Re: [Flightgear-devel] Abnormal surface friction factor

[Wayne B. <kil...@co...>]

For completeness in this discussion.

Erik,

>Just a to make sure: does any other JSBSim aircraft (say the Ercoupe 
which is pretty simple) also have a friction value of 50 on the same 
terrain type?

I checked and it also has 50 in the same cover types.

James,

>Also, you keep suggesting this is a difference between YASim and JSBsim, but the groundractions.cxx file in question is *not* specific to either one, so I wonder if we have some additional confusion going on.

You are correct, I don’t know what I thought I saw with a YASim aircraft. I just checked a few off the shelf, general aviation YASim Pipers and while the friction factors are not exposed in the FDM they exhibit the same strong friction on the same cover types. So again, I was wrong, I don’t know what I thought I saw regarding YASim aircraft. I also didn’t look closely at the code I posted that I thought was referencing YASim and JSBSim. I would need to look at that closer to understand exactly what I was looking at.

Wayne


From: James Turner
Sent: Wednesday, June 14, 2023 6:08 AM
To: FG Developers List
Subject: Re: [Flightgear-devel] Abnormal surface friction factor




On 13 Jun 2023, at 17:12, Wayne Bragg <kil...@co...> wrote:

I was assuming someone knew this code well enough to maybe understand a possible mistake just with this little bit of information,  possibly not.

Unfortunately I think no one knows the code well, we’re all figuring it out from what we see.


Also I submit, the values for land cover friction should be directly tied to material settings, not some amped up value. What other reason would you design this code this way, include a material value for friction only to completely obliterate it later in the code. In this instance, I get you could be trying to adjust for local weather conditions. Where you have a base value for ideal conditions and adjust accordingly, if raining, divide by x. 

My understanding, which is also a bit limited, is the value is not being obliterated, it’s simply being scaled to compensate for some (possible historical) difference in units or definition or similar. (Same as we convert feet to metres in many other places) 

Of course, I don’t know what units the friction is supposed to be in, to know if this understanding is correct, but it’s certainly very common to see such conversion factors being applied when values are read from files. 

Also, you keep suggesting this is a difference between YASim and JSBsim, but the groundractions.cxx file in question is *not* specific to either one, so I wonder if we have some additional confusion going on.

Kind regards,
James