Aerodynamic question for you engineers

Larry Dighera wrote in
:

On Mon, 28 Jan 2008 07:20:09 +0100 (CET), Nomen Nescio
wrote in
:

Your inability to distinguish rotation from translation.

I'm unfamiliar with the term 'translation' as it refers to this issue.
I presume you are referring to one of these definitions:

(1) : a transformation of coordinates in which the new axes are
parallel to the old ones

(2) : uniform motion of a body in a straight line

Can I impose on your MIT BSME to elaborate as to which definition it
is to which you are referring? Thank you.



What, you couldn't find anything in the FARs about it?



Let me (and Merriam-Webster) take a guess first. Rotation is the
action or process of rotating on or as if on an axis or center, and
translation in this case is the movement or change of location of the
CG.

Am I helping or hurting?



You're an idiot.


Bertie

Nomen Nescio wrote in
:

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From:

So, where's the flaw? Does anyone else see the flaw? Is there
a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
identify?

I did identify the flaw.
You can't seem to grasp it.

Is this a serious, enlightening discussion or some sort of
aviation Trivial Pursuit?

A little of both, IF......you actually want to learn something.

Try this one:
You launch a missile at the moon. Halfway there you check it's
free trajectory and find that it will hit dead center in crater X.
Suddenly it explodes into a million pieces, all spinning off into
space in all directions. What happens to the center of mass?

Depends, Plain or peanut center?

Bertie

As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.


no you cant.

an off centre force applied to a body resolves as a couple about the
cg and a force through the CG
the force through the CG pushes it along but the couple result in a
rotation about the cg.


in one of my old engineering texts the gentre of gravity is defined as
the point within a body about which all forces resolve.

the CG isnt a little sticker placed somewhere within a body. it is the
centre of the mass of the thing and it varies with the movement of
parts of that mass, be it evaporation of fuel in a carby which is
burnt and jetisoned overboard as gasses coming out the exhaust or
sweat evaporating from the brow of the pilot.

all forces resolve around the centre of gravity of the body.
it is an inherent attribute of all matter as seen through the eyes of
an engineer.

you really need to go out and fly a little aeroplane with not much
inertia. (like a tailwind) you will see that all gusts and control
inputs result in movements about the CG. it is what makes the
behaviour predictable and the damn things flyable (and the predictive
stress calculations possible)

Stealth Pilot

On Jan 28, 1:10 am, Nomen Nescio wrote:
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From:

So, where's the flaw? Does anyone else see the flaw? Is there
a flaw? Is Nescio here the only one who sees a flaw? A flaw he won't
identify?

I did identify the flaw.
You can't seem to grasp it.

Is this a serious, enlightening discussion or some sort of
aviation Trivial Pursuit?

A little of both, IF......you actually want to learn something.

Try this one:
You launch a missile at the moon. Halfway there you check it's
free trajectory and find that it will hit dead center in crater X. Suddenly
it explodes into a million pieces, all spinning off into space in all directions.
What happens to the center of mass?

I gotcha now. No problem. No more argument.

Dan