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Jumping sailplanes

Printed From: Just Flight Forum
Category: Just Flight Products
Forum Name: Traffic X / Traffic / Traffic 2005
Forum Description: Discussion area for Traffic titles
URL: http://forum.justflight.com/forum_posts.asp?TID=7031
Printed Date: 01 Jun 2020 at 5:29am


Topic: Jumping sailplanes
Posted By: delsutton
Subject: Jumping sailplanes
Date Posted: 06 May 2009 at 5:22pm
Hi All,

I'm pretty new to FSX and I only bought TrafficX a week or so ago. But, I have to say I'm more than happy with it. However, there are a couple of things that are niggling me:

1. Whenever I turn up at an airport where there are sailplanes parked, they seem to jump about a bit and cause a "tyre screech" sound to occur fairly regularly. Am I alone in seen and hearing this, or is there a general issue here? And, if so, is there a patch for it?

2. Whereas I love the AI sounds I find that the tail-off isn't quite there and it ends very suddenly. For example, parked at the end of the runway, listening to a 737 take off sounds fabulous. Then it gradually reduces in intensity until about 2.5Nm out and then the sound suddenly stops, which is unrealistic. Is this just because I have a bog-standard sound card, or is this a general observation (to use the wrong pun!).

Del



Replies:
Posted By: freddy
Date Posted: 07 May 2009 at 4:00am
1. The "jumping" is not only restricted to sailplanes, but to all planes and even airport vehicles. You are not the only one experiencing it. For me, it occurs when I decide I want to view other aircraft and I use the "A" key to view the next aircraft and then "A" again to see the next one and then "A" yet again for the next one after that ... and so on an so forth. As I press the "A" key, I may decide to stop there and view the current aircraft parked on the ramp or at the gate. And, as I am viewing it, it is jumping up and down and doing the tyre screeching thing. If I wait a bit, it "settles down" and the jumping eventually stops. So, it appears to be related to my quickly skipping from one aircraft to the next to the next and not giving the simulator time to fully load all the textures and liveries etc. So when I do decide to stop for a look, FSX seems to want to "catch up". So, I think, it's a kind of "wait for me to catch up, I'm still loading stuff" issue. To be honest I have no real idea. But, nonetheless, you are not the only one who sees this jumping.
 
2. Yes, this is true. And, yes, the sound does end quite suddenly. A little unrealistic. It is not because you have a bog-standard sound card. Make sure you have installed Traffic X Service Pack 1 as it greatly reduces this issue. It doesn't fully fix it, but it does make it a whole lot better than it was before the Service Pack.


Posted By: Soaranden
Date Posted: 01 Jul 2013 at 7:11am
Recently, I downloaded and installed a new freeware user aircraft. I am also using the aircraft as an AI aircraft. When using the aircraft as a user aircraft, there is no jumping. However, initially, when I used the aircraft as an AI aircraft, it showed extreme jumping and burning of tire rubber when making turns. In this case, the solution was to comment out (disable) some lines in the aircraft.cfg file:

[weight_and_balance]

max_gross_weight = 502433
empty_weight = 252499
reference_datum_position = 10, 0, 0
empty_weight_CG_position = -10, 0, 0
empty_weight_pitch_MOI = 18000000
empty_weight_roll_MOI = 5500000
empty_weight_yaw_MOI = 16300000
empty_weight_coupled_MOI = 2048
max_number_of_stations = 5

station_load.0 = 3450, 34.07, 0, -0.24, First
station_load.1 = 3450, -8.69, 0, 0, Business
station_load.2 = 8400, -54.10, 0, 0, Coach
station_load.3 = 7000, 36.44, 0, -5.62, F. Cargo
station_load.4 = 7000, -47.88, 0, -5.62, R. Cargo
// empty_weight_roll_MOI=3839102.000000
// empty_weight_pitch_MOI=19577228.000000
// empty_weight_yaw_MOI=14407840.000000
// empty_weight_coupled_MOI=1000.000000

Since I didn't want to have the above disabled lines disabled when using the aircraft as a user aircraft, I ended up with two installations of the aircraft, using one folder to contain the aircraft that would be used as a user aircraft and using a second folder to contain the aircraft that would be used as AI. I was able to prevent the AI version from appearing on the FSX aircraft menu by renaming the panel folder of the AI version to "ai_panel." (Any name other than "panel" would have worked).

I should note that "MOI" in the above lines is the abbreviation for "Moment of Inertia." Originally, I had tried disabling the first four lines that contained MOI (without disabling the remaining lines that contained MOI), but that had no affect on the jumping.

Not all aircraft.cfg files contain the type of "MOI" lines that I disabled. However, if you experience aircraft jumping and see similar lines in the aircraft's aircraft.cfg file, disabling those lines is worth a try. I didn't try changing the numbers to the right of the equal signs because I was afraid that I would make matters worse, but making changes to those values is an option that could have been tried.

Here's what the FSX SDK has to say about Moments of Inertia:

A moment of inertia (MOI) defines the mass distribution about an axis of an aircraft. A moment of inertia for a particular axis is increased as mass is increased and/or as the given mass is distributed farther from the axis. This is largely what determines the inertial characteristics of the aircraft.

