Multi-Engine Controllability Question

[bLizZuE]

Thanks for teaching me something, yet again.

 

[shdw]

Well, it would, assuming the mass density on the two disks was uniform. The heavier disk would have a greater moment of inertia.

Explain a different way? I'm missing the link between this and the CG example above. Unless you're saying the density distribution, meaning some of the weight increase will occur at x radius off of the rotation point. The result then being some increase in rotational momentum?

 

[tgrayson]

Unless you're saying the density distribution, meaning some of the weight increase will occur at x radius off of the rotation point. The result then being some increase in rotational momentum?

Exactly. That's the intuitive idea behind moment of inertia. Even though for some purposes, we can act as if all the weight were located at the CG, it really isn't there.

 

[Xiph]

Even though for some purposes, we can act as if all the weight were located at the CG, it really isn't there.

For what purposes couldn't you assume that all the weight is at the CG? The way I look at it, if the CG is the sum of all the weights on the airplane, taking into account their position, then you should always be able to consider that all the weight is at the CG.

 

[tgrayson]

you should always be able to consider that all the weight is at the CG.

Anything involving structural integrity, for one. You can't pretend that no other components of the airplane have mass when you're subjecting an airframe to accelerations. If only the CG existed, that's the only area of the airplane that would need to be reinforced.

And the present discussion. To generalize it, pretty much anything to do with they dynamic stability of the airplane, as opposed to the static stability, requires consideration of mass distribution of the airplane. Google on "inertial coupling".

 

[shdw]

Exactly. That's the intuitive idea behind moment of inertia. Even though for some purposes, we can act as if all the weight were located at the CG, it really isn't there.

Ok, driving home I was thinking about it more. It might be easier to not alternate terms, CG = axis of rotation. Replace any mention of cg with "axis of rotation" which is the term used in your formula above and it makes perfect sense. New weight must be applied to the axis of rotation (cg) or the moment of rotation will change.

 

[SuperCubRick]

Great discussion - I don't have my multi engine yet, but when I do I'll be armed with a better understanding of the physics involved thanks to JC,
and especially TGrayson! :beer: