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FM/DM threads Everything about FM/DM in CoD

 
 
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Old 07-17-2012, 05:16 AM
MiG-3U MiG-3U is offline
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Quote:
Originally Posted by CaptainDoggles View Post
You know Crumpp's right about expressing CG as a percentage of MAC. The Datum point doesn't have to be in the same spot for the results to be valid. That's why it's called a datum point.
There is only one accurate reference point for the CoG in the NACA report, the distance of the CoG from the leading edge at the wing root and that is given as 31.4". NACA admits that their measurements for MAC maybe in error and we can easily see that there is error because in the RM2535 the 34% CoG location at the RAE measured MAC is also given same way as distance from the leading edge at the wing root and the value is 2.638' which is 31.656". Even with these values only we can estimate that the real CoG location was about 33.7% at the MAC given by RAE instead 31.4% claimed by NACA (and NACA admited that their value might be wrong).

As we know accurate reference point at the wing root and dimensions for MAC used by RAE and A&AEE and datum line, we can also easily calculate these.

Lenght of the MAC measured by RAE and A&AEE is 78.54" (or 6,54') and position 31.4" behind leading edge at root is 26.4476" at MAC and that means that CoG was at position 33.6741% in the NACA tests using RAE and A&AEE dimensions.

However, British documentation gives CoG values usually as distance from the datum line so we need to make NACA CoG location comparable with these. And that is easy because we know that the datum line is 18.65" behind leading edge at the MAC:

26.4476" - 18.65" = 7.7976"

And this value, 7.8" aft datum line, is comparable with the other sources like A&AEE and RAE tests and loading instructions.

Over and out.
 


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