Tweeking The Flight Models

[Guest Tazkiller]

In the Knowalge Base there is a formula discused to figure moments of inertia.

 

 

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

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

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

 

where g=32.2 (gravity acceleration)

W=gross takeoff weight (lb)

b=span (ft)

d=length (ft)

e=(b+d)/2 and Rx, Ry and Rz are chosen from the following table for the aircraft type.

 

* = Multiplication

^ = Power of

/ = Division

 

Aircraft Type ROLL Rx PITCH Ry YAW Rz

Jet Fighter .266 .346 .400

 

Is this the formula correct.

 

All the specific (aircraft data ini.) entries ask for metric values in all other areas.

 

So in following the formula do i need to change lbs. to kg. and feet to meters. Or leave it in Imperial (U.S.) system terms.

 

Does Gross Takeoff Weight = A Clean Aircraft; ( No External Tanks, Bombs, ect) or exactly as stated.

 

I tend to believe it would have to be a clean aircraft, but i don't know.

 

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[+Fubar512]

Gross take off weight or "Max Gross Takeoff Weight". If it's the latter, that would mean full internal fuel /ammo/ loaded hard points. While we're on the subject, there's also Max Ramp Weight, which can exceed Max Takeoff Weight.

[Guest Tazkiller]

Gross take off weight or "Max Gross Takeoff Weight". If it's the latter, that would mean full internal fuel /ammo/ loaded hard points. While we're on the subject, there's also Max Ramp Weight, which can exceed Max Takeoff Weight.

 

I am seeking defination of terms in the knowledge base.

 

The data ini asks for "Empty Moments Of Inertia"

 

I believe this should be figured by Empty Weight, just as the ini asks for empty mass.

 

The formula Asks for Gross Take Off weight.

 

These are two seprate values.

 

Also, The data ini values are in metric, the formula is in Imperial Units.

 

These are two seprate values and in the Formula yield two different sets of values.

 

Does it read Imperial Moments of Inertia or Metric Moments of Inertia?

 

Also the data ini asks for "Empty" Moments; not "Loaded" as the Knowledge Base Formula instructs.

 

Probably the most esoteric aspect of setting up an aircraft model is the estimation of Moments of Inertia or MOI's.

[+SkippyBing]

As I remember (and it's been a while since I've played around with it) the formula isn't in Imperial per se, it's just the units used to explain it are. e.g. for metric g=9.8 not 32, you use m or cm rather than feet (whichever SF uses, I can't remember), kg not lb. The only problem I had was figuring out what the metric equivalent of the inertia unit was (I seem to remember it's called a slug or something I think it's lbft/m). I think the formula stuff is originally taken from an FS tutorial or some such hence the units, SF is all metric. The moments will be for an empty aircraft as everything you add (fuel, weapons, pilot even) have a mass and a position which means the program can work out the moment of inertia. I think the value for Rx etc. is essentially a best guess for that type of aircraft (I remember seeing a table for different types of a/c somewhere), military aircraft traditionally being tightly packaged, whereas airliners have a low density in the middle (where the pax sit).

Hope that helps.