I calculated them from the figures given in aircraft.cfg in a spreadsheet. I took the mass and multiplying it by the square of the distance from the centre line for each of the items given in the in the fuel and station.load sections. MOI is in slugs so the weight in pounds has to be divided by 32.2. I allowed 6 pound/gallon for fuel.I assume tha FS does much the same.

I just used the formulae against actual values I have for a specific aircraft. Didn't match.Just to be 100% certain... actually tried the new values from the formulae in FS... didn't work. Caused some significant control issues.This 'one size fits all' approach doesn't always work as advertised, so I would urge caution in using this 'template' approach.

mgh, I think your calculations are too simplistic as MOI are rather complicated.However, I'll defer to Dr Roskam to defend his algorithm. I'm not here to defendit, only to pass it on as a means of determining reasonable MOI for aircraft. Nomore no less.Ed brings up an important issue that I alluded to above. Best practice is to firstdetermine MOI then build the air file. The reverse may very well be detrimentalif the MOI are significantly different and I think this is easily seen in how one assigns pitch moments for instance. If, however, Roskam gives very differentresults for a plane he gives moments for than one's you know for sure, then Iwould think he may want to know that. Personally, I've found them very usefuland I would agree with Ed completely that just changing MOI is not a universalcure for anything.Jim

mgh, I think your calculations are too simplistic as MOI are rather complicated.However, I'll defer to Dr Roskam to defend his algorithm. I'm not here to defendit, only to pass it on as a means of determining reasonable MOI for aircraft. Nomore no less.Ed brings up an important issue that I alluded to above. Best practice is to firstdetermine MOI then build the air file. The reverse may very well be detrimentalif the MOI are significantly different and I think this is easily seen in how one assigns pitch moments for instance. If, however, Roskam gives very differentresults for a plane he gives moments for than one's you know for sure, then Iwould think he may want to know that. Personally, I've found them very usefuland I would agree with Ed completely that just changing MOI is not a universalcure for anything.Jim

MOI is quite simple. It is calculated as the mass multiplied by its distance from the relevant axis. In the case of wiong-mounted fuel tanks my calculations assume that fuel in the wing tanks can be treated as a point mass. This is as accurate as is possible in the absence of any information about the shape of the fuel tanks and the distruibution of mass in them. I cannot see how Flight Simulator itself can calculate the loaded MOI in any different way under the circumstances. It can only work with the information actually available.I am not critising Dr Rostrum's algorithm. I have no reason to doubt its accuracy for the specified loaded aircraft. I pointed out that the Kx for the default FSX B747 was close to Dr Rostrum's figures for jet airliners. The point I am making is that MOI does not scale so estimates of empty MOI derived by scaling Dr Rostrums's figures for loaded aircraft cannot be accurate.

I've found that empty weight MOIs and and total weight MOIs are available as variables for XML gauges. Anyone who is keen enough could try to correlate those with the values for MOI and loads in the aircraft.cfg file.

Wooosh! All this (experimental) 'math' went right over my head. But I would like to 'relate' one example of what one can do when your a/c shakes. I recently installed FS2002 and installed the Hughes H-1B taildragger. On loading it sat there shimmering. The cure? Double the values. Done. Did not shimmer, or get 'adversly' affected, by doing this. (So it needed it, and 'had it coming', so I let it 'have it'). Worked perfect. (Look Ma-No Math!).Hughes H-1B: FSDS model by Dave Carroll, for FS2002, 6/19/2001.empty_weight_roll_MOI= 4680.000 //orig=2340.000empty_weight_pitch_MOI= 6300.000 //orig=3156.000empty_weight_yaw_MOI= 9900.000 //orig=4995.000empty_weight_coupled_MOI= 0.000Of course, I don't recommend you do this with larger a/c. Not this drastic, anyway. And I ususally keep the ratio of 'roll=1/pitch=2/yaw=3' for the values. IE: 4/8/12. Works for me 99% of time. Individual MOI's might need 'adjust' due to 'xyz', but the ratio is still a good starting point. What is 'normal'? Reduce to 1/4 and find out, then increase in 20% chunks until it all 'works'.CBNapamule

Wooosh! All this (experimental) 'math' went right over my head. But I would like to 'relate' one example of what one can do when your a/c shakes. I recently installed FS2002 and installed the Hughes H-1B taildragger. On loading it sat there shimmering. The cure? Double the values. Done. Did not shimmer, or get 'adversly' affected, by doing this. (So it needed it, and 'had it coming', so I let it 'have it'). Worked perfect. (Look Ma-No Math!).Hughes H-1B: FSDS model by Dave Carroll, for FS2002, 6/19/2001.empty_weight_roll_MOI= 4680.000 //orig=2340.000empty_weight_pitch_MOI= 6300.000 //orig=3156.000empty_weight_yaw_MOI= 9900.000 //orig=4995.000empty_weight_coupled_MOI= 0.000Of course, I don't recommend you do this with larger a/c. Not this drastic, anyway. And I ususally keep the ratio of 'roll=1/pitch=2/yaw=3' for the values. IE: 4/8/12. Works for me 99% of time. Individual MOI's might need 'adjust' due to 'xyz', but the ratio is still a good starting point. What is 'normal'? Reduce to 1/4 and find out, then increase in 20% chunks until it all 'works'.CBNapamule

A sensible aproach. The flight dynamics of an aircraft depend on the ratio between the MOIs and the aerodynamic forces. Get the ratio right and the dynamics will appear correct.

I was trying to find a good engineering approach to calculating MOIs as well. I bought USAF DATCOM and there was a entire section on how to calculate these figures. I've used them for several models and they work very well. It is a long and involved process so I made it part of my Excel spreadsheet. The big thing is calculating component weights for the aircraft (wing, V. Tail, H. Tail, Engines, etc). I found an interresting paper done at Stanford ( http://adg.stanford.edu/aa241/structures/componentweight.html ) to help out. But some of the constants have to be tweaked until you get ballpark figures for the Empty Weight. I'm thinking this is because the data was from older, mostly-aluminum aircraft where today's aircraft have more composites. But these two items together seem to get me into the right section of the ballpark. :)Scott B.