stay normal to the ground. Now the sum of forces is the weight. why? No problem with convergence but in one case I really do not figure out why the normal forces are not equal to the weight

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Answers

  • Spiros-Foivos Mallios_22475
    Spiros-Foivos Mallios_22475
    Altair Employee
    edited February 29

    Hi Luca,

    Which output/request are you plotting to see the tire vertical forces ?

    Depending on the output request a different coordinate system is used.

    If the TYDEX-C is used then the reference system gets the tire inclination angle (roll or camber) so the sum of the normal force logically won't much the weight of the total system.

    However if you use the TYDEX-W request the z-axis is following the road plane

    You may find more about the Reference systems used on the tire outputs here 

    Coordinate Systems and Output Requests (altair.com)

    Let us know if you have more questions,

    Thanks,

    Spiros Foivos Mallios

  • GTT Adam
    GTT Adam
    Altair Employee
    edited February 29

    Hi Luca,

    I agree with Spiros, in that the discrepancy you are seeing is due to how you are measuring the forces.

    Can you share the model? If not, can you share images or video of the simulation?

     

    Thanks,

    Adam Reid

  • Luca Cavagna_21621
    Luca Cavagna_21621 Altair Community Member
    edited February 29

    Hi guys and thanks for the reply. Yes I am pretty sure the fact the tires assume an angle with respect to the ground is the source of the problem. However i condider the W frame, so the vertical forces I get are normal to the ground by definition.

    Assuming I have two wheels per leg, the total side force on the leg is prescribed as:

    `{ds_mlg_sledge.ny.value}*({ts_0.sys_varsub.sv_cp_force_z.VARVAL}+{ts_1.sys_varsub.sv_cp_force_z.VARVAL})` 

     

    image

    where ny is simply 0.5. sv_cp_force according to the manual should be normal to the ground. Well, If I sum all the xxx_z.VARVAL forces, I do not get the weight. I get the weight only if I use a ball joint for each tire, leaving them stay normal to the ground.

    If the vertical force is wrong, I will apply a wrong side force as well.

    PS sorry Adam I cant share any picture...I hope you can understand. Sorry

    PS2 I checked the value from sv_cp_force_z.VARVAL for each tire and it matches the standard output in the .plt file for forces in W frame.

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