Airfoil Incidence?

[Wing Zero]

I have a picture of a clark Y airfoil section. On it, there is a line shown through the view from the exact point the LE touches to the TE. I assume this is the 0 degree incidence point? Sorry if I sound like a dope! I just don't understand how these foil sections have different centers of lines through them, only thing I could think of was that they are showing zero incidence.

[markschaffin]

The line connecting the LE and the TE is the chord line. This defines the angle of attack of the airfoil. It is always straight. Note that this is rarely the angle where lift is zero.

Mark

[Texas Buzzard]

I have a picture of a clark Y airfoil section. On it, there is a line shown through the view from the exact point the LE touches to the TE. I assume this is the 0 degree incidence point? Sorry if I sound like a dope! I just don't understand how these foil sections have different centers of lines through them, only thing I could think of was that they are showing zero incidence.
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Wing Zero,
You are absolutely correct. You aren't a dope.

In practice when designing models fo over 35 years I have used exactly what you surmised succesfully.

If that Clary-Y was sitting on your flat table, aerodynamically it would nave an incidence relative to the table of about 2 degrees+. :) :)

[Wing Zero]

Thank you very much! :) Now I can move on.

[MarkusN]

There are no absolute rules on this. These are matters of convention. Today the agreed upon consensus is that the chord line ist to be used as the line of reference for the angle of incidence. This has not always been the case, and especially not for sections like the Clark Y. In many cases folks used to use the lower side tangent for the reference line where this was applicable. So if you find references on angle fo incidence, its always sane to check your sources.

[Texas Buzzard]

There are no absolute rules on this. These are matters of convention. Today the agreed upon consensus is that the chord line ist to be used as the line of reference for the angle of incidence. This has not always been the case, and especially not for sections like the Clark Y. In many cases folks used to use the lower side tangent for the reference line where this was applicable. So if you find references on angle fo incidence, its always sane to check your sources.
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You may be absolutely correct - BUT.

In University courses the "convential" is accepted. The wind tunnel usually starts measuring L & D at -4 degrees. You can talk about incedence but wind tunnels have to START from somewhere. And it works.