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Old Mar 23, 2012, 10:48 PM
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United States, OR, Corvallis
Joined Jan 2010
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Question
Wing/Airfoil chord line angle in "level" flight Question

Just curious since I'm thinking about the incidence on the wing(and relative to the horiz stab)on my glider I'm building(Modified 2M Spirit) with a modified fusalage.

***My question is this:
- If, for instance, you have a wing with an airfoil with a raised leading edge with "Philips Entry" sweeping back to the bottom spar, then flat on the bottom from the bottom spar to the trailing edge and a nice "standard"(I know, there isn't really a "standard") curved top (Selig 3010? I think), and if you draw a chord-line from the tip of the leading edge to the tip of the trailing edge, Is that wing generally flying through the air on "that" line during "level" flight? i.e.- Is forward travel of the wing through the air parallel to the chord-line?
- Or, is the wing generally flying with some angle-of-attack of the chord line relative to the airflow? To me, a true glider without a motor seems like it should'nt really be flying with an angle of attack in "level" flight which seems like it would just be shoving into the air and slowing down rapidly.

Yes, I know that without the effect of thermals or slope lift, "level" flight is really going slowly downward to the ground, so I'm calling that "level" for my question at this point.

Thanks - Paul
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Old Mar 24, 2012, 01:10 AM
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Joensuu, Finland
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That really depends on the airfoil and to answer you question a Cl/alpha plot of that particular airfoil would be needed. Typically, but not necessarily, there is a couple of degrees of positive AoA. The drag created in the process is compensated by pull of gravity.
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Old Apr 03, 2012, 12:22 AM
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United States, MA, Waltham
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Let's say you want to fly around at a Cl of 0.6 (a bit on the slow side) and that you're using the 3021. If we go here:
http://www.worldofkrauss.com/foils/967
we can see that the angle of attack for this lift coefficient is a little over 1 degree. We can assume that the tail acts at a very low angle of attack, so in this case the tail incidence ought to be somewhat more than a degree less than the incidence of the wing. What incidence depends on lining up the fuselage with the airflow. Note that for 0 lift you actually have to have a slightly negative angle of attack, and this is true for many cambered sections. In somewhat faster flight with the 3021, you might be at 0 angle of attack and Cl of 0.3

The difference in incidence between the stab and the wing isn't all THAT big a deal because for moderate deviations you can adjust the elevator position to fix it.

Hope this makes sense!
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Old Apr 05, 2012, 10:00 PM
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United States, OR, Corvallis
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Thanks for the Input!

Hey Thanks for responding, I've been slow to respond due to house-related things. And a model rocket. Hopefully I'll get back soon to working my modified Spirit project I have going.

Adios - Paul
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