"Jon S Berndt" <[EMAIL PROTECTED]> wrote:
> On Fri, 13 Feb 2004 10:23:56 -0700
> Russell Suter <[EMAIL PROTECTED]> wrote:
>>Jon S Berndt wrote:
>>>But then, the FDM still has to report where the FDM is in a common
>>>reference frame.
>>
>>Exactly! At my company, we call this the control point and we have
>>standardized on the Empty Weight CG.
> The 3D model designer will likely have no idea where the empty weight
> CG is, nor will they often care. They do know where physical points on
> the aircraft are, however. Additionally, the empty weight CG will be a
> slippery item to standardize on. Does that mean no fuel? No cargo?
> nothing? no stores? the C/D model or the A/B model? etc. The VRP is a
> **solid** point of reference.
I realize that it is useful to agree to the nose as VRP in purpose to
take the load off the 3D model designer to determine the empty weight
CG. This is a valid argument.
Albeit I don't want to hide an argument that speaks for the empty
weight as a VRP which stems from practical experience as an engineer in
say a production environment, where reality 'sometimes' differs from
how things _should_ be in theory. You could call the rule: Reduce
possbile errors already in early stages of design.
Adopted to the current case this means: The longest distance from
whichever CG you take to the edges of the aircraft is _always_ smaller
than the longest distance from the nose to arbitrary edges. This
results in smaller relative 'errors' in case some details don't get
modelled as exact as it probably could be.
Consinder the case as an example that the flight model (not the FDM) is
built upon data from an early design of some aircraft - because access
to this data is easier. On the other hand, the 3D designer takes recent
pictures as a basis for his model, because old picures from the early
design are hard to find. If you take the empty weight as VRP then
chances are that the 3D model fits to the flight model with a smaller
error because probably during development the shape of the nose has
changed, a different radar was chosen (military aircraft), later
production models got a different propeller with a different spinner
(light aircraft) or whatever reason it might have had.
Cheers,
Martin.
--
Unix _IS_ user friendly - it's just selective about who its friends are !
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