The following weight and balance parameters define the MOIs of the empty aircraft, so the values should not reflect fuel, passengers or baggage. The simulation engine determines the total MOIs with these additional, and variable, influences. The units are slugs per foot squared. Omission of a parameter will result in the use of a default value set in the .air file, if one exists.

These values can be estimated with the following formula:

MOI = EmptyWeight x(D^2 / K)

Where:
Pitch
D = Length (feet)
K = 810

Where:
Roll
D = Wingspan (feet)
K = 1870

Where:
Yaw
D = 0.5x (Length+Wingspan)
K = 770


Dan


Posted By: freddy
Date Posted: 02 Jul 2013 at 3:24am
Hmmm. Interesting.

A question ... (I am sure I could look this up myself, but I am at work and also feeling a bit lazy.) Dan, do you know the difference between the TOP MOI lines and the BOTTOM MOI lines? The "titles" appear to be the same? (Highlighted using RED font.)

Which leads to a second question ... why comment out the BOTTOM set of lines, and not the TOP set of lines?

(Perhaps the answer to the initial question lies here? "The simulation engine determines the total MOIs with these additional, and variable, influences." Is that implying that the TOP lines are some kind of TOTALS?)


max_gross_weight = 502433
empty_weight = 252499
reference_datum_position = 10, 0, 0
empty_weight_CG_position = -10, 0, 0
empty_weight_pitch_MOI = 18000000
empty_weight_roll_MOI = 5500000
empty_weight_yaw_MOI = 16300000
empty_weight_coupled_MOI = 2048
max_number_of_stations = 5

station_load.0 = 3450, 34.07, 0, -0.24, First
station_load.1 = 3450, -8.69, 0, 0, Business
station_load.2 = 8400, -54.10, 0, 0, Coach
station_load.3 = 7000, 36.44, 0, -5.62, F. Cargo
station_load.4 = 7000, -47.88, 0, -5.62, R. Cargo
// empty_weight_roll_MOI=3839102.000000
// empty_weight_pitch_MOI=19577228.000000
// empty_weight_yaw_MOI=14407840.000000
// empty_weight_coupled_MOI=1000.000000


Posted By: Soaranden
Date Posted: 02 Jul 2013 at 1:25pm
freddy,

Before I respond to your questions, let me say that I forgot to mention that, in addition to jumping while making turns, my previously jumping AI aircraft had also jumped in place in its parking space as its time for departure neared.

I must say that I am no FSX aircraft expert. Until a few days ago, I knew nothing about MOIs. My desire to try to eliminate the jumping of my AI aircraft necessitated that I do some research. However, I have found no information about the differences between the top and bottom MOI lines. I have looked in the FSX SDK documentation, have searched the FSDeveloper forum, and have searched Microsoft's ESP documentation. I am uncertain as to why commenting out the bottom MOI lines worked and commenting out the top MOI lines didn't work, but I can make a guess. I don't think the top MOI lines are some kind of totals. I'm thinking that the bottom MOI lines might reflect solely the MOI influences of passengers and baggage. From the FSX SDK documentation, it appears that the FSX simulation engine uses MOI values from both the top MOI lines and the bottom MOI lines when calculating MOI totals. Perhaps the reason commenting out the top MOI lines didn't work and commenting out the bottom MOI lines did work, is that the absence of the MOI values of the top MOI lines had less influence upon the MOI totals calculated by the FSX simulation engine than did the absence of the MOI values of the bottom MOI lines. I am just guessing, though.

ADDENDUM:

Some people may use the following words instead of "jumping":

shaking
jittering
bouncing

I am including the above words so that people who search the forum, later, will be able to find this posting.

For the same reason, I am adding the following words:

jump
shake
jitter
bounce

Dan


Posted By: freddy
Date Posted: 03 Jul 2013 at 7:14am
Yes, well, the FSX SDK documentation is, apparently, quite lacking in many areas. Hmmm, you could be correct that the bottom MOI lines might reflect solely the MOI influences of passengers and baggage. Who knows? Since you've achieved the result you are after, the answer to the question probably doesn't really matter. It would have been nice to tuck away in the memory banks though, for better understanding and future reference. Nonetheless, I've made a note of the information you've provided here about MOI, just in case I need to work on correcting some jumping AI planes one day.


Posted By: Soaranden
Date Posted: 03 Jul 2013 at 9:31am
Having been a programmer, I can tell you that the FSX SDK documentation is not all that unusual in its lack of specifics. The documentation provides just enough detail so that the documentation, together with examination of the contents of FSX files (such as the files used for default aircraft, etc.), together with what is learned from trial and error, can enable individuals to do their own development of FSX aircraft, airports, missions, etc. Thankfully, individuals who have done such development, share what they have learned with others. In the process of doing FSX development, individuals have found a few errors in the FSX SDK documentation. However, since only a few errors have been reported, most of the documentation can be trusted.

Moment of inertia (MOI) values aren't the only thing affecting FSX aircraft stability. There are a number of other factors affecting stability. The reason I tried changing MOI, first, rather than changing one of the other factors, is that I thought changing the MOI values would be more likely to yield the aircraft stability that I sought. However, since the aircraft jumping that I experienced could have been the result of one or more of the other factors, I was a bit lucky that my first choice corrected the problem. It's possible that I didn't need to disable all four of the MOI lines, but since doing so achieved the desired outcome, I have no burning desire to do further testing. The aircraft is completely stable.

Dan



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