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Moment Coefficient Values?

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NorthwestJack

Well-Known Member
Joined
Feb 8, 2008
Messages
73
Location
Monroe ,Wa ( close to Seattle)
In designing my airplane I have taken into account all the references to pitching moments which I have encountered in this forum.
For example the penalty for using a highly cambered airfoil or a laminar airfoil gives no significant advantage in lowering Cd because the increase in pitching moment give rise to a large negative lift at the tail which eliminates the advantage of lower drag.
Why do sailplanes use these airfoils then ( ie Eppler and Wortman Sailplane airfoils?)
I am also confused about the Cm values in Abbot-V.D. book.
Example: When I look at the value of Cm a.c. for a Naca 4415 it is around .1
The value for Cm c/4 is given as .4
I would think that there should not be too much difference in position between Cm a.c. ( around .24) and Cm c/4 (at .25).
So why the big difference.
P.S. When I look at my Da Vinci airfoil selector ( computer program) the values for Cm a.c. and for Cm c/4 are very similar.
What Am I missing?
Confused :ermm:
 
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