Re: [Flightgear-devel] Abnormal surface friction factor

[Dany <da...@nu...>]

On 14/06/2023 at 19:44, Dany wrote :
> On 14/06/2023 à 17:25, Dany wrote :
>> /fdm/jsbsim/gear/unit[]/rolling_friction_coeff
>
>> rolling_friction-factor,...
>> We also have (by geodinfo):
>> friction_factor, rolling_friction.
>> With (if I understand well)
>> rolling_friction-factor = rolling_friction / 0.02 = 50 x 
>> rolling_friction.
>
> To be observed:
> rolling_friction_coeff
> rolling_friction-factor = 50 x rolling_friction.
> Possibly too: (by geodinfo):
> rolling_friction.

Hi,

I think I progressively understand the logic (including the 50 factor).

Everything starts from the (geodinfo) rolling_friction.
On asphalt, this (geodinfo) rolling_friction is given the value 0.02. 
Which, multiplied by 50, gives rolling_friction_factor = 1. In this 
case, no correction, the final rolling friction coefficient is the value 
given in the aircraft FDM:
(in the FDM, e.g. <rolling_friction> 0.03 </rolling_friction>)
  /fdm/jsbsim/gear/unit[]/rolling_friction_coeff

On another terrain, the (geodinfo) rolling_friction has another value. 
Let's take "grass": 0.1. Which, multiplied by 50, gives 
rolling_friction_factor = 5. Thus, the final rolling friction 
coefficient = 5 x the value given in the aircraft FDM:
5 x /fdm/jsbsim/gear/unit[]/rolling_friction_coeff
If the aircraft (FDM) initial rolling_friction_coeff = 0.03, it gives 
0.03 x 5 = 0.15 for the final rolling friction coefficient.

Example: C172P, 180hp. Full thrust at rest: 549 lbs.
If the aircraft weight is 2000 lbs, it will not move if the final 
rolling friction coefficient is greater than 549 / 2000 = 0.275.

If the aircraft will not move on a terrain, the first (and easier) thing 
is to check if the (geodinfo) rolling_friction is not the culprit.

Is it true? Please fix me otherwise, thank you.

Dany