# FSX-MS Moments of Inertia (MOI)

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I'm trying to upgrade an aircraft (alabeo's da42 to da42-VI) but I'm stuck in the aircraft configuration file, section [weight_and_balance].  Aircraft Configuration File

I need to calculate the MOI for Pitch, Roll and Yaw and formula according to the above link is MOI = EmptyWeight * (D^2 / K), it looks like I need to know the length which is 28ft and 1 inch and the wingspan which is 44 feet and 4 inch.

The D and the K values are in the link above, I'm not a math or calculus person though and I don't know how to solve it.  Any help calculating the values for these 3 MOIs would be greatly appreciated.  Thanks in advance!

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I forgot to mention the empty weight for this aircraft which is 3,109 lbs

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idude,

I did the MOI calculations two different ways for you.  Try them both and see which one seems to best fit your project.

First I did them using the formula in the ESP you linked which, as mentioned, is very "generic" in nature and a good starting point.  Using that formula here are the values given your weight, length, and wingspan data...

MOIPitch=3009.20494

MOIRoll=3267.191406

MOIYaw=5280.88774

These values will get you close, but I have been using a different method to calculate MOIs for a number of years ever since I discovered a "talking paper" on it by a gentlemen named Tom Goodrick.  For the life of me I cannot find the link to his paper, but I did copy and save the pdf to my documents folder, so I constantly return to it to use his method instead of the ESP method and the aircraft almost without fail will fly better.  First a little background so you understand why I use his method.

Tom Goodrick collaborated with Professor Roskam who taught aeronautical engineering at Kansas State University.  Professor Roskam has written several papers on the topic and gives lectures to NASA and to many aerospace companies, including and especially Beechcraft and Cessna. His method is based on dimensionless radii of gyration for basic aircraft types which are given in tables he publishes.  Based on his formula these are the "revised" values I got...

MOIPitch=2048.390333

MOIRoll=3207.050522

MOIYaw=5026.476015

Notice one thing, though, MOIP is less than MOIR is less than MOIY in both sets of figures.  Tom Goodrick, using Professor Roskam's knowledge suggested that you should almost always see MOIR < MOIP < MOIY which is not the case here.  However, I suspect this is because the wingspan is almost double that of the length in the aircraft.

Anyway, hope this helps.  Try both sets of data and see how she flies...

Randy

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Thank you so much, Randy, I will definitely try both and let you know which one feels better.

Ed

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Here is a original post by Tom Goodrick: http://bgaforums.proboards.com/thread/269/calculating-mois

You need to google "Tom Goodrick moments of inertia" and you are going to find a lot of links with his document.

There is a also another great document about MSFS flight dynamics by Yves Guillaume: https://www.fsdeveloper.com/forum/resources/flight-dynamics-in-msfs-v1-0.169/

I used a lot both of them

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Posted (edited)

@idude,  Let us know how they compare idude, if you will.  I, personally, am curious.

@lodestar,  Thanks for the link to Tom's MOI post.  Much appreciated.  The second link to an FSDeveloper post speaks of a pdf document to download, but I don't see a download link anywhere.  Probably just me "looking" but not "seeing". 😁

Randy

Edited by Ramjett
To point comments to the correct poster

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55 minutes ago, Ramjett said:

Just click on the orange "download" button in the right upper corner  If you don't see it, try different browser

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59 minutes ago, Ramjett said:

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13 hours ago, Ramjett said:

@Ramjett  I sure will.

Ed

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Quote

Ha Ha, got it.  I had scrolled down to read the post and didn't scroll back up far enough.  Thanks guys.

Randy

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On 6/13/2018 at 9:12 PM, Ramjett said:

@idude,  Let us know how they compare idude, if you will.  I, personally, am curious.

Randy

2

Hi Randy,

As of now, I'm more inclined to use the first set of MOI calculations,  I feel that the second set produces a little too much lift, as I go down the runway the aircraft just goes airborne without any "flaps" or me touching the stick which is not the case with the first set!  Does it make sense to you?

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It does and I completely understand, but that still begs the question of which set of numbers is "more correct" in emulating the real world aircraft?  What you need to have to "complete" your model and make your choice is input from someone who has flown a real world DA42 or at the least an entry from the DA42 POH that explains takeoff settings.  For instance is a "no flaps" takeoff on a longer runway a normal procedure?  What was your trim setting vs a normal "takeoff trim" setting?

I get the impression you are using numbers that give the aircraft model a typical "flight simulator" profile and not a typical "real world" profile because using flaps and trim is "the way it's done" in flight simulator.

Regardless, I'm glad the numbers worked, no matter which set you used.  Glad I could be of help.

Randy

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Posted (edited)

Finding someone who has flown this airplane would be a good idea but I have a feeling that is going to be hard.

The latest manual for this aircraft is here

Edited by idude

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