REFLECTOR: MODIFICATIONS

[Jim Sower]

Tom Martino wrote:

> 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 believe it's also a misquote.  The discussion concerned inertia coupling.

>  
>
> I think what people mean to say is:  "When the CM gets too far astray 
> from the CG envelope, the object become unstable".
>
I don't believe anyone said that.  Re-read the relevant posts more slowly.

>  
>
> 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".
>
True as far as it goes, but the discussion concerned two situations.  In 
the first case, the mass of the airplane is concentrated close to the 
CG, like two kids sitting on the fulcrum of a teeter totter..  In the 
second, the mass (like a particularly heavy engine and the compensating 
lead ballast in the nose) is concentrated farther from the CG - like 
kids on the end of the teeter totter.  CG is in the same point, but 
dynamic behavior is different. 

>  
>
> 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".
>
You are describing a *static *condition - no inertial forces.  The 
discussion was about inertial coupling - a *dynamic* phenomenon.

>  
>
> 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!
>
Assuming aft CG where control forces are light, snatch in a big bunch of 
aft stick.
In the first case above (mass concentrated near CG), rotation will tend 
stop when you run out of pitch authority.
But in the second case, with mass concentrations farther from the CG, 
the rotational velocity of the mass concentration*S* give them much more 
rotational momentum that will tend to keep the airplane rotating past 
where aerodynamic influences would normally stop it.

>  
>
> 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.
>
As far as ground handling, the airplane rotates about the point where 
the main mounts touch the runway.  This point being *aft* of the CG, it 
takes more canard lift to rotate about this point than about the CG.  As 
soon as the airplane breaks ground, it is suddenly rotating around the 
CG and instantly requires less canard lift to rotate.  That is why 
canards (particularly Velocitys, where the main mounts were deliberately 
mounted farther aft than EZs, Cozys, etc.) want to over-rotate on takeoff. 
But that is a static situation that doesn't involve inertia ... Jim S.

>  
>
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