REFLECTOR: Re: Statics & Dynamics - Center of Gravity

Tom Martino tmartino at troubleshooter.com
Tue Jan 4 20:05:05 CST 2005


I believe flying "real smooth" is good for most Velocity aircraft ...
they all have heavy engines out back, relative to their mass up front.
However, balance is not achieved by objects (such as batteries vs
engines) ... balance is achieved through weight and moments.  
 
Angular momentum is the product of mass, distance and velocity as they
relate to the center of rotation.  It would indeed be unbalanced with no
counterweight or mass.  But as with any plane "in balance" (ie, placing
CG within proper limits) there is a counterweight.  Any stability or
"static" nature of things can be disturbed by not rotating smoothly.  
 
Am I missing something?
 
-----Original Message-----
From: KeithHallsten [mailto:KeithHallsten at quiknet.com] 
Sent: Tuesday, January 04, 2005 5:46 PM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Re: Statics & Dynamics - Center of Gravity
 
Center of mass and center of gravity are two names for the same thing.  
 
However, the distribution of mass can give a different polar moment of
inertia even if the overall mass and the center of gravity is the same.
This will result in more angular momentum at a given rate of rotation
when the mass is distributed farther from the center of mass.
Therefore, the potential for over-rotating is more significant when you
have a heavy engine balanced by, say, a heavy battery tucked into the
nose.  
 
As long as you fly real smooth, with low angular accelerations, you have
a "quasi-static" situation, in which the current forces are most
significant.  However, as angular accelerations increase, the dynamic
effects become more pronounced and statics alone cannot explain the
behavior.
 
Keith Hallsten, Mechanical Engineer & Civil Engineer
   
	----- Original Message ----- 
	From: Tom Martino <mailto:tmartino at troubleshooter.com>  
	To: reflector at tvbf.org 
	Sent: Tuesday, January 04, 2005 11:38 AM
	Subject: REFLECTOR: MODIFICATIONS
	 
	I don't profess to be a genius, but I do have extensive
education in physics and related sciences ... and what I am reading on
this forum about "Center of Gravity" (CG) and "Center of Mass" (CM) is
not accurate.
	 
	The following statement is inaccurate:  "When the center of mass
gets too far away from the center of gravity, it makes the object
unstable".
	 
	I think what people mean to say is:  "When the CM gets too far
astray from the CG envelope, the object become unstable".
	 
	Physics dictates that the CG must be the same as CM when weights
and moments are factored by their distances from a common reference
point.  They are one and the same ... also referred to as the
"Centroid".
	 
	A CG Envelope is determined by the Mean Aerodynamic Cord (which
is the average distance from the leading and trailing edge of the air
foil).  The middle of this is the center of lift.  
	 
	If you picture the plane being lifted on a string at its center
of lift ... the CG takes on simple meaning.  If forward ... the object
will lean forward, if aft, it will lean aft.
	 
	Heavy weight on one end or the other has nothing to do with it,
so long as the CG has been calculated properly and falls within the safe
envelope.  That is why it is called "Center of Gravity".
	 
	Sudden, jerky movements can disrupt the stability but does not
make it inherently unstable.  However, when you are close to the ground,
you may need more time to recover than the ground will allow!
	 
	In other words ... as you get farther away from the center of
the envelope, the recovery becomes more difficult but it is almost never
dangerous unless close to the ground or "out of balance".
	 
	As far as ground handling, the center of rotation is the only
thing that matters at lower speeds.  As speed increases you shift over
to center of lift and ascend.  When the CG is aft in the envelope, the
transition could cause a prop strike if not rotated properly.
	 
	
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