REFLECTOR: Re: Statics & Dynamics - Center of Gravity

Jorge Bujanda jbujanda at dslextreme.com
Tue Jan 4 20:24:11 CST 2005


True.  Although those who like Martinis would probably argue the 
toothpick part.
I like olives...



Tom Martino wrote:

>Mass closer to CG stabilizes faster when disturbed.  Momentum causes
>extended mass (away from CG) to take longer to find equilibrium.  But
>the nature of most airplanes is such that mass is usually extended a
>good distance from CG.  Therefore "jerky" movement or uneven rotations
>can cause stability problems.  That is why the most important thin in a
>plane is balance and structure to support the extended moments.
>
>By the way ... two olives in a Martini (at the bottom of the glass) will
>not help stability if you have to drink it to get them.  It would be
>more stable to keep them on a toothpick.  
>
> 
>
>-----Original Message-----
>From: Jorge Bujanda [mailto:jbujanda at dslextreme.com] 
>Sent: Tuesday, January 04, 2005 6:44 PM
>To: Velocity Aircraft Owners and Builders list
>Subject: Re: REFLECTOR: Re: Statics & Dynamics - Center of Gravity
>
>Keith/Tom
>
>Excuse my ignorance but I have much to learn about airplanes and 
>aerodynamics. For those like me who only were exposed to physics in high
>
>school, I want to try to have this one clear in terms I can understand. 
>Is the following a true statement based on your comments?
>
>When abruptly disturbed, the structure formed by two olives in the 
>middle of a skewer hanging by a thread from the center of gravity is 
>more stable, or more prone to be returned to its original position, than
>
>the structure of the same two olives (same mass) located one at each end
>
>of the skewer, for any given disturbing force? In other words, mass 
>closer to the CG is better than distant from the CG?
>Please clarify. Thanks
>
>Regards,
>Jorge Bujanda
>XL FG
>Palmdale, CA
>
>"Make your best case... not the best case."
>"Spare the noise... convince through silence."
>
>
>
>
>KeithHallsten wrote:
>
>  
>
>>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 <mailto: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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