## Pitching Moment of Inertia

## Pitching Moment of Inertia

(OP)

Hi everyone,

Just wonder if anyone could explain me with an example how to calculate the Pitching Moment of Inertia on aircraft.

Aircraft Details;

Weight 50t

fuselage length 40m

wing span 37m

c.g from nose 20m

Ry 0.38 for 2engine cargo aircraft

Any other missing information please make up. I tried and looked everywhere but Im still confused. Please Help.

Thanks a lot

Just wonder if anyone could explain me with an example how to calculate the Pitching Moment of Inertia on aircraft.

Aircraft Details;

Weight 50t

fuselage length 40m

wing span 37m

c.g from nose 20m

Ry 0.38 for 2engine cargo aircraft

Any other missing information please make up. I tried and looked everywhere but Im still confused. Please Help.

Thanks a lot

## RE: Pitching Moment of Inertia

If the former is correct then Iyy=50*(.38*40)^2*1000 kg m^2

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Pitching Moment of Inertia

The R value comes in a form of Rx Ry and Rz with a dash above them. As I am only required to find pitch moment therefore I only need Ry.

The formula I found in the text book to calculate moment of inertia is;

Ixx=....

Iyy=(L^2*W*R^2)/(4g) L=fuselage length W=weight

Izz=...

I have only gave the formula for Iyy as I guess its the one that need to be use for pitching moment of inertia.

As I have read few books, I found out that the moment of inertia has to do something with the centre of gravity. However, the equation I gave you above has nothing to do with aircraft centre of gravity.

Please show me the solution with an example if you can.

Thanks a lot

## RE: Pitching Moment of Inertia

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Pitching Moment of Inertia

What if I do not have Ry value. What formula should I use instead or how do I calculate Ry.

Thanks a lot

## RE: Pitching Moment of Inertia

Then Ry^2=(pitching moment of inertia)/(total mass)/L^2

This is high school physics, are you sure you are in the right course?

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Pitching Moment of Inertia

is this a student problem ? the numbers are too "round", it's unlikely that the airplane CG would be in the middle of the fuselage.

Ry looks like radius of gyration, as explained by the OP.

why the "4" in "4g" ?

another day in paradise, or is paradise one day closer ?

## RE: Pitching Moment of Inertia

Greg Locock

## RE: Pitching Moment of Inertia

Ry is the radius of gryration about the CG, but if you assumed it was about the nose, then there'd be a shift squared term (a 4 times factor ... bd^3/12 vs bd^3/3)

another day in paradise, or is paradise one day closer ?

## RE: Pitching Moment of Inertia

Iyy=M*(Ry*L)^2

whereas they have used

Iyy=M*(Ry*L/2)^2

The confusion comes because Ry is not a radius, it is a scale number, and so its values depends on the equation you use. Both are valid but theirs results in bigger numbers for Ry, and it seems more natural to scale radius than diameter I suppose.

Greg Locock

## RE: Pitching Moment of Inertia

another day in paradise, or is paradise one day closer ?