EmptyInertia calculation

[FrankD]

hello modders,

 

 

EmptyInertia parameter being the initial step stone of any Flight Model, I would very like to get its calculation right and for that, I would like to receive your opinion on my below computing.

 

From the SFeditNotes, its sources and a post by NeverEnough , I know that:

 

 

- EmptyInertia parameter is the base for the engine to compute inertia of the aircraft from take-off to the end of the flight, in the Y (left/right), X (Fore/aft) and Z (up/down) directions.

 

- the values can be determined with the following formulas elaborated by Dr Jan Roskam:

 

Inertia (Y Direction) Iy= (W/g)*(Ry*d/2)^2

Inertia (X Direction) Ix= (W/g)*(Rx*b/2)^2

Inertia (Z Direction) Iz= (W/g)*(Rz*e/2)^2

 

where:

 

g = gravitational acceleration (m.s-2 I assume)

W = EmptyMass (Kg, as entered in the data.ini)

b = ReferenceSpan (m, as entered in the data.ini)

d = ReferenceLength (m, as entered in the data.ini)

e =(b+d)/2

 

Ry, Rx and Rz are average values for a Jet Fighter: 0.346;0.266;0.400

 

(* = Multiplication, ^ = Power of, / = Division)

 

 

So:

 

G = 9.81

W = 13473.79

b = 10.65

d = 19.58

e = 15.115

 

(F-105F, SF2V modded)

 

Inertia (Y Direction) Iy= (13473.79/9.81)*(0.346*19.58/2)^2 = 15759.35

Inertia (X Direction) Ix= (13473.79/9.81)*(0.266*10.65/2)^2 = 2755.65

Inertia (Z Direction) Iz= (13473.79/9.81)*(0.400*15.115/2)^2 = 12551.54

 

 

Now, what annoy me is that it's so far from what we actually have in the game:

 

EmptyInertia=136874.8,21369.7,180869.1

So, is my calculation wrong? Or is it the radius (R* figures) that are?

 

If, hopefully, my calculation is sound, and that the radius are the weakness, where to find more accurate one... for all of our aircrafts? (I'm a big fan of normalization, as you will discover... soon™)

 

 

Thanks for your attention and TIA for your inputs.

[+streakeagle]

W is weight not mass W = m * g, so W/g = m = mass in kg.

I bet that will change your numbers a bit :)

[Fubar512]

Averages mean little in this case.

 

For example, one cannot equate an A-4 to an F-105, so the use of averages works against one in that case. Also, how does one compensate for an aircraft with a high-polar moment of inertia versus one that has a low-polar one? What of modern fighters, which almost to the very last, are tail heavy by comparison to their predecessors from 40 years ago?

 

My point is that moments of inertia need to be calculated and tailored for every model.

[FrankD]

W is weight not mass W = m * g, so W/g = m = mass in kg.

I bet that will change your numbers a bit :)

 

Good catch Streakeagle! That would makes my estimation closer to the rest of the figures.

Averages mean little in this case.

 

For example, one cannot equate an A-4 to an F-105, so the use of averages works against one in that case. Also, how does one compensate for an aircraft with a high-polar moment of inertia versus one that has a low-polar one? What of modern fighters, which almost to the very last, are tail heavy by comparison to their predecessors from 40 years ago?

 

My point is that moments of inertia need to be calculated and tailored for every model.

 

 

You are, of course, right, it would never be more than an approximation but is a tailored compution possible for all aicrafts? (especially if one don't have access to the model)

 

 

 

Thank you both for your input.

Edited March 14, 2011 by FrankD

[FrankD]

Gents,

 

I found a moment of inertia expressed in "slug-ft^²".

 

As slug is an imperial unit of measure, I would like to convert it to a metric measure, but I do ignore what is the equivalent unit of measure and, last but not least, how to convert "slug-ft^²" to it.

 

I tried to convert it to Kg per m/s by multiplying it by 14.5939 (a slug in Kg) and by then dividing the result by 0.3048 but, as you can guess, that was a no go as I obtained way to high figures.

Edit: I bet that it didn't work, moron I am! It have to be read "slug per square foot"

 

I found an online converter but there's still difficulty for me.

 

What is the equivalent metric unit of measure, at least the one we are actually using in game?

 

Would it be "kilogram square meter" or "kilogram force meter square sec."?

 

Would any educated soul around know the way and in the mood for sharing it?

 

TIA

Glad that TW's forum is back, here's the answer from TK:

The inertia is not in slugs (thats not even correct unit for inertia' date=' inertia in english unit should be slug-ft^2), it should be in kg-m^2. To convert from slug-ft^2, you just need to convert slug-to-kg, times ft-to-m, times ft-to-m, whatever that comes out to be :)[/size']

TK

--

Well, me, pleased to meet me. We should work together more often, don't you think?

(Yeah, you got it, it's my new trick to drink two beers and pretend that I worth it!)

Edited March 17, 2011 by FrankD

[+Monty CZ]

Hello I think TK uses SI units for all ingame calculations

http://en.wikipedia.org/wiki/International_System_of_Units

so by that estimation is it kg . m2

Monty CZ