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Aug 01, 2001, 09:28 PM
Registered User
imagine drawing the longest line possible from the front to the back of the chord profile. This is the line you should use.

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Aug 01, 2001, 11:11 PM
Registered User

The center line is a line that runs from the center of the leading edge radius to the trailing edge. That's the line you use for setting up the wing/stab relationship.

Bottom of airfoil etc.. means nothing.
Picture a dowel forming the leading edge of the chord, the point on the dowel farthest forward is the "center of the leading edge radius" that you use to draw the line to the point of the trailing edge.


Aug 02, 2001, 12:26 AM
Registered User

Designing in 0 deg. incidence

Would someone please clarify 0 deg. incidence for me in layman's terms? For example- say I wanted to design a plane with a clark y airfoil where the wing and stabilizer were both at 0 deg. incidence.

When I look at the clark y airfoil in Profili, there is a line called the center line that runs chord-wise. This line is not parallel to the bottom edge of the airfoil. Would I use this line as my 0 deg. reference, would I use the bottom edge of the airfoil as my 0 deg. reference, or is there some other way to figure the 0 deg. incidence of this airfoil?

What if I was using a semi-symmetrical airfoil- how would I find the 0 deg. reference then?

Aug 02, 2001, 12:30 AM
Registered User
Foxtrot's Avatar
You would use the line that runs from the airfoil leading edge to trailing edge as your reference line not the bottom of the airfoil. At zero degrees that line and the line and the stabalizer would be parallel to each other. Hope this helps. You're thinking along the right lines it seems. This stuff was boggling my mind a couple of months ago too.

Aug 02, 2001, 12:59 AM
Registered User
Check the thread 'small chord'. You'll find a nice diagram in there, courtesy of Sparky Paul, explaining the same thing to me.

Aug 02, 2001, 09:05 AM
Registered User
Jim Ryan's Avatar
A couple of clarifications: People talk in terms of using the center line of the foil section or drawing a line through the longest axis of the foil. That will get you close.

Every airfoil has a "zero lift line". As the name implies, this is the angle of attack at which the foil is generating no lift. On a Clark Y, the zero lift line is (if memory serves) just under -3 degrees.

So to build a model with zero incidence, you want the zero lift line of the wing and stab to be parallel to the thrust line (which may also be the datum line).


Aug 02, 2001, 01:34 PM
Registered User
Sabrejock's Avatar
And because none of us is perfect, take a tip from the Bridi designs and always plan for a tiny bit of positive incidence. If you get it slightly wrong, no big deal. If you try for zero and get it even a tiny bit wrong, you have a can of worms. Tex.