means is the center of pressure doesn't move back and forward much on the airfoil which produces a low pitching moment and makes the airfoil easier to trim.
My friend, Harry Riblett, is a mechanical engineer, a former president of EAA Chapter 240, has built a Starduster Too and a CUBy and is currently working on an original design. We have some airfoil ideas that we would like to share with you from Harry. First is the concept of the camber line or mean line - the line that is equidistant between the upper and lower surfaces of an airfoil. On a symmetric airfoil it is, of course, a straight line. There can be a great deal of curve in the leading edge of an airfoil and a good example of this is the forward loaded 230XX mean line below or it can be aft loaded as the GAW-1 and GAW-2 series are. In the 230 family the 23012 has an extremely forward loaded mean line and very low trim drag - highly desirable. What this means is the center of pressure doesn't move back and forward much on the airfoil which produces a low pitching moment and makes the airfoil easier to trim. Due to the geometry of the 23012, it has a sharp stall, excellent for aerobatic aircraft such as the Stephens Akro, the Super Acro Sport and others. A sharp stall is not to be desired on a non-aerobatic aircraft. Its outstanding features are low center of pressure travel and low trim drag. The GAW series has an extremely aft loaded mean line and due to this the
drag below a coefficient of lift of .6 is very high.
Camber 230XX
GAW 1-2 Aft Loaded .——-——Forward Loaded
A feature of the GAW-1 and GAW-2 series of airfoils is the trailing edge cusp. Some of the users of these airfoils have reflexed the trailing edge up at cruising speed for better performance. The cusp is difficult to build and does cause occasional problems in getting proper aileron response. It seems that ailerons like to have flat surfaces on top and bottom surfaces. When conventionally designed airfoils are modified for flat surfaces in the control area, this is usually designated by "A". Elimination of the cusp provides for deeper rear spar depth and easier construction, better control effectiveness and lighter control forces.
Typical "Cusp" on Trailing Edge
Harry's airfoils have several outstanding features that he has specifically designed into them. The first is the elimination of high pitching moment caused by excessive movement of the center of pressure fore and aft on the airfoil. Some airfoils have movement of the pitching moment so severe that at low angles of attack the center of pressure is very near the trailing edge. To make it easier for calculations, the old center of pressure movement was translated to coefficient of moment in modern airfoil notation. However, the relationship between center of pressure movement and coefficient of moment as
used today is clearly explained in Harry's book. This tendency of the
center of pressure to move around causes the trim drag at the elevator and horizontal stabilizer to vary considerably and it is an important factor in airfoil
% Chord RA MAY 1001
100
selection.
Ben Owen
What Harry did was to calculate (for an average aircraft) trim drag and add it into the overall drag of the airfoil. By this process it is easy to see that the airfoil you might select for low coefficient of drag is not the airfoil that you would select for total drag reduction. Many of these low drag airfoils have excessive trim drag which raises the total drag higher than you would experience with one of Harry's airfoils designed for minimum movement of the center of pressure or low coefficient of moment. This concept of trim drag is a highly important one and rarely is addressed in other literature available to and understandable by a layman. The below camber or mean line drawing is exaggerated in the vertical dimension but you can easily compare the camber or mean line profile. This particular camber or mean line loads the airfoil to 50% of the chord and is designated as a (a = .5) mean line airfoil. Riblett Mean Line 0/0
Camber
L.E.
Camber or Mean Line
T.E.
The system he used to analyze and develop the airfoils is the computer program "Airfoil ii" known as the "Eppler Program." The appendix to Harry's recent book on "GA Airfoils" also has an added portion covering ultralight airfoils, airfoils for fabric wings and airfoils for low to high performance from 12% to 18% thick. In addition to marketing the book (see ad in classified section), he also provides assistance to designers and invaluable free advice. You may contact Harry Riblett at 416 Riblett Lane, Wilmington, DE 19808, 302/994-0479.
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