I am not an expert on airfoils, but I suspect the problem is because the zero lift line on the airfoil is most likely not parallel with the bottom of the two spars. I don't know exactly where it is on the airfoils in question, but someone else on the list should know.
I did look in one of the aircraft design books I have and it states that you should draw a chord line from the trailing edge to the middle of the leading edge, then you draw a perpendicular line at the half chord position from the top surface of the airfoil to the bottom surface of the airfoil. Then you find the center of this line. Draw another line from the trailing edge through this center point and that is your zero lift line that you measure the angle of incidence from. Typically that gives you a line from the trailing edge to a point somewhere above the leading edge. Now I am not sure weather this is really good for every airfoil or if it is just a rule of thumb that gets you close for most of them, but the moral of this story is that there is a correct line somewhere for that particular airfoil that may, or more likely, may not be parallel with the bottoms of the spars. robert tallini wrote: Hi gang, here I go again, I am going to risk the possibility of another personal attack on my building know-how by asking for help in a problem I have with the KR plans. In checking the plans, Drawing #2 for the KR2, [this sheet is included , as you know ] in plans for the KR2S I checked the angle of incidence by running a line from the bottom of the two spars and extending it till it intersected with the extended chord line of the rib. Measuring 48" from the intersection the distance between the thrust line, and the angle of incidence of the wing could be determined in inches. As most of you agree, the plus 3.5 degrees incidence is excessive. had intended to reduce this regardless of the airfoil I used. However, I wanted to see how much I would have to shim the lower longeron at Stations G & H. To my surprise my measurements indicate the angle of incidence, if you follow the plans exactly, give a 7 degree angle, not 3.5. I arrived at this conclusion by multiplying the 48" chord by pi. Finding the circumference, I divided by 360 to get the inch equivilent to a degree. Multiplying this by 3.5 I would get the angular difference between the chord line of the rib and the thrust line of the fuselage in inches. Let me digress for a moment, I KNOW that ALL of you are perfectly familiar with finding the relationship between degrees and inches. I just felt that letting all of YOU know that I do, too. It might finesse aspersions re. my I.Q. I believe, with all the KR's that have been, or are being built, someone checked this out. The only mention in the manual or the plans is to use the top longerons as reference. But the attach points on the lower longerons are parallel at Stations E and H at 20" to the top longerons. Could it be possible that a "strictly plans built" KR2 or 2S could be flying with a 7 degree angle of inc. My question is, as Bill O'Reilly would say, where have I gone wrong Is my geometry that bad, should I abandon all hope of building a KR2S and as has been suggested, start learning to be a GOOD pastry chef? Happy New Year to everyone, Bob Tallini _______________________________________________ see KRnet list details at http://www.krnet.org/instructions.html
