Offset 0x11BA (g force) wrong?

[Jens82]

Hello Mr. Dowson,

 

I am currently facing a problem with the flight recording software I am currently developing.

 

According FSUIPC4 Offset Status.pdf the offset 0x11B8 is a copy of 0x11BA on touchdown. Some users reported that they have a value of less than 1.0g at the same time the on ground flag changes to 'on ground', which is not possible in real life as a touchdown is a positive acceleration (and therefore greater than 1.0g). I checked the saved flight recordings and 11BA provides a value of about 0.96g (times 624 for the raw value) in 0x11BA so 0x11B8 is the same on the time the on ground flag changes. The next time values are read the value in 0x11BA is at about 1.59g, which is a realistic value.

 

Maybe you could check if you find any problems here or can provide any support or information on this issue?

 

Thanks in advance & Greetings,

Jens

 

 

[Pete Dowson]

According FSUIPC4 Offset Status.pdf the offset 0x11B8 is a copy of 0x11BA on touchdown. Some users reported that they have a value of less than 1.0g at the same time the on ground flag changes to 'on ground', which is not possible in real life as a touchdown is a positive acceleration (and therefore greater than 1.0g).

 

11B8 is simply populated at the same time as 11BA except when the on-ground flag (0366) is set. Because notifications from Simconnect are aynchronous, the on-ground and G-force values may not correspond exactly in time, so initially the 11B8 copy may refer to a bounce (hence -ve value). However, there is code in FSUIPC to keep updating 11B8 even after the "on-ground" flag is set, but only whilst it is increasing. This should get over the problem of bounce, but it does mean you may need to have a short delay after seeing on-ground before trusting 11B8. If you are reading both offsets together, don't. Only read 11B8 when you have separately read 0366.

 

Pete

[Jens82]

Hello Pete,

 

thanks for the reply.

 

So, the "true" g force at touchdown is the value of 11B8 several seconds after touchdown? I mean, if I read the value at a later point (for example when the runway is left after landing) the value of 11B8 will be the correct one?

 

Does 030C (vertical speed at touchdown) behave the same way?

 

Greetings,

Jens

[Jens82]

One more question:

How can I get the vertical speed and g force of the first touchdown?

[Pete Dowson]

So, the "true" g force at touchdown is the value of 11B8 several seconds after touchdown? I mean, if I read the value at a later point (for example when the runway is left after landing) the value of 11B8 will be the correct one?

 

I repeat, once the "on ground" flag is set the value will only change it if increases. This will only occur during the few milliseconds after touchdown, or more if it's a big bounce.

 

Does 030C (vertical speed at touchdown) behave the same way?

 

Not sure. Ask me again tomorrow and I'll look at the code.

 

One more question:

How can I get the vertical speed and g force of the first touchdown?

 

There's an offset providing vertical speed at touchdown, and you know the one for G-Force. That's as close as I know how to get.  How would you suggest?

 

Pete

 

[Jens82]

I repeat, once the "on ground" flag is set the value will only change it if increases. This will only occur during the few milliseconds after touchdown, or more if it's a big bounce.

Ok, I just wanted to confirm that. Thanks.

 

Not sure. Ask me again tomorrow and I'll look at the code.

 

Thanks :smile:

There's an offset providing vertical speed at touchdown, and you know the one for G-Force. That's as close as I know how to get.  How would you suggest?

Well, in case of a bounce, on ground will switch back to "air" and therefore the values of g-force and vertical speed at touchdown will be overwritten at next touchdown. I was wondering about a way to ignore those bounces.

[Pete Dowson]

Well, in case of a bounce, on ground will switch back to "air" and therefore the values of g-force and vertical speed at touchdown will be overwritten at next touchdown. I was wondering about a way to ignore those bounces.

 

In the case of a really big bounce I don't see that you can -- more like a touch and go, repeated! ;-).

 

I suppose I could ignore any further changes from off-ground to on-ground for a few seconds. I've no idea what sort of time to allow though ... have you? It would be an easy change to make (FSUIPC4 only, I'm not changing FSUIPC3).

 

Pete

[Jens82]

I suppose I could ignore any further changes from off-ground to on-ground for a few seconds. I've no idea what sort of time to allow though ... have you? It would be an easy change to make (FSUIPC4 only, I'm not changing FSUIPC3).

I was working on this issue yesterday and I think the best way to do it is to have that in the client program. Frozen values wouldn't help if one wants the values for the succeeding bounces. So this highly depends on what the client programmer wants to achieve.

I am thinking about solving that with a combination of the on ground flag to "count" the bounces and try to get the values for each bounce:

- ON GND set -> Get the values of rate and g force on touchdown as long as landing has finished or ON GND is not set anymore.

- ON GND set again -> Touchdown values will be overwritten, so get the values until landing has finished or ON GND is not set anymore.

And so on...

As, as you have stated before, the on gnd flags and touchdown values get updated async, there will be a small error (in case time between two readouts is too large to cover the changes) but I think this will work for my program. It reads all the values I need about 8-10 times a second, which works pretty nice without a significant performance hit.

[Pete Dowson]

I was working on this issue yesterday and I think the best way to do it is to have that in the client program. 

 

Okay. Let me know how it works out. If folks bounce they it should be recorded in any case! ;-)

 

Pete

[Jens82]

Hello Pete,

my client now checks the on ground flag during landing roll phase and during rollout the rate- and g-force- values of the last touchdown are continously updated. This should give good results and the best values for each touchdown. However, I was not able to force several touchdowns on one landing yet :smile:

Greetings,

Jens

[Pete Dowson]

my client now checks the on ground flag during landing roll phase and during rollout the rate- and g-force- values of the last touchdown are continously updated. This should give good results and the best values for each touchdown. However, I was not able to force several touchdowns on one landing yet :smile:

 

Do you mean you aren't able to get a bounce, or that you do but they aren't recorded?

 

Pete

[Jens82]

I wasn't able to get a bounce, yet :smile:

[mgh]

If FS models the aircraft suspension then  there can be a lower value than 1.0g even with the aircraft on the wheels. 

[Jens82]

Well, are there any fs models that do simulate suspension?

[mgh]

Practically all models have suspension. The details are given in the [contact_points] section of the aircraft.cfg which show the Static Compression, Ratio of Maximum Compression to Static Compression, and Damping Ratio each item of landing gear..